[Pixman] [PATCH] Improve handling of tangent circles

[Siarhei Siamashka]

On Friday 14 January 2011 17:30:52 Andrea Canciani wrote:
> Soren suggested extending composite, i.e. computing a "reference
> gradient" in double precision with any algorithm and comparing its
> results with those from the pixman implementation.
> This might help with different fp implementations, but I'm afraid that
> it would not solve the problem completely, unless the algorithm used
> to compute the gradient is exactly the same.
> 
> Do you have any idea on what could be done to make such a test
> more numerically stable? I've been thinking about blurring or things
> like that, but I'm afraid that they would also miss off-by-ones and
> other real bugs.

In my opinion, for radial gradient tests it makes sense focusing at different
parts of the implementation separately. The gradient walker part is reasonably 
simple and can be tested independently (on linear gradients for example, or via
some other methods).

So for the start, I would try making some test, which does not use
interpolation between stop colors at all. Something like the following, or
maybe arrangement of stops could be different:
0% - black
10% - black
10% - white
20% - white
20% - black
30% - black
...

Next get two radial gradient pictures, trying different, probably random, inner
and outer circles positions and radii. First picture done with a reference
high precision implementation, and another with a performance optimized one.
And then, for example, for each 3x3 uniformly colored pixel area in the
reference picture, require that the pixel representing the center
of this area also has the same color in another picture. That would
provide precision requirement of being roughly one pixel off at most.
That's just an untested idea, some variation or different criteria may
be possible. Filter (if it applies to gradients), should be just set to
nearest.

> PS: A simple non-gtk version of the test might be an interesting
> addition to notice division by zero. Maybe it is appropriate for
> gradient-crash-test.

Yes, I think extending gradient-crash-test with a test for division by zero
problem would be great.

-- 
Best regards,
Siarhei Siamashka