gpu: add Framebuffer.ReadPixels

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/backend.go

@@ -127,6 +127,7 @@ type Framebuffer interface {
 	Bind()
 	Invalidate()
 	Release()
+	ReadPixels(src image.Rectangle, pixels []byte) error
 }
 
 type Timer interface {

gpu/gl/backend.go

@@ -589,6 +589,16 @@ func (b *gpuBuffer) BindIndex() {
 	b.backend.funcs.BindBuffer(ELEMENT_ARRAY_BUFFER, b.obj)
 }
 
+func (f *gpuFramebuffer) ReadPixels(src image.Rectangle, pixels []byte) error {
+	glErr(f.funcs)
+	f.Bind()
+	if len(pixels) < src.Dx()*src.Dy() {
+		return errors.New("unexpected RGBA size")
+	}
+	f.funcs.ReadPixels(src.Min.X, src.Min.Y, src.Dx(), src.Dy(), RGBA, UNSIGNED_BYTE, pixels)
+	return glErr(f.funcs)
+}
+
 func (f *gpuFramebuffer) Bind() {
 	f.funcs.BindFramebuffer(FRAMEBUFFER, f.obj)
 }

gpu/gl/gl.go

@@ -139,6 +139,7 @@ type Functions interface {
 	GetUniformLocation(p Program, name string) Uniform
 	InvalidateFramebuffer(target, attachment Enum)
 	LinkProgram(p Program)
+	ReadPixels(x, y, width, height int, format, ty Enum, data []byte)
 	RenderbufferStorage(target, internalformat Enum, width, height int)
 	ShaderSource(s Shader, src string)
 	TexImage2D(target Enum, level int, internalFormat int, width, height int, format, ty Enum, data []byte)