Getting Started

pathogen-lang is a language that extends SVG path syntax with variables, expressions, control flow, and functions. It compiles to standard SVG path data that works in any browser or graphics application.

Your First Path

Try this simple example in the playground:

define ViewBox(0, 0, 200, 200);
define default PathLayer('main-path-layer') ${
  fill: #bbb;
  stroke: #222;
  stroke-width: 1;
};

// A simple rectangle using variables
let size = 50
let x = 10
let y = 10

M x y
h size
v size
h calc(-size)
Z

Every Pathogen program starts with a define ViewBox declaring the canvas, and typically one or more layer definitions describing how strokes and fills should look.

This creates a rectangle by:

1. Moving to position (10, 10)
2. Drawing a horizontal line of length 50
3. Drawing a vertical line of length 50
4. Drawing a horizontal line back
5. Closing the path

Why pathogen-lang?

SVG paths are powerful but writing them by hand is tedious:

// Standard SVG - repetitive coordinates
M 20 20 L 80 20 L 80 80 L 20 80 Z
M 100 20 L 160 20 L 160 80 L 100 80 Z
M 180 20 L 240 20 L 240 80 L 180 80 Z

With pathogen-lang, you can use variables and loops:

// pathogen-lang - DRY and readable
let size = 60
for (i in 0..3) {
  rect(calc(20 + i * 80), 20, size, size)
}

Key Features

Variables

Store and reuse values:

let width = 200
let height = 100
let centerX = calc(width / 2)

Expressions with calc()

Use math in path commands:

let r = 50
M calc(100 - r) 100
L calc(100 + r) 100

Loops

Repeat patterns easily:

for (i in 0..10) {
  circle(calc(20 + i * 30), 100, 10)
}

Functions

Define reusable shapes:

fn square(x, y, size) {
  rect(x, y, size, size)
}

square(10, 10, 50)
square(70, 10, 50)

Built-in Shapes

Common shapes are included:

circle(100, 100, 50)
rect(10, 10, 80, 60)
polygon(100, 100, 40, 6)  // hexagon
star(100, 100, 50, 25, 5)

Next Steps

• Syntax Reference - Learn all the language features
• Standard Library - Explore built-in functions
• Examples - See practical patterns and recipes

Syntax Reference

pathogen-lang is a superset of SVG path syntax that adds variables, expressions, control flow, functions, and path blocks.

Path Commands

All standard SVG path commands are supported:

Uppercase commands use absolute coordinates; lowercase use relative coordinates.

M 0 0 L 100 100 Z

Variables

Declare variables with let:

let width = 200;
let height = 100;
let centerX = 100;

Use variables directly in path commands:

let x = 50;
let y = 75;
M x y L 100 100

Note: Single letters that are path commands (M, L, C, etc.) cannot be used as variable names.

Strings and Template Literals

String values use double quotes:

let name = "World";

Template literals use backticks with ${expression} interpolation:

let greeting = `Hello ${name}!`;          // "Hello World!"
let msg = `Score: ${2 + 3}`;             // "Score: 5"
let pos = `(${ctx.position.x}, ${ctx.position.y})`;

Template literals are the sole string construction mechanism — the + operator stays strictly numeric. String equality works with == and !=:

let mode = "dark";
if (mode == "dark") { /* ... */ }
if (mode != "light") { /* ... */ }

.length

Returns the number of characters in the string:

let str = `Hello`;
log(str.length);  // 5

.empty()

Returns 1 (truthy) if the string has no characters, 0 (falsy) otherwise:

let str = ``;
if (str.empty()) {
  // string is empty
}

Index Access

Access individual characters by zero-based index using [expr]:

let str = `Hello`;
let first = str[0];   // "H"
let last = str[4];     // "o"

Out-of-bounds access throws an error.

.split()

Splits a string into an array of individual characters:

let str = `abc`;
let chars = str.split();  // ["a", "b", "c"]
for (ch in chars) {
  log(ch);
}

.append(value)

Returns a new string with the given value appended to the end:

let str = `Hello`;
let result = str.append(` World`);  // "Hello World"

.prepend(value)

Returns a new string with the given value prepended to the beginning:

let str = `World`;
let result = str.prepend(`Hello `);  // "Hello World"

.includes(substring)

Returns 1 (truthy) if the string contains the given substring, 0 (falsy) otherwise:

let str = `Hello World`;
if (str.includes(`World`)) {
  // found it
}

.slice(start, end)

Returns a substring from start (inclusive) to end (exclusive). Negative indices count from the end:

let str = `Hello World`;
let sub = str.slice(0, 5);    // "Hello"
let end = str.slice(6, 11);   // "World"
let last3 = str.slice(-3, 11); // "rld"

Color Literals

Hex color codes and CSS color functions are first-class expressions:

let c = #cc0000;                      // 6-digit hex
let c = #f00;                         // 3-digit shorthand
let c = #cc000080;                    // 8-digit with alpha
let c = rgb(255, 0, 0);              // CSS color function
let c = hsl(0, 100%, 50%);           // % is literal inside parens
let c = oklch(0.6 0.15 30);          // any CSS color space
let lighter = (#cc0000).lighten(20%); // method chaining via parens

See the Color documentation for full details.

Percent Suffix

The % suffix converts a number to a fraction: 50% becomes 0.5.

let half = 50%;          // 0.5
let third = 33.3%;       // 0.333
let c = (#ff0000).lighten(20%);  // lighten by 0.2

Disambiguation: 20% (no space) is a percent literal (= 0.2). 20 % 5 (with spaces) is the modulus operator (= 0).

Expressions with calc()

For mathematical expressions, wrap them in calc():

let r = 50;
M calc(100 - r) 100
L calc(100 + r) 100

Supported Operators

Operator precedence follows standard mathematical conventions.

Style Blocks

Style blocks are CSS-like key-value maps wrapped in ${ }. They're used for layer styles but are also first-class values — you can store them in variables, merge them, and read their properties.

Literals

let styles = ${
  stroke: #cc0000;
  stroke-width: 3;
  fill: none;
};

Each property is a name: value; declaration. Values are try-evaluated as expressions — if the value parses as a valid expression (like a variable reference or calc()), its result is used. Otherwise the raw string is kept (e.g., rgb(...), #hex).

Merge (<<)

The << operator merges two values of the same type. The right side overrides the left on key conflicts:

// Style blocks
let base = ${ stroke: red; stroke-width: 2; };
let merged = base << ${ stroke-width: 4; fill: blue; };
// Result: stroke: red, stroke-width: 4, fill: blue

// Objects
let a = { x: 1, y: 2 };
let b = a << { y: 99, z: 3 };
// Result: {x: 1, y: 99, z: 3}

Multiple merges can be chained: a << b << c. See also Objects — Merging.

Property Access

Use dot notation with camelCase names to read kebab-case properties:

let s = ${ stroke-width: 4; };
let sw = s.strokeWidth;  // "4" (reads 'stroke-width')

Property values are always strings.

Usage in Layers

Style blocks are used in layer definitions and can be passed as per-element styles on text() and tspan(). See Layers for full details.

Null

The null literal represents the absence of a value. It is returned by pop() and shift() on empty arrays, and can be used in variable assignments and conditionals.

let x = null;

Truthiness

null is falsy in conditionals:

let x = null;
if (x) {
  // not reached
} else {
  M 0 0  // this branch runs
}

Equality

null is only equal to itself:

if (x == null) { /* x is null */ }
if (x != null) { /* x has a value */ }

null == 0 evaluates to 0 (false) — null is distinct from zero.

Error Behavior

Using null in arithmetic or as a path argument throws a descriptive error:

let x = null;
let y = x + 1;     // Error: Cannot use null in arithmetic expression
M x 0               // Error: Cannot use null as a path argument

Booleans

The true and false keywords represent boolean values. They are a semantic subtype of number — true is 1, false is 0 — but display as true/false in logs and template literals.

let flag = true;
let check = false;

Numeric Equivalence

Booleans participate in arithmetic as their numeric values:

true + 1     // 2
true + true  // 2
false + 1    // 1
true == 1    // true
false == 0   // true

Display

Booleans display as true or false, and comparisons return booleans:

log(true);       // true
log(5 > 3);      // true
log(1 > 5);      // false
log(`${true}`);  // true

Truthiness

false is falsy (like 0 and null); true is truthy:

if (true) { /* runs */ }
if (false) { /* skipped */ }

let result = 5 > 3;  // true (BooleanValue)
if (result) { /* runs */ }

Logical Operators

!true         // false
!false        // true
true && false // false
false || true // true

Arc Flags

Booleans can be used directly as arc flag arguments, converting to 1/0 in the SVG output:

let largeArc = true;
let sweep = false;
M 0 0 A 50 50 0 largeArc sweep 100 0
// → M 0 0 A 50 50 0 1 0 100 0

Enums

Built-in Enums

Pathogen provides built-in enums for gradient and geometry properties. Enum members resolve to the string values accepted by these properties:

topo.easing = Easing.Smoothstep;       // equivalent to 'smoothstep'
grad.interpolation = Interpolation.OKLCH;

Enum values are interchangeable with their string equivalents:

Easing.Linear == 'linear'  // true

User-Defined Enums

Define custom enums with enum:

// Auto-valued — member name lowercased to a string
enum Symmetry { None, Bilateral, Radial, Rotational }
log(Symmetry.Bilateral);  // bilateral

// Explicit string values
enum Season { Spring = 'vernal', Summer = 'estival' }

// Explicit typed values — number, angle, color, boolean
enum Angle { Quarter = 90deg, Half = 180deg, Full = 360deg }
enum Palette { Primary = #0066ff, Accent = #ff6600, Muted = #999 }
enum Weight { Thin = 1, Normal = 2, Bold = 4 }
enum Toggle { On = true, Off = false }

Auto-valued members always produce the lowercase string of the member name. Other types require an explicit = value.

Enum members are accessed with dot notation and can be used in conditionals:

let d = Dir.Up;
if (d == 'up') { M 10 20 }

Points

Points represent 2D coordinates and provide geometric operations for SVG path construction.

Constructor

Create a point with Point(x, y):

let center = Point(200, 200);
let origin = Point(0, 0);

Properties

let pt = Point(100, 200);
M pt.x pt.y           // M 100 200
L calc(pt.x + 10) pt.y  // L 110 200

Methods

All angles are in radians, consistent with the standard library.

.translate(dx, dy)

Returns a new point offset by the given deltas:

let pt = Point(100, 100);
let moved = pt.translate(10, -20);  // Point(110, 80)

.polarTranslate(angle, distance)

Returns a new point offset by angle and distance:

let pt = Point(100, 100);
let moved = pt.polarTranslate(0, 50);     // Point(150, 100)
let up = pt.polarTranslate(-0.5pi, 30);   // 30 units upward

.midpoint(other)

Returns the midpoint between two points:

let a = Point(0, 0);
let b = Point(100, 100);
let mid = a.midpoint(b);  // Point(50, 50)

.lerp(other, t)

Linear interpolation between two points. t=0 returns this point, t=1 returns the other:

let a = Point(0, 0);
let b = Point(100, 200);
let quarter = a.lerp(b, 0.25);  // Point(25, 50)

.rotate(angle, origin)

Rotates this point around a center point:

let pt = Point(100, 0);
let center = Point(0, 0);
let rotated = pt.rotate(90deg, center);  // Point(0, 100) approximately

.distanceTo(other)

Returns the Euclidean distance between two points:

let a = Point(0, 0);
let b = Point(3, 4);
log(a.distanceTo(b));  // 5

.angleTo(other)

Returns the angle in radians from this point to another:

let a = Point(0, 0);
let b = Point(1, 0);
log(a.angleTo(b));  // 0 (pointing right)

.offset(other)

Returns an object with dx and dy properties representing the vector from this point to other. Useful for applying the same relative displacement to multiple points:

let ref = Point(200, 200);
let target = Point(100, 300);
let off = ref.offset(target);
// off.dx = -100, off.dy = 100

// Apply the same offset to a different point
let other = Point(50, 75);
M calc(other.x + off.dx) calc(other.y + off.dy)

Display

log() shows points in a readable format:

let pt = Point(100, 200);
log(pt);  // Point(100, 200)

Template Literals

Points display as Point(x, y) when interpolated in template literals:

let pt = Point(42, 99);
let msg = `position: ${pt}`;  // "position: Point(42, 99)"

Arrays

Arrays hold ordered collections of values. Elements can be numbers, strings, style blocks, other arrays, or null.

Literals

let empty = [];
let nums = [1, 2, 3];
let mixed = [10, "hello", [4, 5]];

Spread (...)

Use the spread operator to expand an array's elements into another array literal:

let a = [1, 2, 3];
let b = [0, ...a, 4, 5];     // [0, 1, 2, 3, 4, 5]
let c = [...a, ...b];         // combine two arrays

Spread works anywhere inside an array literal and can be mixed with regular elements:

let head = [10, 20];
let tail = [40, 50];
let full = [...head, 30, ...tail];  // [10, 20, 30, 40, 50]

Index Access

Access elements by zero-based index using [expr]:

let list = [10, 20, 30];
let first = list[0];         // 10
let second = list[1];        // 20
M list[0] list[1]            // M 10 20

Out-of-bounds access throws an error.

.length

Returns the number of elements:

let list = [1, 2, 3];
log(list.length);  // 3

.empty()

Returns 1 (truthy) if the array has no elements, 0 (falsy) otherwise:

let list = [];
if (list.empty()) {
  // list is empty
}

Methods

.push(value)

Appends a value to the end. Returns the new length.

let list = [1, 2];
let len = list.push(3);  // list is now [1, 2, 3], len is 3

.pop()

Removes and returns the last element. Returns null if the array is empty.

let list = [1, 2, 3];
let last = list.pop();   // last is 3, list is now [1, 2]
let empty = [];
let x = empty.pop();     // x is null

.unshift(value)

Prepends a value to the start. Returns the new length.

let list = [2, 3];
list.unshift(1);  // list is now [1, 2, 3]

.shift()

Removes and returns the first element. Returns null if the array is empty.

let list = [1, 2, 3];
let first = list.shift();  // first is 1, list is now [2, 3]

.slice(start, end?)

Returns a new array containing elements from start to end (inclusive). Negative indexes count from the end. If end is omitted, returns from start to the end of the array.

