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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build !android
// +build !android
// Package glwidget provides a widget containing a GL ES framebuffer.
package glwidget
import (
// GL is a widget that maintains an OpenGL ES context.
// The Draw function is responsible for configuring the GL viewport
// and for publishing the result to the widget by calling the Publish
// method when the frame is complete. A typical draw function:
// func(w *glwidget.GL) {
// w.Ctx.Viewport(0, 0, w.Rect.Dx(), w.Rect.Dy())
// w.Ctx.ClearColor(0, 0, 0, 1)
// w.Ctx.Clear(gl.COLOR_BUFFER_BIT)
// // ... draw the frame
// w.Publish()
// }
// The GL context is separate from the one used by the gldriver to
// render the window, and is only used by the glwidget package during
// initialization and for the duration of the Publish call. This means
// a glwidget user is free to use Ctx as a background GL context
// concurrently with the primary UI drawing done by the gldriver.
type GL struct {
Ctx gl.Context
draw func(*GL)
framebuffer gl.Framebuffer
tex gl.Texture
dst *image.RGBA
origin image.Point
// NewGL creates a GL widget with a Draw function called when painted.
func NewGL(drawFunc func(*GL)) *GL {
// TODO: use the size of the monitor as a bound for texture size.
const maxWidth, maxHeight = 4096, 3072
glctx, err := gldriver.NewContext()
if err != nil {
panic(fmt.Sprintf("glwidget: %v", err)) // TODO: return error?
w := &GL{
Ctx: glctx,
draw: drawFunc,
w.tex = w.Ctx.CreateTexture()
w.Ctx.BindTexture(gl.TEXTURE_2D, w.tex)
w.Ctx.TexImage2D(gl.TEXTURE_2D, 0, gl.RGBA, maxWidth, maxHeight, gl.RGBA, gl.UNSIGNED_BYTE, nil)
w.Ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR)
w.Ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR)
w.Ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE)
w.Ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE)
w.framebuffer = w.Ctx.CreateFramebuffer()
w.Ctx.BindFramebuffer(gl.FRAMEBUFFER, w.framebuffer)
w.Ctx.FramebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, w.tex, 0)
// TODO: delete the framebuffer, texture, and gl.Context.
// TODO: explicit or finalizer cleanup?
w.Wrapper = w
return w
func (w *GL) PaintBase(ctx *node.PaintBaseContext, origin image.Point) error {
if w.Rect.Empty() {
return nil
w.dst = ctx.Dst
w.origin = origin
w.dst = nil
return nil
// Publish renders the default framebuffer of Ctx onto the area of the
// window occupied by the widget.
func (w *GL) Publish() {
if w.dst == nil {
panic("glwidget: no destination, Publish called outside of Draw")
// TODO: draw the widget texture directly into the window framebuffer.
m := image.NewRGBA(image.Rect(0, 0, w.Rect.Dx(), w.Rect.Dy()))
w.Ctx.PixelStorei(gl.PACK_ALIGNMENT, 1)
w.Ctx.ReadPixels(m.Pix, 0, 0, w.Rect.Dx(), w.Rect.Dy(), gl.RGBA, gl.UNSIGNED_BYTE)
draw.Draw(w.dst, w.Rect.Add(w.origin), m, image.Point{}, draw.Over)