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Drawing Pixels is Hard

Way harder than it should be.

Back in 2009 when I first started to work on what would become my HTML5 game engine Impact, I was immediately presented with the challenge of scaling the game screen while maintaining crisp, clean pixels. This sounds like an easy problem to solve – after all Flash did this from day one and "retro" games are a big chunk of the market, especially for browser games, so it really should be supported – but it's not.

Let's say I have a game with an internal resolution of 320×240 and I want to scale it up 2x to 640×480 when presented on a website. With the HTML5 Canvas element, there are essentially two different ways to do this.

a) Creating the Canvas element in the scaled up resolution (640×480) and draw all images at twice the size:

var canvas = document.createElement('canvas');
canvas.width = 640;
canvas.width = 480;

var ctx = canvas.getContext('2d');
ctx.scale( 2, 2 );
ctx.drawImage( img, 0, 0 );

b) Using CSS to scale the Canvas – In my opinion this is the cleaner way to do it. It nicely decouples the internal canvas size from the size at which it is presented:

var canvas = document.createElement('canvas');
canvas.width = 320;
canvas.width = 240;
canvas.style.width = '640px';
canvas.style.width = '480px';

var ctx = canvas.getContext('2d');
ctx.drawImage( img, 0, 0 );

Both methods have a problem though – they use a bilinear (blurry) filtering instead of nearest-neighbor (pixel repetition) when scaling.

For the internal scaling approach (method a), you can set the context's imageSmoothingEnabled property to false in order to have crisp, nearest-neighbor scaling. This has been supported in Firefox for a few years now, but Chrome only just recently implemented it and it is currently unsupported in Safari (including Mobile Safari) and Internet Explorer (test case).

When doing the scaling in CSS (method b), you can use the image-rendering CSS property to specify the scaling algorithm the browser should use. This works well in Firefox and Safari, but all other browsers simply ignore it for the Canvas element (test case).

Of course Internet Explorer is the only browser that currently doesn't support any of these methods.

Not having crisp scaling really bothered me when I initially started to work on Impact. Keep in mind that at the time no browser supported either of the two methods described above. So I experiment a lot to find a solution.

And I found one. It's incredibly backwards and really quite sad: I do the scaling in JavaScript. Load the pixel data of each image, loop through all pixels and copy and scale the image, pixel by pixel, into a larger canvas then throw away the original image and use this larger canvas as the source for drawing instead.

var resize = function( img, scale ) {
    // Takes an image and a scaling factor and returns the scaled image

    // The original image is drawn into an offscreen canvas of the same size
    // and copied, pixel by pixel into another offscreen canvas with the 
    // new size.

    var widthScaled = img.width * scale;
    var heightScaled = img.height * scale;

    var orig = document.createElement('canvas');
    orig.width = img.width;
    orig.height = img.height;
    var origCtx = orig.getContext('2d');
    origCtx.drawImage(img, 0, 0);
    var origPixels = origCtx.getImageData(0, 0, img.width, img.height);

    var scaled = document.createElement('canvas');
    scaled.width = widthScaled;
    scaled.height = heightScaled;
    var scaledCtx = scaled.getContext('2d');
    var scaledPixels = scaledCtx.getImageData( 0, 0, widthScaled, heightScaled );

    for( var y = 0; y < heightScaled; y++ ) {
        for( var x = 0; x < widthScaled; x++ ) {
            var index = (Math.floor(y / scale) * img.width + Math.floor(x / scale)) * 4;
            var indexScaled = (y * widthScaled + x) * 4;
            scaledPixels.data[ indexScaled ] = origPixels.data[ index ];
            scaledPixels.data[ indexScaled+1 ] = origPixels.data[ index+1 ];
            scaledPixels.data[ indexScaled+2 ] = origPixels.data[ index+2 ];
            scaledPixels.data[ indexScaled+3 ] = origPixels.data[ index+3 ];
        }
    }
    scaledCtx.putImageData( scaledPixels, 0, 0 );
    return scaled;
}

This worked surprisingly well and has been the easiest way to scale up pixel-style games in Impact from day one. The scaling is only done once when the game first loads, so the performance hit isn't that bad, but you still notice the longer load times on mobile devices or when loading big images. After all, it's a stupidly costly operation do to, even in native code. We usually use GPUs for stuff like that.

All in all, doing the scaling in JavaScript is not the "right" solution, but the one that works for all browsers.

Or rather worked for all browsers.

Meet the retina iPhone

When Apple introduced the iPhone 4, it was the first device with a retina display. The pixels on the screen are so small, that you can't discern them. This also means, that in order to read anything on a website at all, this website has to be scaled up 2x.

