693539 - Canvas .toDataURL should throw when invalid mimetype specified.

Status: RESOLVED INVALID

(Core :: Graphics: Canvas2D, defect)

Description

[Justin Dolske [:Dolske]]

Over in bug 693099 we added support for saving a frame of a HTML5 <video> as a JPEG. Except what we were saving wasn't actually a JPEG, it was actually a PNG. Oops.

The culprit was this call:

  canvas.toDataURL("image/jpg", "")

Spot the flaw? ... Yep, should be "image/jpeg".

Canvas should throw when given an invalid mimetype to help avoid this kind of problem. We might also want to consider allowing "image/jpg" for JPEG, though I expect there are MIME purists who will have words to say about that. ;-)

Comment 1

[Masatoshi Kimura [:emk]]

User agents supposed to ignore unknown types.
http://dev.w3.org/html5/spec/the-canvas-element.html#a-serialization-of-the-image-as-a-file
> User agents must support PNG ("image/png"). User agents may support other types.
> If the user agent does not support the requested type, it must create the file
> using the PNG format. [PNG]

Comment 2

[:Ms2ger (he/him; ⌚ UTC+1/+2)]

Invalid as filed. A console warning could be added, of course.

Comment 3

[Ed Morley [:emorley]]

Would it still be useful to modify saveImageURL (http://mxr.mozilla.org/mozilla-central/source/toolkit/content/contentAreaUtils.js#145) such that if Canvas .toDataURL has fallen back to PNG, the filename passed from nsContextMenu.js gets adjusted from JPG to PNG?

Comment 4

[Justin Dolske [:Dolske]]

Yuck, that seems like a rather unfortunate part of the spec. Sigh.