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Re: [Public WebGL] Compositing with HTML

On Sat, Jan 9, 2010 at 2:26 AM, Chris Marrin <cmarrin@apple.com> wrote:
> I'm confused.  Clamping to 1 doesn't guarantee that it's a legal
> premultiplied color, does it?  Or are you saying that the 2d canvas, svg and
> css take non-premultiplied colors as input, clamp all the channels to 1 and
> convert them to pre-mulitplied behind the scenes?
> I'm not sure what they actually do, but I know that the spec says 3 useful
> things:
> [...]
> 2) Colors in the buffer (when read back) are premultiplied

Where do you see this? getImageData explicitly says "Pixels must be
returned as non-premultiplied alpha values". The conceptual model
intended throughout the whole 2d context is that colours are
non-premultiplied [0, 1] RGBA values - an implementation is free to
store premultiplied values internally, but that's invisible to the API
(except via the loss of precision). Some implementations, e.g. Opera's
(at least in the past), store non-premultiplied values internally and
never have to do conversions (though it'll need more multiplications
for its Porter-Duff operators).

When you composite with "lighter" (plus), the (non-premultiplied)
colour components should be clamped to 1, so it can never be
oversaturated. (I believe all implementations do this. The spec could
probably be made clearer, though.)

Philip Taylor

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