opengl: Refactoring: Hide projection matrix inside opengl package

internal/graphicscommand/command.go

@@ -227,20 +227,16 @@ func (c *drawImageCommand) String() string {
 
 // Exec executes the drawImageCommand.
 func (c *drawImageCommand) Exec(indexOffsetInBytes int) error {
-	// On some environments, viewport size must be within the framebuffer size.
-	// e.g. Edge (#71), Chrome on GPD Pocket (#420), macOS Mojave (#691).
-	// Use the same size of the framebuffer here.
-	if err := c.dst.image.SetViewport(); err != nil {
-		return err
-	}
-
+	// TODO: Is it ok not to bind any framebuffer here?
 	if c.nindices == 0 {
 		return nil
 	}
-	proj := c.dst.image.ProjectionMatrix()
-	dw, dh := c.dst.Size()
-	sw, sh := c.src.Size()
-	opengl.UseProgram(c.mode, proj, c.src.image, dw, dh, sw, sh, c.color, c.filter)
+
+	c.dst.image.SetAsDestination()
+	c.src.image.SetAsSource()
+	if err := opengl.UseProgram(c.mode, c.color, c.filter); err != nil {
+		return err
+	}
 	opengl.GetContext().DrawElements(c.nindices, indexOffsetInBytes)
 
 	// glFlush() might be necessary at least on MacBook Pro (a smilar problem at #419),

internal/opengl/image.go

@@ -85,7 +85,11 @@ func (i *Image) Delete() {
 	}
 }
 
-func (i *Image) SetViewport() error {
+func (i *Image) SetAsDestination() {
+	theOpenGLState.destination = i
+}
+
+func (i *Image) setViewport() error {
 	if err := i.ensureFramebuffer(); err != nil {
 		return err
 	}
@@ -104,7 +108,7 @@ func (i *Image) Pixels() ([]byte, error) {
 	return p, nil
 }
 
-func (i *Image) ProjectionMatrix() []float32 {
+func (i *Image) projectionMatrix() []float32 {
 	if i.framebuffer == nil {
 		panic("not reached")
 	}
@@ -127,3 +131,7 @@ func (i *Image) ensureFramebuffer() error {
 func (i *Image) TexSubImage2D(p []byte, x, y, width, height int) {
 	theContext.texSubImage2D(i.textureNative, p, x, y, width, height)
 }
+
+func (i *Image) SetAsSource() {
+	theOpenGLState.source = i
+}

internal/opengl/program.go

@@ -130,6 +130,9 @@ type openGLState struct {
 	lastColorMatrixTranslation []float32
 	lastSourceWidth            int
 	lastSourceHeight           int
+
+	source      *Image
+	destination *Image
 }
 
 var (
@@ -265,15 +268,40 @@ func BufferSubData(vertices []float32, indices []uint16) {
 	c.elementArrayBufferSubData(indices)
 }
 
-func UseProgram(mode graphics.CompositeMode, proj []float32, src *Image, dstW, dstH, srcW, srcH int, colorM *affine.ColorM, filter graphics.Filter) {
+func UseProgram(mode graphics.CompositeMode, colorM *affine.ColorM, filter graphics.Filter) error {
+	destination := theOpenGLState.destination
+	if destination == nil {
+		panic("destination image is not set")
+	}
+	source := theOpenGLState.source
+	if source == nil {
+		panic("source image is not set")
+	}
+
+	// On some environments, viewport size must be within the framebuffer size.
+	// e.g. Edge (#71), Chrome on GPD Pocket (#420), macOS Mojave (#691).
+	// Use the same size of the framebuffer here.
+	if err := destination.setViewport(); err != nil {
+		return err
+	}
+	proj := destination.projectionMatrix()
+	dw, dh := destination.width, destination.height
+	sw, sh := source.width, source.height
+
 	GetContext().blendFunc(mode)
-	theOpenGLState.useProgram(proj, src.textureNative, dstW, dstH, srcW, srcH, colorM, filter)
+	theOpenGLState.useProgram(proj, dw, dh, sw, sh, colorM, filter)
+
+	theOpenGLState.source = nil
+	theOpenGLState.destination = nil
+	return nil
 }
 
 // useProgram uses the program (programTexture).
-func (s *openGLState) useProgram(proj []float32, texture textureNative, dstW, dstH, srcW, srcH int, colorM *affine.ColorM, filter graphics.Filter) {
+func (s *openGLState) useProgram(proj []float32, dstW, dstH, srcW, srcH int, colorM *affine.ColorM, filter graphics.Filter) {
 	c := GetContext()
 
+	texture := theOpenGLState.source.textureNative
+
 	var program program
 	switch filter {
 	case graphics.FilterNearest: