[Pixman] [cairo] Supersampling - 1st attempt

Soeren Sandmann sandmann at daimi.au.dk
Tue Aug 10 20:25:20 PDT 2010

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.


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