[whatwg] Enabling LCD Text and antialiasing in canvas

Rik Cabanier cabanier at gmail.com
Thu Feb 14 19:44:13 PST 2013

On Fri, Feb 15, 2013 at 2:21 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'.

Yes, that's pretty much my proposal. Except putImageData could detect it
but that might be too hard for authors to figure out.

> 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?

I think that would be very helpful.

So, if you clear (or matte) the canvas, the flag becomes true but if you do
an operator that removes alpha, it will become false.

> Rob
> --
> Wrfhf pnyyrq gurz gbtrgure naq fnvq, “Lbh xabj gung gur ehyref bs gur
> Tragvyrf ybeq vg bire gurz, naq gurve uvtu bssvpvnyf rkrepvfr nhgubevgl
> bire gurz. Abg fb jvgu lbh. Vafgrnq, jubrire jnagf gb orpbzr terng nzbat
> lbh zhfg or lbhe freinag, naq jubrire jnagf gb or svefg zhfg or lbhe fynir
> — whfg nf gur Fba bs Zna qvq abg pbzr gb or freirq, ohg gb freir, naq gb
> tvir uvf yvsr nf n enafbz sbe znal.” [Znggurj 20:25-28]

More information about the whatwg mailing list