[FFmpeg-devel] Using dsp.fdct()

Loren Merritt lorenm
Sun Apr 20 09:19:37 CEST 2008

On Sat, 19 Apr 2008, Ramiro Polla wrote:
> Michael Niedermayer wrote:
>> PS: yes you do need to run a idct over it if you want to use it as reference
>> frame otherwise decoder and encoder will have different reference frames.
> And this leaves me in a spot where I have more doubts than before asking =)
> The current code compares both frames with their original content (well,
> besides that luma plane pre-processing, but I'm not sure how long I'll
> keep that). I imagine this scenario where copying the idct over the
> original content could be worse (untested, but sounds like a good scenario):
> If I have a low quality encoding, the idct output will have lots of
> differences from the original content. If I compare the current frame's
> content with the idct output from the previous one, the differences
> might be higher than the threshold, so it will encode the new image,
> when it could have just used the previous one instead.

The reconstructed content of the previous frame is what you'll get if 
you code a skip in the current frame. So either the reconstructed 
reference frame is too different from the current input, in which case you 
should code the block, or the reconstructed reference frame is good 
enough, in which case either it'll pass your threshold or you should fix 
your threshold.
corollary: the skip threshold should be at least as large as the typical 
quantization error at your current quantizer.

--Loren Merritt

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