[whatwg] Enabling LCD Text and antialiasing in canvas

Rik Cabanier cabanier at gmail.com
Thu Feb 14 09:32:25 PST 2013

On Thu, Feb 14, 2013 at 2:59 AM, Stephen White <senorblanco at chromium.org>wrote:

> On Wed, Feb 13, 2013 at 11:35 PM, Robert O'Callahan <robert at ocallahan.org>wrote:
>> On Thu, Feb 14, 2013 at 5:16 PM, Rik Cabanier <cabanier at gmail.com> wrote:
>>> Looking at the WebKit implementation, I'm unsure how 'opaque' can
>>> implemented for accelerated canvas. It might work with non-accelerated
>>> canvas but would have to run some experiments.
>>> I also look at mozilla's Core Graphics implementation and unless I'm
>>> missing something, it doesn't have special code to handle 'opaque'. When do
>>> you use this parameter?
>> CanvasRenderingContext2D::GetSurfaceFormat is part of the process. That
>> selects a surface format that is passed down to the graphics layer when
>> creating the canvas surface. It's true that we don't currently do anything
>> with that when drawing with CoreGraphics. That would need to be cleaned up
>> before we started promoting this feature.
>> Now that you mention it, having to modify the definition of compositing
>> is a bit of a bummer for the 'opaque' attribute approach. I think we could
>> do everything we want using your approach --- internally keeping a flag to
>> indicate whether the alpha values of the canvas are all 1, setting it when
>> the canvas is filled with a solid color and clearing it when non-over
>> drawing (or clear()) are used. Let's try that!
> 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.

You only have to do that if you use compositing modes such as source-in
In addition, you could detect at low cost that the CanvasImageSource is
opaque so some compositing modes would not reset.

> 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.

Yes, that is worrisome. It seems another flag is in ordre.

> We could consider separating the two concepts again.  In an earlier
> thread, there was an attempt to automatically determine all the places
> where it's safe to enabled subpixel AA, but that seemed to result in a
> complex implementation, with all cases still not being covered (such as
> canvas-to-canvas drawImage()). The other alternative is programmatic
> control over subpixel AA, using a context attribute. That was the first
> thing that Justin proposed in the earlier thread, and would be my
> preference as well (a fully-loaded footgun:  you can shoot yourself with
> it, but the behaviour and performance characteristics are very easy to
> understand and spec).  But there didn't seem to be agreement around that
> either.

Can you refresh me on the issue with that? Is the cost too high?

> Which is how I ended up at the "moz-opaque" flag.  it restricts canvas to
> the subset of operations which result in a 1 alpha in the backing store, to
> allow optimizations and the use of subpixel AA.  I think that is actually
> quite a useful subset (generally, the subset that doesn't need destination
> alpha).  I believe the same thing can be achieved in WebGL by setting the
> "alpha" attribute to false in WebGLContextAttributes.

My fear is that this is too disruptive as it changes how canvas compositing
It might also not be implemented easily on all platforms (especially the
ones that use the operating system to draw)

>> But I think "matte" is unnecessarily obscure. How about adding a
>> clear(DOMString) method that does a 'copy' of the color to the entire
>> canvas buffer? The color could default to rgba(0,0,0,0).
>> 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]

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