[cairo] Pixel consistency? (testing)

[Ian Britten]

M Joonas Pihlaja wrote:

>> - When producing images, is all the rendering controlled through
>>    cairo(mm) and pixman (And System calls)?  Are there any other
>>    factors that might alter the output (eg: Configuration options,
>>    64-bit, other dependencies, etc)?
 >
> The main controlling factor is the backend you choose, but even then 
> there will probably be differences across versions of all the 
> components involved.

Sorry for not being completely clear, but for now I was focusing
solely on the Cairo image surface.
[ Admittedly, I have similar questions regarding the other backends,
but I'll defer them for now ... ]

>> - Can anyone offer any insight into how they approached their
>>    testing and these sorts of issues?  Currently, I'm just looking
>>    at doing a straight RGB(A) comparison of the pixels, so any
>>    difference will be reported as a failure.
> 
> It's all a bit 
> fragile however and it's still very easy to provoke test suite 
> failures with small differences in how things like antialising or 
> filtering are done.

Ya, that's what I'm encountering when trying to simply do a 'diff' on
the images.
If the main cause of the differences is stuff like antialiasing, I
wonder what the behaviour would be like if my tests were done with
AA turned off?  Obviously the output wouldn't be as nice, but maybe
it would be more robust?  For my testing, I'm mostly concerned that
various attributes/settings were propagated+used correctly.  Whether
the output is 'pretty' would seem to be almost secondary to ensuring
the rendering was done correctly.  However, I'm assuming that someone
already considered this for your own tests, and dismissed it as
unsuitable for some reason ...

Many thanks for the information and suggestions!
Ian