[Pixman] [cairo] Supersampling - 1st attempt

[Terrence Cole]

On Wed, 2010-08-11 at 05:25 +0200, Soeren Sandmann wrote:
> Benjamin Otte <otte at redhat.com> writes:
> 
> > On Wed, 2010-08-11 at 04:41 +0200, Soeren Sandmann wrote:
> > > The main problem with moving to floating point is that all the test
> > > suites will break. We'll need to figure out of to update that, but
> > > fundamentally, I don't see a problem moving to floating point
> > > unconditionally.
> > > 
> > From a Cairo POV, that is just updating the ref images and looking funny
> > at people that use an "old" pixman. We have never been very
> > backwards-compatibility sensitive in our test suite.
> 
> Right, backwards-compatibility is mostly irrelevant. The problem is
> that floating point doesn't necessarily produce bit-exact results from
> machine to machine. (Even if you assume bit-exact output from IEEE
> 754, we'll likely still want to use instructions that have more or
> less internal precision, or compile with -ffast-math or whatever).
> 
> Basically, the test suites need to understand tolerances. That may not
> be too difficult for cairo, but for the pixman test suite, it means we
> can't really base it on CRC checks anymore (although those may still
> be useful to catch unexpected rendering changes). Instead, we might
> want to allow pixman_implementations to be turned on and off
> dynamically so that their outputs can be compared to each other.

I had the same testing problem with a floating point imaging library
that I am building currently.  I ended up using ImageMagick's compare
utility with -fuzz set to an appropriately small number.  It's fairly
slow, but very good about smoothing over platform related issues.  And,
of course, it was very fast to implement.

> Soren
> 

Cheers,
Terrence