Canvas is a Go drawing library based on OpenGL or using software rendering that is very similar to the HTML5 canvas API


Go canvas GoDoc

Canvas is a pure Go library that provides drawing functionality as similar as possible to the HTML5 canvas API. It has nothing to do with HTML or Javascript, the functions are just made to be approximately the same.

Most of the functions are supported, but it is still a work in progress. The library aims to accept a lot of different parameters on each function in a similar way as the Javascript API does.

Whereas the Javascript API uses a context that all draw calls go to, here all draw calls are directly on the canvas type. The other difference is that here setters are used instead of properties for things like fonts and line width.

OpenGL backend

The OpenGL backend is intended to provide decent performance. Obviously it will not be able to rival hand coded OpenGL for a given purpose, but for many purposes it will be enough. It can also be combined with hand coded OpenGL.

Software backend

The software backend can also be used if no OpenGL context is available. It will render into a standard Go RGBA image.

There is experimental MSAA anti-aliasing, but it doesn't fully work properly yet. The best option for anti-aliasing currently is to render to a larger image and then scale it down.

SDL/GLFW convenience packages

The sdlcanvas and glfwcanvas subpackages provide a very simple way to get started with just a few lines of code. As the names imply they are based on the SDL library and the GLFW library respectively. They create a window for you and give you a canvas to draw with.

OS support

Both the OpenGL and software backends work on the following operating systems:

  • Linux
  • Windows
  • macOS
  • Android
  • iOS

Using gomobile to build a full Go app using gomobile build now works by using an offscreen texture to render to and then rendering that to the screen. See the example in examples/gomobile. The offscreen texture is necessary since gomobile automatically creates a GL context without a stencil buffer, which this library requires.


Look at the example/drawing package for some drawing examples.

Here is a simple example for how to get started:

package main

import (


func main() {
	wnd, cv, err := sdlcanvas.CreateWindow(1280, 720, "Hello")
	if err != nil {
	defer wnd.Destroy()

	wnd.MainLoop(func() {
		w, h := float64(cv.Width()), float64(cv.Height())
		cv.FillRect(0, 0, w, h)

		for r := 0.0; r < math.Pi*2; r += math.Pi * 0.1 {
			cv.SetFillStyle(int(r*10), int(r*20), int(r*40))
			cv.MoveTo(w*0.5, h*0.5)
			cv.Arc(w*0.5, h*0.5, math.Min(w, h)*0.4, r, r+0.1*math.Pi, false)

		cv.Arc(w*0.5, h*0.5, math.Min(w, h)*0.4, 0, math.Pi*2, false)

The result:

Implemented features

These features should work just like their HTML5 counterparts, but there are likely to be a lot of edge cases where they don't work exactly the same way.

  • beginPath
  • closePath
  • moveTo
  • lineTo
  • rect
  • arc
  • arcTo
  • quadraticCurveTo
  • bezierCurveTo
  • stroke
  • fill
  • clip
  • save
  • restore
  • scale
  • translate
  • rotate
  • transform
  • setTransform
  • fillText
  • measureText
  • textAlign
  • textBaseline
  • fillStyle
  • strokeText
  • strokeStyle
  • linear gradients
  • radial gradients
  • image patterns with repeat and transform
  • lineWidth
  • lineEnd (square, butt, round)
  • lineJoin (bevel, miter, round)
  • miterLimit
  • lineDash
  • getLineDash
  • lineDashOffset
  • global alpha
  • drawImage
  • getImageData
  • putImageData
  • clearRect
  • shadowColor
  • shadowOffset(X/Y)
  • shadowBlur
  • isPointInPath
  • isPointInStroke
  • self intersecting polygons

Missing features

  • globalCompositeOperation
  • imageSmoothingEnabled
  • textBaseline hanging and ideographic (currently work just like top and bottom)
  • Zoom / Magnify + MouseWheel

    Zoom / Magnify + MouseWheel

    Hi, Based on your event example -- project at:

    Updated sdlcanvas to include MouseWheelEvent:

    	MouseWheel func(x, y int)
    		case *sdl.MouseWheelEvent:
    			if wnd.MouseWheel != nil {
    					wnd.MouseWheel(int(e.X), int(e.Y))
    					handled = true

    then included it in project:

    	wnd.MouseWheel = func(x, y int) {
    		action = 1
    		if y == 1 {
    			cscale /= cmultiplier
    		if y == -1 {
    			cscale *= cmultiplier
    		cv.Scale(cscale, cscale)
    		println("y:  ", y)

    this does zoom (my code is buggy..) what I've drawn with lines but the Images (from PutImageData) are not zoomed. I believe this is by design based on this article:

    Is it possible to include a function which scales the whole canvas/window?

    opened by 5k3105 5
  • [suggestion] Video support?

    [suggestion] Video support?

    Hello, I think it would be great if this lib had some way to play a video on the canvas. Maybe something like in HTML5 Canvas where you can drawImage a video, or something even better w/ sound support? Thoughts?

    opened by SuperDog83 4
  • Lines remain on canvas after screen clear

    Lines remain on canvas after screen clear

    Example both here: and here:

    Artifacts remain on screen after scale/clear/redraw. This is windows 7.

    opened by 5k3105 4
  • Slow text rendering?

    Slow text rendering?

    Hello! I have a somewhat text-heavy use case and I think I'm noticing that text rendering seems to be pretty slow -- with a couple hundred characters on the screen I get about 20fps and it only goes down from there.

    I glanced through the code and it seems it's building up a bitmap of the text to draw every frame with image.Draw, and then copying that to the destination buffer. Is that right? Is there some caching or a fast-path somewhere I can exploit?

    opened by JohnEmhoff 3
  • Error building on macOS

    Error building on macOS

    I'm running the examples on macOS Mojave 10.14.3, and all throw me the same error:

    ld: framework not found OpenGLES
    clang: error: linker command failed with exit code 1 (use -v to see invocation)

    After changing line 19 of backend/gogl/gl/package.go from

    // #cgo darwin           LDFLAGS: -framework OpenGLES


    // #cgo darwin           LDFLAGS: -framework OpenGL

    the examples now run successfully. As Apple's OpenGL ES documentation only mentions iOS and tvOS, but no macOS, should OpenGL be used to build on macOS?

    opened by beta 3
  • Anti-aliasing



    Do you have an option to apply an anti-aliasing filter (like described here:

    I'm just trying to display a rectangle like the white line below: anti_alias

    It can be seen here that there is no sufficient anti-aliasing.

    Thank you !

    opened by VincentHoorpah 3
  • Rect shadow broken

    Rect shadow broken

    Hello again, Was just experimenting with rect shadows and found out that:

    • Rendering shadows is really slow (and uses 100% of my GPU)
    • There is a bug when you call DrawImage before rendering the rect

    For the bug, here's a screenshot:


    what's excepted:


    Here's the code used to draw the rect

    cnv.Rect(this.X, this.Y, this.W, this.H)
    opened by SuperDog83 3
  • What's the proper way to use RGBA for something like SetFillStyle?

    What's the proper way to use RGBA for something like SetFillStyle?

    Every thing I've tried (while studying the parseColor function, line 83 color.go) it chokes on the Alpha number.

    I can't quite figure out what type it expects for that fourth value.

    I would love to just send it a string like SetFillStyle("rgba(240, 236, 206, .5)") and it looks like it's supposed to be able to do that, but it doesn't seem to work.

    Sending it a string like "rgb(240, 236, 206)" DOES work, so I'm thinking there might be a problem with line 209 on color.go.