Note: Array .slice() uses inclusive end indexes, while string .slice() uses exclusive end indexes (matching JavaScript string behavior).

let arr = [10, 20, 30, 40, 50];

let mid = arr.slice(1, 3);    // [20, 30, 40] — indices 1, 2, 3
let tail = arr.slice(3);      // [40, 50]     — from index 3 to end
let last2 = arr.slice(-2);    // [40, 50]     — last 2 elements
let head = arr.slice(0, -2);  // [10, 20, 30, 40] — up to second-to-last

.map {|item| ... } / .map {|item, index, arrayRef| ... }

Transforms each element using a trailing block, returning a new array. Use return to specify the mapped value. If no return is executed, the element maps to null.

The block receives up to three parameters:

• item — the current element
• index (optional) — the zero-based index
• arrayRef (optional) — a reference to the original array

let prices = [10, 25, 50];
let doubled = prices.map {|price|
  return calc(price * 2);
};
// doubled is [20, 50, 100]

// Block body supports full language features
let labels = [1, 2, 3].map {|n|
  let prefix = `item-`;
  return `${prefix}${n}`;
};
// labels is ["item-1", "item-2", "item-3"]

Use the index parameter for position-aware transforms:

let items = [10, 20, 30];
let indexed = items.map {|val, i|
  return calc(val + i);
};
// indexed is [10, 21, 32]

Use the array reference for look-ahead or look-behind:

let arr = [1, 2, 3, 4];
let pairs = arr.map {|item, idx, ref|
  if (idx < ref.length - 1) {
    return calc(item + ref[idx + 1]);
  }
  return item;
};
// pairs is [3, 5, 7, 4]

The block has access to variables from the enclosing scope:

let offset = 100;
let shifted = [1, 2, 3].map {|x|
  return calc(x + offset);
};
// shifted is [101, 102, 103]

.reduce(initialValue) {|accumulator, item, index, arrayRef| ... }

Iterates the array, threading an accumulator through each step. The initialValue argument sets the starting accumulator. The block must return the new accumulator value; if no return is executed, the accumulator becomes null.

The block receives up to four parameters:

• accumulator — the current accumulated value
• item (optional) — the current element
• index (optional) — the zero-based index
• arrayRef (optional) — a reference to the original array

let sum = [1, 2, 3, 4].reduce(0) {|acc, n|
  return calc(acc + n);
};
// sum is 10

let csv = ['a', 'b', 'c'].reduce('') {|acc, s, i|
  if (i == 0) { return s; }
  return `${acc},${s}`;
};
// csv is "a,b,c"

On an empty array, reduce returns initialValue unchanged:

let result = [].reduce(42) {|acc, n| return calc(acc + n); };
// result is 42

.mapSlice(length)

Returns a new array where each element is a sub-array (slice) of length elements starting at that element's index. Near the end of the array, slices are shorter as they extend past the bounds.

let arr = [1, 2, 3, 4];
let slices = arr.mapSlice(2);
// slices is [[1, 2], [2, 3], [3, 4], [4]]

let triples = [10, 20, 30, 40, 50].mapSlice(3);
// triples is [[10, 20, 30], [20, 30, 40], [30, 40, 50], [40, 50], [50]]

Reference Semantics

Arrays are passed by reference. Mutations through one binding are visible through all others:

let a = [1, 2, 3];
let b = a;
b.push(4);
log(a.length);  // 4 — same underlying array

For-Each Iteration

Iterate over array elements with for (item in list):

let points = [10, 20, 30];
for (p in points) {
  M p 0
}
// Produces: M 10 0 M 20 0 M 30 0

Destructure to get both item and index with for ([item, index] in list):

let sizes = [5, 10, 15];
for ([size, i] in sizes) {
  circle(calc(i * 40 + 20), 50, size)
}

Iterating over an empty array produces no output.

Destructuring

Extract array elements into individual variables with destructuring in let declarations:

let [a, b, c] = [1, 2, 3];
log(a);  // 1
log(b);  // 2
log(c);  // 3

If the array has more elements than bindings, extras are silently ignored:

let [first, second] = [10, 20, 30, 40];
// first is 10, second is 20 — 30 and 40 ignored

If the array has fewer elements than bindings, missing values are null:

let [x, y, z] = [1, 2];
// x is 1, y is 2, z is null

Use the rest pattern (...name) to collect remaining elements into a new array:

let [head, ...tail] = [1, 2, 3, 4, 5];
// head is 1, tail is [2, 3, 4, 5]

let [only, ...rest] = [42];
// only is 42, rest is []

The rest pattern must be the last binding in the destructuring pattern.

Angle Units

Numbers can have angle unit suffixes for convenience:

let angle = 90deg;
M sin(45deg) cos(45deg)

// Equivalent to:
let angle = rad(90);
M sin(rad(45)) cos(rad(45))

The pi suffix multiplies the number by π. This is especially convenient for polar coordinates and angles expressed as fractions of π:

let quarter = 0.25pi;   // π/4
let half = 0.5pi;       // π/2
let full = 2pi;          // 2π
M sin(0.25pi) cos(0.25pi)

The pi suffix participates in angle unit mismatch checking: calc(0.25pi + 5) throws an error, while calc(90deg + 0.5pi) is allowed (both have angle units).

Note: The pi suffix only works on numeric literals. For expressions or variables, use mpi(x) (see Standard Library).

For Loops

Repeat path commands with for:

for (i in 0..10) {
  L calc(i * 20) calc(i * 10)
}

The range 0..10 includes both endpoints (0 through 10, giving 11 iterations).

Descending Ranges

Ranges automatically count down when start > end:

// Countdown from 5 to 1
for (i in 5..1) {
  M calc(i * 20) 0
}
// Produces: M 100 0 M 80 0 M 60 0 M 40 0 M 20 0

Nested Loops

for (row in 0..2) {
  for (col in 0..2) {
    circle(calc(col * 50 + 25), calc(row * 50 + 25), 10)
  }
}

This creates a 3x3 grid (rows 0, 1, 2 and cols 0, 1, 2).

Conditionals

Use if, else if, and else for conditional path generation:

let size = 100;

if (size > 75) {
  M 0 0 L 100 100
} else if (size > 50) {
  M 0 0 L 75 75
} else {
  M 0 0 L 50 50
}

You can chain as many else if blocks as needed. Comparison results are numeric: 1 for true, 0 for false.

Functions

Defining Functions

Create reusable path generators with fn:

fn square(x, y, size) {
  rect(x, y, size, size)
}

Calling Functions

square(10, 10, 50)
square(70, 10, 50)

Functions can call other functions and use all language features.

Comments

Line comments start with //:

// This is a comment
let x = 50;  // inline comment
M x 0

Path Context (ctx)

When using compileWithContext(), a ctx object tracks the current drawing state:

M 10 20
L 30 40
L calc(ctx.position.x + 10) ctx.position.y  // L 40 40

ctx Properties

How Position Updates

• M/m: Sets position and subpath start
• L/l, H/h, V/v: Updates position to endpoint
• C/c, S/s, Q/q, T/t: Updates position to curve endpoint
• A/a: Updates position to arc endpoint
• Z/z: Returns to subpath start

Lowercase (relative) commands add to current position; uppercase (absolute) set it directly.

log() Function

Use log() to inspect the context during evaluation:

M 10 20
log(ctx)           // Logs full context as JSON
log(ctx.position)  // Logs just position object
log(ctx.position.x) // Logs just the x value
L 30 40

The logs are captured in the logs array returned by compileWithContext().

Example: Drawing Relative to Current Position

M 100 100
L 150 150
// Continue from current position
L calc(ctx.position.x + 50) ctx.position.y
L ctx.position.x calc(ctx.position.y + 50)
Z

Complete Example

// Draw a grid of circles with varying sizes
let cols = 5;
let rows = 5;
let spacing = 40;

for (row in 0..rows) {
  for (col in 0..cols) {
    let x = calc(col * spacing + 20);
    let y = calc(row * spacing + 20);
    let r = calc(5 + col + row);
    circle(x, y, r)
  }
}

Standard Library Reference

pathogen-lang includes built-in functions for math operations and common SVG shapes.

Math Functions

Trigonometry

All trigonometric functions use radians.

// Draw a point on a circle
let angle = 0.5;
let r = 50;
M calc(100 + cos(angle) * r) calc(100 + sin(angle) * r)

Angle Conversion

// Use degrees instead of radians
let angle = rad(45);
M calc(cos(angle) * 50) calc(sin(angle) * 50)

Exponential & Logarithmic

Rounding

Utility

Interpolation & Clamping

// Interpolate between two positions
let t = 0.5;
M calc(lerp(0, 100, t)) calc(lerp(0, 50, t))

// Clamp a value
let x = clamp(150, 0, 100);  // Result: 100

Constants

// Draw a semicircle
let r = 50;
for (i in 0..20) {
  let angle = calc(i / 20 * PI());
  L calc(100 + cos(angle) * r) calc(100 + sin(angle) * r)
}

Random

Note: Random functions are not deterministic. Each call produces a different value.

Cycler

A Cycler wraps an array and cycles through it sequentially via .pick(), returning to the beginning after reaching the end. Useful for deterministic round-robin assignment of colors, layer names, styles, etc.

Cycler(array, shuffle?)

Creates a cycler from an array. If the optional shuffle argument is truthy, the array is shuffled once at construction (the shuffled order is stable across all cycles).

let c = Cycler(['red', 'green', 'blue']);
c.pick()  // 'red'
c.pick()  // 'green'
c.pick()  // 'blue'
c.pick()  // 'red' (wraps around)

// Shuffled cycler — stable order across wraps
let r = Cycler(['a', 'b', 'c'], true);

.pick()

Returns the next element in the cycle, advancing the internal index. Wraps around to the beginning after the last element.

.length

Returns the number of items in the cycler.

let c = Cycler([1, 2, 3]);
log(c.length);  // 3

PolarVector

A PolarVector represents a direction and distance in polar coordinates. It is used to define bezier control point positions relative to anchor points — you specify "which direction and how far" rather than computing absolute x, y coordinates.

PolarVector(angle, distance)

Creates a polar vector. Angle is in radians (use rad() or deg suffix for degrees).

let pv = PolarVector(0.25 * PI(), 30);
let pv2 = PolarVector(rad(45), 30);      // equivalent

.angle

Returns the angle in radians.

.distance

Returns the distance.

.turn(deltaAngle)

Returns a new PolarVector with the angle rotated by deltaAngle. Distance is unchanged.

let pv = PolarVector(0, 20);
let turned = pv.turn(0.5 * PI());  // angle is now π/2, distance still 20

.scale(factor)

Returns a new PolarVector with the distance multiplied by factor. Angle is unchanged.

let pv = PolarVector(0, 20);
let wider = pv.scale(1.5);  // angle still 0, distance is now 30

.mirror()

Returns a new PolarVector with the angle rotated by π (180°). Distance is unchanged. This is the key operation for achieving C1 (smooth) continuity when chaining bezier curves — the outgoing handle mirrors the incoming handle.

let pv = PolarVector(0.25 * PI(), 20);
let mirrored = pv.mirror();  // angle is now 1.25π, distance still 20

Path Functions

These functions generate complete path segments.

circle(cx, cy, r)

Draws a circle centered at (cx, cy) with radius r.

circle(100, 100, 50)

Output: A full circle using two arc commands.

rect(x, y, width, height)

Draws a rectangle.

rect(10, 10, 80, 60)

roundRect(x, y, width, height, radius)

Draws a rectangle with rounded corners.

roundRect(10, 10, 80, 60, 10)

polygon(cx, cy, radius, sides)

Draws a regular polygon.

polygon(100, 100, 50, 6)  // Hexagon
polygon(100, 100, 50, 8)  // Octagon

star(cx, cy, outerRadius, innerRadius, points)

Draws a star shape.

star(100, 100, 50, 25, 5)  // 5-pointed star

line(x1, y1, x2, y2)

Draws a line segment.

line(0, 0, 100, 100)

arc(rx, ry, rotation, largeArc, sweep, x, y)

Draws an arc to (x, y). This is a direct wrapper around the SVG A command.

M 50 100
arc(50, 50, 0, 1, 1, 150, 100)

quadratic(x1, y1, cx, cy, x2, y2)

Draws a quadratic bezier curve from (x1, y1) to (x2, y2) with control point (cx, cy).

quadratic(0, 100, 50, 0, 100, 100)

cubic(x1, y1, c1x, c1y, c2x, c2y, x2, y2)

Draws a cubic bezier curve.

cubic(0, 100, 25, 0, 75, 0, 100, 100)

polarCubicBezier(start, pv1, pv2, end)

Draws a cubic bezier curve where control points are defined as polar vectors relative to the start and end points. start and end are Point values; pv1 and pv2 are PolarVector values.

• pv1 — direction and distance from start to the first control point
• pv2 — direction and distance from end to the second control point

let a = Point(0, 100);
let b = Point(100, 100);
polarCubicBezier(a, PolarVector(rad(-60), 40), PolarVector(rad(-120), 40), b)

Output: m (relative move) followed by c (relative cubic) — matches the spline function convention.

PolarVector methods compose naturally for handle manipulation:

let handle = PolarVector(rad(-45), 30);
// Symmetric curve: mirror the handle for the other end
polarCubicBezier(a, handle, handle.mirror(), b)

// Wider version: scale the handle distance
polarCubicBezier(a, handle.scale(1.5), handle.mirror().scale(1.5), b)

moveTo(x, y)

Returns a move command. Useful inside functions.

moveTo(50, 50)

lineTo(x, y)

Returns a line command.

lineTo(100, 100)

closePath()

Returns a close path command.

closePath()

cubicSpline(points)

Draws a chain of cubic bezier curves with explicit tangent angle and handle length at each point. Adjacent curves share a common tangent direction at join points, guaranteeing G1 (smooth) continuity.

Point schema:

cubicSpline([
  { x: 0, y: 100, angle: 0, exit: 30 },
  { x: 50, y: 0, angle: 0, entry: 20, exit: 25 },
  { x: 100, y: 100, angle: 0, entry: 30 }
])

Output: m (relative move) followed by one c (relative cubic) command per segment. A single-point array emits only m. All spline functions use relative commands so they work naturally inside path blocks.

quadSpline(start, points, end)

Draws a chain of quadratic bezier curves with implicit angle derivation. Only the start point specifies an explicit angle; intermediate points derive their tangent angle from the geometry of the previous control point.

Start: { x, y, angle, exit } Intermediate: { x, y, exit } End: { x, y }

quadSpline(
  { x: 0, y: 0, angle: 0, exit: 30 },
  [{ x: 60, y: 0, exit: 30 }],
  { x: 120, y: 0 }
)

Output: m followed by one q (relative quadratic) command per segment.

clippedQuadSpline(start, points, end)

Extends quadSpline by splitting the implicit shared control point into two cubic control points using time-based fractions (exitTime/entryTime). This allows dampening curve eccentricity while preserving the quadratic geometry.

