[FFmpeg-soc] Differing

[Ramiro Polla]

On Wed, Jul 22, 2009 at 4:48 AM, Michael Niedermayer<michaelni at gmx.at> wrote:
> On Wed, Jul 22, 2009 at 01:58:16AM -0300, Ramiro Polla wrote:
>> >From what I understood and from some tests I've done, this creates
>> some rather ugly last lines at the far right of the image. The output
>> also differs for C, mmx, and mmx2, and it's not under BITEXACT.
>>
>> To remove this adjustment and have the same output for all
>> configurations, wouldn't it only be necessary to:
>> - adjust mmx and mmx2 filters to only work up to the last input pixels
>> it can before reading past the end.
>> - run the C code on the last pixels if necessary.
>
> that would need a benchmark, after all the fast_bilinear case is intended
> to be fast ...

fast_bilinear has different output with different optimizations not
only because they treat the width differently to not read past the
end, but because the algorithms they use are different:

C luma, x86 luma and chroma:
src[xx] * (128 - xalpha) + src[xx+1] * (xalpha)
C chroma:
src[xx] * (127 - xalpha) + src[xx+1] * (xalpha)
MMX2 luma and chroma:
src[xx] * (127 - xalpha) + src[xx+1] * (xalpha + 1)

I would prefer if they all gave the same output, but as you mention
fast_bilinear is intended to be fast (I would suppose just for testing
purposes). It is also not under BITEXACT.

What do you think is the best to do:
1- make them bitexact (will slow them down a bit, some more than others)
2- choose one of the 3 algorithms and use it under bitexact (falling
back to slower code if needed)
3- document that fast_bilinear has no intention of being bitexact (and
in fact ignores that flag)

Ramiro Polla