Requirement:
Now: Draw on a Canvas, and hit Save (store Canvas state/drawing offline - but NOT as image).
Later: Open up the Canvas with previously saved drawing showing, and continue to draw again.
For drawing we normally use code as follows:
canvas = document.getElementById('can');
ctx = canvas.getContext("2d");
...
ctx.beginPath();
ctx.moveTo(prevX, prevY);
ctx.lineTo(currX, currY);
....
In order to restore Canvas state later - exporting to Image does not help.
I want to restore the Canvas to it's original state to continue editing the drawing at a later date.
I guess, the Canvas context has to be exported and stored offline - how?
Your best shot here is to use a Proxy that will both store the draw commands and perform the drawings.
Since the browser support for Proxy is very bad (only FF as of today), you'll have to build the Proxy yourself, either by using nosuchmethod, or by building a new brand new WatchedContext Class out of the Context2D.
I took the last solution (WatchedContext Class) for this short demo :
function WatchedContext(hostedCtx) {
this.commands= [];
Context2dPrototype = CanvasRenderingContext2D.prototype;
for (var p in Context2dPrototype ) {
this[p] = function(methodName) {
return function() {
this.commands.push(methodName, arguments);
return Context2dPrototype[methodName].apply(hostedCtx, arguments);
}
}(p);
}
this.replay=function() {
for (var i=0; i<this.commands.length; i+=2) {
var com = this.commands[i];
var args = this.commands[i+1];
Context2dPrototype[com].apply(hostedCtx, args);
}
}
}
Obviously you might need some other method (start/stop recording, clear, ...)
Just a small example of use :
var cv = document.getElementById('cv');
var ctx=cv.getContext('2d');
var watchedContext=new WatchedContext(ctx);
// do some drawings on the watched context
// --> they are performed also on the real context
watchedContext.beginPath();
watchedContext.moveTo(10, 10);
watchedContext.lineTo(100, 100);
watchedContext.stroke();
// clear context (not using the watched context to avoid recording)
ctx.clearRect(0,0,100,1000);
// replay what was recorded
watchedContext.replay();
You can see here :
http://jsbin.com/gehixavebe/2/edit?js,output
That the replay does work, and the line is re-drawn as a result of replaying the stored commands.
For storing offline you can either store the commands locally using localStorage or store them remotely on a server an use AJAX calls or similar.
Related
We're working on a project where people can be in a chat room with their webcams, and they can grab a snapshot of someone's cam at that moment, do some annotations on top of it, and then share that modified picture as if it was their own webcam (like sharing a whiteboard).
Capturing the webcam stream into a canvas element where it can be edited was relatively easy. Finding the canvas element on our page and doing a .getContext('2d') on it,
Used an open library to add editing tools to it. Grabbing a stream from that canvas was done like so:
var canvasToSend = document.querySelector('canvas');
var stream = canvasToSend.captureStream(60);
var room = osTwilioVideoWeb.getConnectedRoom();
var mytrack = null;
room.localParticipant.publishTrack(stream.getTracks()[0]).then((publication) => {
mytrack = publication.track;
var videoElement = mytrack.attach();
});
This publishes the stream alright, but the first frame will not get sent unless you draw something else on the canvas. Let's say you drew 2 circles and then hit Share, the stream will start but will not be shown on the recipients' side unless you draw a line, or another circle, or anything. It seems like it needs a frame change for it to send data over.
I was able to force this with developer tools by doing something like context.fill();, but when I tried adding this after the publishing function, even in a then()... no luck.
Any ideas on how to force this "refresh" to happen?
So it seems it is expected behavior (and thus would make my FF buggy).
From the specs about the frame request algorithm:
A new frame is requested from the canvas when frameCaptureRequested is true and the canvas is painted.
Let's put some emphasis on the "and the canvas as been painted". This means that we need both these conditions, and while captureStream itself, or its frameRate argument ellapsing or a method like requestFrame would all set the frameCaptureRequested flag to true, we still need the new painting...
The specs even have a note stating
This algorithm results in a captured track not starting until something changes in the canvas.
And Chrome indeed seems to generate an empty CanvasCaptureMediaStreamTrack if the call to captureStream has been made after the canvas has been painted.
const ctx = document.createElement('canvas')
.getContext('2d');
ctx.fillRect(0,0,20,20);
// let's request a stream from before it gets painted
// (in the same frame)
const stream1 = ctx.canvas.captureStream();
vid1.srcObject = stream1;
// now let's wait that a frame ellapsed
// (rAF fires before next painting, so we need 2 of them)
requestAnimationFrame(()=>
requestAnimationFrame(()=> {
const stream2 = ctx.canvas.captureStream();
vid2.srcObject = stream1;
})
);
<p>stream initialised in the same frame as the drawings (i.e before paiting).</p>
<video id="vid1" controls autoplay></video>
<p>stream initialised after paiting.</p>
<video id="vid2" controls autoplay></video>
So to workaround this, you should be able to get a stream with a frame by requesting the stream from the same operation as a first drawing on the canvas, like stream1 in above example.
