[theora-dev] VP3 IDCT

Michael Niedermayer michaelni at gmx.at
Wed Mar 5 17:12:13 PST 2003


On Wednesday 05 March 2003 18:26, Dan Miller wrote:
> I believe they are.  At some point I replaced them with my own functions
> off the standard texts and I recall there wasn't any noticeable visual
> problem.  However I can't remember if they were bit-for-bit accurate.
> With any DCT/IDCT implementation, the issue of any loss of precision etc.
> that might cause the encoder's reference decoding to drift from the
> decoder's has to be considered.  I know we have run into problems like this
> when porting decode to different platforms.  The only true document for now
> (until I get my spec together) is the code, so you will have to look at it
> or at least compare your output to the output of the present decoder.  It's
> intended to be a pure 32-bit integer implementation if that helps.
btw, does theora/vp3 have a maximum allowed keyframe (or co-located non-skiped 
non-intra macroblock) limit like mpeg* ? IMHO it should have one, otherwise 
different decoders with different IDCTs will drift too much like u mentioned

another related issue is the dequantizer, mpeg1/2/4/h263 does various weird 
oddifications to reduce the idct drift, i havnt found anything like this in 
the vp3 source, but possibly i missed it
anyway, as soon as melansons implementation is finished we can try vp3 with a 
few different IDCTs (& DCTs) ...


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