From 3fd231367f4a316c9474a8ba9ca6e129a97fe272 Mon Sep 17 00:00:00 2001
From: Egon Elbre <egonelbre@gmail.com>
Date: Sat, 12 Mar 2022 14:11:56 +0200
Subject: [PATCH] internal/stroke: optimize arc drawing

Arc with a small angle doesn't need many segments.

Updates: https://todo.sr.ht/~eliasnaur/gio/313
Signed-off-by: Egon Elbre <egonelbre@gmail.com>
---
 gpu/internal/rendertest/refs/TestPaintArc.png | Bin 1390 -> 1747 bytes
 .../TestStrokedPathCoincidentControlPoint.png | Bin 1918 -> 1895 bytes
 .../rendertest/refs/TestTexturedStroke.png    | Bin 390 -> 692 bytes
 .../refs/TestTexturedStrokeClipped.png        | Bin 390 -> 692 bytes
 internal/stroke/stroke.go                     |  19 ++++++++++++++----
 op/clip/clip.go                               |   4 +---
 6 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/gpu/internal/rendertest/refs/TestPaintArc.png b/gpu/internal/rendertest/refs/TestPaintArc.png
index f432914fa9ce36b4c608d9ff49869a8760512776..9ebbe5bf42e98ce685e9e3a90285e621108660f3 100644
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literal 390
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diff --git a/gpu/internal/rendertest/refs/TestTexturedStrokeClipped.png b/gpu/internal/rendertest/refs/TestTexturedStrokeClipped.png
index 637c93294a8284b771fee032ee2368d5662cef83..8da06b39b2a953f7c9df1f377a08d1998fa326a5 100644
GIT binary patch
literal 692
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literal 390
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diff --git a/internal/stroke/stroke.go b/internal/stroke/stroke.go
index 741e56a1..a115dc79 100644
--- a/internal/stroke/stroke.go
+++ b/internal/stroke/stroke.go
@@ -91,9 +91,8 @@ func (qs *StrokeQuads) lineTo(pt f32.Point) {
 }
 
 func (qs *StrokeQuads) arc(f1, f2 f32.Point, angle float32) {
-	const segments = 16
 	pen := qs.pen()
-	m := ArcTransform(pen, f1.Add(pen), f2.Add(pen), angle, segments)
+	m, segments := ArcTransform(pen, f1.Add(pen), f2.Add(pen), angle)
 	for i := 0; i < segments; i++ {
 		p0 := qs.pen()
 		p1 := m.Transform(p0)
@@ -546,7 +545,19 @@ func strokePathRoundCap(qs *StrokeQuads, hw float32, pivot, n0 f32.Point) {
 //   cubic Bezier curves", L. Maisonobe
 // An electronic version may be found at:
 //  http://spaceroots.org/documents/ellipse/elliptical-arc.pdf
-func ArcTransform(p, f1, f2 f32.Point, angle float32, segments int) f32.Affine2D {
+func ArcTransform(p, f1, f2 f32.Point, angle float32) (transform f32.Affine2D, segments int) {
+	const segmentsPerCircle = 16
+	const anglePerSegment = 2 * math.Pi / segmentsPerCircle
+
+	s := angle / anglePerSegment
+	if s < 0 {
+		s = -s
+	}
+	segments = int(math.Ceil(float64(s)))
+	if segments <= 0 {
+		segments = 1
+	}
+
 	var rx, ry, alpha float64
 	if f1 == f2 {
 		// degenerate case of a circle.
@@ -605,7 +616,7 @@ func ArcTransform(p, f1, f2 f32.Point, angle float32, segments int) f32.Affine2D
 	// Before applying the rotation matrix rot, transform the coordinates
 	// to a frame centered to the ellipse (and warped into a unit circle), then rotate.
 	// Finally, transform back into the original frame.
-	return inv.Mul(rot).Mul(ref)
+	return inv.Mul(rot).Mul(ref), segments
 }
 
 func dist(p1, p2 f32.Point) float64 {
diff --git a/op/clip/clip.go b/op/clip/clip.go
index d251d7a0..3ac02fe8 100644
--- a/op/clip/clip.go
+++ b/op/clip/clip.go
@@ -286,9 +286,7 @@ func (p *Path) QuadTo(ctrl, to f32.Point) {
 // The sign of angle determines the direction; positive being counter-clockwise,
 // negative clockwise.
 func (p *Path) ArcTo(f1, f2 f32.Point, angle float32) {
-	const segments = 16
-	m := stroke.ArcTransform(p.pen, f1, f2, angle, segments)
-
+	m, segments := stroke.ArcTransform(p.pen, f1, f2, angle)
 	for i := 0; i < segments; i++ {
 		p0 := p.pen
 		p1 := m.Transform(p0)