Start: { x, y, angle, exit, exitTime } Intermediate: { x, y, exit, exitTime, entryTime } End: { x, y, entryTime }

• exitTime = 1, entryTime = 1: mathematically equivalent to quadratic
• exitTime = 0.5, entryTime = 0.5: control points at half arm length (moderate dampening)
• exitTime = 0, entryTime = 0: linear segments

clippedQuadSpline(
  { x: 0, y: 0, angle: 0, exit: 100, exitTime: 0.5 },
  [],
  { x: 200, y: 0, entryTime: 0.5 }
)

Output: m followed by one c (relative cubic) command per segment — not q.

Grid Functions

These functions generate complete grid patterns as path segments. Each accepts a GridPatternType enum (or string) that controls the visual style:

Not to be confused with the Grid() constructor — that's a data container for 2D values mapped to canvas coordinates (flow fields, heatmaps, sampling). The functions below produce SVG path data for visual lattices.

squareGrid(type, x, y, width, height, cellSize)

Generates a square grid pattern within the bounding rectangle starting at (x, y).

• type — GridPatternType enum value or string ('shape', 'dot', 'intersection', 'partial')
• x, y — Top-left origin of the grid
• width, height — Bounding dimensions
• cellSize — Side length of each square cell

The grid contains floor(width / cellSize) columns and floor(height / cellSize) rows. Extra space is ignored.

gridLayer.apply {
  squareGrid(GridPatternType.Shape, 0, 0, 200, 200, 20);
}

triangleGrid(type, x, y, width, height, cellSize)

Generates an equilateral triangle grid. cellSize is the triangle height (altitude). Triangles have flat bases with alternating up/down orientation.

gridLayer.apply {
  triangleGrid(GridPatternType.Shape, 0, 0, 200, 200, 20);
}

The triangle side length is derived from the height: side = 2 * cellSize / sqrt(3).

hexagonGrid(type, x, y, width, height, cellSize, orientation?)

Generates a hexagonal grid. cellSize is the flat-to-flat height of each hexagon.

• orientation — Optional. HexagonOrientation.Edge (default, flat-top) or HexagonOrientation.Vertex (pointy-top)

// Flat-top hexagons (default)
gridLayer.apply {
  hexagonGrid(GridPatternType.Shape, 0, 0, 200, 200, 20);
}

// Pointy-top hexagons
gridLayer.apply {
  hexagonGrid(GridPatternType.Shape, 0, 0, 200, 200, 20, HexagonOrientation.Vertex);
}

Usage with Layers and Transforms

Grid functions return path data and are typically used inside layer.apply {} blocks. Rotation and styling are handled via the layer:

let gridStyles = ${ stroke: #88f; stroke-width: 0.25; fill: none; };
let gridLayer = PathLayer('grid') << gridStyles;

gridLayer.ctx.transform.rotate.set(0.125pi);
gridLayer.apply {
  squareGrid(GridPatternType.Partial, 0, 0, 400, 400, 20);
}

A convenience wrapper for one-line grid drawing:

fn drawGridToLayer(layer, gridFn, type, angle, x, y, w, h, s) {
  layer.ctx.transform.rotate.set(angle);
  layer.apply { gridFn(type, x, y, w, h, s); }
}

Context-Aware Functions

These functions use the current path context (position, tangent direction) to generate path segments. They maintain path continuity and are ideal for building complex shapes programmatically.

Polar Movement

polarPoint(angle, distance)

Returns a point at a polar offset from current position. Does not emit any path commands.

M 100 100
let p = polarPoint(0, 50);
L p.x p.y  // Line to (150, 100)

polarOffset(angle, distance)

Returns {x, y} coordinates at a polar offset. Similar to polarPoint.

polarMove(angle, distance)

Emits a line command (L) moving in the specified direction. Updates position but draws a visible line.

M 100 100
polarMove(0, 50)  // Draws line to (150, 100)

polarLine(angle, distance)

Emits a line command (L) in the specified direction. Same as polarMove.

M 100 100
polarLine(45deg, 70.7)  // Draws line diagonally

Arc Functions

arcFromCenter(dcx, dcy, radius, startAngle, endAngle, clockwise)

Draws an arc defined by center offset and angles. Returns {point, angle} with endpoint and tangent.

• dcx, dcy: Offset from current position to arc center
• radius: Arc radius
• startAngle, endAngle: Start and end angles in radians
• clockwise: 1 for clockwise, 0 for counter-clockwise

Warning: If current position doesn't match the calculated arc start point, a line segment (L) will be drawn to the arc start. For guaranteed continuous arcs, use arcFromPolarOffset.

M 50 50
arcFromCenter(50, 0, 50, 180deg, 270deg, 1)
// Center at (100, 50), arc from (50, 50) to (100, 100)

arcFromPolarOffset(angle, radius, angleOfArc)

Draws an arc where the center is at a polar offset from current position. The current position is guaranteed to be on the circle, so only an A command is emitted (no M or L). Returns {point, angle} with endpoint and tangent.

• angle: Direction from current position to arc center (radians)
• radius: Arc radius
• angleOfArc: Sweep angle (positive = clockwise, negative = counter-clockwise)

This function is ideal for creating continuous curved paths because it never emits extra line segments.

M 100 100
arcFromPolarOffset(0, 50, 90deg)
// Center at (150, 100), sweeps 90° clockwise
// Ends at (150, 50)

Comparison with arcFromCenter:

Heading Control

Angles follow SVG coordinate conventions: 0 is rightward, positive angles rotate clockwise (toward the positive y-axis, which points down in SVG).

heading(angle)

Sets the heading to an absolute angle. No command is emitted and the cursor does not move. This enables tangentArc and tangentLine immediately after M without needing a dummy segment like h 0.01.

M 50 100
heading(0)           // Set heading to rightward
tangentArc(20, 90deg) // Works immediately — no dummy segment needed

Inside path blocks, heading() avoids the offset artifacts that h 0.01 causes with z closePath:

let cLike = @{
  heading(0)
  tangentArc(20, 90deg)
  tangentArc(20, -90deg)
  z  // Closes cleanly to start — no tiny offset
};

turn(delta)

Adds delta to the current heading (relative change). Requires an existing heading — either from heading() or from a previous drawing command. Negative deltas turn counter-clockwise.

M 50 100
heading(0)          // Start heading rightward
turn(90deg)         // Now heading downward
tangentLine(30)     // Draws 30px down

After drawing commands:

M 0 0  L 50 0      // Heading is 0 (rightward)
turn(45deg)         // Heading is now 45°
tangentLine(20)     // Continues at 45°

ctx.heading

The current heading (read-only), readable via the context object. Set by heading(), turn(), or any drawing command that establishes direction. M (moveTo) clears the heading.

M 0 0  L 50 0
log(ctx.heading)   // 0 (rightward)
heading(90deg)
log(ctx.heading)   // π/2 (downward)
M 200 200
log(ctx.heading)   // undefined (M clears the heading)

Tangent Functions

These functions continue from the current heading. Any path command that establishes a direction — including native SVG commands (L, H, V, C, S, Q, T, A, Z) and stdlib path functions — sets a heading that tangentLine and tangentArc can follow.

You can also set the heading explicitly with heading(), adjust it with turn(), or read it via ctx.heading.

M (moveTo) clears the heading since a move does not establish a direction.

tangentLine(length)

Draws a line continuing in the tangent direction from the previous command.

arcFromPolarOffset(0, 50, 90deg)
tangentLine(30)  // Continues in the arc's exit direction

After native SVG commands:

M 50 100  L 150 100
tangentLine(30)  // Continues rightward to (180, 100)

tangentArc(radius, sweepAngle)

Draws an arc continuing tangent to the previous command.

arcFromPolarOffset(0, 50, 90deg)
tangentArc(30, 45deg)  // Smooth continuation with a smaller arc

After native SVG commands:

M 50 100  L 150 100
tangentArc(30, 90deg)  // Smooth arc curving down from the line's endpoint

Color

The Color type provides first-class color manipulation in OKLCH color space. See the full Color documentation for constructor forms, methods, properties, and examples.

let c = Color('#e63946');
let lighter = c.lighten(0.2);
let comp = c.complement();

CSSVar

The CSSVar type creates CSS custom property references (var()) for use in style blocks. See the full CSSVar documentation for constructor forms, properties, and examples.

let fg = CSSVar('--foreground', '#333');
define PathLayer('main') ${ stroke: fg; }

Using Functions Inside calc()

Math functions can be used inside calc():

M calc(sin(0.5) * 100) calc(cos(0.5) * 100)
L calc(lerp(0, 100, 0.5)) calc(clamp(150, 0, 100))

Path functions are called at the statement level:

circle(100, 100, calc(25 + 25))  // calc() inside arguments

ViewBox

Every Pathogen program renders into an SVG with a viewBox. The define ViewBox statement specifies that viewBox directly in code, making the program self-contained and reproducible across the CLI, playground, and VS Code preview.

Syntax

define ViewBox(originX, originY, width, height);

The four arguments become the SVG viewBox="originX originY width height" attribute on the root <svg> element. The root width and height attributes default to the same width and height values.

define ViewBox(0, 0, 200, 200);
M 50 50 L 150 150

Renders to <svg viewBox="0 0 200 200" width="200" height="200">…</svg>.

Arguments

Arguments are expressions and may use variables, calc(), or any other expression form:

let W = 400;
let H = 300;
define ViewBox(0, 0, W, H);

define ViewBox(0, 0, calc(100 * 4), calc(100 * 3));

Negative Origin

originX and originY may be negative — useful for centering geometry around (0, 0):

define ViewBox(-100, -100, 200, 200);
M -50 -50 L 50 50

Default ViewBox

If a program contains no define ViewBox statement, the viewBox defaults to 0 0 200 200.

Placement

define ViewBox is a top-level statement. It may appear anywhere among other top-level statements, but not inside a layer().apply { } block, a path block, or a text block.

// OK
define ViewBox(0, 0, 200, 200);
define default PathLayer('main') ${ stroke: #222; };
M 0 0 L 200 200

// Error: ViewBox must appear at top level
layer('main').apply {
  define ViewBox(0, 0, 200, 200);
  M 0 0
}

Errors

The compiler rejects:

• Duplicate define ViewBox — only one viewBox per program.
• Zero or negative width or height — these are invalid SVG dimensions.
• default modifier — define default ViewBox(…) is not allowed; only PathLayer and TextLayer accept default.
• Non-numeric arguments — every argument must evaluate to a finite number.

Precedence with the CLI

The CLI accepts --viewBox, --width, and --height flags. When the source contains a define ViewBox statement, the source wins; the CLI flags are used only when the source does not define a viewBox:

This lets inline -e snippets supply a viewBox via the CLI while persistent programs declare it in source.

Why source, not configuration

Storing viewBox in source code (rather than in workspace metadata, comments, or external configuration) keeps a program self-contained: copying the code anywhere reproduces the same image. It also lets editor tooling (completion, hover, formatting) reason about the viewBox the same way it reasons about any other statement.

Related

• Layers — the rest of the define family (PathLayer, TextLayer, GroupLayer)
• CLI — using --viewBox/--width/--height flags alongside source-defined viewBox

Layers

Layers let you output multiple <path> elements from a single program, each with its own styles and independent pen tracking.

See also define ViewBox for declaring the SVG viewBox — a sibling define-family statement that controls the canvas dimensions.

Defining Layers

Use define to create a named layer with a style block:

define PathLayer('outline') ${
  stroke: #cc0000;
  stroke-width: 3;
  fill: none;
}

Layer names must be unique strings. The style block uses CSS/SVG property syntax — any SVG presentation attribute works (stroke, fill, opacity, stroke-dasharray, etc.).

Breaking change: Style blocks now use ${ } syntax instead of { }. Update existing layer definitions: { stroke: red; } → ${ stroke: red; }.

Default Layer

Mark one layer as default to receive all bare path commands (commands outside any layer().apply block):

define default PathLayer('main') ${
  stroke: #333;
  stroke-width: 2;
  fill: none;
}

// These commands go to 'main' automatically
M 10 10
L 90 10
L 90 90
Z

Without a default layer, bare commands go to an implicit unnamed layer.

Writing to Layers

Use layer('name').apply { ... } to send commands to a specific layer:

define PathLayer('grid') ${
  stroke: #ddd;
  stroke-width: 0.5;
}

define default PathLayer('shape') ${
  stroke: #333;
  stroke-width: 2;
  fill: none;
}

// Draw a grid on the 'grid' layer
layer('grid').apply {
  for (i in 0..10) {
    M calc(i * 20) 0
    V 200
    M 0 calc(i * 20)
    H 200
  }
}

// These go to 'shape' (the default)
M 40 40
L 160 40
L 100 160
Z

Context Isolation

Each layer has its own pen position. Commands in one layer don't affect another layer's ctx:

define default PathLayer('a') ${ stroke: red; }
define PathLayer('b') ${ stroke: blue; }

M 100 100    // layer 'a' position: (100, 100)

layer('b').apply {
  M 50 50    // layer 'b' position: (50, 50)
}

// Back in layer 'a', position is still (100, 100)
L 200 200

Accessing Layer Context

Use layer('name').ctx to read a layer's pen state from anywhere:

define default PathLayer('main') ${ stroke: #333; }
define PathLayer('markers') ${ stroke: red; fill: red; }

M 50 50
L 150 80
L 100 150

// Draw markers at the main layer's current position
layer('markers').apply {
  let px = layer('main').ctx.position.x
  let py = layer('main').ctx.position.y
  circle(px, py, 4)
}

Available context properties:

Dynamic Layer Names

Layer names can be expressions, including variables:

let target = 'overlay'
define PathLayer(target) ${ stroke: blue; }

layer(target).apply {
  M 0 0 L 100 100
}

Dynamic Layer Creation

Layers can also be created as first-class values using PathLayer() and TextLayer() constructor expressions. This allows storing layers in variables, appending styles after creation, and using .apply { } directly on the variable.

