[cairo] four-channel alpha and destination alpha

[Keith Packard]

Around 10 o'clock on Feb 1, Owen Taylor wrote:

> And the obvious question was "how do you compute the right value for the A
> channel of the mask".

Xft uses the middle sub-pixel (green, in the existing cases), mostly 
because it doesn't matter for normal usage.

> - You get super-luminous pixels like the above mentioned 
>   88AA0000. pixman handles that OK, but it's conceivable that
>   hardware-accelerated compositing paths would have difficulty.

I think any hardware that can handle GL will be able to manage this 
without trouble; it can happen with a regular ADD operation.

-keith


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