I'm using glfx.js to edit my image but when I'm trying to get that image's data using the toDataURL()
function I get a blank image (width the same size as the original image).
The strange thing is that in Chrome the script works perfect.
What I want to mention is that the image is loaded in canvas
using the onload event:
img.onload = function(){
try {
canvas = fx.canvas();
} catch (e) {
alert(e);
return;
}
// convert the image to a texture
texture = canvas.texture(img);
// draw and update canvas
canvas.draw(texture).update();
// replace the image with the canvas
img.parentNode.insertBefore(canvas, img);
img.parentNode.removeChild(img);
}
Also my image's path is on the same domain;
The problem (in Firefox) is when i hit the save button. Chrome returns the expected result but Firefox return this:
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAA7YAAAIWCAYAAABjkRHCAAAHxklEQVR4nO3BMQEAAADCoPVPbQZ/oAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
... [ lots of A s ] ...
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACAzwD6aAABkwvPRgAAAABJRU5ErkJggg==
What could cause this result and how can I fix it?
Most likely there's some async event between the time you draw to the canvas and the time you call toDataURL
. By default the canvas is cleared after every composite. Either prevent the canvas from being cleared by creating the WebGL context with preserveDrawingBuffer: true
as in
var gl = canvas.getContext("webgl", {preserveDrawingBuffer: true});
or (recommended) make sure toDataURL is called before exiting whatever event you're using to render. For example if you do this
function render() {
drawScene();
requestAnimationFrame(render);
}
render();
And somewhere else do this
someElement.addEventListener('click', function() {
var data = someCanvas.toDataURL();
});
Those 2 events, the animation frame
, and the click
are not in sync and the canvas may be cleared between calling them. Note: The canvas won't appear cleared as it's double buffered but the buffer toDataURL and other commands that effect that buffer are looking at is cleared.
The solution is either use preserveDrawingBuffer
or make your call to toDataURL
inside the same event as rendering. For example, without enabling preserveDrawingBuffer
(recommended):
function render() {
drawScene();
requestAnimationFrame(render);
}
render();
someElement.addEventListener('click', function() {
drawScene(); // draw at the same time (same event, same macrotask)
var data = someCanvas.toDataURL();
});
or
var captureFrame = false;
function render() {
drawScene();
if (captureFrame) {
captureFrame = false;
var data = someCanvas.toDataURL();
...
}
requestAnimationFrame(render);
}
render();
someElement.addEventListener('click', function() {
captureFrame = true;
});
And for example if you enabled preserveDrawingBuffer
:
function render() {
drawScene();
requestAnimationFrame(render);
}
render();
someElement.addEventListener('click', function() {
// drawScene(); // no longer needed
var data = someCanvas.toDataURL();
});
What's the point of preserveDrawingBuffer: false
which is the default? It can be significantly faster, especially on mobile to not have to preserve the drawing buffer. Another way to look at it is the browser needs 2 copies of your canvas. The one you're drawing to and the one it's displaying. It has 2 ways to deal with these 2 buffers. (A) double buffer. Let you draw to one, display the other, swap the buffers when you're done rendering which is inferred from exiting any event that issued draw commands (B) Copy the contents of the buffer you're drawing to do the buffer that's being displayed. Swapping is much faster than copying. So, swapping is the default. It's up to the browser what actually happens. The only requirement is that if preserveDrawingBuffer
is false
that the drawing buffer get cleared after a composite (which is yet another async event and therefore unpredictable) if preserveDrawingBuffer
is true
then it must copy so that the drawingbuffer's contents is preserved.
Note that once a canvas has a context it will always have the same context. So in other words let's say you change the code that initializes the WebGL context but you still want to set preserveDrawingBuffer: true
There are at least 2 ways.
since the code later will end up with the same context.
<script>
document.querySelector('#somecanvasid').getContext(
'webgl', {preserveDrawingBuffer: true});
</script>
<script src="script/that/will/use/somecanvasid.js"></script>
Because you've already created a context for that canvas whatever script comes after will get the same context.
getContext
<script>
HTMLCanvasElement.prototype.getContext = function(origFn) {
return function(type, attributes) {
if (type === 'webgl') {
attributes = Object.assign({}, attributes, {
preserveDrawingBuffer: true,
});
}
return origFn.call(this, type, attributes);
};
}(HTMLCanvasElement.prototype.getContext);
</script>
<script src="script/that/will/use/webgl.js"></script>
In this case any webgl context created after augmenting the getContext
will have preserveDrawingBuffer
set to true.