Or, you could redraw the canvas context over itself (assuming it is a 2d context) by calling ctx.drawImage(ctx.canvas,0,0) after having set its globalCompositeOperation to 'copy' to avoid transparency issues.
const ctx = document.createElement('canvas')
.getContext('2d');
ctx.font = '15px sans-serif';
ctx.fillText('if forced to redraw it should work', 20, 20);
// produce a silent stream again
requestAnimationFrame(() =>
requestAnimationFrame(() => {
const stream = ctx.canvas.captureStream();
forcePainting(stream);
vid.srcObject = stream;
})
);
// beware will work only for canvas intialised with a 2D context
function forcePainting(stream) {
const ctx = (stream.getVideoTracks()[0].canvas ||
stream.canvas) // FF has it wrong...
.getContext('2d');
const gCO = ctx.globalCompositeOperation;
ctx.globalCompositeOperation = 'copy';
ctx.drawImage(ctx.canvas, 0, 0);
ctx.globalCompositeOperation = gCO;
}
<video id="vid" controls autoplay></video>
I wonder, how can I obtain any WebGL program instance (WebGLProgram) from any desired WebGL context?
To fetch the WebGL context is NOT a problem. You are searching the DOM of the current page for the canvas element using document.getElementsByTagName() or document.getElementById(), if you know the exact canvas id:
let canvas = document.getElementById( "canvasId" );
let context = canvas.getContext( "webgl" );
Here we fetch the current context as I suppose, but if I want to get some shader parameters or get certain value from already running vertex/fragment shader - I need to have a WebGL program, which is associated with the current WebGL rendering context.
But I can't find any method in WebGL API like context.getAttachedProgram() or context.getActiveProgram().
So what is the way get the active WebGL program which is used for the rendering process?
Maybe, there is some special WebGL parameter?
There is no way to get all the programs or any other resources from a WebGL context. If the context is already existing the best you can do is look at the current resources with things like gl.getParameter(gl.CURRENT_PROGRAM) etc..
What you can do instead is wrap the WebGL context
var allPrograms = [];
someContext.createProgram = (function(oldFunc) {
return function() {
// call the real createProgram
var prg = oldFunc.apply(this, arguments);
// if a program was created save it
if (prg) {
allPrograms.push(prg);
}
return prg;
};
}(someContext.createProgram));
Of course you'd need to wrap gl.deleteProgram as well to remove things from the array of all programs.
someContext.deleteProgram = (function(oldFunc) {
return function(prg) {
// call the real deleteProgram
oldFunc.apply(this, arguments);
// remove the program from allPrograms
var ndx = allPrograms.indexOf(prg);
if (ndx >= 0) {
allPrograms.splice(ndx, 1);
}
};
}(someContext.deleteProgram));
These are the techniques used by things like the WebGL Inspector and the WebGL Shader Editor Extension.
If you want to wrap all contexts you can use a similar technique to wrap getContext.
HTMLCanvasElement.prototype.getContext = (function(oldFunc) {
return function(type) {
var ctx = oldFunc.apply(this, arguments);
if (ctx && (type === "webgl" || type === "experimental-webgl")) {
ctx = wrapTheContext(ctx);
}
return ctx;
};
}(HTMLCanvasElement.prototype.getContext));
gl.getParameter(gl.CURRENT_PROGRAM). Check out https://www.khronos.org/files/webgl/webgl-reference-card-1_0.pdf pg 2 to the right.
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 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();
}, false);
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
var captureFrame = false;
function render() {
drawScene();
if (captureFrame) {
captureFrame = false;
var data = someCanvas.toDataURL();
...
}
requestAnimationFrame(render);
}
render();
someElement.addEventListener('click', function() {
captureFrame = true;
}, false);
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.
find the canvas first, get a context on it
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.
augment 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.
Ive been trying to make the canvas save between page refreshes using html5 local storage, but the canvas always gets back to blank when i refresh the page.
HTML
<canvas onload="loadCanvas()" onClick="canvas(event)" id="myCanvas" width="400" height="400">
Your browser does not support the canvas element
</canvas>
Javascript:
function saveCanvas() {
var c = document.getElementById("myCanvas"),
ctx = c.toDataURL();
if (typeof (localStorage) !== "undefined") {
localStorage.setItem('myCanvas', ctx);
} else {
document.getElementById("save").innerHTML.dataURL = "Local Storage not supported";
}
}
function loadCanvas() {
var image = localStorage.getItem('myCanvas');
document.getElementById('myCanvas').src = image;
}
saveCanvas function gets called when something has been drawed in the canvas
Anyone knows what the problem is?
It has been solved, onLoad did not work in canvas
localStorage can only save so much, ie. in most browsers 5 mb and in others less.
Another caveat is that each char stored takes 2 bytes due to unicoding so the storage is in reality only half of this in the practical sense. There is no guarantee about size as this is not defined by the standard - 5 mb is only a suggestion so browsers can use any size.
