gpu/internal/rendertest: add regression test for issue 331

References: https://todo.sr.ht/~eliasnaur/gio/331
Signed-off-by: Andy Balholm <andy@balholm.com>

gpu/internal/rendertest/clip_test.go

@@ -185,6 +185,28 @@ func TestStrokedPathZeroWidth(t *testing.T) {
 	})
 }
 
+func TestStrokedPathCoincidentControlPoint(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		p := new(clip.Path)
+		p.Begin(o)
+		p.MoveTo(f32.Pt(70, 20))
+		p.CubeTo(f32.Pt(70, 20), f32.Pt(70, 110), f32.Pt(120, 120))
+		p.LineTo(f32.Pt(20, 120))
+		p.LineTo(f32.Pt(70, 20))
+		cl := clip.Stroke{
+			Path:  p.End(),
+			Width: 20,
+		}.Op().Push(o)
+
+		paint.Fill(o, black)
+		cl.Pop()
+	}, func(r result) {
+		r.expect(0, 0, transparent)
+		r.expect(70, 20, colornames.Black)
+		r.expect(70, 90, transparent)
+	})
+}
+
 func TestPathReuse(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		var path clip.Path

gpu/internal/rendertest/util_test.go

@@ -195,7 +195,7 @@ func verifyRef(t *testing.T, img *image.RGBA, frame int) (ok bool) {
 		for y := bnd.Min.Y; y < bnd.Max.Y; y++ {
 			exp := ref.RGBAAt(x, y)
 			got := img.RGBAAt(x, y)
-			if !colorsClose(exp, got) {
+			if !colorsClose(exp, got) || !alphaClose(exp, got) {
 				t.Error("not equal to ref at", x, y, " ", got, exp)
 				return false
 			}
@@ -209,6 +209,11 @@ func colorsClose(c1, c2 color.RGBA) bool {
 	return yiqEqApprox(c1, c2, delta)
 }
 
+func alphaClose(c1, c2 color.RGBA) bool {
+	d := int8(c1.A - c2.A)
+	return d > -5 && d < 5
+}
+
 // yiqEqApprox compares the colors of 2 pixels, in the NTSC YIQ color space,
 // as described in:
 //