[whatwg] Enabling LCD Text and antialiasing in canvas

Stephen White senorblanco at chromium.org
Fri Feb 15 07:35:42 PST 2013

On Thu, Feb 14, 2013 at 10:21 PM, Robert O'Callahan <robert at ocallahan.org>wrote:

> On Thu, Feb 14, 2013 at 11:59 PM, Stephen White <senorblanco at chromium.org>wrote:
>> I think this is difficult to do in the general case, such as
>> putImageData() or drawImage() or patterns, since you would need to examine
>> all the pixels of the source image to determine if they contain non-1
>> alpha.  This would be cost-prohibitive.
>> If it's just for the purposes of optimization, you could be conservative
>> and simply clear the flag when there's the potential for (but not certainty
>> of) non-1 alpha.  But if you're also using that flag to allow subpixel AA,
>> the behaviour becomes quite unpredictable for the web developer and hard to
>> spec crisply.
> What if we just said
> a) non-over operators clear the subpixel AA flag
> b) putImageData clears the subpixel AA flag
> Both of these are infrequently used AFAIK. Authors could work around most
> cases of b) by doing putImageData to another canvas and compositing it into
> the destination with 'over'.

Even with these constraints, I don't think we can guarantee that it's safe
to use LCD AA text. Once you've drawn with LCD AA text, even if it's safe
at the time of drawing, there's no guarantee that it will be safe later.
 For instance, if you then drawImage() that canvas rotated into another
canvas, or even just full-page-zoom it, you'll see colour fringing. Or
apply CSS 2D or 3D transforms. There are also existing apps which use
canvas for 2D text glyphs, and then transform and place them rotated in
WebGL. Those will show colour fringing. Even within canvas, there may be a
way to break it if the LCD AA text is drawn first and a dest-alpha
compositing mode is used overtop (I haven't verified this though).

So I'm starting to think that LCD AA text really has to be opt-in, to avoid
breaking existing content. By opting it, you're agreeing that these
artifacts are acceptable for your app. For example, you know that even if
you're going to do a canvas-to-canvas draw, you're always going to draw at
1:1 scale and no rotation.


> How bad would that be? Would it help if we added an attribute to the
> canvas to let authors detect whether the subpixel AA flag is set?
> Rob
> --
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