Constructor Expression

let myLayer = PathLayer('unique-name') ${ stroke: red; fill: none; };
myLayer.apply { M 0 0 L 100 100 }

The style block is optional:

let myLayer = PathLayer('unique-name');
myLayer.apply { M 0 0 }

Style Mutation with <<

The << operator on a layer reference merges styles in place and returns the reference for chaining:

let l = PathLayer('outline');
l << ${ stroke: red; } << ${ fill: blue; };
l.apply { M 0 0 L 100 100 }
// l.styles: stroke: red, fill: blue

Explicit .styles Property

Read or replace a layer's styles via the .styles property:

let l = PathLayer('outline') ${ stroke: red; };

// Read: returns a StyleBlockValue copy
let s = l.styles;
log(s.stroke)  // "red"

// Write: replaces all styles
l.styles = l.styles << ${ fill: blue; };

TextLayer Constructor

let labels = TextLayer('labels') ${ font-size: 14; font-family: monospace; };
labels.apply { text(50, 45)`Start` }

Accessing Layer Properties

Dynamic layers support the same properties as layer() references:

Coexistence with define

Both approaches work together. The define syntax supports the default modifier; dynamic constructors do not:

define default PathLayer('main') ${ stroke: #333; fill: none; }
let overlay = PathLayer('overlay') ${ stroke: red; };

M 10 10 L 90 90          // goes to 'main' (default)
overlay.apply { M 50 50 L 60 60 }

// layer() function works for both:
layer('overlay').apply { M 70 70 }

Layers render in definition order regardless of how they were created.

Style Properties

Style properties map directly to SVG presentation attributes. Common properties:

Each property is a semicolon-terminated declaration:

define PathLayer('dashed') ${
  stroke: #0066cc;
  stroke-width: 2;
  stroke-dasharray: 8 4;
  fill: none;
}

Output Format

When using the JavaScript API, compile() returns a structured result:

import { compile } from 'pathogen-lang';

const result = compile(`
  define default PathLayer('bg') ${
    stroke: #ddd;
    fill: none;
  }
  define PathLayer('fg') ${
    stroke: #333;
    stroke-width: 2;
    fill: none;
  }

  M 0 0 H 100 V 100 H 0 Z

  layer('fg').apply {
    M 20 20 L 80 80
  }
`);

// result.layers is an array of LayerOutput:
// [
//   {
//     name: 'bg',
//     type: 'path',
//     data: 'M 0 0 H 100 V 100 H 0 Z',
//     styles: { stroke: '#ddd', fill: 'none' },
//     isDefault: true
//   },
//   {
//     name: 'fg',
//     type: 'path',
//     data: 'M 20 20 L 80 80',
//     styles: { stroke: '#333', 'stroke-width': '2', fill: 'none' },
//     isDefault: false
//   }
// ]

Programs without any define statements produce a single implicit layer:

compile('M 0 0 L 100 100').layers
// [{ name: 'default', type: 'path', data: 'M 0 0 L 100 100', styles: {}, isDefault: true }]

Full Example

A multi-layer illustration with a background grid, main shape, and annotation markers:

// Layer definitions
define PathLayer('grid') ${
  stroke: #e0e0e0;
  stroke-width: 0.5;
}

define default PathLayer('shape') ${
  stroke: #333333;
  stroke-width: 2;
  fill: none;
  stroke-linejoin: round;
}

define PathLayer('points') ${
  stroke: #cc0000;
  fill: #cc0000;
}

// Grid
layer('grid').apply {
  for (i in 0..10) {
    M calc(i * 20) 0  V 200
    M 0 calc(i * 20)  H 200
  }
}

// Shape (goes to default layer)
let cx = 100
let cy = 100
let r = 60
let sides = 6

for (i in 0..sides) {
  let angle = calc(i * 360 / sides - 90)
  let x = calc(cx + r * cos(radians(angle)))
  let y = calc(cy + r * sin(radians(angle)))
  if (i == 0) { M x y } else { L x y }
}
Z

// Mark each vertex
layer('points').apply {
  for (i in 0..sides) {
    let angle = calc(i * 360 / sides - 90)
    let x = calc(cx + r * cos(radians(angle)))
    let y = calc(cy + r * sin(radians(angle)))
    circle(x, y, 3)
  }
}

TextLayer

TextLayers produce SVG <text> elements instead of <path> elements.

Defining a TextLayer

define TextLayer('labels') ${
  font-size: 14;
  font-family: monospace;
  fill: #333;
}

text() — Two Forms

Inline form — simple text content:

layer('labels').apply {
  text(50, 45)`Start`
  text(150, 75, 30deg)`End`    // rotation uses angle units (deg/rad/pi)
}

Block form — mixed text runs and tspan children:

layer('labels').apply {
  text(10, 180) {
    `Hello `
    tspan(0, 0, 30deg)`world`
    ` and more`
  }
}

The block form maps to SVG's mixed content model: <text x="10" y="180">Hello <tspan rotate="30">world</tspan> and more</text>

Note: 30deg in the source becomes rotate="30" (degrees) in SVG output.

tspan() — Only Inside text() Blocks

tspan()`content`                   // no offset
tspan(dx, dy)`content`             // with offsets
tspan(dx, dy, 45deg)`content`      // with offsets and rotation

Position arguments (x, y, dx, dy) are plain numbers. Rotation follows the standard angle unit convention — bare numbers are radians, use deg/rad/pi suffixes for explicit units. Content is always a template literal.

Template Literals

Template literals use backtick syntax with ${expression} interpolation. They work everywhere — text content, log messages, variable values:

let name = "World"
let x = `Hello ${name}!`              // "Hello World!"
let msg = `Score: ${2 + 3}`           // "Score: 5"
log(`Position: ${ctx.position.x}`)    // in log messages

Template literals are the sole string construction mechanism — + stays strictly numeric. String equality (==/!=) works for conditionals:

let mode = "dark"
if (mode == "dark") { /* ... */ }
if (mode != "light") { /* ... */ }

TextLayer Output Format

const result = compile(`
  define TextLayer('labels') ${ font-size: 14; fill: #333; }
  layer('labels').apply {
    text(50, 45)\`Start\`
    text(10, 180) {
      tspan()\`Multi-\`
      tspan(0, 16)\`line\`
    }
  }
`);

// result.layers[0]:
// {
//   name: 'labels',
//   type: 'text',
//   data: 'Start Multi-line',
//   textElements: [
//     { x: 50, y: 45, children: [{ type: 'run', text: 'Start' }] },
//     { x: 10, y: 180, children: [
//       { type: 'tspan', text: 'Multi-' },
//       { type: 'tspan', text: 'line', dx: 0, dy: 16 },
//     ]},
//   ],
//   styles: { 'font-size': '14', fill: '#333' },
//   isDefault: false,
// }

Restrictions

• text() can only be used inside a layer().apply block targeting a TextLayer
• tspan() can only appear inside a text() { } block
• Path commands (M, L, etc.) cannot be used inside a TextLayer apply block
• If a TextLayer is the default layer, bare path commands will throw an error

Style Blocks

Style blocks are first-class values that can be stored in variables, merged, and accessed via dot notation.

Style Block Literals

let styles = ${
  stroke-dasharray: 0.01 20;
  stroke-linecap: round;
  stroke-width: 8.4;
};

Merge Operator (<<)

The << operator merges two style blocks, with the right side overriding the left:

let base = ${ stroke: red; stroke-width: 2; };
let merged = base << ${ stroke-width: 4; fill: blue; };
// merged has: stroke: red, stroke-width: 4, fill: blue

Property Access

Use dot notation with camelCase to read kebab-case properties:

let styles = ${ stroke-width: 4; };
let sw = styles.strokeWidth;  // reads 'stroke-width' → "4"

Expression Evaluation in Values

Style block values are try-evaluated: if a value parses and evaluates as an expression, its result is used. Otherwise the raw string is kept:

let dynamic = ${
  font-size: calc(12 + 15);       // evaluates to "27"
  stroke-width: randomRange(2, 8); // evaluates to a random number
  stroke: rgb(232, 74, 166);       // kept as raw string
  fill: #996633;                   // kept as raw string
};

Layer Definitions with Style Expressions

Layer definitions accept any expression that evaluates to a style block:

let baseStyles = ${ stroke: red; stroke-width: 2; };
define PathLayer('main') baseStyles << ${ fill: none; }

Per-Element Styles on Text and Tspan

Pass style blocks as the 4th argument to text() or tspan():

let bold = ${ font-weight: bold; };
layer('labels').apply {
  text(10, 20, 0, bold)`Hello`
  text(50, 80) {
    tspan(0, 0, 0, ${ fill: red; })`colored`
  }
}

Transforms

Apply SVG matrix transformations (translate, rotate, scale) at the layer level. Transforms are set via method calls on ctx.transform and rendered as SVG transform attributes on the output elements.

Translate

define PathLayer('shape') ${ stroke: #333; fill: none; }

layer('shape').ctx.transform.translate.set(50, 50)

layer('shape').apply {
  M 0 0 L 100 0 L 100 100 Z
}
// Output: <path d="..." transform="translate(50, 50)"/>

Rotate

Angles are in radians (consistent with polar commands). Use deg suffix for degrees:

layer('shape').ctx.transform.rotate.set(45deg)         // around origin
layer('shape').ctx.transform.rotate.set(45deg, 50, 50) // around (50, 50)

Scale

layer('shape').ctx.transform.scale.set(2, 2)             // uniform scale
layer('shape').ctx.transform.scale.set(2, 2, 50, 50)     // scale around (50, 50)

Reset

layer('shape').ctx.transform.translate.reset()  // clear translate only
layer('shape').ctx.transform.rotate.reset()     // clear rotate only
layer('shape').ctx.transform.scale.reset()      // clear scale only
layer('shape').ctx.transform.reset()            // clear all transforms

Read Access

layer('shape').ctx.transform.translate.x    // 0 if not set
layer('shape').ctx.transform.translate.y
layer('shape').ctx.transform.rotate.angle   // 0 if not set
layer('shape').ctx.transform.scale.x        // 1 if not set (default scale)
layer('shape').ctx.transform.scale.y        // 1 if not set

Default Context (No Layers)

When no layers are defined, use ctx.transform directly:

ctx.transform.translate.set(25, 25)
ctx.transform.rotate.set(45deg)
M 0 0 L 100 0

Inside Apply Blocks

Inside a layer().apply block, ctx refers to the active layer's context:

layer('shape').apply {
  ctx.transform.translate.set(10, 20)
  M 0 0 L 50 50
}

Combined Transforms

When multiple transforms are set, they are applied in SVG order: translate → rotate → scale (translate applied last visually):

layer('shape').ctx.transform.translate.set(10, 20)
layer('shape').ctx.transform.rotate.set(90deg)
layer('shape').ctx.transform.scale.set(2, 2)
// Output: transform="translate(10, 20) rotate(90) scale(2, 2)"

Transform Convenience Properties

Style blocks support individual transform properties as an alternative to transform: ... or the imperative API. These work on PathLayer, GroupLayer, and TextLayer:

define PathLayer('p') ${
  translate-x: 50;
  translate-y: 100;
  scale-x: 2;
  scale-y: 2;
  rotate: 0.25pi;
}
// Output: transform="translate(50, 100) rotate(45) scale(2, 2)"

Shorthands for translate and scale accept comma-separated values:

define PathLayer('p') ${ translate: 50, 100; scale: 2, 3; }
// Output: transform="translate(50, 100) scale(2, 3)"

Single-value scale uses the same value for both axes:

define PathLayer('p') ${ scale: 2; }
// Output: transform="scale(2, 2)"

The rotate value is an expression in radians (angle units like deg and pi work normally):

define PathLayer('p') ${ rotate: 45deg; }
// Output: transform="rotate(45)"

Precedence: An explicit transform property overrides convenience properties. Convenience properties override imperative ctx.transform calls. The individual translate-x/translate-y properties override the translate shorthand (and similarly for scale).

Convenience properties are removed from the output styles — they only affect the transform attribute.

Per-Layer Isolation

Each layer has independent transforms — setting a transform on one layer does not affect others:

define PathLayer('a') ${ stroke: red; }
define PathLayer('b') ${ stroke: blue; }

layer('a').ctx.transform.translate.set(10, 10)
layer('b').ctx.transform.scale.set(2, 2)
// Layer 'a' gets translate(10, 10), layer 'b' gets scale(2, 2)

GroupLayer

GroupLayers map to SVG <g> elements and organize child layers via .append(). They support transforms through style blocks and the imperative ctx.transform API, but do not support apply blocks.

Definition

// Define a group with styles
let panel = GroupLayer('panel') ${ opacity: 0.8; };

// Or with define (cannot be default)
define GroupLayer('panel') ${ opacity: 0.8; }

GroupLayers cannot be the default layer — define default GroupLayer(...) is an error.

Adding Children with .append()

Use .append(ref1, ref2, ...) to add layers as children of a group. All arguments must be layer references:

let panel = GroupLayer('panel') ${};
let bg = PathLayer('bg') ${ fill: #eee; };
bg.apply { rect(0, 0, 200, 200) }

let label = TextLayer('label') ${ font-size: 14; fill: #333; };
label.apply { text(10, 20)`Panel Title` }

// Append children to group
panel.append(bg, label)

Output SVG:

<g>
  <path d="..." fill="#eee" .../>
  <text x="10" y="20" font-size="14" fill="#333">Panel Title</text>
</g>

Appended layers are removed from the top-level output and rendered inside the group.

Nesting Groups

Groups can contain other groups, up to a maximum nesting depth of 10:

let inner = GroupLayer('inner') ${};
let child = PathLayer('child') ${};
child.apply { M 5 5 }
inner.append(child)

let outer = GroupLayer('outer') ${};
outer.append(inner)

Transforms

GroupLayers support both style block transforms and imperative transforms:

// Style block transform
let panel = GroupLayer('panel') ${ transform: translate(50, 100); };

// Imperative transform
panel.ctx.transform.rotate.set(0.785)
panel.ctx.transform.scale.set(2, 2)

When both are present, the style block transform takes precedence.

Moving Layers Between Groups

Appending a layer that already belongs to another group moves it. A warning log is emitted:

let g1 = GroupLayer('g1') ${};
let g2 = GroupLayer('g2') ${};
let child = PathLayer('child') ${};
g1.append(child)  // child is in g1
g2.append(child)  // child moves to g2, warning logged

No Apply Blocks

GroupLayers do not support .apply blocks. Use .append() to add children:

// This is an error:
// g.apply { M 0 0 }

// Use .append() instead:
g.append(myPath)

Limitations

• No nesting apply blocks — layer().apply blocks cannot be nested inside each other
• Layer order — layers render in definition order (first defined = bottom)
• GroupLayer nesting — maximum depth of 10 levels
• PathLayer transforms only — transforms are currently available on PathLayers and GroupLayers via ctx.transform; TextLayer transform support can be added later

Path Blocks

Path Blocks let you define reusable, introspectable paths without immediately drawing them. A PathBlock captures relative path commands and exposes metadata (length, vertices, endpoints) for positioning other elements relative to the path.

