On Tue, Apr 01, 2008, Diego Biurrun wrote: > > Does anyone know why the output of the SSE2 IDCT is different from the > > one of the MMXEXT IDCT ? Since the algorithm is supposed to be the same, > > they should have exactly identical results. This not being the case, it > > may indicate that some extra approximations are used, and the SSE2 code > > may not pass the ieee 1180 conformance test. > > How much of a problem is this given that an IDCT is always a lossy > operation?
IDCT is lossy but MPEG-2 specifies that it mustn't be too lossy. So we must check that the output is within the allowed boundaries. If the results are identical to the ones of the MMXEXT version, then there is no need to do the check since the MMXEXT has already been checked to be compliant. However, the results are different, so the checks must be done again. This is really not my area of expertise, I've asked to people who know about this but haven't had any answer yet (this is a good opportunity to remind them :). But if you speak x86 asm fluently (which I don't), maybe you can spot what's the reason why the results are different while implementing the same algorithm. -- Christophe Massiot. ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://ad.doubleclick.net/clk;164216239;13503038;w?http://sf.net/marketplace _______________________________________________ Libmpeg2-devel mailing list Libmpeg2-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/libmpeg2-devel
