shareable: Use the shared backend again in some conditions

Fixes #633

internal/shareable/shareable.go

@@ -91,13 +91,20 @@ type Image struct {
 
 	backend *backend
 
-	node *packing.Node
+	node        *packing.Node
+	sharedCount int
 }
 
 func (i *Image) isShared() bool {
 	return i.node != nil
 }
 
+func (i *Image) IsSharedForTesting() bool {
+	backendsM.Lock()
+	defer backendsM.Unlock()
+	return i.isShared()
+}
+
 func (i *Image) ensureNotShared() {
 	if i.backend == nil {
 		i.allocate(false)
@@ -121,6 +128,36 @@ func (i *Image) ensureNotShared() {
 	}
 }
 
+func (i *Image) forceShared() {
+	if i.backend == nil {
+		i.allocate(false)
+		return
+	}
+
+	if i.isShared() {
+		return
+	}
+
+	if !i.shareable() {
+		panic("not reached")
+	}
+
+	newI := NewImage(i.width, i.height)
+	pixels := make([]byte, 4*i.width*i.height)
+	for y := 0; y < i.height; y++ {
+		for x := 0; x < i.width; x++ {
+			c := i.at(x, y).(color.RGBA)
+			pixels[4*(x+i.width*y)] = c.R
+			pixels[4*(x+i.width*y)+1] = c.G
+			pixels[4*(x+i.width*y)+2] = c.B
+			pixels[4*(x+i.width*y)+3] = c.A
+		}
+	}
+	newI.replacePixels(pixels)
+	i.dispose(false)
+	*i = *newI
+}
+
 func (i *Image) region() (x, y, width, height int) {
 	if i.backend == nil {
 		panic("not reached")
@@ -145,6 +182,8 @@ func (i *Image) QuadVertices(sx0, sy0, sx1, sy1 int, a, b, c, d, tx, ty float32)
 	return graphicsutil.QuadVertices(w, h, sx0+dx, sy0+dy, sx1+dx, sy1+dy, a, b, c, d, tx, ty)
 }
 
+const ReshareCount = 10
+
 func (i *Image) DrawImage(img *Image, vertices []float32, indices []uint16, colorm *affine.ColorM, mode opengl.CompositeMode, filter graphics.Filter) {
 	backendsM.Lock()
 	defer backendsM.Unlock()
@@ -168,12 +207,24 @@ func (i *Image) DrawImage(img *Image, vertices []float32, indices []uint16, colo
 	}
 
 	i.backend.restorable.DrawImage(img.backend.restorable, vertices, indices, colorm, mode, filter)
+
+	i.sharedCount = 0
+	if !img.isShared() && img.shareable() {
+		img.sharedCount++
+		if img.sharedCount >= ReshareCount {
+			img.forceShared()
+			img.sharedCount = 0
+		}
+	}
 }
 
 func (i *Image) ReplacePixels(p []byte) {
 	backendsM.Lock()
 	defer backendsM.Unlock()
+	i.replacePixels(p)
+}
 
+func (i *Image) replacePixels(p []byte) {
 	if i.disposed {
 		panic("shareable: the image must not be disposed")
 	}
@@ -191,7 +242,10 @@ func (i *Image) ReplacePixels(p []byte) {
 func (i *Image) At(x, y int) color.Color {
 	backendsM.Lock()
 	defer backendsM.Unlock()
+	return i.at(x, y)
+}
 
+func (i *Image) at(x, y int) color.Color {
 	if i.backend == nil {
 		return color.RGBA{}
 	}
@@ -271,12 +325,16 @@ func NewImage(width, height int) *Image {
 	}
 }
 
+func (i *Image) shareable() bool {
+	return i.width <= maxSize && i.height <= maxSize
+}
+
 func (i *Image) allocate(shareable bool) {
 	if i.backend != nil {
 		panic("not reached")
 	}
 
-	if !shareable || i.width > maxSize || i.height > maxSize {
+	if !shareable || !i.shareable() {
 		i.backend = &backend{
 			restorable: restorable.NewImage(i.width, i.height, false),
 		}

internal/shareable/shareable_test.go

@@ -88,6 +88,10 @@ func TestEnsureNotShared(t *testing.T) {
 	vs := img3.QuadVertices(0, 0, size/2, size/2, 1, 0, 0, 1, size/4, size/4)
 	is := graphicsutil.QuadIndices()
 	img4.DrawImage(img3, vs, is, nil, opengl.CompositeModeCopy, graphics.FilterNearest)
+	want := false
+	if got := img4.IsSharedForTesting(); got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
 
 	for j := 0; j < size; j++ {
 		for i := 0; i < size; i++ {
@@ -107,3 +111,76 @@ func TestEnsureNotShared(t *testing.T) {
 	// This bug was fixed by 03dcd948.
 	img4.DrawImage(img3, vs, is, nil, opengl.CompositeModeCopy, graphics.FilterNearest)
 }
+
+func TestReshared(t *testing.T) {
+	const size = 16
+
+	img0 := NewImage(size, size)
+	defer img0.Dispose()
+	img0.ReplacePixels(make([]byte, 4*size*size))
+
+	img1 := NewImage(size, size)
+	defer img1.Dispose()
+	img1.ReplacePixels(make([]byte, 4*size*size))
+	want := true
+	if got := img1.IsSharedForTesting(); got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+
+	img2 := NewImage(size, size)
+	defer img2.Dispose()
+	pix := make([]byte, 4*size*size)
+	for j := 0; j < size; j++ {
+		for i := 0; i < size; i++ {
+			pix[4*(i+j*size)] = byte(i + j)
+			pix[4*(i+j*size)+1] = byte(i + j)
+			pix[4*(i+j*size)+2] = byte(i + j)
+			pix[4*(i+j*size)+3] = byte(i + j)
+		}
+	}
+	img2.ReplacePixels(pix)
+
+	// Use img1 as a render target.
+	vs := img2.QuadVertices(0, 0, size, size, 1, 0, 0, 1, 0, 0)
+	is := graphicsutil.QuadIndices()
+	img1.DrawImage(img2, vs, is, nil, opengl.CompositeModeCopy, graphics.FilterNearest)
+	want = false
+	if got := img1.IsSharedForTesting(); got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+
+	// Use img1 as a render source.
+	for i := 0; i < ReshareCount-1; i++ {
+		img0.DrawImage(img1, vs, is, nil, opengl.CompositeModeCopy, graphics.FilterNearest)
+		want := false
+		if got := img1.IsSharedForTesting(); got != want {
+			t.Errorf("got: %v, want: %v", got, want)
+		}
+	}
+
+	for j := 0; j < size; j++ {
+		for i := 0; i < size; i++ {
+			want := color.RGBA{byte(i + j), byte(i + j), byte(i + j), byte(i + j)}
+			got := img1.At(i, j).(color.RGBA)
+			if got != want {
+				t.Errorf("got: %v, want: %v", got, want)
+			}
+		}
+	}
+
+	img0.DrawImage(img1, vs, is, nil, opengl.CompositeModeCopy, graphics.FilterNearest)
+	want = true
+	if got := img1.IsSharedForTesting(); got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+
+	for j := 0; j < size; j++ {
+		for i := 0; i < size; i++ {
+			want := color.RGBA{byte(i + j), byte(i + j), byte(i + j), byte(i + j)}
+			got := img1.At(i, j).(color.RGBA)
+			if got != want {
+				t.Errorf("got: %v, want: %v", got, want)
+			}
+		}
+	}
+}