[whatwg] Enabling LCD Text and antialiasing in canvas

Stephen White senorblanco at chromium.org
Thu Feb 14 02:59:20 PST 2013

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.

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.

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

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.


> 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
> --
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