You are getting the image as a PNG as this is the default format of toDataURL(). If the produced data-uri is too large (which is likely here as base-64 adds 33% to the size + a small header) the save will truncate or fail depending on the browser.
This is most likely (as you don't state size of canvas or the resulting data-uri) why your canvas is blank when you try to reload the data-uri as it would be invalid.
You can try to save as JPEG instead:
dataUri = c.toDataURL('image/jpeg', 0.5); /// 0.5 is quality, higher is better
If this does not work then you will need to look into other local storage mechanisms such as Indexed DB (where you can request a storage quota) or File API (but this is only supported in Chrome at this moment). There is also the deprecated Web SQL which will be around for still a while.
Update
Also try to move your onload from canvas element to window:
window.onload = function() {
var image = localStorage.getItem('myCanvas');
document.getElementById('myCanvas').src = image;
}
Note: you cannot set a src on a canvas element (as the ID from your code here suggest as well as your example code show). You need an image element for that. When you set a src on an image you also need to use the onload handler on the image, so an example could be:
window.onload = function() {
var img = new Image;
img.onload = function() {
var ctx = document.getElementById('myCanvas').getContext('2d');
ctx.drawImage(img, 0, 0);
/// call next step here...
}
img.src = localStorage.getItem('myCanvas');
}
Usually I suggest (and others did too in this thread) people store their drawings as points and shape types in an array as objects which then is serialized to a string which you instead store in localStorage. It involves a little bit more code in the render stage (which you need anyways to update canvas when it is blanked for some reason) but is worth it.
I'm using some javascript to allow users to dynamically load a sketch on click to a canvas element using:
Processing.loadSketchFromSources('canvas_id', ['sketch.pde']);
If I call Processing.loadSketchFromSources(...) a second (or third...) time, it loads a second (or third...) .pde file onto the canvas, which is what I would expect.
I'd like for the user to be able to click another link to load a different sketch, effectively unloading the previous one. Is there a method I can call (or a technique I can use) to check if Processing has another sketch running, and if so, tell it to unload it first?
Is there some sort of Processing.unloadSketch() method I'm overlooking? I could simply drop the canvas DOM object and recreate it, but that (1) seems like using a hammer when I need a needle, and (2) it results in a screen-flicker that I'd like to avoid.
I'm no JS expert, but I've done my best to look through the processing.js source to see what other functions may exist, but I'm hitting a wall. I thought perhaps I could look at Processing.Sketches.length to see if something is loaded already, but simply pop'ing it off the array doesn't seem to work (didn't think it would).
I'm using ProcessingJS 1.3.6.
In case someone else comes looking for the solution, here's what I did that worked. Note that this was placed inside a closure (not included here for brevity) -- hence the this.launch = function(), blah blah blah... YMMV.
/**
* Launches a specific sketch. Assumes files are stored in
* the ./sketches subdirectory, and your canvas is named g_sketch_canvas
* #param {String} item The name of the file (no extension)
* #param {Array} sketchlist Array of sketches to choose from
* #returns true
* #type Boolean
*/
this.launch = function (item, sketchlist) {
var cvs = document.getElementById('g_sketch_canvas'),
ctx = cvs.getContext('2d');
if ($.inArray(item, sketchlist) !== -1) {
// Unload the Processing script
if (Processing.instances.length > 0) {
// There should only be one, so no need to loop
Processing.instances[0].exit();
// If you may have more than one, then use this loop:
for (i=0; i < Processing.instances.length; (i++)) {
// Processing.instances[i].exit();
//}
}
// Clear the context
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.clearRect(0, 0, cvs.width, cvs.height);
// Now, load the new Processing script
Processing.loadSketchFromSources(cvs, ['sketches/' + item + '.pde']);
}
return true;
};
I'm not familiar with Processing.js, but the example code from the site has this:
var canvas = document.getElementById("canvas1");
// attaching the sketchProc function to the canvas
var p = new Processing(canvas, sketchProc);
// p.exit(); to detach it
So in your case, you'll want to keep a handle to the first instance when you create it:
var p1 = Processing.loadSketchFromSources('canvas_id', ['sketch.pde']);
When you're ready to "unload" and load a new sketch, I'm guessing (but don't know) that you'll need to clear the canvas yourself:
p1.exit();
var canvas = document.getElementById('canvas_id');
var context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
// Or context.fillRect(...) with white, or whatever clearing it means to you
Then, from the sound of things, you're free to attach another sketch:
var p2 = Processing.loadSketchFromSources('canvas_id', ['sketch2.pde']);
Again, I'm not actually familiar with that library, but this appears straightforward from the documentation.
As of processing.js 1.4.8, Andrew's accepted answer (and the other answers I've found in here) do not seem to work anymore.
This is what worked for me:
var pjs = Processing.getInstanceById('pjs');
if (typeof pjs !== "undefined") {
pjs.exit();
}
var canvas = document.getElementById('pjs')
new Processing(canvas, scriptText);
where pjs is the id of the canvas element where the scrips is being run.