Syntax

let myPath = @{
  v 20
  h 30
  v -20
};

@{ opens a Path Block, } closes it. The body contains relative path commands, control flow, variables, and function calls. The result is a PathBlock value — no path commands are emitted.

Drawing a Path Block

Use .draw() to emit the path's commands at the current cursor position:

let shape = @{ v 20 h 20 v -20 z };

M 10 10
shape.draw()     // emits: v 20 h 20 v -20 z
M 50 50
shape.draw()     // reuse at a different position

draw() advances the cursor to the path's endpoint and returns a ProjectedPath with absolute coordinates.

Assigning the draw result

let shape = @{ v 20 h 20 };
M 10 10
let proj = shape.draw();
// proj.startPoint = Point(10, 10)
// proj.endPoint = Point(30, 30)

Drawing at a specific position

Use .drawTo(x, y) to emit M x y followed by the path's commands in a single call. This combines positioning and drawing — no separate M command needed.

let shape = @{ v 20 h 20 v -20 z };

shape.drawTo(10, 10)     // emits: M 10 10 v 20 h 20 v -20 z
shape.drawTo(50, 50)     // reuse at a different position

drawTo() returns a ProjectedPath with absolute coordinates, just like draw():

let shape = @{ v 20 h 30 };
let proj = shape.drawTo(10, 10);
// proj.startPoint = Point(10, 10)
// proj.endPoint = Point(40, 30)

drawTo() also works on ProjectedPath values — it re-positions the projected path to the new origin:

let shape = @{ h 50 v 30 };
let proj = shape.project(0, 0);
proj.drawTo(100, 100)    // emits: M 100 100 h 50 v 30

Projecting Without Drawing

Use .project(x, y) to compute absolute coordinates without emitting commands or moving the cursor:

let shape = @{ v 20 h 30 };
let proj = shape.project(10, 10);
// proj.startPoint = Point(10, 10)
// proj.endPoint = Point(40, 30)
// No path commands emitted, cursor unchanged

Properties

PathBlock

ProjectedPath

Same properties as PathBlock but with absolute coordinates.

subPathCommands entries

Each entry in subPathCommands is an object with:

{
  command: "v",           // lowercase command letter
  args: [20],             // numeric arguments
  start: Point(0, 0),     // cursor before command
  end: Point(0, 20)       // cursor after command
}

Control Flow Inside Path Blocks

Variables, for loops, foreach loops, if statements, and function calls all work inside path blocks:

let zigzag = @{
  for (i in 0..4) {
    v 10
    h calc(i % 2 == 0 ? 10 : -10)
  }
};

Context-aware functions like arcFromPolarOffset, tangentLine, and tangentArc work against the block's temporary path context.

Accessing Outer Variables

Path blocks can read variables from enclosing scope:

let size = 20;
let box = @{ v size h size v calc(-size) z };

First-Class Values

PathBlocks can be passed as function arguments and returned from functions:

fn makeStep(dx, dy) {
  return @{ h dx v dy };
}

let step = makeStep(10, 5);
M 0 0
step.draw()    // emits: h 10 v 5

Using Path Metadata

Access path properties for layout calculations:

let segment = @{ v 20 h 30 };

// Use length to create a matching horizontal line
let total = segment.length;       // 50

// Use endpoint for positioning
let end = segment.endPoint;       // Point(30, 20)
M end.x end.y                     // Position at path endpoint

Restrictions

Path blocks enforce these rules at runtime:

1. Relative commands only — All path commands must be lowercase (m, l, h, v, etc.). Uppercase (absolute) commands throw an error.
2. No layer definitions — define PathLayer/TextLayer is not allowed
3. No layer apply blocks — layer().apply { } is not allowed
4. No text statements — text() / tspan() are not allowed
5. No nesting — Path blocks cannot contain other @{ } expressions
6. No draw/project inside blocks — Calling .draw() or .project() inside a path block throws an error

Parametric Sampling

Parametric sampling lets you query points, tangent directions, and normal directions at any position along a path. The parameter t is a fraction from 0 (start) to 1 (end) measured by arc length.

These methods work on both PathBlock values and ProjectedPath values.

get(t) → Point

Returns the point at arc-length fraction t along the path.

let p = @{ v 50 h 100 };
let mid = p.get(0.5);       // Point roughly at distance 75 along path
M mid.x mid.y               // position at midpoint

tangent(t) → { point, angle }

Returns the point and tangent angle (radians) at fraction t. The angle is the direction of travel.

let p = @{ v 50 h 100 };
let tan = p.tangent(0.0);
log(tan.point);              // Point(0, 0)
log(tan.angle);              // ~1.5708 (π/2, pointing down)

normal(t) → { point, angle }

Returns the point and left-hand normal angle at fraction t. The normal angle equals the tangent angle minus π/2.

let p = @{ h 100 };
let n = p.normal(0.5);
log(n.point);                // Point(50, 0)
log(n.angle);                // ~-1.5708 (pointing up — left-hand normal of rightward path)

partition(n) → OrientedPoint[]

Divides the path into n equal-length segments, returning n + 1 oriented points (endpoints inclusive). Each oriented point has point, angle, and t properties.

let p = @{ h 100 };
let pts = p.partition(4);    // 5 points at x = 0, 25, 50, 75, 100
for (op in pts) {
  log(op.point.x, op.angle, op.t);
}
// t values: 0, 0.25, 0.5, 0.75, 1

Sampling on ProjectedPath

Projected paths return absolute coordinates:

let p = @{ h 100 };
let proj = p.project(10, 20);
let mid = proj.get(0.5);    // Point(60, 20) — offset by projection origin

Curve Support

Sampling works on all command types including cubic/quadratic Bézier curves and arcs. Curves use arc-length parameterization so that t = 0.5 always represents the geometric midpoint, not the parametric midpoint.

Transforms

Transforms create new paths from existing ones — reversing direction, computing bounding boxes, and constructing parallel paths. These methods work on both PathBlock values and ProjectedPath values.

reverse() → PathBlock / ProjectedPath

Returns a new path with reversed direction of travel. The reversed path starts where the original ended and ends where the original started.

let p = @{ h 50 v 30 };
let r = p.reverse();
log(r.endPoint);             // Point(-50, -30) — reversed from original
M 100 100
r.draw()                     // draws the path in reverse

Smooth commands (S/T) are automatically converted to their explicit forms (C/Q) before reversal. Closed paths (ending with z) preserve closure.

let closed = @{ h 30 v 30 h -30 z };
let rev = closed.reverse();  // reversed, still ends with z

boundingBox() → { x, y, width, height }

Returns the axis-aligned bounding box of the path. Accounts for Bézier curve extrema and arc extrema — not just endpoints.

let p = @{ c 0 -40 50 -40 50 0 };
let bb = p.boundingBox();
log(bb.y);                    // negative — curve extends above endpoints
log(bb.width, bb.height);    // full extent of the curve

For a straight-line path the bounding box matches the endpoint coordinates:

let line = @{ h 100 };
let bb = line.boundingBox();
// bb = { x: 0, y: 0, width: 100, height: 0 }

intersects(geometry) → Boolean

AABB overlap test — returns true if this path's bounding box overlaps the argument's bounding box. Works on both PathBlock and ProjectedPath values.

Accepted arguments:

let a = @{ h 60 v 40 h -60 z };
let b = @{ h 40 v 30 h -40 z };

// Overlapping — both start at origin
let projA = a.project(0, 0);
let projB = b.project(10, 10);
log(projA.intersects(projB));        // true

// Non-overlapping
let projC = b.project(200, 200);
log(projA.intersects(projC));        // false

Testing against a rectangle object:

let shape = @{ h 50 v 50 h -50 z };
let proj = shape.project(10, 10);
log(proj.intersects({x: 0, y: 0, width: 100, height: 100}));    // true
log(proj.intersects({x: 200, y: 200, width: 10, height: 10}));  // false

Works on unprojected PathBlocks too (bounding box computed from relative coordinates):

let a = @{ h 60 v 40 h -60 z };
let b = @{ h 40 v 30 h -40 z };
log(a.intersects(b));                // true (both at origin)

intersectionPoints(geometry) → Array<Point>

Returns the intersection points between this path's bounding box edges and the geometry's line segments. Works on both PathBlock and ProjectedPath values.

Accepted arguments:

let box = @{ h 100 v 100 h -100 z };
let line = @{ m -10 50 h 120 };

let projBox = box.project(0, 0);
let projLine = line.project(0, 0);
let pts = projBox.intersectionPoints(projLine);
// pts contains the points where the line crosses the box's bounding box edges

Non-overlapping paths return an empty array:

let a = @{ h 50 v 50 h -50 z };
let b = @{ h 10 v 10 h -10 z };
let projA = a.project(0, 0);
let projB = b.project(200, 200);
let pts = projA.intersectionPoints(projB);
log(pts.length);                     // 0

offset(distance) → PathBlock / ProjectedPath

Creates a parallel path offset by distance units. Positive values offset to the left of the travel direction, negative to the right.

let p = @{ h 60 v 40 };
let outer = p.offset(5);     // 5 units left of travel
let inner = p.offset(-5);    // 5 units right of travel

Offset preserves curve types — cubic Béziers produce offset cubics, arcs produce offset arcs with adjusted radii. Segment joins use miter joins with a limit of 4× the offset distance.

let curve = @{ c 0 -40 50 -40 50 0 };
let parallel = curve.offset(3);
M 0 50
curve.draw()
M 0 50
parallel.draw()              // parallel curve 3 units to the left

mirror(angle) → PathBlock / ProjectedPath

Reflects the path across a line through the start point at the given angle. The angle uses standard language units (radians).

let p = @{ h 60 v 40 };
let m = p.mirror(0.5pi);       // reflect across vertical axis → goes left
M 100 100
m.draw()

Common angles:

• mirror(0) — horizontal axis (y → -y)
• mirror(0.5pi) — vertical axis (x → -x)
• mirror(0.25pi) — diagonal (swaps x and y)

Mirror preserves path length and curve types. Arc commands have their sweep flag flipped (reflection reverses chirality) and their rotation parameter adjusted.

let curve = @{ c 0 -40 50 -40 50 0 };
let flipped = curve.mirror(0);
M 0 50
curve.draw()
M 0 50
flipped.draw()               // curve reflected below the axis

rotateAtVertexIndex(index, angle) → PathBlock / ProjectedPath

Rotates the path around the vertex at index (from the .vertices array) by angle radians. PathBlockValue results are normalized to (0, 0) start.

let p = @{ h 50 v 50 };
// p.vertices = [Point(0,0), Point(50,0), Point(50,50)]
let r = p.rotateAtVertexIndex(1, 0.5pi);  // rotate around corner
M 10 10
r.draw()

The index must be a non-negative integer within range. The rotation preserves path length and curve types. Arc commands have their rotation parameter adjusted.

// Create a radial pattern by rotating around the first vertex
let arm = @{ h 50 v 10 };
for (i in 0..5) {
  let angle = calc(i * 2 * 3.14159265358979 / 6);
  let r = arm.rotateAtVertexIndex(0, angle);
  M 100 100
  r.draw()
}

scale(sx, sy) → PathBlock / ProjectedPath

Scales the path from its start point. sx scales x-coordinates, sy scales y-coordinates.

let p = @{ h 50 v 30 };
let doubled = p.scale(2, 2);      // endPoint (100, 60)
let wide = p.scale(3, 1);         // endPoint (150, 30)
let flipped = p.scale(-1, 1);     // mirror across y-axis

Uniform scaling (sx == sy) preserves shape and scales arc radii proportionally. Non-uniform scaling (sx != sy) performs full ellipse eigendecomposition to compute new arc radii and rotation. Negative scale values flip the arc sweep flag (reflection reverses chirality).

let arc = @{ a 25 25 0 0 1 50 0 };
let wide = arc.scale(2, 1);       // stretched elliptical arc
let big = arc.scale(3, 3);        // uniform: radii tripled

subPath(startT, endT) → PathBlock

Extracts the geometric portion of a path between two arc-length fractions. Both startT and endT must be between 0 and 1. Always returns a PathBlock (normalized to (0, 0) origin), even when called on a ProjectedPath.

let p = @{ h 100 v 100 };
let first = p.subPath(0, 0.5);    // first half of the path
let second = p.subPath(0.5, 1);   // second half of the path
M 10 10
first.draw()
M 10 10
second.draw()                      // visually reconstructs the original

If startT > endT, the result is reversed (equivalent to .subPath(endT, startT).reverse()):

let p = @{ h 100 };
let rev = p.subPath(1, 0);        // full path, reversed direction

Use .get() on the ProjectedPath to find the absolute position, then .draw() the extracted PathBlock:

let p = @{ h 100 v 50 };
let proj = p.project(10, 20);
let start = proj.get(0.2);
let sub = proj.subPath(0.2, 0.8);  // PathBlock, normalized to (0,0)
M start.x start.y
sub.draw()                          // draws the middle 60% at the right position

Edge cases:

• subPath(0, 1) returns approximately the original path
• subPath(t, t) returns an empty PathBlock (not an error)
• Works with all command types including curves and arcs

let curve = @{ c 0 -40 50 -40 50 0 };
let front = curve.subPath(0, 0.5);
M 0 50
curve.draw()
M 0 80
front.draw()                        // first half of the Bézier curve

Transforms on ProjectedPath

Projected paths return results in absolute coordinates:

let p = @{ h 100 };
let proj = p.project(10, 20);
let bb = proj.boundingBox();
// bb.x = 10, bb.y = 20 — absolute coordinates

let rev = proj.reverse();
log(rev.startPoint);         // Point(110, 20) — starts at original end

For mirror() on a ProjectedPath, the mirror line passes through the projection's start point. For rotateAtVertexIndex(), the rotation center is the absolute vertex position.

let p = @{ h 50 };
let proj = p.project(100, 100);
let m = proj.mirror(0.5pi);
// Mirrors across vertical line through (100, 100)
// startPoint stays at (100, 100), endPoint moves to (50, 100)

For scale() on a ProjectedPath, the scale center is the projection's start point:

let p = @{ h 50 v 30 };
let proj = p.project(10, 20);
let s = proj.scale(2, 2);
// startPoint stays at (10, 20), endPoint moves to (110, 80)

Concatenation (<<)

The << operator joins two PathBlocks end-to-end. The right path's relative commands continue from where the left path ends.

let a = @{ h 50 };
let b = @{ v 30 };
let c = calc(a << b);               // endPoint (50, 30)
M 10 10
c.draw()                             // draws "h 50 v 30"

Chaining works naturally since << is left-associative and the result is a PathBlock:

let a = @{ h 50 };
let b = @{ v 30 };
let d = calc(a << b << a);          // endPoint (100, 30)

Self-concatenation repeats the path:

let p = @{ h 50 };
let doubled = calc(p << p);         // endPoint (100, 0)

Concatenated paths support all PathBlock methods — draw, project, sampling, and transforms:

let combined = calc(a << b);
let rev = combined.reverse();
let mid = combined.get(0.5);

The << operator also works for style block merging. The operand types must match — mixing PathBlocks and style blocks throws an error.

