internal/graphicsdriver: replace Pixels with ReadPixels

Now preparing a byte slice is the caller's responsibility.

internal/graphicscommand/command.go

@@ -605,11 +605,9 @@ type pixelsCommand struct {
 
 // Exec executes a pixelsCommand.
 func (c *pixelsCommand) Exec(indexOffset int) error {
-	p, err := c.img.image.Pixels()
+	if err := c.img.image.ReadPixels(c.result); err != nil {
-	if err != nil {
 		return err
 	}
-	c.result = p
 	return nil
 }
 

internal/graphicscommand/image.go

@@ -165,18 +165,19 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageNum]*Image, offsets [gra
 	theCommandQueue.EnqueueDrawTrianglesCommand(i, srcs, offsets, vertices, indices, clr, mode, filter, address, dstRegion, srcRegion, shader, uniforms, evenOdd)
 }
 
-// Pixels returns the image's pixels.
+// ReadPixels reads the image's pixels.
-// Pixels might return nil when OpenGL error happens.
+// ReadPixels returns an error when an error happens in the graphics driver.
-func (i *Image) Pixels() ([]byte, error) {
+func (i *Image) ReadPixels(buf []byte) error {
 	i.resolveBufferedReplacePixels()
 	c := &pixelsCommand{
-		img: i,
+		img:    i,
+		result: buf,
 	}
 	theCommandQueue.Enqueue(c)
 	if err := theCommandQueue.Flush(); err != nil {
-		return nil, err
+		return err
 	}
-	return c.result, nil
+	return nil
 }
 
 func (i *Image) ReplacePixels(pixels []byte, x, y, width, height int) {
@@ -222,8 +223,9 @@ func (i *Image) Dump(path string, blackbg bool, rect image.Rectangle) error {
 	}
 	defer f.Close()
 
-	pix, err := i.Pixels()
+	w, h := i.InternalSize()
-	if err != nil {
+	pix := make([]byte, 4*w*h)
+	if err := i.ReadPixels(pix); err != nil {
 		return err
 	}
 

internal/graphicsdriver/graphics.go

@@ -80,7 +80,7 @@ type Image interface {
 	ID() ImageID
 	Dispose()
 	IsInvalidated() bool
-	Pixels() ([]byte, error)
+	ReadPixels(buf []byte) error
 	ReplacePixels(args []*ReplacePixelsArgs)
 }
 

internal/graphicsdriver/metal/graphics_darwin.go

@@ -1186,15 +1186,18 @@ func (i *Image) syncTexture() {
 	cb.WaitUntilCompleted()
 }
 
-func (i *Image) Pixels() ([]byte, error) {
+func (i *Image) ReadPixels(buf []byte) error {
+	if got, want := len(buf), 4*i.width*i.height; got != want {
+		return fmt.Errorf("metal: len(buf) must be %d but %d at ReadPixels", want, got)
+	}
+
 	i.graphics.flushIfNeeded(false)
 	i.syncTexture()
 
-	b := make([]byte, 4*i.width*i.height)
+	i.texture.GetBytes(&buf[0], uintptr(4*i.width), mtl.Region{
-	i.texture.GetBytes(&b[0], uintptr(4*i.width), mtl.Region{
 		Size: mtl.Size{Width: i.width, Height: i.height, Depth: 1},
 	}, 0)
-	return b, nil
+	return nil
 }
 
 func (i *Image) ReplacePixels(args []*graphicsdriver.ReplacePixelsArgs) {

internal/graphicsdriver/opengl/context_desktop.go

@@ -162,12 +162,10 @@ func (c *context) bindFramebufferImpl(f framebufferNative) {
 	gl.BindFramebufferEXT(gl.FRAMEBUFFER, uint32(f))
 }
 
