From 1106d90f1102faeecaaafac2b78c50bbd7741cde Mon Sep 17 00:00:00 2001
From: Sebastien Binet <s@sbinet.org>
Date: Tue, 10 Nov 2020 15:46:10 +0000
Subject: [PATCH] gpu,op/clip: implement stroked paths with round caps

Signed-off-by: Sebastien Binet <s@sbinet.org>
---
 gpu/stroke.go                                 |  34 ++++++++++++++++++
 internal/rendertest/clip_test.go              |  26 ++++++++++++++
 .../refs/TestStrokedPathBevelRound.png        | Bin 0 -> 3407 bytes
 op/clip/stroke.go                             |   5 +++
 4 files changed, 65 insertions(+)
 create mode 100644 internal/rendertest/refs/TestStrokedPathBevelRound.png

diff --git a/gpu/stroke.go b/gpu/stroke.go
index 75e9189e..2c6ecea4 100644
--- a/gpu/stroke.go
+++ b/gpu/stroke.go
@@ -28,6 +28,7 @@ import (
 
 	"gioui.org/f32"
 	"gioui.org/internal/ops"
+	"gioui.org/op"
 	"gioui.org/op/clip"
 )
 
@@ -45,6 +46,10 @@ type strokeState struct {
 
 type strokeQuads []strokeQuad
 
+func (qs *strokeQuads) pen() f32.Point {
+	return (*qs)[len(*qs)-1].quad.To
+}
+
 func (qs *strokeQuads) lineTo(pt f32.Point) {
 	end := (*qs)[len(*qs)-1].quad.To
 	*qs = append(*qs, strokeQuad{
@@ -56,6 +61,27 @@ func (qs *strokeQuads) lineTo(pt f32.Point) {
 	})
 }
 
+func (qs *strokeQuads) arc(f1, f2 f32.Point, angle float32) {
+	var (
+		p clip.Path
+		o = new(op.Ops)
+	)
+	p.Begin(o)
+	p.Move(qs.pen())
+	beg := len(o.Data())
+	p.Arc(f1, f2, angle)
+	end := len(o.Data())
+	raw := o.Data()[beg:end]
+
+	for qi := 0; len(raw) >= (ops.QuadSize + 4); qi++ {
+		quad := ops.DecodeQuad(raw[4:])
+		raw = raw[ops.QuadSize+4:]
+		*qs = append(*qs, strokeQuad{
+			quad: quad,
+		})
+	}
+}
+
 // split splits a slice of quads into slices of quads grouped
 // by contours (ie: splitted at move-to boundaries).
 func (qs strokeQuads) split() []strokeQuads {
@@ -409,6 +435,8 @@ func strokePathCap(sty clip.StrokeStyle, qs *strokeQuads, hw float32, pivot, n0
 		strokePathFlatCap(qs, hw, pivot, n0)
 	case clip.SquareCap:
 		strokePathSquareCap(qs, hw, pivot, n0)
+	case clip.RoundCap:
+		strokePathRoundCap(qs, hw, pivot, n0)
 	default:
 		panic("impossible")
 	}
@@ -433,3 +461,9 @@ func strokePathSquareCap(qs *strokeQuads, hw float32, pivot, n0 f32.Point) {
 	qs.lineTo(corner2)
 	qs.lineTo(end)
 }
+
+// strokePathRoundCap caps the start or end of a path with a round cap.
+func strokePathRoundCap(qs *strokeQuads, hw float32, pivot, n0 f32.Point) {
+	c := pivot.Sub(qs.pen())
+	qs.arc(c, c, math.Pi)
+}
diff --git a/internal/rendertest/clip_test.go b/internal/rendertest/clip_test.go
index c20c6dc0..9a1aef01 100644
--- a/internal/rendertest/clip_test.go
+++ b/internal/rendertest/clip_test.go
@@ -129,6 +129,32 @@ func TestStrokedPathBevelFlat(t *testing.T) {
 	})
 }
 
+func TestStrokedPathBevelRound(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		const width = 2.5
+		sty := clip.StrokeStyle{
+			Cap: clip.RoundCap,
+		}
+
+		p := new(clip.Path)
+		p.Begin(o)
+		p.Move(f32.Pt(10, 50))
+		p.Line(f32.Pt(10, 0))
+		p.Arc(f32.Pt(10, 0), f32.Pt(20, 0), math.Pi)
+		p.Line(f32.Pt(10, 0))
+		p.Line(f32.Pt(10, 10))
+		p.Arc(f32.Pt(0, 30), f32.Pt(0, 30), 2*math.Pi)
+		p.Line(f32.Pt(-20, 0))
+		p.Quad(f32.Pt(-10, -10), f32.Pt(-30, 30))
+		p.Stroke(width, sty).Add(o)
+
+		paint.Fill(o, colornames.Red)
+	}, func(r result) {
+		r.expect(0, 0, colornames.White)
+		r.expect(10, 50, colornames.Red)
+	})
+}
+
 func TestStrokedPathBevelSquare(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		const width = 2.5
diff --git a/internal/rendertest/refs/TestStrokedPathBevelRound.png b/internal/rendertest/refs/TestStrokedPathBevelRound.png
new file mode 100644
index 0000000000000000000000000000000000000000..36ba27a261a4fc6104e03999c2ac4a7be1f50908
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diff --git a/op/clip/stroke.go b/op/clip/stroke.go
index c85fd49b..b693435f 100644
--- a/op/clip/stroke.go
+++ b/op/clip/stroke.go
@@ -20,4 +20,9 @@ const (
 	// and left-hand sides of a stroked path with a half square of length
 	// the stroked path's width.
 	SquareCap
+
+	// RoundCap caps stroked paths with a round cap, joining the right-hand and
+	// left-hand sides of a stroked path with a half disc of diameter the
+	// stroked path's width.
+	RoundCap
 )