mipmap: Bug fix: DrawTriangles with a scale-only color matrix should be optimized

Fixes #1101

image_test.go

@@ -1902,3 +1902,112 @@ func TestImageReplacePixelsOnSubImage(t *testing.T) {
 		}
 	}
 }
+
+func TestImageDrawTrianglesWithColorM(t *testing.T) {
+	const w, h = 16, 16
+	dst0, _ := NewImage(w, h, FilterDefault)
+	dst1, _ := NewImage(w, h, FilterDefault)
+	src, _ := NewImage(w, h, FilterDefault)
+	src.Fill(color.White)
+
+	vs0 := []Vertex{
+		{
+			DstX:   0,
+			DstY:   0,
+			SrcX:   0,
+			SrcY:   0,
+			ColorR: 1,
+			ColorG: 1,
+			ColorB: 1,
+			ColorA: 1,
+		},
+		{
+			DstX:   w,
+			DstY:   0,
+			SrcX:   w,
+			SrcY:   0,
+			ColorR: 1,
+			ColorG: 1,
+			ColorB: 1,
+			ColorA: 1,
+		},
+		{
+			DstX:   0,
+			DstY:   h,
+			SrcX:   0,
+			SrcY:   h,
+			ColorR: 1,
+			ColorG: 1,
+			ColorB: 1,
+			ColorA: 1,
+		},
+		{
+			DstX:   w,
+			DstY:   h,
+			SrcX:   w,
+			SrcY:   h,
+			ColorR: 1,
+			ColorG: 1,
+			ColorB: 1,
+			ColorA: 1,
+		},
+	}
+	op := &DrawTrianglesOptions{}
+	op.ColorM.Scale(0.2, 0.4, 0.6, 0.8)
+	is := []uint16{0, 1, 2, 1, 2, 3}
+	dst0.DrawTriangles(vs0, is, src, op)
+
+	vs1 := []Vertex{
+		{
+			DstX:   0,
+			DstY:   0,
+			SrcX:   0,
+			SrcY:   0,
+			ColorR: 0.2,
+			ColorG: 0.4,
+			ColorB: 0.6,
+			ColorA: 0.8,
+		},
+		{
+			DstX:   w,
+			DstY:   0,
+			SrcX:   w,
+			SrcY:   0,
+			ColorR: 0.2,
+			ColorG: 0.4,
+			ColorB: 0.6,
+			ColorA: 0.8,
+		},
+		{
+			DstX:   0,
+			DstY:   h,
+			SrcX:   0,
+			SrcY:   h,
+			ColorR: 0.2,
+			ColorG: 0.4,
+			ColorB: 0.6,
+			ColorA: 0.8,
+		},
+		{
+			DstX:   w,
+			DstY:   h,
+			SrcX:   w,
+			SrcY:   h,
+			ColorR: 0.2,
+			ColorG: 0.4,
+			ColorB: 0.6,
+			ColorA: 0.8,
+		},
+	}
+	dst1.DrawTriangles(vs1, is, src, nil)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst0.At(i, j)
+			want := dst1.At(i, j)
+			if got != want {
+				t.Errorf("At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}

internal/mipmap/mipmap.go

@@ -132,7 +132,7 @@ func (m *Mipmap) DrawImage(src *Mipmap, bounds image.Rectangle, geom GeoM, color
 	}
 
 	cr, cg, cb, ca := float32(1), float32(1), float32(1), float32(1)
-	if colorm.ScaleOnly() {
+	if colorm != nil && colorm.ScaleOnly() {
 		body, _ := colorm.UnsafeElements()
 		cr = body[0]
 		cg = body[5]
@@ -166,6 +166,23 @@ func (m *Mipmap) DrawImage(src *Mipmap, bounds image.Rectangle, geom GeoM, color
 }
 
 func (m *Mipmap) DrawTriangles(src *Mipmap, vertices []float32, indices []uint16, colorm *affine.ColorM, mode driver.CompositeMode, filter driver.Filter, address driver.Address) {
+	// TODO: Use a mipmap? (#909)
+
+	if colorm != nil && colorm.ScaleOnly() {
+		body, _ := colorm.UnsafeElements()
+		cr := body[0]
+		cg := body[5]
+		cb := body[10]
+		ca := body[15]
+		colorm = nil
+		const n = graphics.VertexFloatNum
+		for i := 0; i < len(vertices)/n; i++ {
+			vertices[i*n+8] *= cr
+			vertices[i*n+9] *= cg
+			vertices[i*n+10] *= cb
+			vertices[i*n+11] *= ca
+		}
+	}
 	m.orig.DrawTriangles(src.orig, vertices, indices, colorm, mode, filter, address)
 	m.disposeMipmaps()
 }