Canvas drawing and Retina display: doable? - iphone

Question

Working with phoneGap implementing drawing with Canvas. The catch we've run into is that canvas expects specific pixel dimensions. This is fine except that the iPhone 4's Retina display, from a CSS/Webkit POV is still 320px wide, even though technically there are 640 actual screen pixels.
Is there anyway to accommodate the retina display using Canvas on Webkit while preserving compatibility with non-retina displays?

Answer 1

I sat with the same problem last week and discovered how to solve it -
var canvas = document.querySelector('canvas');
var ctx = canvas.getContext('2d');
if (window.devicePixelRatio > 1) {
var canvasWidth = canvas.width;
var canvasHeight = canvas.height;
canvas.width = canvasWidth * window.devicePixelRatio;
canvas.height = canvasHeight * window.devicePixelRatio;
canvas.style.width = canvasWidth + "px";
canvas.style.height = canvasHeight + "px";
ctx.scale(window.devicePixelRatio, window.devicePixelRatio);
}
Full code on gist, demo on jsfiddle

Answer 2

There is a drop-in polyfill that will take care of most basic drawing operations for you, and remove the ambiguity between browsers that handle this automatically for you (safari) and others that don't.
https://github.com/jondavidjohn/hidpi-canvas-polyfill
You simply include it before your drawing code and it should give you decent retina support automatically.

Answer 3

I couldn't find anywhere else on the internet suggesting this so I figured it out. If you have a full screen canvas and you want it to be the actual amount of pixels that the device has, just remove this line from your HTML:
<meta name='viewport' content='width=device-width' />
Don't set the viewport at all. Then you just do:
canvas.width = innerWidth
canvas.height = innerHeight
canvas.style.width = innerWidth+'px'
canvas.style.height = innerHeight+'px'
That will use the full screen resolution of the device.
A pixel will always be a pixel.
You don't need to scale.
And getImageData() will give the same values that you see.

Answer 4

WebCode (http://www.webcodeapp.com) is a vector drawing app that generates JavaScript HTML5 Canvas code for you. The code is Retina-compatible, you can check out how they do it.

Answer 5

There is a very good polyfill by TJ Holowaychuk:
https://www.npmjs.com/package/autoscale-canvas

Answer 6

EDIT: Just noticed I posted the wrong link for the demo!
Retina (or other hdpi display) canvas resolution is definitely possible. There is a working example here:
http://spencer-evans.com/share/github/canvas-resizer/
I've also bumped into this a few times. The accepted answer code is essentially correct, but you could also use a library solution. I whipped one up to handle intelligent canvas re-sizing to make the element more responsive and higher resolution (as shown in the demo).
https://github.com/swevans/canvas-resizer