gpu,op/clip: implement dashed stroked paths

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

op/clip/stroke.go

@@ -2,10 +2,22 @@
 
 package clip
 
+import (
+	"encoding/binary"
+	"math"
+
+	"gioui.org/internal/opconst"
+	"gioui.org/op"
+)
+
 // Stroke represents a stroked path.
 type Stroke struct {
 	Path  PathSpec
 	Style StrokeStyle
+
+	// Dashes specify the dashes of the stroke.
+	// The empty value denotes no dashes.
+	Dashes DashSpec
 }
 
 // Op returns a clip operation representing the stroke.
@@ -13,6 +25,7 @@ func (s Stroke) Op() Op {
 	return Op{
 		path:   s.Path,
 		stroke: s.Style,
+		dashes: s.Dashes,
 	}
 }
 
@@ -57,3 +70,49 @@ const (
 	// RoundJoin joins path segments with a round segment.
 	RoundJoin
 )
+
+// Dash records dashes' lengths and phase for a stroked path.
+type Dash struct {
+	ops   *op.Ops
+	macro op.MacroOp
+	phase float32
+	size  uint8 // size of the pattern
+}
+
+func (d *Dash) Begin(ops *op.Ops) {
+	d.ops = ops
+	d.macro = op.Record(ops)
+	// Write the TypeAux opcode
+	data := ops.Write(opconst.TypeAuxLen)
+	data[0] = byte(opconst.TypeAux)
+}
+
+func (d *Dash) Phase(v float32) {
+	d.phase = v
+}
+
+func (d *Dash) Dash(length float32) {
+	if d.size == math.MaxUint8 {
+		panic("clip: too large dash pattern")
+	}
+	data := d.ops.Write(4)
+	bo := binary.LittleEndian
+	bo.PutUint32(data[0:], math.Float32bits(length))
+	d.size++
+}
+
+func (d *Dash) End() DashSpec {
+	c := d.macro.Stop()
+	return DashSpec{
+		spec:  c,
+		phase: d.phase,
+		size:  d.size,
+	}
+}
+
+// DashSpec describes a dashed pattern.
+type DashSpec struct {
+	spec  op.CallOp
+	phase float32
+	size  uint8 // size of the pattern
+}