draw2dgl/gc.go
func (p *Painter) Paint(ss []raster.Span, done bool) {
	//gl.Begin(gl.LINES)
	sslen := len(ss)
	clenrequired := sslen * 8
	vlenrequired := sslen * 4
	if clenrequired >= (cap(p.colors) - len(p.colors)) {
		p.Flush()

		if clenrequired >= cap(p.colors) {
			p.vertices = make([]int32, 0, vlenrequired+(vlenrequired/2))
			p.colors = make([]uint8, 0, clenrequired+(clenrequired/2))
		}
	}
	vi := len(p.vertices)
	ci := len(p.colors)
	p.vertices = p.vertices[0 : vi+vlenrequired]
	p.colors = p.colors[0 : ci+clenrequired]
	var (
		colors   []uint8
		vertices []int32
	)
	for _, s := range ss {
		a := uint8((s.Alpha * p.ca / M16) >> 8)

		colors = p.colors[ci:]
		colors[0] = p.cr
		colors[1] = p.cg
		colors[2] = p.cb
		colors[3] = a
		colors[4] = p.cr
		colors[5] = p.cg
		colors[6] = p.cb
		colors[7] = a
		ci += 8
		vertices = p.vertices[vi:]
		vertices[0] = int32(s.X0)
		vertices[1] = int32(s.Y)
		vertices[2] = int32(s.X1)
		vertices[3] = int32(s.Y)
		vi += 4
	}
}
func (p *Painter) Paint(ss []raster.Span, done bool) {
	//gl.Begin(gl.LINES)
	sslen := len(ss)
	clenrequired := sslen * 8
	vlenrequired := sslen * 4
	if clenrequired >= (cap(p.colors) - len(p.colors)) {
		p.Flush()

		if clenrequired >= cap(p.colors) {
			p.vertices = make([]int32, 0, vlenrequired+(vlenrequired/2))
			p.colors = make([]uint8, 0, clenrequired+(clenrequired/2))
		}
	}
	vi := len(p.vertices)
	ci := len(p.colors)
	p.vertices = p.vertices[0 : vi+vlenrequired]
	p.colors = p.colors[0 : ci+clenrequired]
	var (
		colors   []uint8
		vertices []int32
	)
	for _, s := range ss {
		a := uint8((s.Alpha * p.ca / M16) >> 8)

		colors = p.colors[ci:]
		colors[0] = p.cr
		colors[1] = p.cg
		colors[2] = p.cb
		colors[3] = a
		colors[4] = p.cr
		colors[5] = p.cg
		colors[6] = p.cb
		colors[7] = a
		ci += 8
		vertices = p.vertices[vi:]
		vertices[0] = int32(s.X0)
		vertices[1] = int32(s.Y)
		vertices[2] = int32(s.X1)
		vertices[3] = int32(s.Y)
		vi += 4
	}
}
// Paint satisfies the Painter interface by painting ss onto an image.RGBA.