stroke

Overview ¶

Package stroke provides stroke expansion algorithms for converting stroked paths to filled outlines.

This package implements CPU-side stroke expansion following tiny-skia and kurbo patterns. The algorithm converts a path with stroke style into a filled path suitable for GPU rasterization.

Algorithm Overview ¶

Stroke expansion works by building two parallel offset paths:

• Forward path: Offset by +width/2 perpendicular to the tangent
• Backward path: Offset by -width/2 perpendicular to the tangent

The final filled path is constructed by:

1. Forward path goes forward
2. End cap connects forward to backward
3. Backward path is reversed
4. Start cap connects backward to forward and closes

Line Caps ¶

Line caps define the shape of stroke endpoints:

• LineCapButt: Flat cap ending exactly at the endpoint
• LineCapRound: Semicircular cap with radius = width/2
• LineCapSquare: Square cap extending width/2 beyond the endpoint

Line Joins ¶

Line joins define how stroke segments connect:

• LineJoinMiter: Sharp corner (limited by miter limit)
• LineJoinRound: Circular arc at corners
• LineJoinBevel: Straight line across the corner

Usage ¶

style := stroke.Stroke{
    Width:      2.0,
    Cap:        stroke.LineCapRound,
    Join:       stroke.LineJoinMiter,
    MiterLimit: 4.0,
}

expander := stroke.NewStrokeExpander(style)
expander.SetTolerance(0.1) // Optional: adjust curve flattening

verbs := []stroke.PathVerb{stroke.VerbMoveTo, stroke.VerbLineTo, stroke.VerbLineTo}
coords := []float64{0, 0, 100, 0, 100, 100}

outVerbs, outCoords := expander.Expand(verbs, coords)

Performance ¶

The expander is optimized for typical use cases:

• Simple lines: ~2 microseconds
• Complex paths (100 segments): ~50-60 microseconds
• Cubic curves are flattened to line segments based on tolerance

References ¶

The algorithm is based on:

• tiny-skia (Rust): path/src/stroker.rs
• kurbo (Rust): src/stroke.rs

See GPU-STK-001 in docs/dev/kanban/ for the full task specification.

Package stroke provides stroke expansion algorithms for converting stroked paths to filled outlines.

This package implements CPU-side stroke expansion following tiny-skia and kurbo patterns. The algorithm converts a path with stroke style into a filled path suitable for GPU rasterization.

Key algorithm insight: A stroke is converted to a FILL path where:

• The outer offset path goes forward
• The inner offset path is reversed
• Line caps connect the endpoints
• Line joins connect the segments