Html5 Canvas Limitations - javascript

I have been unsuccessfully trying to find documentation that details the limitations regarding the Html5 Canvas Element such as what is the largest image file size it can load etc. The reason I ask is that I have been trying to resize image sizes ranging from 50kb to 2.0mb through ctx.scale(), yet it has been horribly inconsistent in that for the same image sometimes ctx.drawImage() will be successful and other times unsuccessful (unsuccessful being no re-scaled image appears in the canvas).
I have also placed console.log(base64) to monitor the result of var base64 = canvas.toDataURL() and have noticed that when successfully resized the resized base64 will be quite a long string as expected and when unsuccessfully resized a string will still appear yet be relatively short and outputs a blank image.
Does this have something to do with memory limitations and the unsuccessful strings wrapping around themselves? If so, what are the memory limitations imposed on the canvas element?

First:
Hard limitations would depend on the browser, not the canvas API.
Even then, browsers are always trying to improve that performance, so that number would always be changing.
But with WebGL and Canvas being used to make games, texture atlases / sprite atlases are HUGE .jpg/.png files.
Chances are very, very good that your images are smaller, and I've frequently used 4MB/5MB/16MB images in canvas for demonstrations.
A huge image (or dozens of them) might crash the tab, if it's big enough, but until that time, canvas hasn't really complained to me.
Second:
There are security-limitations.
Editing photos in canvas comes down to what browser you're on, and whether the file is on the same domain as your website or not.
Third:
When you say that "large files don't work, but they do sometimes..."
...that leads me to believe that your image-loading method is faulty.
If you do something like this:
var canvas = document.createElement("canvas"),
context = canvas.getContext("2d"),
img = new Image();
img.src = "//mydomain.com/myimg.png";
context.drawImage(img, 0, 0, img.width, img.height);
...or anything else which isn't either event-based or callback-based,
then your problem has nothing to do with canvas and has everything to do with callbacks, and that you're trying to draw the image to the canvas before the image is done loading.
If your browser has already cached a copy of the large image, or if a small image only takes a fraction of a second to download, then there's no problem.
If you try downloading an 18MB image, and draw it to the canvas as soon as you set the url for the image, then you're going to get a blank screen, because it hasn't finished loading.
Instead, you need to wait for the image to finish loading, and then draw it to the canvas when it's ready.
var canvas = document.createElement("canvas"),
context = canvas.getContext("2d"),
image = new Image();
image.onload = function () {
var img = this,
width = img.width,
height = img.height;
canvas.width = width;
canvas.height = height;
context.drawImage(img, 0, 0, width, height);
document.body.appendChild(canvas);
};
image.src = "//mydomain.com/path-to-really-huge-image.png";
Now it could be a 16MP image. Nothing will happen until the file is done loading.
Then, the onload event fires, and does the rest of the setup.
You could make it more abstract, and turn it into a nifty program, but it feels like this is the key piece you might be missing.

Related

Is it possible to draw a canvas thats bigger than the current screen?

I'm drawing a image to a canvas, and when doing so the image gets downscaled to the canvas(which makes it lose quality) even though the image is the same size as the canvas, thats because the img does a good job scaling down the actual img that in reality has a bigger naturalheight and naturalwidth. I know there is possible ways to make this quality better, however i have no need of actually showing this canvas to the user/no need of downscaling. Therefore am i wondering if there is any way to drawImage that is bigger than the screen and hold it somewhere? Heard someone mention a box object or a camera object somewhere but couldn't really get use of that information only.
Question, is it possible to draw a canvas bigger than the screen? in that case how?
This is the code im working with atm
var image = document.getElementById('insertedImg');
var height = $("#insertedImg").height();
var width = $("#insertedImg").width();
var c=document.getElementById("canvass");
var ctx=c.getContext("2d");
c.height=height;
c.width=width;
ctx.drawImage(image,0,0,width,height);
Use an offscreen canvas, you just need to create a new canvas and set its height and width accordingly. Then you can append it to an element or export the image as a base64 string or whatever you need.
//canvas is not visible unless appended to a DOM element
var canvas = document.createElement('canvas');
canvas.width = $("#insertedImg").height();
canvas.height = $("#insertedImg").width();
var ctx = canvas.getContext('2d');
//do the drawing, etc.
ctx.drawImage(...);
//export the image
var img = canvas.toDataURL("image/png");