    I don't know if it's a bug or if I'm just stupid.

    opened by InterwebCounty 2
  • Blank red screen macOS

    Blank red screen macOS

    Hi, I have this blank red screen when I try to render something on screen. Weirdly, the only things that are rendered properly are images (DrawImage) This is a screenshot from the drawing example Screenshot 2020-06-01 at 14 59 42 I'm using go version go1.14 darwin/amd64 macOS Catalina Version 10.15.4 Do you guys have any idea what might be a problem?

    opened by crooliusz 1
  • Fix a panic issue of loading an image from cache

    Fix a panic issue of loading an image from cache

    When calling LoadImage(src interface{}) with a byte array, there will be a panic:

    panic: runtime error: hash of unhashable type []uint8

    that is caused by visiting the map structure of the image cache using the content of the image (byte array) as the key. In order to avoid this issue, we added a condition before visiting the map.

    See the changes. Thanks.

    opened by kanle-hotstar 1
  • Memory Leak in DrawImage()

    Memory Leak in DrawImage()

    If I leave the code bellow running for a while, I can see it has a memory leak... If I replace the DrawImage call with PutImageData, the leakage goes away. It happens with when using glfwcanvas and sdlcanvas.

    OS: macOS Mohave OpenGL: native? (didn't have to install anything) SDL installed with brew install sdl2

    package main
    import (
    var (
    	first = true
    	bla   = time.Now()
    	limg  *image.RGBA
    func clientScreen() (image.Image, int64) {
    	if !first && rand.Float64() < 0.99 {
    		return limg, bla.UnixNano()
    	first = false
    	rgba := image.NewRGBA(image.Rect(0, 0, 800, 800))
    	for y := 0; y < 800; y++ {
    		for x := 0; x < 800; x++ {
    			rgba.Set(x, y, color.RGBA{
    				R: byte(rand.Intn(256)),
    				G: byte(rand.Intn(256)),
    				B: byte(rand.Intn(256)),
    				A: 255,
    	bla = time.Now()
    	limg = rgba
    	return rgba, bla.UnixNano()
    func main() {
    	win, cv, err := glfwcanvas.CreateWindow(800, 800, "hello")
    	if err != nil {
    	var lastRefresh int64
    	win.MainLoop(func() {
    		// This call returns a new image and the last timestamp it was updated
    		img, lastUpdate := clientScreen()
    		// If there were no changes, terminate loop
    		if lastRefresh == lastUpdate {
    		// Process screen updates
    		// cv.PutImageData(img.(*image.RGBA), 0, 0)
    		cv.DrawImage(img, 0, 0)
    		lastRefresh = lastUpdate
    opened by deluan 1
  • support font.Face drawing // font fallbacks

    support font.Face drawing // font fallbacks

    hello o/ my reasoning for this is that i wanted to use this multiface package to support font fallback but realized i couldnt use it since canvas basically only supports truetype.Font and the multiface package implements font.Face

    i'm not sure how hard (or easy) this would be. if you have an alternate way to be able to draw text with fallbacks fonts that would be fine too !!

    opened by TorchedSammy 0
  • Call CreateWindow function failed on WSL2

    Call CreateWindow function failed on WSL2

    I used canvas in WSL2 on win10.

    When I call sdlcanvas.CreateWindow, it failed with error:

    Error initializing SDL: No available video device

    Then I switched to glfwcanvas.CreateWindow

    it also emit an error:

    2022/01/17 15:27:59 PlatformError: X11: The DISPLAY environment variable is missing
    panic: NotInitialized: The GLFW library is not initialized
    opened by yunnysunny 3
  • How to save canvas as png with  transparent background

    How to save canvas as png with transparent background

    I want save canvas as png photo. But the background is always black.

                    fDst, err := os.Create("out2.png")
    		if err != nil {
    		defer fDst.Close()
    		err = png.Encode(fDst, cv.GetImageData(0, 0, cv.Width(), cv.Height()))
    		if err != nil {
    opened by t42ji2ji 1
  • Fix for #29: Failed to compile vertex shader

    Fix for #29: Failed to compile vertex shader

    This pulls in the changes @tfriedel6 suggested in the discussion in #29.