Chamfers

Chamfers cut corners by replacing a vertex with a straight line segment. The incoming and outgoing edges are trimmed by the specified distance, and a line connects the two trim points.

chamfer(distance) → PathBlock / ProjectedPath

Chamfers all corner vertices with equal distance on both sides:

let box = @{ h 60 v 40 h -60 z };
let chamfered = box.chamfer(8);
M 10 10
chamfered.draw()

chamfer(d1, d2) → PathBlock / ProjectedPath

Asymmetric chamfer — d1 is the trim distance on the incoming edge, d2 on the outgoing edge:

let box = @{ h 60 v 40 h -60 z };
let asym = box.chamfer(5, 15);
M 10 10
asym.draw()

chamferAtVertex(index, distance) → PathBlock / ProjectedPath

Chamfers a single vertex by index (from the .vertices array):

let box = @{ h 60 v 40 h -60 z };
// box.vertices: Point(0,0), Point(60,0), Point(60,40), Point(0,40)
let oneCorner = box.chamferAtVertex(1, 10);
M 10 10
oneCorner.draw()

chamferAtVertex(index, d1, d2) → PathBlock / ProjectedPath

Asymmetric chamfer at a single vertex:

let box = @{ h 60 v 40 h -60 z };
let asym = box.chamferAtVertex(2, 5, 15);
M 10 10
asym.draw()

Edge cases

If the chamfer distance exceeds the available edge length, it is clamped to the edge length and a warning is logged. If the vertex index is out of range, an error is thrown.

Chamfers work with all command types — lines, curves, and arcs. For curves, the trim operation uses arc-length parameterization to find the exact split point.

Fillets

Fillets round corners by replacing a vertex with a circular arc. The incoming and outgoing edges are trimmed, and an arc tangent to both edges is inserted.

Scope: Line-line junctions only. At curve junctions, the fillet is skipped and a warning is logged.

fillet(radius) → PathBlock / ProjectedPath

Fillets all corner vertices with the given radius:

let box = @{ h 60 v 40 h -60 z };
let rounded = box.fillet(8);
M 10 10
rounded.draw()

filletAtVertex(index, radius) → PathBlock / ProjectedPath

Fillets a single vertex:

let box = @{ h 60 v 40 h -60 z };
let oneRound = box.filletAtVertex(1, 12);
M 10 10
oneRound.draw()

If the radius is too large for the available edge length, it is clamped and a warning is logged. If the vertex index is out of range, an error is thrown.

Elliptical Fillets

Elliptical fillets replace a corner with an elliptical arc instead of a circular one, allowing for more expressive corner shapes.

Scope: Line-line junctions only (same as circular fillets).

ellipticalFillet(rx, ry) → PathBlock / ProjectedPath

Fillets all corners with an elliptical arc of radii rx and ry:

let box = @{ h 60 v 40 h -60 z };
let eFilleted = box.ellipticalFillet(12, 6);
M 10 10
eFilleted.draw()

ellipticalFillet(rx, ry, rotation) → PathBlock / ProjectedPath

Elliptical fillet with a rotated ellipse (rotation in radians, default 0):

let box = @{ h 60 v 40 h -60 z };
let rotated = box.ellipticalFillet(12, 6, 0.3);
M 10 10
rotated.draw()

ellipticalFilletAtVertex(index, rx, ry) → PathBlock / ProjectedPath

Elliptical fillet at a single vertex:

let box = @{ h 60 v 40 h -60 z };
let one = box.ellipticalFilletAtVertex(1, 15, 8);
M 10 10
one.draw()

ellipticalFilletAtVertex(index, rx, ry, rotation) → PathBlock / ProjectedPath

Elliptical fillet at a single vertex with rotation:

let box = @{ h 60 v 40 h -60 z };
let one = box.ellipticalFilletAtVertex(2, 15, 8, 0.5);
M 10 10
one.draw()

Boolean Operations

Boolean operations combine two closed paths using set operations. Both paths must be closed (end with z or have coincident start and end points). The result preserves original curve types — no linearization.

See also: Standard Library path functions for creating shapes to use with boolean operations.

union(other) → PathBlock

Combines two paths into their union (outer boundary):

let a = @{ circle(30) };
let b = @{ circle(30) };
let combined = a.project(50, 50).union(b.project(70, 50));

difference(other) → PathBlock

Subtracts other from the path:

let plate = @{ circle(40) };
let hole = @{ circle(15) };
let result = plate.project(50, 50).difference(hole.project(50, 50));

intersection(other) → PathBlock

Returns only the overlapping region:

let a = @{ circle(30) };
let b = @{ circle(30) };
let overlap = a.project(50, 50).intersection(b.project(70, 50));

xor(other) → PathBlock

Returns the symmetric difference — everything in either path but not both:

let a = @{ circle(30) };
let b = @{ circle(30) };
let exclusive = a.project(50, 50).xor(b.project(70, 50));

Requirements and behavior

• Both paths must be closed. Open paths throw an error.
• The other argument can be a PathBlock or ProjectedPath.
• Multi-component results produce multiple subpaths (M...z M...z).
• All curve types (lines, cubics, quadratics, arcs) are preserved through the operation.
• Results are always returned as PathBlock values (normalized to (0, 0) origin).

Font Integration

Font integration lets you convert text characters into PathBlock values — turning each glyph into vector paths you can draw, transform, sample, and combine with boolean operations.

@font Directive

The @font directive declares a font for use in the program. It must appear at the top level (not inside a function or block).

@font "Inter";
@font "Roboto Mono" 700;
@font "./fonts/CustomFont.ttf";

Syntax:

@font "family-or-path" [weight];

Font loading by environment:

• CLI: Loads from local file paths (relative to source file) or searches system font directories (/Library/Fonts, /System/Library/Fonts, ~/Library/Fonts on macOS; equivalent paths on Linux/Windows)
• Playground: Fetches from Google Fonts CDN automatically

The directive is declarative metadata — the host environment loads fonts before compilation begins. If a font cannot be found, a warning is logged and compilation continues.

PathBlock.fromGlyph(text, styles)

Converts text into an array of PathBlock values — one per character. Each PathBlock contains the glyph's vector outline as relative path commands.

@font "Inter";

let glyphs = PathBlock.fromGlyph("A", ${ font-family: Inter; font-size: 48; });

M 50 100
glyphs[0].draw()

Arguments:

Returns: Array of PathBlock values. Each element has all standard PathBlock properties and methods (draw(), project(), get(), tangent(), boundingBox(), scale(), boolean operations, etc.).

@font "Inter";
let styles = ${ font-family: Inter; font-size: 48; };
let glyphs = PathBlock.fromGlyph("Hi", styles);
log(glyphs.length);    // 2

advanceWidth

Each glyph PathBlock has an .advanceWidth property — the horizontal distance to advance the cursor after drawing the glyph. This enables manual text layout:

@font "Inter";
let styles = ${ font-family: Inter; font-size: 48; };
let glyphs = PathBlock.fromGlyph("Hello", styles);

let x = 10;
let y = 100;
for (g in glyphs) {
  M x y
  g.draw()
  let x = calc(x + g.advanceWidth);
}

Space characters return an empty PathBlock (no path commands) but still have a non-zero advanceWidth.

contours

Glyphs with multiple contours (e.g., "O" has an outer ring and inner hole) can be decomposed with the .contours property. This returns an array of PathBlock values, one per contour:

@font "Inter";
let styles = ${ font-family: Inter; font-size: 48; };
let glyphs = PathBlock.fromGlyph("O", styles);
let contours = glyphs[0].contours;
log(contours.length);              // 2 (outer + inner)

for (c in contours) {
  c.drawTo(100, 100)
}

Each contour is a closed PathBlock with all standard properties and methods.

Error cases

TextBlock

TextBlock is a composition-first text primitive that lets you compose, measure, and position text before drawing it. This is essential for diagrams and schematics where labels must be positioned relative to geometry without overlapping.

TextBlock parallels PathBlock: both follow the pattern compose -> measure -> position -> draw.

Quick Overview

// Compose text at relative coordinates
let label = &{
  text(0, 14)`Title`
  text(0, 30)`Subtitle`
} << ${ font-size: 14; fill: #333; };

// Measure before placing
let bb = label.boundingBox();

// Project into absolute coordinates
let placed = label.project(50, 100);

// Draw to a TextLayer
define TextLayer('labels') ${}
layer('labels').apply {
  placed.draw();
}

Syntax

TextBlock uses the &{ } sigil:

let t = &{
  text(x, y)`content`
  text(x, y) {
    tspan()`first`
    tspan(0, 16)`second`
  }
};

Inside a text block you can use:

• text() statements — the core text elements
• let, for, if — control flow for dynamic content
• User-defined functions — called as expressions

Not allowed inside text blocks:

• Path commands (M, L, etc.)
• Layer definitions or apply blocks
• Nested text blocks

Types

TextBlockValue

Created by the &{ } expression. All coordinates are relative to origin (0, 0).

ProjectedTextValue

Created by .project(), .drawTo(), .polarProject(), or .translate(). Contains text elements with absolute coordinates and tracks the projection origin.

Methods

TextBlockValue

ProjectedTextValue

Properties

TextBlockValue

ProjectedTextValue

Style Merging

Use the << operator to merge styles into a TextBlock:

let t = &{ text(0, 16)`Hello` } << ${ font-size: 24; fill: #333; };

This sets block-level styles that apply to all elements unless overridden by element-level styles.

BBoxAnchor Enum

The BBoxAnchor enum provides named positions on a bounding box:

BBoxAnchor.TopLeft      BBoxAnchor.Top      BBoxAnchor.TopRight
BBoxAnchor.Left         BBoxAnchor.Center   BBoxAnchor.Right
BBoxAnchor.BottomLeft   BBoxAnchor.Bottom   BBoxAnchor.BottomRight

Used with .anchor() and .polarProject().

Font Metrics

TextBlock uses built-in character width tables for bounding box estimation:

• Sans-serif (default): per-character widths approximating Arial/Helvetica
• Serif: per-character widths approximating Times New Roman
• Monospace: uniform character width approximating Courier New

Set the font category via the font-family style property. Accuracy is ~85-90% for Latin text, sufficient for layout decisions.

Font metrics respect:

• font-size (default 16)
• font-family (category detection)
• font-weight (bold applies ~6% width increase)
• letter-spacing
• tspan dx/dy offsets

Polar Projection

Place text along a polar vector with anchor alignment:

let label = &{ text(0, 14)`Node A` } << ${ font-size: 14; };

// Place label 80px from center at 45 degrees, anchored at center-left
let placed = label.polarProject(100, 100, 45deg, 80, BBoxAnchor.Left);

The anchor determines which point of the text's bounding box is placed at the target location. For example, BBoxAnchor.Left means the left-center of the text bbox lands on the polar target point.

Intersection Detection

Check if text bounding boxes overlap to avoid label collisions:

let label1 = (&{ text(0, 14)`First` } << ${ font-size: 14; }).project(50, 50);
let label2 = (&{ text(0, 14)`Second` } << ${ font-size: 14; }).project(55, 55);

if (label1.intersects(label2)) {
  // Labels overlap — adjust position
  label2 = label2.translate(0, 20);
}

.intersects() accepts:

• Another ProjectedTextValue (AABB overlap test)
• A ProjectedPathValue (bbox-edge vs path-segment intersection)
• An object with {x, y, width, height} (AABB overlap test)

Text to Path Conversion

When you need text that renders identically without requiring fonts — or when you want to apply path transforms and boolean operations to text — .toPathBlock() converts glyph outlines into vector geometry. After conversion, the text is no longer a text element: it's path geometry that can be filled, stroked, scaled, mirrored, and combined with boolean operations like any other PathBlock.

This is different from PathBlock.fromGlyph(), which returns an array of per-character PathBlocks. .toPathBlock() returns a single PathBlock containing all glyphs from the entire TextBlock, already laid out according to element positions, tspan offsets, and letter-spacing.

Requirements:

• Fonts must be loaded via @font directive or compile options
• font-family must be set in the TextBlock's styles
• Only available on TextBlockValue (not ProjectedTextValue)

@font "./fonts/Baumans-Regular.ttf";

let tb = &{
  text(0, 20)`Hello`
  text(0, 40)`World`
} << ${ font-family: Baumans-Regular; font-size: 24; };

let pb = tb.toPathBlock();

define PathLayer('text-as-path') ${ fill: #333; stroke: none; }
layer('text-as-path').apply {
  pb.drawTo(20, 20);
}

The resulting PathBlock is normalized to a (0, 0) origin, so .drawTo(x, y) places the text geometry at absolute coordinates (x, y). Space characters advance the cursor without generating outline commands.

Since the result is a standard PathBlock, you can chain any PathBlock operation:

// Scale the text geometry down to 60%
let small = pb.scale(0.6, 0.6);

// Mirror the text horizontally
let flipped = pb.mirror(0);

// Use text as a boolean punch — cut text out of a rectangle
let plate = @{ h 200 v 60 h -200 z }.project(10, 10);
let cutout = plate.difference(pb.project(20, 20));

Per-tspan style overrides (font-family, font-size) are respected, allowing mixed fonts within a single PathBlock output.

Code Snippet Blocks

For diagrams that need to show source code alongside visual output — tutorials, blog schematics, API documentation — .toCodeSnippetBlock() generates a self-contained code block as SVG layers with Pathogen-aware syntax highlighting.

.toCodeSnippetBlock(name [, fontSize, padding]) transforms a TextBlock containing code text into a styled GroupLayer.

