[cairo] Cairo gradients and pre-mul data

[David Turner]

Hello Federico,

Federico Mena Quintero a écrit :
> Since I'm too lazy to figure out the math...
>
> What's the premultiplied equivalent of a non-premult gradient from
> (1, 0, 0, 0) to (1, 1, 1, 0)?  I.e. transparent red to solid white.  On
> the GIMP this gives me a transparent-to-white gradient with a red tinge
> in the middle section.
>
>   
I suppose you mean (1,0,0,0) to (1,1,1,1), with (R,G,B,A) notation, right ?

In one case, you interpolate all channels, THEN perform the ALPHA 
multiplication,
the result:   transparent "color" at the start, 50% transparent *pink* 
at the middle,
and solid white at the end (e.g. Cairo and GIMP)

In the other case, you perform the ALPHA multiplication BEFORE the 
interpolation,
the result: transparent "color" at the start, 50% *gray* at the middle, 
and solid
white at the end (when using pre-multiplied gradient stops)

that's because pre-multipication loses all original color information 
when alpha=0
(which is another way of saying that a fully transparent pixel has no color)

Clearly, using pre-multiplied gradient stops has some problems. I'll add 
that I measured
the performance cost of alpha premultiplication on each pixel, and that 
it is not very
important compared to other gradient computations.

Hope this helps,

- David Turner
- The FreeType Project  (www.freetype.org)


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