A rendering engine library for Go supporting both SDL2 and WebAssembly (HTML Canvas) targets.
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package canvas
import (
// Texture can hold on to cached image textures.
type Texture struct {
data string // data:image/png URI
image js.Value // DOM image element
canvas js.Value // Warmed up canvas element
ctx2d js.Value // 2D drawing context for the canvas.
width int
height int
// StoreTexture caches a texture from a bitmap.
func (e *Engine) StoreTexture(name string, img image.Image) (render.Texturer, error) {
var (
fh = bytes.NewBuffer([]byte{})
imageSize = img.Bounds().Size()
width = imageSize.X
height = imageSize.Y
// Encode to PNG format.
if err := png.Encode(fh, img); err != nil {
return nil, err
var dataURI = "data:image/png;base64," + base64.StdEncoding.EncodeToString(fh.Bytes())
tex := &Texture{
data: dataURI,
width: width,
height: height,
// Preheat a cached Canvas object.
canvas := js.Global().Get("document").Call("createElement", "canvas")
canvas.Set("width", width)
canvas.Set("height", height)
tex.canvas = canvas
ctx2d := canvas.Call("getContext", "2d")
tex.ctx2d = ctx2d
// Load as a JS Image object.
image := js.Global().Call("eval", "new Image()")
image.Call("addEventListener", "load", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
ctx2d.Call("drawImage", image, 0, 0)
return nil
image.Set("src", tex.data)
tex.image = image
// Cache the texture in memory.
e.textures[name] = tex
return tex, nil
// Size returns the dimensions of the texture.
func (t *Texture) Size() render.Rect {
return render.NewRect(int32(t.width), int32(t.height))
// LoadTexture recalls a cached texture image.
func (e *Engine) LoadTexture(name string) (render.Texturer, error) {
if tex, ok := e.textures[name]; ok {
return tex, nil
return nil, errors.New("no bitmap data stored for " + name)
// Copy a texturer bitmap onto the canvas.
func (e *Engine) Copy(t render.Texturer, src, dist render.Rect) {
tex := t.(*Texture)
// e.canvas.ctx2d.Call("drawImage", tex.image, dist.X, dist.Y)
e.canvas.ctx2d.Call("drawImage", tex.canvas, dist.X, dist.Y)