    Also added GetGlobalAlpha() to the Canvas to allow apps a way to set a new global alpha based on the existing global alpha.

    opened by richardwilkes 6
  • Failed to compile vertex shader

    Failed to compile vertex shader

    Trying to use this on the latest macOS release (Big Sur) and when running, I get the following error message:

    failed to compile vertex shader:
                                      ERROR: 0:2: '' :  #version required and missing.
                                      ERROR: 0:2: 'attribute' : syntax error: syntax error

    I'm using an OpenGLView directly, rather than some other framework, like glfw or sdl. Is there some special setup I need to do in order to get things to work that those libraries are doing (I'm assuming they work, but I haven't yet tried them to verify)?

    For reference, here are some code snippets showing the setup being done for the view:

    When creating the view:

    NSViewPtr nsNewOpenGLView() {
    	NSOpenGLPixelFormatAttribute attr[] = {
    		NSOpenGLPFAOpenGLProfile, NSOpenGLProfileVersion3_2Core,
    		NSOpenGLPFAColorSize,     24,
    		NSOpenGLPFADepthSize,     16,
    		// Opt-in to automatic GPU switching. CGL-only property.
    	id pixFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attr];
    	NSRect frame = NSMakeRect(0, 0, 0, 0);
    	OpenGLView *view = [[OpenGLView alloc] initWithFrame:frame pixelFormat:pixFormat];
    	[view setWantsBestResolutionOpenGLSurface:YES];
    	[view setWantsLayer:YES];
    	return (NSViewPtr)view;

    The override for prepareOpenGL:

    -(void)prepareOpenGL {
    	[super prepareOpenGL];
    	// Bind a default VBA to emulate OpenGL ES 2.
    	GLuint defVBA;
    	glGenVertexArrays(1, &defVBA);
    opened by richardwilkes 6
  • v0.12.1(Jun 2, 2020)

  • v0.12.0(May 28, 2020)

  • v0.11.1(Mar 17, 2020)

  • v0.11.0(Feb 28, 2020)

  • v0.10.0(Feb 11, 2020)

    • High DPI scaling now works in the sdlcanvas and glfwcanvas packages. If the window is scaled by some factor, the canvas size will be different, and the actual content scaling is left up to the application.
    • Updated SDL and GLFW versions
    • Fixed polygon triangulation in some cases
    • Fixed Arc and ArcTo scaling
    Source code(tar.gz)
    Source code(zip)
  • v0.9.3(Dec 18, 2019)

  • v0.9.2(Dec 4, 2019)

  • v0.9.1(Oct 1, 2019)

  • v0.9.0(May 13, 2019)

  • v0.8.5(May 12, 2019)

  • v0.8.4(Apr 30, 2019)

  • v0.8.3(Apr 30, 2019)

    • Image patterns now support transformations and repeat types
    • Canvas was sometimes not activated properly when using multiple canvases or combining with other GL code
    Source code(tar.gz)
    Source code(zip)
  • v0.8.2(Apr 30, 2019)

  • v0.8.1(Apr 30, 2019)

  • v0.8.0(Apr 30, 2019)

  • v0.7.2(Apr 30, 2019)

  • v0.7.1(Apr 30, 2019)

    • Line strokes are now scaled and transformed correctly
    • Added an ImagePattern type and a CreatePattern function (still missing repeat and transform)
    Source code(tar.gz)
    Source code(zip)
  • v0.7.0(Apr 30, 2019)

    • Restuctured the code to have a single frontend and multiple backends, although currently only Go-GL and an automatically generated xmobile GL backend are available. This should in theory make it possible to write a software backend as well, or maybe Vulkan or some other libraries.

    • Got rid of the OpenGL interface and used Go-GL for Android and iOS. I also tried using it for Shiny and gomobile, but for some reason once the xmobile GL context is created, Go-GL no longer works, so those use the xmobile GL backend.

    • SetLineEnd is now SetLineCap

    Source code(tar.gz)
    Source code(zip)
Thomas Friedel
Thomas Friedel
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