[whatwg] Enabling LCD Text and antialiasing in canvas

Rik Cabanier cabanier at gmail.com
Wed Feb 13 21:54:11 PST 2013

On Thu, Feb 14, 2013 at 3:35 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!
> 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).

Yes, that would work.
Drawbacks are:
- you can't draw onto a transparent backdrop and apply matting.
- if you use a compositing mode that changes the canvas alpha to something
else than 1, you can't optimize the drawing any more.

Both these are caused because 'clear' removes the canvas content.

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