-func (c *context) framebufferPixels(f *framebuffer, width, height int) []byte {
+func (c *context) framebufferPixels(buf []byte, f *framebuffer, width, height int) {
 	gl.Flush()
 	c.bindFramebuffer(f.native)
-	pixels := make([]byte, 4*width*height)
+	gl.ReadPixels(0, 0, int32(width), int32(height), gl.RGBA, gl.UNSIGNED_BYTE, gl.Ptr(buf))
-	gl.ReadPixels(0, 0, int32(width), int32(height), gl.RGBA, gl.UNSIGNED_BYTE, gl.Ptr(pixels))
-	return pixels
 }
 
 func (c *context) framebufferPixelsToBuffer(f *framebuffer, buffer buffer, width, height int) {

internal/graphicsdriver/opengl/context_js.go

@@ -236,7 +236,7 @@ func (c *context) bindFramebufferImpl(f framebufferNative) {
 	gl.bindFramebuffer.Invoke(gles.FRAMEBUFFER, js.Value(f))
 }
 
-func (c *context) framebufferPixels(f *framebuffer, width, height int) []byte {
+func (c *context) framebufferPixels(buf []byte, f *framebuffer, width, height int) {
 	gl := c.gl
 
 	c.bindFramebuffer(f.native)
@@ -244,8 +244,7 @@ func (c *context) framebufferPixels(f *framebuffer, width, height int) []byte {
 	l := 4 * width * height
 	p := jsutil.TemporaryUint8ArrayFromUint8Slice(l, nil)
 	gl.readPixels.Invoke(0, 0, width, height, gles.RGBA, gles.UNSIGNED_BYTE, p)
-
+	copy(buf, uint8ArrayToSlice(p, l))
-	return uint8ArrayToSlice(p, l)
 }
 
 func (c *context) framebufferPixelsToBuffer(f *framebuffer, buffer buffer, width, height int) {

internal/graphicsdriver/opengl/context_mobile.go

@@ -148,14 +148,12 @@ func (c *context) bindFramebufferImpl(f framebufferNative) {
 	c.ctx.BindFramebuffer(gles.FRAMEBUFFER, uint32(f))
 }
 
-func (c *context) framebufferPixels(f *framebuffer, width, height int) []byte {
+func (c *context) framebufferPixels(buf []byte, f *framebuffer, width, height int) {
 	c.ctx.Flush()
 
 	c.bindFramebuffer(f.native)
 
-	pixels := make([]byte, 4*width*height)
+	c.ctx.ReadPixels(buf, 0, 0, int32(width), int32(height), gles.RGBA, gles.UNSIGNED_BYTE)
-	c.ctx.ReadPixels(pixels, 0, 0, int32(width), int32(height), gles.RGBA, gles.UNSIGNED_BYTE)
-	return pixels
 }
 
 func (c *context) framebufferPixelsToBuffer(f *framebuffer, buffer buffer, width, height int) {

internal/graphicsdriver/opengl/image.go

@@ -60,13 +60,13 @@ func (i *Image) setViewport() error {
 	return nil
 }
 
-func (i *Image) Pixels() ([]byte, error) {
+func (i *Image) ReadPixels(buf []byte) error {
 	if err := i.ensureFramebuffer(); err != nil {
-		return nil, err
+		return err
 	}
 
-	p := i.graphics.context.framebufferPixels(i.framebuffer, i.width, i.height)
+	i.graphics.context.framebufferPixels(buf, i.framebuffer, i.width, i.height)
-	return p, nil
+	return nil
 }
 
 func (i *Image) framebufferSize() (int, int) {

internal/restorable/image.go

@@ -497,8 +497,8 @@ func (i *Image) makeStaleIfDependingOnShader(shader *Shader) {
 
 // readPixelsFromGPU reads the pixels from GPU and resolves the image's 'stale' state.
 func (i *Image) readPixelsFromGPU() error {
-	pix, err := i.image.Pixels()
+	pix := make([]byte, 4*i.width*i.height)
-	if err != nil {
+	if err := i.image.ReadPixels(pix); err != nil {
 		return err
 	}
 	i.basePixels = Pixels{}
@@ -626,8 +626,8 @@ func (i *Image) restore() error {
 
 	if len(i.drawTrianglesHistory) > 0 {
 		i.basePixels = Pixels{}
-		pix, err := gimg.Pixels()
+		pix := make([]byte, 4*w*h)
-		if err != nil {
+		if err := gimg.ReadPixels(pix); err != nil {
 			return err
 		}
 		i.basePixels.AddOrReplace(pix, 0, 0, w, h)