[theora-dev] VP3 IDCT

Dan Miller dan at on2.com
Thu Mar 6 13:11:24 PST 2003

ahh, I see.  Unfortunately, I cannot recommend changing the spec to fit the application.  I feel pretty strongly about this, as does everyone who had to deal with the problem in MPEG1, MPEG2, and (?) MPEG4.  Going forward, I think you'll find that the newer algorithms (Including upcoming MPEG-4 part 10 aka JVT, H264, AVC) specify the forward and reverse transforms to bitwise accuracy.
Note that you might be able to get away with using ffmpeg's routines for compression (FDCT), as long as you make sure that the reference decoder within the compression engine (IDCT) is precisely equivalent to the spec.  It's the IDCT that causes drift (note there is no FDCT in the decoder, so it is 'non-normative', ie not really part of the spec in a formal sense) - dan

        -----Original Message----- 
        From: Mike Melanson [mailto:melanson at pcisys.net] 
        Sent: Thu 3/6/2003 3:33 PM 
        To: theora-dev at xiph.org 
        Subject: RE: [theora-dev] VP3 IDCT

        On Thu, 6 Mar 2003, Dan Miller wrote:
	> Look, someone can go ahead and create a non-spec decoder that might look OK, but I think we should discourage it.
                Right now, Michael and I are simply trying to establish how easily
        the VP3 algorithm can be adapted to fit into ffmpeg. The codebase, as you
        may know, already has a large body of code for inverting DCT very quickly
        with i386 SIMD sets, PPC AltiVec, Sun MediaLib, and even on Alpha and PS2
        (MIPS?) CPUs.
                -Mike Melanson
        --- >8 ----
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