[whatwg] Enabling LCD Text and antialiasing in canvas

Robert O'Callahan robert at ocallahan.org
Thu Feb 14 19:21:20 PST 2013

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'.

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?

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