JavaScript image opacity is being ignored

I'm trying to display an image via JavaScript and set its opacity, but the opacity I've specified is getting ignored. I've tried all the latest browsers, but am mainly using IE11. The image I specify displays fine and I can position, scale, and rotate it without any problem. I've done extensive searches on this site and others to try to identify the problem but so far haven't had any luck. I've also tried rebooting my PC and using another PC to check the results. Nothing seems to help. Even trying a PNG file.
Here's my code:
var imageObj = new Image();
imageObj.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
imageObj.onload = function () {
imageObj.style.filter = "alpha(opacity=20)";
imageObj.style.opacity = 0.20;
ctx.drawImage(imageObj, 10, 10);
};
Oh, prior to this code, the background color was set to pure white (#ffffff). I tried playing around with the background color, but it seemed to make no difference.
Try context.imageSmoothingEnabled = true;. The canvas element has a global alpha attribute that lets you apply partial transparency to anything you draw. You can see more here.
The CSS opacity is applied to the Node and not to the image data, this means that when you draw to your canvas' context, this meta information is forgotten.
Instead, you can use context.globalAlpha
imageObj.onload = function () {
var x = ctx.globalAlpha;
ctx.globalAlpha = 0.2;
ctx.drawImage(imageObj, 10, 10);
ctx.globalAlpha = x;
};
Beware though, this may not be safe if you're expecting to draw a lot asynchronously, so you may even want to create a second canvas for fixing the alpha, then export/import into the canvas you're really interested in, getImageData, putImageData.
Another option could always be to use an image format which supports an alpha channel, such as PNG and just serve the images at the desired opacity. This would cut down on processing, but may increase bandwidth usage (really depends on the image contents to how well each format compresses the data, e.g. JPEG is designed for photos/real world images whereas a PNG may better compress an artificial image with no noise).

Saving a canvas drawing larger than the physical screen

I'm dynamically drawing a floorplan through canvas which you can scroll through up/down left/right but I would like to save the whole image of the floorplan for other uses. I know I can scale the floorplan down and capture the image but I need it to be in a higher resolution than the actual screen I'm capturing it on.
I'm currently using FileSaver.js to save the canvas as a bitmap because it's super easy.
Is this possible?
It's hard to tell without more information, but you can create a larger canvas (of the size you need it to be) and let it hidden. Then when you want to capture, you re-draw everything into that canvas and save your picture from it instead of the one that is displayed.
So you don't need to save the canvas, but the image.
I don't know your FileSaver.js, but if it cannot save an image directly,
putting the image inside a new canvas is easy :
function getCanvasFromImage(img) {
var cv = document.createElement('canvas');
cv.width = img.width; cv.height = img.height;
cv.getContext('2d').putImage(img, 0, 0);
return cv;
}

Taking advantage of foreground and background canvases in HTML5

Right now I have one large canvas where everything is drawn. The canvas is created directly in the JS file:
var canvas = document.createElement ("canvas");
var ctx = canvas.getContext("2d");
canvas.width = document.width;
canvas.height = document.height;
document.body.appendChild(canvas);
It resizes itself to the browser window.
I have a gradient background being drawn onto the one canvas along with all the other elements. The colors for the background are randomly generated at each game mode change (eg when the main menu is toggled to the game, then the level end screen, etc). Currently I'm drawing these onto the canvas like this:
var grad1 = ctx.createRadialGradient(canvas.width / 2, canvas.height / 2, 0, canvas.width / 2, canvas.height / 2, 500);
grad1.addColorStop(0, bgGradStop1);
grad1.addColorStop(1, bgGradStop2);
ctx.fillStyle = grad1;
ctx.fillRect(0, 0, canvas.width, canvas.height);
I read numerous times that having a separate canvas for the background is better for performance. Is there anything specific that I need to do to make this effective or can I simply create another canvas and draw the background using the same exact code, only modified to use the other canvas, like this:
var grad1 = ctxBg.createRadialGradient(canvasBg.width / 2, canvasBg.height / 2, 0, canvasBg.width / 2, canvasBg.height / 2, 500);
grad1.addColorStop(0, bgGradStop1);
grad1.addColorStop(1, bgGradStop2);
ctxBg.fillStyle = grad1;
ctxBg.fillRect(0, 0, canvasBg.width, canvasBg.height);
Or does getting any performance benefit involve some totally different method of using the second canvas?
Also, if it really is just a matter of doing the exact same thing but on a different canvas, would there be any benefit to also for example moving the HUD text to its own canvas while other entities are moving? Even separating various types of entities onto their own canvases? How far does the benefit of multiple canvases actually stretch?
Or does getting any performance benefit involve some totally different method of using the second canvas?
You've got the right idea of how to do it, though I don't think a second canvas is necessary in your case.
The important thing from a performance perspective is that you don't want to have to keep constructing and filling the gradient. If all you're doing in your draw loop is:
ctx.fillStyle = grad1;
ctx.fillRect(0, 0, canvas.width, canvas.height);
Then you should be pretty swell here. I don't think having a second canvas in the background will help you much after that. There might be a slight performance boost, but who wants to have to keep track of additional DOM elements if you don't have to?
It's a little hard to test the performance of having a second canvas behind your main one, but instead of having two large canvases in the DOM one similar alternative is to draw the gradient an in-memory canvas and always call ctx.drawImage(inMemCan, 0, 0); instead of setting the gradient and drawing it. Drawing images is known to be fast, and using two canvases this way is probably close to the speed you could expect from two on-page canvases (and hopefully it would be faster).
So we could test drawing the gradient from an in-memory canvas to your main canvas versus drawing the plain old gradient. Here's a simple test with a large canvas:
http://jsperf.com/gradient-vs-immemcan
They're pretty equal it seems. If this one thing is the only thing in your background I wouldn't worry about it. There are probably bigger performance fish for you to fry first.
I'd save the benefit of multiple canvases for when a relatively complicated background updates rarely but independently of the foreground. If your background was instead made with 30 gradients and some paths, then using a second canvas (or an in-memory canvas to cache an image) would give you a sizable boost.
It resizes itself to the browser window.
Just a reminder that the full screen API works pretty well in webkit and firefox if you want to look into it in the future.

