gpu,op/clip: implement stroked paths

Flat and Square caps are implemented.
Bevel joins are implemented.

Round caps, Round joins and Miter joins are left for another PR.

Signed-off-by: Sebastien Binet <s@sbinet.org>

op/clip/clip.go

@@ -39,6 +39,8 @@ func (p *Path) Pos() f32.Point { return p.pen }
 type Op struct {
 	call   op.CallOp
 	bounds image.Rectangle
+	width  float32     // Width of the stroked path, 0 for outline paths.
+	style  StrokeStyle // Style of the stroked path, 0 for outline paths.
 }
 
 func (p Op) Add(o *op.Ops) {
@@ -50,6 +52,8 @@ func (p Op) Add(o *op.Ops) {
 	bo.PutUint32(data[5:], uint32(p.bounds.Min.Y))
 	bo.PutUint32(data[9:], uint32(p.bounds.Max.X))
 	bo.PutUint32(data[13:], uint32(p.bounds.Max.Y))
+	bo.PutUint32(data[17:], math.Float32bits(p.width))
+	data[21] = uint8(p.style.Cap)
 }
 
 // Begin the path, storing the path data and final Op into ops.
@@ -321,6 +325,26 @@ func (p *Path) Outline() Op {
 	}
 }
 
+// Stroke returns a stroked path with the specified width
+// and configuration.
+// If the provided width is <= 0, the path won't be stroked.
+func (p *Path) Stroke(width float32, sty StrokeStyle) Op {
+	if width <= 0 {
+		// Explicitly discard the macro to ignore the path.
+		p.macro.Stop()
+		return Op{
+			call: op.Record(p.ops).Stop(),
+		}
+	}
+
+	c := p.macro.Stop()
+	return Op{
+		call:  c,
+		width: width,
+		style: sty,
+	}
+}
+
 // Rect represents the clip area of a pixel-aligned rectangle.
 type Rect image.Rectangle
 

op/clip/stroke.go

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package clip
+
+// StrokeStyle describes how a stroked path should be drawn.
+// StrokeStyle zero value draws a Bevel-joined and Flat-capped stroked path.
+type StrokeStyle struct {
+	Cap StrokeCap
+}
+
+// StrokeCap describes the head or tail of a stroked path.
+type StrokeCap uint8
+
+const (
+	// FlatCap caps stroked paths with a flat cap, joining the right-hand
+	// and left-hand sides of a stroked path with a straight line.
+	FlatCap StrokeCap = iota
+
+	// SquareCap caps stroked paths with a square cap, joining the right-hand
+	// and left-hand sides of a stroked path with a half square of length
+	// the stroked path's width.
+	SquareCap
+)