Capture HTML Canvas as gif/jpg/png/pdf?

Question

Is it possible to capture or print what's displayed in an html canvas as an image or pdf?

I'd like to generate an image via canvas, and be able to generate a png from that image.

Answer 1

Original answer was specific to a similar question. This has been revised:

const canvas = document.getElementById('mycanvas')
const img    = canvas.toDataURL('image/png')

with the value in IMG you can write it out as a new Image like so:

document.getElementById('existing-image-id').src = img

or

document.write('<img src="'+img+'"/>');

Answer 2

HTML5 provides Canvas.toDataURL(mimetype) which is implemented in Opera, Firefox, and Safari 4 beta. There are a number of security restrictions, however (mostly to do with drawing content from another origin onto the canvas).

So you don't need an additional library.

e.g.

 <canvas id=canvas width=200 height=200></canvas>
 <script>
      window.onload = function() {
          var canvas = document.getElementById("canvas");
          var context = canvas.getContext("2d");
          context.fillStyle = "green";
          context.fillRect(50, 50, 100, 100);
          // no argument defaults to image/png; image/jpeg, etc also work on some
          // implementations -- image/png is the only one that must be supported per spec.
          window.location = canvas.toDataURL("image/png");
      }
 </script>

Theoretically this should create and then navigate to an image with a green square in the middle of it, but I haven't tested.

Answer 3

I thought I'd extend the scope of this question a bit, with some useful tidbits on the matter.

In order to get the canvas as an image, you should do the following:

var canvas = document.getElementById("mycanvas");
var image = canvas.toDataURL("image/png");

You can use this to write the image to the page:

document.write('<img src="'+image+'"/>');

Where "image/png" is a mime type (png is the only one that must be supported). If you would like an array of the supported types you can do something along the lines of this:

var imageMimes = ['image/png', 'image/bmp', 'image/gif', 'image/jpeg', 'image/tiff']; //Extend as necessary 
var acceptedMimes = new Array();
for(i = 0; i < imageMimes.length; i++) {
    if(canvas.toDataURL(imageMimes[i]).search(imageMimes[i])>=0) {
        acceptedMimes[acceptedMimes.length] = imageMimes[i];
    }
}

You only need to run this once per page - it should never change through a page's lifecycle.

If you wish to make the user download the file as it is saved you can do the following:

var canvas = document.getElementById("mycanvas");
var image = canvas.toDataURL("image/png").replace("image/png", "image/octet-stream"); //Convert image to 'octet-stream' (Just a download, really)
window.location.href = image;

If you're using that with different mime types, be sure to change both instances of image/png, but not the image/octet-stream. It is also worth mentioning that if you use any cross-domain resources in rendering your canvas, you will encounter a security error when you try to use the toDataUrl method.