How to optimize animation on canvas? HTML 5

I am faced with a problem that slows down animation on a canvas, as various pictures move left, right, up and down. I need advice on how to optimize the animation.
It is important that the animation works on all main browsers, in particular: Chrome, Firefox and Internet Explorer.
Is it possible to optimize the animation? Maybe put a delay on the drawing? Thank you in advance.
In javascript you can use the setInterval and setTimeout functions to create delays and throttle the frame rate.
for instance if you wanted to make your drawing loop approximately 30 FPS you could have some code that looks like this:
function draw(){
var canvas = document.getElementById('myCanvas');
//create the image object
var img = new Image();
//set the image object's image file path
var img.src = "images/myImage.png"
//check to see that our canvas exists
if( canvas.getContext )
{
//grab the context to draw to.
var ctx = cvs.getContext('2d');
//clear the screen to a white background first
//NOTE to make this faster you can clear just the parts
//of the canvas that are moving instead of the whole thing
//every time. Check out 'Improving HTML5 Canvas Performance'
//link mention in other post
ctx.fillStyle="rgb(255,255,255)";
ctx.fillRect (0, 0,512, 512);
//DO ALL YOUR DRAWING HERE....
//draw animation
ctx.drawImage(img, 0, 0);
}
//Recalls this draw update 30 times per second
setTimeout(draw,1000/30);
}
This will help you control how much processing time the animation is taking. Thus if you find that your animation is going too slow you can increase the frame rate by changing the denominator '30' to something like '60' fps or anything that really works well for your program.
As far as optimizing goes in addition to setTimeout() the way you draw your animations is critical too. Try to load all your images before you render them this is called "Preloading". That is if you have a bunch of different images for each animated cell then before you call your draw function load all the images into an array of images like so:
var loadedImages = new Array();
loadedImages[0] = new Image();
loadedImages[0].src = "images/animationFrame1.png";
loadedImages[1] = new Image();
loadedImages[1].src = "images/animationFrame2.png";
loadedImages[2] = new Image();
loadedImages[2].src = "images/animationFrame3.png";
loadedImages[3] = new Image();
loadedImages[3].src = "images/animationFrame4.png";
you could even put them in a hash if it makes sense for you app where instead of
loadedImages[0] = new Image();
loadedImages[0].src = "images/animationFrame1.png";
you do this
loadedImages['frame1'] = new Image();
loadedImages['frame1'].src = "images/animationFrame1.png";
once you have all your images loaded you references them for drawing by doing calling them like so:
//Using the integer array
ctx.drawImage(loadedImages[0], 0, 0);
//OR
//Using the stringed hash
ctx.drawImage(loadedImages['frame1'], 0, 0);
You want to separate your loading process from your rendering process because loading images is process intensive thus will slow your animations down if you are loading things while rendering.
That is not to say that you can't ever load anything while rendering, but instead just be conscience that this will slow animation speed down.
Here is an article on preloading images.
There is another post on here which talks about consistent animation speed on all browsers here
Note the link posted by the green checked answer
Other things to be noted are making sure to only clearing and redrawing bounding boxes as mentioned in the post with HTML 5 canvas optimization. That link has some really good techniques to be conscience of while developing canvas animations.
Here are some frame works as well which might come in handy to cross compare what you are doing to what other engines are doing.
Hope some of this helps. I am new to javascript (only started coding with it about 2 weeks ago) and so there could be better ways to do things but these are the things I have found thus far.
window.requestAnimationFrame() is one sure way to make your animation run smoother.
https://developer.mozilla.org/en/DOM/window.mozRequestAnimationFrame
(cross browser http://paulirish.com/2011/requestanimationframe-for-smart-animating/ )
However it doesn't fix the possible problems with your drawing code which was missing from the question.

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