Arguments:

• name (string) — name for the GroupLayer
• fontSize (number, optional) — code font size, default 10
• padding (number, optional) — padding around code, default 12

Returns: LayerReference to a GroupLayer containing:

• {name}-bg — PathLayer with dark background (#1e293b), border (#334155), and rounded corners
• {name}-code — TextLayer with per-token syntax-highlighted tspan elements

The name must not collide with existing layer names (including the -bg and -code suffixed names).

let code = &{
  text(0, 0)`// Shape layer with styles
define PathLayer('main') \${ fill: #3b82f6; }

let shape = rect(0, 0, 80, 60);
layer('main').apply {
  shape.drawTo(50, 50);
}`
};

let snippet = code.toCodeSnippetBlock('my-snippet', 10, 12);
snippet << ${ translate-x: 400; translate-y: 100; };

Escaping ${ in Code Text

Template literals in Pathogen treat ${ as a string interpolation sequence. If your code text contains literal ${ (e.g., style blocks), escape the dollar sign with a backslash:

// ✗ This fails — ${ triggers interpolation
let code = &{ text(0,0)`let s = ${ fill: red; }` };

// ✓ Escape the dollar sign
let code = &{ text(0,0)`let s = \${ fill: red; }` };

@{ and &{ do not need escaping — they pass through template literals as plain text. Only ${ requires the \$ escape.

Syntax Highlighting Palette

Keywords and builtins share the same color (#c084fc) — both represent language-level constructs.

The code text is automatically normalized: common leading whitespace is removed (dedent), blank leading/trailing lines are trimmed, and tabs are converted to 2 spaces. Indentation within the code (for blocks, loops, conditionals) is preserved and rendered via x-coordinate offsets.

Examples

Label placement around a shape

define PathLayer('shape') ${ stroke: #333; fill: none; }
define TextLayer('labels') ${ font-size: 12; fill: #666; }

let shape = @{ l 80 0 l 0 60 l -80 0 z };

// Place labels at compass positions around the shape
let top = &{ text(0, 12)`Top` } << ${ font-size: 12; };
let right = &{ text(0, 12)`Right` } << ${ font-size: 12; };

layer('shape').apply { shape.drawTo(60, 70); }

layer('labels').apply {
  top.polarProject(100, 100, -90deg, 50, BBoxAnchor.Bottom).draw();
  right.polarProject(100, 100, 0, 60, BBoxAnchor.Left).draw();
}

Dynamic labels with collision avoidance

define TextLayer('labels') ${ font-size: 11; }

let points = [
  { x: 50, y: 50, name: "A" },
  { x: 55, y: 65, name: "B" },
  { x: 120, y: 50, name: "C" },
];

let placed = [];
layer('labels').apply {
  for (pt in points) {
    let label = &{ text(0, 11)`${pt.name}` } << ${ font-size: 11; };
    let proj = label.project(pt.x + 5, pt.y);

    // Check against all previously placed labels
    let ok = true;
    for (prev in placed) {
      if (proj.intersects(prev)) {
        ok = false;
      }
    }

    if (ok) {
      proj.draw();
      placed.push(proj);
    } else {
      // Try below instead
      let alt = label.project(pt.x + 5, calc(pt.y + 15));
      alt.draw();
      placed.push(alt);
    }
  }
}

Color Type

The Color type provides first-class color manipulation in OKLCH color space. Colors are resolved at compile time to concrete CSS values.

Color Literals

Hex color codes are first-class expressions — no quotes or Color() wrapper needed:

let c = #cc0000;                      // 6-digit hex → ColorValue
let c = #f00;                         // 3-digit shorthand
let c = #cc000080;                    // 8-digit with alpha
let c = #f008;                        // 4-digit with alpha

Color literals support method chaining via parentheses:

let lighter = (#cc0000).lighten(20%); // 20% → 0.2
let faded = (#0066ff).alpha(50%);     // 50% → 0.5

Color() accepts color literals as pass-through (no-op for backwards compatibility):

let c = Color(#cc0000);               // same as: let c = #cc0000;

CSS Color Function Literals

CSS color functions are first-class expressions with raw capture (content between parens is captured as-is):

let c = rgb(255, 0, 0);
let c = rgba(255, 0, 0, 0.5);
let c = hsl(0, 100%, 50%);           // % inside parens is literal
let c = hsla(0, 100%, 50%, 0.5);
let c = oklch(0.6 0.15 30);
let c = oklch(0.6 0.15 30 / 0.5);    // / for alpha is literal
let c = oklab(0.6 -0.1 0.15);
let c = hwb(0 0% 0%);
let c = lab(50 40 59.5);
let c = lch(50 64 30);

Method chaining works directly:

let lighter = rgb(255, 0, 0).lighten(20%);

Note: CSS color function names (rgb, hsl, oklch, etc.) are effectively reserved — they always produce color literals, even if a user-defined function of the same name exists.

Constructor

The Color() wrapper is still available for string-based construction and named colors:

let c = Color('#e63946');              // hex (3, 6, or 8 digit)
let c = Color('red');                  // named CSS color (all 148)
let c = Color('rgb(255, 0, 0)');       // rgb/rgba
let c = Color('hsl(0, 100%, 50%)');    // hsl/hsla
let c = Color('oklch(0.6 0.15 30)');   // oklch
let c = Color(0.6, 0.15, 30);         // direct OKLCH (L, C, H)
let c = Color(0.6, 0.15, 30, 0.5);    // OKLCH + alpha
let c = Color(#cc0000);               // pass-through (accepts ColorValue)

All input formats are converted to OKLCH internally for perceptually uniform manipulation.

Properties

Read-only properties for inspecting color values:

let c = Color('#e63946');
log(c.hex);        // #e63946
log(c.lightness);  // ~0.52
log(c.hue);        // ~27
log(c.a);          // 1

Methods

All methods return a new Color — they never mutate the original.

Lightness

let c = Color('#e63946');
let lighter = c.lighten(0.2);   // increase L by 0.2
let darker = c.darken(0.15);    // decrease L by 0.15
log(lighter.hex);  // lighter red
log(darker.hex);   // darker red

Saturation

let c = Color('#e63946');
let vivid = c.saturate(1.5);     // multiply chroma by 1.5
let muted = c.desaturate(0.5);   // multiply chroma by 0.5

Alpha

let c = Color('#e63946');
let semi = c.alpha(0.5);
log(semi.css);  // rgba(230, 57, 70, 0.5)

Hue

let c = Color('#e63946');
let shifted = c.hueShift(180);   // shift hue by 180°
let comp = c.complement();       // shorthand for hueShift(180)

Mixing

Mix two colors in OKLCH space:

let a = Color('#e63946');
let b = Color('#457b9d');
let mid = a.mix(b, 0.5);         // 50/50 mix
let mostly_a = a.mix(b, 0.2);    // 80% a, 20% b

Method Chaining

Methods return new Colors, so they chain naturally:

let c = Color('#e63946')
  .lighten(0.1)
  .desaturate(0.8)
  .alpha(0.9);

Color Harmonies

Generate sets of harmonious colors based on color theory. All harmony methods return an array of Colors, preserving lightness, chroma, and alpha.

.analogous(angle?)

Returns 3 colors: [hue - angle, self, hue + angle]. Default angle: 30.

let c = Color('#e63946');
let colors = c.analogous();       // 3 colors at -30°, 0°, +30°
let wide = c.analogous(45);       // wider spread at ±45°

.triadic()

Returns 3 colors evenly spaced at 120° intervals: [self, hue + 120, hue + 240].

let c = Color('#e63946');
let colors = c.triadic();

.tetradic()

Returns 4 colors evenly spaced at 90° intervals: [self, hue + 90, hue + 180, hue + 270].

let c = Color('#e63946');
let colors = c.tetradic();

.splitComplementary(angle?)

Returns 3 colors: [self, hue + 180 - angle, hue + 180 + angle]. Default angle: 30.

let c = Color('#e63946');
let colors = c.splitComplementary();     // flanks of complement at ±30°
let narrow = c.splitComplementary(15);   // tighter split

Using Harmonies

Harmony methods return arrays, so use for-each to iterate:

let c = Color('#e63946');
for ([color, i] in c.triadic()) {
  define PathLayer(`p${i}`) ${ fill: color; stroke: none; }
  layer(`p${i}`).apply { circle(calc(50 + i * 60), 100, 25) }
}

Complete Example

A full color swatch showcase demonstrating base methods, harmonies, palettes, and derived colors across multiple tiers. Uses CSSVar-backed Colors so that changing --base-color or --accent-color in the playground's CSS var panel reactively updates every swatch. Connecting lines show how colors flow from a single base color through transformations.

Canvas: 600 × 700 viewBox with four sections flowing top-to-bottom.

// ═══════════════════════════════════════════════════════════
// Color Swatch Showcase — full demo of Color manipulation,
// harmonies, palettes, and derived colors
// ═══════════════════════════════════════════════════════════

let base = Color(CSSVar('--base-color', '#e63946'));
let accent = Color(CSSVar('--accent-color', '#457b9d'));

// ── Tier 0: Base Methods ──────────────────────────────────

let lighter   = base.lighten(0.15);
let darker    = base.darken(0.15);
let vivid     = base.saturate(1.4);
let muted     = base.desaturate(0.5);
let shifted   = base.hueShift(60);
let comp      = base.complement();
let semi      = base.alpha(0.6);
let mixed     = base.mix(accent, 0.5);

// ── Tier 1: Harmonies ────────────────────────────────────

let analog  = base.analogous();
let triad   = base.triadic();
let tetrad  = base.tetradic();
let split   = base.splitComplementary();

// ── Tier 1b: Palettes ────────────────────────────────────

let ramp   = Color.palette(base, 5);
let interp = Color.palette(base, accent, 5);

// ── Tier 2: Derived Colors ───────────────────────────────

let tri1       = triad[1];
let tri1Light  = tri1.lighten(0.15);
let tri1Dark   = tri1.darken(0.15);
let tri1Vivid  = tri1.saturate(1.4);

let rampMid      = ramp[2];
let rampShifted  = rampMid.hueShift(60);
let rampComp     = rampMid.complement();
let rampAlpha    = rampMid.alpha(0.5);

// ═══════════════════════════════════════════════════════════
// Layers
// ═══════════════════════════════════════════════════════════

define PathLayer('connectors') ${
  stroke: #999;
  stroke-width: 1;
  fill: none;
}

define TextLayer('section-labels') ${
  font-family: system-ui, sans-serif;
  font-size: 13;
  font-weight: bold;
  fill: #555;
}

define TextLayer('labels') ${
  font-family: system-ui, sans-serif;
  font-size: 9;
  fill: #777;
  text-anchor: middle;
}

// ── Swatch sizing ────────────────────────────────────────

let sx = 64;
let sp = 96;
let sw = 36;
let sh = 36;
let sr = 6;

// ═══════════════════════════════════════════════════════════
// Tier 0: Base Method Swatches
// ═══════════════════════════════════════════════════════════

// Row 1: base, lighten, darken, saturate, desaturate
let row1 = [base, lighter, darker, vivid, muted];
let row1names = ['base', 'lighten', 'darken', 'saturate', 'desat'];
for ([color, i] in row1) {
  let x = calc(sx + i * sp);
  define PathLayer(`t0r1_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`t0r1_${i}`).apply { roundRect(calc(x - sw / 2), calc(50 - sh / 2), sw, sh, sr) }
}

// Row 2: hueShift, complement, alpha, mix, accent
let row2 = [shifted, comp, semi, mixed, accent];
let row2names = ['hueShift', 'compl.', 'alpha', 'mix', 'accent'];
for ([color, i] in row2) {
  let x = calc(sx + i * sp);
  define PathLayer(`t0r2_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`t0r2_${i}`).apply { roundRect(calc(x - sw / 2), calc(115 - sh / 2), sw, sh, sr) }
}

// Section header
layer('section-labels').apply {
  text(10, 20)`Base Methods`
}

// Row 1 labels
layer('labels').apply {
  for ([name, i] in row1names) {
    text(calc(sx + i * sp), calc(50 + sh / 2 + 12))`${name}`
  }
}

// Row 2 labels
layer('labels').apply {
  for ([name, i] in row2names) {
    text(calc(sx + i * sp), calc(115 + sh / 2 + 12))`${name}`
  }
}

// Section divider
layer('connectors').apply {
  M 10 170
  L 590 170
}

// ═══════════════════════════════════════════════════════════
// Tier 1: Harmonies
// ═══════════════════════════════════════════════════════════

layer('section-labels').apply {
  text(10, 195)`Harmonies`
}

let hsx = 160;
let hsp = 55;
let hsw = 30;
let hsh = 30;
let hsr = 5;

// Row 3: analogous
for ([color, i] in analog) {
  let x = calc(hsx + i * hsp);
  define PathLayer(`analog_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`analog_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(220 - hsh / 2), hsw, hsh, hsr)
  }
}

// Row 4: triadic
for ([color, i] in triad) {
  let x = calc(hsx + i * hsp);
  define PathLayer(`triad_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`triad_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(275 - hsh / 2), hsw, hsh, hsr)
  }
}

// Row 5: tetradic
for ([color, i] in tetrad) {
  let x = calc(hsx + i * hsp);
  define PathLayer(`tetrad_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`tetrad_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(330 - hsh / 2), hsw, hsh, hsr)
  }
}

// Row 6: splitComplementary
for ([color, i] in split) {
  let x = calc(hsx + i * hsp);
  define PathLayer(`split_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`split_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(385 - hsh / 2), hsw, hsh, hsr)
  }
}

// Harmony row labels
define TextLayer('hlabels') ${
  font-family: system-ui, sans-serif;
  font-size: 10;
  fill: #888;
}
layer('hlabels').apply {
  text(30, 224)`analogous`
  text(30, 279)`triadic`
  text(30, 334)`tetradic`
  text(30, 389)`splitComp.`
}

// Section divider
layer('connectors').apply {
  M 10 420
  L 590 420
}

// ═══════════════════════════════════════════════════════════
// Tier 1b: Palettes
// ═══════════════════════════════════════════════════════════

layer('section-labels').apply {
  text(10, 445)`Palettes`
}

// Row 7: lightness ramp
for ([color, i] in ramp) {
  let x = calc(hsx + i * hsp);
  define PathLayer(`ramp_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`ramp_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(470 - hsh / 2), hsw, hsh, hsr)
  }
}

// Row 8: interpolation
for ([color, i] in interp) {
  let x = calc(hsx + i * hsp);
  define PathLayer(`interp_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`interp_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(525 - hsh / 2), hsw, hsh, hsr)
  }
}

