Jonathon Bolster

web developer, programmer, geek

Grabbing image data from an external source to stuff into a canvas element

I was playing with Remy Sharp's collaborative drawing example after he posted it on Twitter.

Drew a couple of pictures and realised that I'm not as handy at drawing with the mouse as I used to be - so then I figured it would be better if I just put a random picture of a unicorn in. So I whacked open Developer Tools and tried to directly draw an image on the canvas element:

var img = document.createElement("img");
img.src = "";

Can you see the problem? Pub-quiz points if you correctly guessed 'Cross Domain Security'. It was perfectly fine to chuck the picture into the canvas without a problem cropping up. The issue happens when the code tried to call 'getDataURL' for transmission. That's the no no.

So what did I do? I cheated a little bit - browsing to the picture I wanted, getting the dataURL and copying it across to the original browser window.


var canvas = document.createElement("canvas");
canvas.width = 50;
canvas.height= 50;

// Random image from Google Image search for unicorn - don't judge me!
var img = document.createElement("img");
img.src = "";

var ctx = canvas.getContext("2d");

var dataStr = canvas.toDataURL("image/png");

The window URL changes to give me my data URL (which I've put as the src attribute for the image tag below:

Base 64 encoded image of a girl from glee

Last thing to do is set the encoded data as the src rather than the external image URL in the code above:

var img = document.createElement("img");
img.src = "data:image/jpeg;base64,/9j/4AAQSkZJR....";

No more security problems - but we did cheat! It's a far from perfect solution (and far from perfect coding) but it works for what I wanted to do. I wanted to manually put external image data on to a canvas element and do it manually.