Re: Alpha compositing in gnustep-back

[Jeff Teunissen]

Adam Fedor wrote:

> Thanks. I added this.
> 
> Jeff Teunissen wrote:
> > XGBitmap's _pixmap_combine_alpha() and _bitmap_combine_alpha()
> > functions don't quite work right. Specifically, the colors are all
> > wrong.
> >
> > For some reason, the colors seem to be scaled by the value of alpha
> > (127/127/127/127 becomes 63/63/63/127 somewhere). In my spare time,
> > I've been working on them to make them generate correct output.
> 
> Well, colors are supposed to be scaled by alpha. Thats how alpha works.

Actually, changing the alpha on a pixel should never change the values of
that pixel's component colors -- only how they are drawn. Alpha is solely
a measure of opacity, not of color value.

For example, compositing a white pixel onto a white background (regardless
of alpha) should always result in a white pixel. If the source pixels are
scaled by alpha, then the lower alpha is (to an extent -- as alpha
approaches 1/3, the pixel begins to grow lighter), the darker the result
will be.

Final output should combine the original source pixel values with the
destination, according to alpha and the fraction of the source/dest pixels
that contribute to the result -- dependant on composition algorithm, of
course.

> But perhaps when you combine two colors together they need to be
> unscaled.

It's the combining that should change what the colors are. Until that
point, colors ought to be invariant.

> > As part of trying to understand the functions, I partially rewrote the
> > fast paths for them. The code is shorter, but it may be slower since
> > the math is done in floating point.
> 
> It's about 10% slower in my tests. But that may not be accurate.

The salient change can be applied to the original code, with some small
changes. My rewrite of that particular code was so that I could easily
show correctness.

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