So Apple introduced the devicePixelRatio. It's the ratio of real hardware pixels to CSS pixels. The iPhone 4 has a device pixel ratio of 2, i.e. one CSS pixel is displayed with 2 hardware pixels on the screen.

This also means that the following canvas element will be automatically scaled up to 640×480 hardware pixels on a retina device, when drawn on a website. Its internal resolution, however, still is 320×240.

<canvas width="320" height="240">

This automatic scaling again happens with the bilinear (blurry) filtering by default.

So, in order to draw at the native hardware resolution, you'd have to do your image scaling in JavaScript as usual but with twice the scaling factor, create the canvas with twice the internal size and then scale it down again using CSS.

Or, in recent Safari's, use the image-rendering: -webkit-optimize-contrast; CSS property. Nice!

This certainly makes things a bit more complicated, but devicePixelRatio was a sane idea. It makes sense.

Meet the retina MacBook Pro

For the new retina MacBook Pro (MBP), Apple had another idea. Instead of behaving in the same way as Mobile Safari on the iPhone, Safari for the retina MBP will automatically create a canvas element with twice the internal resolution than you requested. In theory, this is quite nice if you only want to draw shapes onto your canvas - they will automatically be in retina resolution. However, it significantly breaks drawing images.

Consider this Canvas element:

<canvas width="320" height="240"></canvas>

On the retina MBP, this will actually create a Canvas element with an internal resolution of 640×480. It will still behave as if it had an internal resolution of 320×240, though. Sort of.

This ingenious idea is called backingStorePixelRatio and, you guessed it, for the retina MBP it is 2. It's still 1 for the retina iPhone. Because… yeah…

(Paul Lewis recently wrote a nice article about High DPI Canvas Drawing, including a handy function that mediates between the retina iPhone and MBP and always draws in the native resolution)

Ok, so what happens if you now draw a 320×240 image to this 320×240 Canvas that in reality is a 640×480 Canvas? Yep, the image will get scaled using bilinear (blurry) filtering. Granted, if it wouldn't use bilinear filtering, this whole pixel ratio dance wouldn't make much sense. The problem is, there's no opt-out.

Let's say I want to analyze the colors of an image. I'd normally just draw the image to a canvas element retrieve an array of pixels from the canvas and then do whatever I want to do with them. Like this:

ctx.drawImage( img, 0, 0 );
var pixels = ctx.getImageData( 0, 0, img.width, img.height );
// do something with pixels.data...

On the retina MBP you can't do that anymore. The pixels that getImageData() returns are interpolated pixels, not the original pixels of the image. The image you have drawn to the canvas was first scaled up, to meet the bigger backing store and then scaled down again when retrieved through getImageData(), because getImageData() still acts as if the canvas was 320×240.

Fortunately, Apple also introduced a new getImageDataHD() method to retrieve the real pixel data from the backing store. So all you'd have to do is draw your image to the canvas with half the size, in order to draw it at the real size. Confused yet?

var ratio = ctx.webkitBackingStorePixelRatio || 1;
ctx.drawImage( img, 0, 0, img.width/ratio, img.height/ratio );

var pixels = null;
if( ratio != 1 ) {
    pixels = ctx.webkitGetImageDataHD( 0, 0, img.width, img.height );
}
else {
    pixels = ctx.getImageData( 0, 0, img.width, img.height );
}

(Did I say it's called getImageDataHD()? I lied. You gotta love those vendor prefixes. Imagine how nice it would be if there also was a moz, ms, o and a plain variant!)

The "Good" News

Ok, take a deep breath, there are only 3 different paths you have to consider when drawing sharp pixels on a scaled canvas.

The CSS image-rendering property and the Canvas' imageSmoothingEnabled really make things a bit easier, but it would be nice if they were universally supported. Especially Safari is in desperate need for imageSmoothingEnabled-support, with all the crazy retina stuff they have going on.

Let me also go on record saying that backingStorePixelRatio was a bad idea. It would have been a nice opt-in feature, but it's not a good default. A comment from Jake Archibald on Paul Lewis' article tells us why:

<canvas> 2D is a bitmap API, it's pixel dependent. An api that lets you query individual pixels shouldn't be creating pixels you don't ask for.

Apple's backingStorePixelRatio completely breaks the font rendering in Impact, makes games look blurry and breaks a whole bunch of other apps that use direct pixel manipulation. But at least Apple didn't have to update all their dashboard widgets for retina resolution. How convenient!

Update September 18th 2012: To demonstrate the bug in Safari, I build another test case and filed a report with Apple.

Thursday, September 13th 2012
— Dominic Szablewski, @phoboslab