func (c *drawImageCommand) Exec(context *opengl.Context, indexOffsetInBytes int) error {
	f, err := c.dst.createFramebufferIfNeeded(context)
	if err != nil {
		return err
	}
	if err := f.setAsViewport(context); err != nil {
		return err
	}
	context.BlendFunc(c.mode)

	n := c.quadsNum()
	if n == 0 {
		return nil
	}
	_, h := c.dst.Size()
	proj := f.projectionMatrix(h)
	p := &programContext{
		state:            &theOpenGLState,
		program:          theOpenGLState.programTexture,
		context:          context,
		projectionMatrix: proj,
		texture:          c.src.texture.native,
		colorM:           c.color,
	}
	if err := p.begin(); err != nil {
		return err
	}
	// TODO: We should call glBindBuffer here?
	// The buffer is already bound at begin() but it is counterintuitive.
	context.DrawElements(opengl.Triangles, 6*n, indexOffsetInBytes)
	return nil
}

func shader(c *opengl.Context, id shaderId) string {
	str := shaders[id]
	if !c.GlslHighpSupported() {
		str = strings.Replace(str, "highp ", "", -1)
		str = strings.Replace(str, "lowp ", "", -1)
	}
	return str
}

func newFramebufferFromTexture(context *opengl.Context, texture *texture) (*framebuffer, error) {
	native, err := context.NewFramebuffer(opengl.Texture(texture.native))
	if err != nil {
		return nil, err
	}
	return &framebuffer{
		native: native,
	}, nil
}

func (i *Image) Pixels(context *opengl.Context) ([]uint8, error) {
	// Flush the enqueued commands so that pixels are certainly read.
	if err := theCommandQueue.Flush(context); err != nil {
		return nil, err
	}
	f, err := i.createFramebufferIfNeeded(context)
	if err != nil {
		return nil, err
	}
	return context.FramebufferPixels(f.native, i.width, i.height)
}

func (a *arrayBufferLayout) enable(c *opengl.Context, program opengl.Program) {
	for _, p := range a.parts {
		c.EnableVertexAttribArray(program, p.name)
	}
	total := a.totalBytes()
	offset := 0
	for _, p := range a.parts {
		c.VertexAttribPointer(program, p.name, p.num, p.dataType, p.normalize, total, offset)
		offset += p.dataType.SizeInBytes() * p.num
	}
}

func (q *commandQueue) Flush(context *opengl.Context) error {
	q.m.Lock()
	defer q.m.Unlock()
	// glViewport must be called at least at every frame on iOS.
	context.ResetViewportSize()
	for _, g := range q.commandGroups() {
		n := 0
		for _, c := range g {
			switch c := c.(type) {
			case *drawImageCommand:
				n += len(c.vertices)
			}
		}
		vertices := make([]float32, 0, n)
		for _, c := range g {
			switch c := c.(type) {
			case *drawImageCommand:
				vertices = append(vertices, c.vertices...)
			}
		}
		if 0 < len(vertices) {
			context.BufferSubData(opengl.ArrayBuffer, vertices)
		}
		// NOTE: WebGL doesn't seem to have Check gl.MAX_ELEMENTS_VERTICES or gl.MAX_ELEMENTS_INDICES so far.
		// Let's use them to compare to len(quads) in the future.
		if maxQuads < len(vertices)*opengl.Float.SizeInBytes()/QuadVertexSizeInBytes() {
			return fmt.Errorf("len(quads) must be equal to or less than %d", maxQuads)
		}
		numc := len(g)
		indexOffsetInBytes := 0
		for _, c := range g {
			if err := c.Exec(context, indexOffsetInBytes); err != nil {
				return err
			}
			if c, ok := c.(*drawImageCommand); ok {
				n := len(c.vertices) * opengl.Float.SizeInBytes() / QuadVertexSizeInBytes()
				indexOffsetInBytes += 6 * n * 2
			}
		}
		if 0 < numc {
			// Call glFlush to prevent black flicking (especially on Android (#226) and iOS).
			context.Flush()
		}
	}
	q.commands = []command{}
	return nil
}

func (i *Image) IsInvalidated(context *opengl.Context) bool {
	return !context.IsTexture(i.texture.native)
}

func (f *framebuffer) setAsViewport(context *opengl.Context) error {
	width := viewportSize
	height := viewportSize
	return context.SetViewport(f.native, width, height)
}

func (a *arrayBufferLayout) disable(c *opengl.Context, program opengl.Program) {
	// TODO: Disabling should be done in reversed order?
	for _, p := range a.parts {
		c.DisableVertexAttribArray(program, p.name)
	}
}

func (a *arrayBufferLayout) newArrayBuffer(c *opengl.Context) opengl.Buffer {
	return c.NewBuffer(opengl.ArrayBuffer, a.totalBytes()*4*maxQuads, opengl.DynamicDraw)
}

func (s *openGLState) reset(context *opengl.Context) error {
	if err := context.Reset(); err != nil {
		return err
	}
	s.lastProgram = zeroProgram
	s.lastProjectionMatrix = nil
	s.lastColorMatrix = nil
	s.lastColorMatrixTranslation = nil

	if s.arrayBuffer != zeroBuffer {
		context.DeleteBuffer(s.arrayBuffer)
	}
	if s.indexBufferQuads != zeroBuffer {
		context.DeleteBuffer(s.indexBufferQuads)
	}
	if s.programTexture != zeroProgram {
		context.DeleteProgram(s.programTexture)
	}

	shaderVertexModelviewNative, err := context.NewShader(opengl.VertexShader, shader(context, shaderVertexModelview))
	if err != nil {
		panic(fmt.Sprintf("graphics: shader compiling error:\n%s", err))
	}
	defer context.DeleteShader(shaderVertexModelviewNative)

	shaderFragmentTextureNative, err := context.NewShader(opengl.FragmentShader, shader(context, shaderFragmentTexture))
	if err != nil {
		panic(fmt.Sprintf("graphics: shader compiling error:\n%s", err))
	}
	defer context.DeleteShader(shaderFragmentTextureNative)

	s.programTexture, err = context.NewProgram([]opengl.Shader{
		shaderVertexModelviewNative,
		shaderFragmentTextureNative,
	})
	if err != nil {
		return err
	}

	s.arrayBuffer = theArrayBufferLayout.newArrayBuffer(context)

	indices := make([]uint16, 6*maxQuads)
	for i := uint16(0); i < maxQuads; i++ {
		indices[6*i+0] = 4*i + 0
		indices[6*i+1] = 4*i + 1
		indices[6*i+2] = 4*i + 2
		indices[6*i+3] = 4*i + 1
		indices[6*i+4] = 4*i + 2
		indices[6*i+5] = 4*i + 3
	}
	s.indexBufferQuads = context.NewBuffer(opengl.ElementArrayBuffer, indices, opengl.StaticDraw)

	return nil
}