// Palette row labels
layer('hlabels').apply {
  text(30, 474)`palette(c,5)`
  text(30, 529)`palette(a,b,5)`
}

// Section divider
layer('connectors').apply {
  M 10 560
  L 590 560
}

// ═══════════════════════════════════════════════════════════
// Tier 2: Derived Colors
// ═══════════════════════════════════════════════════════════

layer('section-labels').apply {
  text(10, 583)`Derived Colors`
}

let dsx = 260;
let dsp = 70;

// Row 9: triadic[1] → lighten, darken, saturate
define PathLayer('tri1_parent') ${ fill: tri1; stroke: #666; stroke-width: 1; }
layer('tri1_parent').apply {
  roundRect(calc(hsx - hsw / 2), calc(605 - hsh / 2), hsw, hsh, hsr)
}

let derived1 = [tri1Light, tri1Dark, tri1Vivid];
let derived1names = ['lighten', 'darken', 'saturate'];
for ([color, i] in derived1) {
  let x = calc(dsx + i * dsp);
  define PathLayer(`d1_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`d1_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(605 - hsh / 2), hsw, hsh, hsr)
  }
}

// Row 10: ramp[2] → hueShift, complement, alpha
define PathLayer('ramp2_parent') ${ fill: rampMid; stroke: #666; stroke-width: 1; }
layer('ramp2_parent').apply {
  roundRect(calc(hsx - hsw / 2), calc(660 - hsh / 2), hsw, hsh, hsr)
}

let derived2 = [rampShifted, rampComp, rampAlpha];
let derived2names = ['hueShift', 'compl.', 'alpha'];
for ([color, i] in derived2) {
  let x = calc(dsx + i * dsp);
  define PathLayer(`d2_${i}`) ${ fill: color; stroke: #ccc; stroke-width: 0.5; }
  layer(`d2_${i}`).apply {
    roundRect(calc(x - hsw / 2), calc(660 - hsh / 2), hsw, hsh, hsr)
  }
}

// Derived row labels
layer('hlabels').apply {
  text(30, 609)`triad[1] →`
  text(30, 664)`ramp[2] →`
}

// Derived swatch labels
layer('labels').apply {
  for ([name, i] in derived1names) {
    text(calc(dsx + i * dsp), calc(605 + hsh / 2 + 12))`${name}`
  }
  for ([name, i] in derived2names) {
    text(calc(dsx + i * dsp), calc(660 + hsh / 2 + 12))`${name}`
  }
}

// ── Connecting lines (reactivity chain) ──────────────────

layer('connectors').apply {
  // Vertical from base swatch down to tier 1 divider
  M sx calc(50 + sh / 2)
  L sx 170

  // Connector: triadic[1] down to derived row 9
  M calc(hsx + 1 * hsp) calc(275 + hsh / 2)
  L calc(hsx + 1 * hsp) calc(605 - hsh / 2 - 5)
  L hsx calc(605 - hsh / 2 - 5)
  L hsx calc(605 - hsh / 2)

  // Arrow: parent → derived in row 9
  M calc(hsx + hsw / 2) 605
  L calc(dsx - hsw / 2) 605

  // Connector: ramp[2] down to derived row 10
  M calc(hsx + 2 * hsp) calc(470 + hsh / 2)
  L calc(hsx + 2 * hsp) calc(660 - hsh / 2 - 5)
  L hsx calc(660 - hsh / 2 - 5)
  L hsx calc(660 - hsh / 2)

  // Arrow: parent → derived in row 10
  M calc(hsx + hsw / 2) 660
  L calc(dsx - hsw / 2) 660
}

Compile with: pathogen-lang --output-svg-file=swatches.svg --viewBox="0 0 600 700" --width="600" --height="700"

Static Methods

Color.mix(c1, c2, ratio)

Mix two colors at a given ratio (0 = all c1, 1 = all c2):

let a = Color('#e63946');
let b = Color('#457b9d');
let mid = Color.mix(a, b, 0.5);

Color.palette(color, n)

Generate a lightness ramp of n colors from dark (L=0.15) to light (L=0.95), preserving hue and chroma:

let c = Color('#e63946');
let shades = Color.palette(c, 5);   // 5 shades from dark to light

Color.palette(c1, c2, n)

Generate n evenly interpolated colors between two colors:

let a = Color('#e63946');
let b = Color('#457b9d');
let gradient = Color.palette(a, b, 7);   // 7-step gradient

n must be an integer >= 2.

Color.lightDark(light, dark)

Create a theme-aware color that uses CSS light-dark() in style output:

let fg = Color.lightDark(Color('#333'), Color('#eee'));
// Style output: light-dark(#333333, #eeeeee)

Works with CSSVar-backed colors for full customizability:

let fg = Color.lightDark(
  Color(CSSVar('--fg-light', '#333')),
  Color(CSSVar('--fg-dark', '#eee'))
);
// Style output: light-dark(var(--fg-light, #333), var(--fg-dark, #eee))

Both arguments must be Colors. At compile time, .hex, .lightness, and other properties resolve to the light variant. Method calls (.lighten(), .hueShift(), etc.) operate on the light variant and lose the light-dark semantics.

@property Declarations

When you create a Color(CSSVar('--name', fallback)), the compiler automatically collects a CSS @property declaration for that custom property. This enables browsers to interpolate the property in transitions and animations.

The collected declarations appear in CompileResult.cssProperties and are emitted as a <style> block in CLI SVG output:

<svg ...>
  <style>
    @property --base-color {
      syntax: "<color>";
      inherits: true;
      initial-value: #e63946;
    }
  </style>
  ...
</svg>

Only Color-typed CSSVars produce @property declarations — plain CSSVar('--width', 2) does not. When the same variable name appears multiple times, the first occurrence wins.

Style Block Auto-Conversion

Colors auto-convert to CSS strings when used in style blocks:

let primary = Color('#e63946');
let light = primary.lighten(0.2);

layer PathLayer('main') ${
  stroke: primary;
  fill: light;
}

This outputs stroke="#e63946" and fill as the lightened hex value — no .css property needed.

Template Literals

Colors display as Color(#hex) in template literals and log():

let c = Color('#e63946');
log(c);              // Color(#e63946)
log(`color: ${c}`);  // color: Color(#e63946)

Roundtrip Fidelity

Standard CSS colors roundtrip exactly:

let c = Color('#ff0000');
log(c.hex);  // #ff0000

Named Colors

All 148 CSS named colors are supported:

let c = Color('coral');
let c = Color('dodgerblue');
let c = Color('mediumseagreen');

Named color lookup is case-insensitive.

Gradients

Gradients define SVG paint servers (<linearGradient> and <radialGradient>) that can be used as fill or stroke values on layers.

LinearGradient

Create a linear gradient with an ID and coordinates defining the gradient axis:

let fade = LinearGradient('fade', 0, 0, 1, 1) {|g|
  g.stop(0, Color('#e63946'));
  g.stop(0.5, Color('#f4a261'));
  g.stop(1, Color('#2a9d8f'));
};

Constructor signature: LinearGradient(id, x1, y1, x2, y2) — coordinates are in objectBoundingBox units by default (0–1 range).

RadialGradient

Create a radial gradient with an ID, center point, and radius:

let glow = RadialGradient('glow', 0.5, 0.5, 0.5) {|g|
  g.stop(0, Color('#ffffff'));
  g.stop(1, Color('#000000').alpha(0));
};

Constructor signature: RadialGradient(id, cx, cy, r) — optional focal point: RadialGradient(id, cx, cy, r, fx, fy).

Trailing Block Syntax

Both constructors accept a trailing block {|g| ... } where g is bound to the newly created gradient. Use g.stop(offset, color) inside the block to add color stops:

• offset — a number from 0 to 1 (position along the gradient axis)
• color — any Color value (Color('#hex'), Color('named'), OKLCH constructor, etc.)

The block is optional — you can create an empty gradient and add stops later or use .inherit() to derive from another gradient.

let empty = LinearGradient('empty', 0, 0, 1, 0);

Using Gradients in Styles

Reference a gradient in fill or stroke style properties. The compiler automatically wraps the gradient ID as url(#id):

let g = LinearGradient('sunset', 0, 0, 1, 0) {|g|
  g.stop(0, Color('#e63946'));
  g.stop(1, Color('#2a9d8f'));
};

define PathLayer('bg') ${ fill: g; stroke: none; }

layer('bg').apply {
  M 0 0 L 200 0 L 200 200 L 0 200 Z
}

This produces fill="url(#sunset)" on the output <path> element, with a <linearGradient id="sunset"> in <defs>.

Gradient Attributes

Set optional attributes via property assignment after creation:

let g = LinearGradient('repeat-fade', 0, 0, 0.25, 0) {|g|
  g.stop(0, Color('#e63946'));
  g.stop(1, Color('#2a9d8f'));
};

g.spreadMethod = 'repeat';
g.gradientUnits = 'userSpaceOnUse';
g.gradientTransform = 'rotate(45)';

Color Interpolation

Control how colors transition between stops using the interpolation property.

OKLCh Interpolation

Set interpolation = 'oklch' for perceptually uniform transitions. The compiler expands stops at compile time using OKLCh color mixing, avoiding the muddy midpoints common with sRGB interpolation:

let smooth = LinearGradient('smooth', 0, 0, 1, 0) {|g|
  g.stop(0, Color('#e63946'));
  g.stop(1, Color('#2a9d8f'));
};
smooth.interpolation = 'oklch';
smooth.steps = 12;  // 12 intermediate stops per unit offset (default: 10)

The steps property controls the density of generated intermediate stops. Higher values produce smoother transitions but increase SVG output size. The compiler:

1. Iterates adjacent stop pairs
2. Generates ceil(steps * offsetSpan) - 1 intermediate stops between each pair
3. Uses mixColors() for shortest-arc hue interpolation in OKLCh space
4. Always preserves the original stops at their exact offsets

linearRGB Interpolation

Set interpolation = 'linearRGB' for physically linear color transitions. This uses the native SVG color-interpolation attribute — no stop expansion is needed:

let physical = LinearGradient('physical', 0, 0, 1, 0) {|g|
  g.stop(0, Color('#ff0000'));
  g.stop(1, Color('#0000ff'));
};
physical.interpolation = 'linearRGB';

This emits color-interpolation="linearRGB" on the gradient element. The browser handles the interpolation natively.

Default (sRGB)

When interpolation is not set (or set to 'srgb'), the browser's default sRGB interpolation is used. No additional attributes or stop expansion occur.

Reactive Gradient Stops

Use Color(CSSVar(...)) in gradient stops to create live-updating gradients that respond to CSS custom property changes:

let accent = Color(CSSVar('--accent', '#e63946'));
let reactive = LinearGradient('reactive', 0, 0, 1, 0) {|g|
  g.stop(0, accent);            // → stop-color="var(--accent, #e63946)"
  g.stop(1, Color('#2a9d8f'));
};

The compiler preserves the var() reference in the stop-color attribute, allowing the gradient to update when the custom property changes at runtime.

CSSVar stops are skipped during OKLCh expansion — since their actual color is determined at runtime, the compiler cannot interpolate them at compile time. Non-CSSVar stops adjacent to CSSVar stops will not have intermediate stops generated between them.

Pattern Paint Server

Create a tiling pattern with an ID, position, and tile dimensions:

let dot = @{ circle(10, 10, 3) };
let dots = Pattern('dots', 0, 0, 20, 20) {|p|
  p.append(dot, ${ fill: Color('#e63946'); });
};
dots.patternUnits = 'userSpaceOnUse';

Constructor signature: Pattern(id, x, y, width, height) — defines the tile origin and size.

Pattern Methods

Use .append(pathBlock, styles?) inside the trailing block to add path elements to the pattern. This works the same way as Mask.append():

• pathBlock — a PathBlock (@{ ... }) or ProjectedPath
• styles — optional style block for the path element

let line = @{ m 0 0 l 20 20 };
let hatch = Pattern('hatch', 0, 0, 20, 20) {|p|
  p.append(line, ${ stroke: Color('#999'); stroke-width: 1; });
};

Pattern Properties

Using Patterns in Styles

Reference a pattern in fill or stroke style properties, just like gradients:

define PathLayer('bg') ${ fill: dots; stroke: none; }
layer('bg').apply { M 0 0 L 200 0 L 200 200 L 0 200 Z }

This produces fill="url(#dots)" on the output <path> element.

Pattern SVG Output

<defs>
  <pattern id="dots" x="0" y="0" width="20" height="20" patternUnits="userSpaceOnUse">
    <path d="M 7 10 A 3 3 0 0 1 13 10 A 3 3 0 0 1 7 10" fill="#e63946"/>
  </pattern>
</defs>

Conic Gradient

Create a conic (angular) gradient with an ID and center point:

let wheel = ConicGradient('wheel', 100, 100) {|g|
  g.stop(0, Color('#e63946'));
  g.stop(0.33, Color('#2a9d8f'));
  g.stop(0.66, Color('#264653'));
  g.stop(1, Color('#e63946'));
};

Constructor signature: ConicGradient(id, cx, cy) — center coordinates in user space.

Conic gradients use the same .stop(offset, color) method as linear and radial gradients. Stops map to the angular sweep: offset 0 is the start angle, offset 1 is the end angle.

Conic Gradient Properties

Angle Units Required

The from and to properties require an angle unit suffix on literal numbers:

gauge.from = 135deg;     // degrees → converted to radians
gauge.to = 2.356rad;     // radians (used as-is)
gauge.from = 0.75pi;     // multiples of π

gauge.from = 135;        // ERROR: requires angle unit. Use 135deg

Computed expressions and function results are accepted without unit checks (they are assumed to already be in radians):

gauge.from = rad(135);   // OK — rad() returns radians

Partial Sweep

Set from and to for arcs less than (or greater than) a full revolution:

// Gauge: 270° arc with gap at bottom
let gauge = ConicGradient('gauge', 100, 100) {|g|
  g.stop(0, Color('#2a9d8f'));
  g.stop(0.5, Color('#e9c46a'));
  g.stop(1, Color('#e63946'));
};
gauge.from = 135deg;
gauge.to = 405deg;

Direction

direction controls which way colors sweep within the arc:

• 'cw' (default) — colors flow clockwise from from to to
• 'ccw' — colors flow counter-clockwise (stop offsets are reversed)

let reversed = ConicGradient('rev', 100, 100) {|g|
  g.stop(0, Color('#000'));
  g.stop(1, Color('#fff'));
};
reversed.direction = 'ccw';

Spread Modes

spread controls what happens outside the [from, to] arc for partial sweeps: