[whatwg] Enabling LCD Text and antialiasing in canvas

[Robert O'Callahan]

On Tue, Mar 12, 2013 at 8:23 AM, Stephen White <senorblanco at chromium.org>wrote:

> On Mon, Mar 11, 2013 at 2:56 PM, Robert O'Callahan <robert at ocallahan.org>wrote:
>
>> On Tue, Mar 12, 2013 at 7:53 AM, Stephen White <senorblanco at chromium.org>wrote:
>>
>>> All other canvas functionality behaves as normal, including operations
>>> which modify the alpha values of the backing store.  However, any such
>>> transparency values will be ignored when compositing the canvas into the
>>> page, and the canvas will be treated as if every pixel has an alpha of 1.0.
>>>
>>
>> That would mean getImageData can return non-1.0 alpha values, which is
>> probably not what you want to implement.
>>
>
> That's what Firefox/Linux does (in fact, it always seems to return 0.0
> alpha from getImageData()).
>

We definitely shouldn't spec that! And I'm pretty sure that behavior would
vary across Firefox platforms. But we need to have consistent behavior here.

I considered three options:
>
> 1)  Prevent non-1.0 alpha ever getting into the canvas.  At a minimum,
> this would require the following:
>
>    - For putImageData, apply premultiplication, then write 1.0 alpha into
>    the canvas.
>    - Change initialization and clearRect() to clear to opaque black
>    instead of transparent black.
>    - Modify all canvas compositing modes to leave destination alpha
>    unchanged
>
> The latter is easy to do in OpenGL and CoreGraphics, but hard to do in
> Skia, and hard to do in accelerated CoreGraphics (IOSurfaces don't seem to
> support any opaque formats, although I could be wrong -- that was just from
> an hour or so of experimentation).  I'm not sure about Cairo.
>

You can always implement it slowly using readback. I think we should just
spec this, and maybe note that authors shouldn't use non-over operators on
opaque canvases. Over time we'll probably find a way to make it fast
everywhere.

Rob
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