[M4IF Technotes] Mismatch of IDCT algorithms

[Gary Sullivan]

Christoph,
Klaus Diepold was also discussing IDCT mismatch problems recently.
You might want to check with him.
+> 1.) To which extend is the intra refresh every 132 frames required ?
I believe it is required.
+> 2.) Can an MPEG-4 bitstream exceeding the 132-refresh-rule actually
be
+>     valid ?
I believe the answer is No.
+> 3.) Does the MoMoSys reference codec really ignore this refresh,
+>     and if so, why ?
I don't know.  Could be a bug.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: Christoph Stadler [mailto:Christoph.Stadler   dynapel.de] 
+> Sent: Monday, January 21, 2024 7:40 AM
+> To: technotes   lists.m4if.org
+> Subject: [M4IF Technotes] Mismatch of IDCT algorithms
+> 
+> 
+> 
+> Dear MPEG-4 community,
+> 
+> I am currently concerned with the accumulation of mismatch errors
+> when repeatedly adding residual images during MPEG-4 video decoding.
+> 
+> The MPEG-4 Visual standard (e.g. document N4350 ISO/IEC 14496-2) says
+> that every macroblock is required to be 'intra' coded at least every
+> 132 reference VOPs.
+> But this requirement is somewhat hidden as 'Note 1' of the 
+> (normative)
+> 'Appendix A'.
+> And the MoMuSys reference codec seems to ignore this rule too.
+> 
+> I understand that IEEE 1180-1990 demands a limitation of the 
+> number of
+> residuals that may be added even with a conform IDCT. And our tests
+> show, that violation of this can cause a severe degradation in image
+> quality if encoder and decoder use different (but conform) IDCT
+> algorithms.
+> 
+> So there are my questions :
+> 1.) To which extend is the intra refresh every 132 frames required ?
+> 2.) Can an MPEG-4 bitstream exceeding the 132-refresh-rule 
+> actually be
+>     valid ?
+> 3.) Does the MoMoSys reference codec really ignore this refresh,
+>     and if so, why ?
+> 
+> Thanks in advance and
+> Best regards,
+> 
+>         Christoph Stadler
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