Lair Of The Multimedia Guru

• no loop filter to reduce blocking artefacts, H.263 upon which MPEG4 is based had one, and the thing people most often complain about when looking at MPEG4 videos are blocking artefacts, why cant postprocessing filters cover this up? well its much harder to remove multiple overlapping and randomliy shifted bocking artefacts instead of ones which are exactly at a 8×8 grid and only around macroblocks with specific parameters …
• block boundary mirroring for qpel, this is AFAIK needed because there is no deblocking loop filter, if mirroring wouldnt be done then the filter would cross many more edges created by blocking artefacts and create a random mess, mirroring also reduces the number of bytes which need to be fetched for each block, ok but why am i complaining? well its a mess to implement and it forces the encoder to do redundant calculations as each block needs things mirrored differently
• all the startcode avoidance and redundant marker bits, there are really just 2 possibilities, either the stream is undamaged in which case we dont need to search for startcodes unless we try to seek in raw mpeg4 (.m4v NOT .mp4!) or it is damaged in which case the data can contain anything anyway, now if all this mess was for seeking in raw mpeg4 why are there no reliably timestamps? additionally all non-MPEG/ITU codecs work perfectly fine without this startcode emulation prevention thing
• OBMC is in the spec, theres a bit in the header to enable it but oops its not allowed in ANY profile
• IDCT see prevous blog entry
• GMC is very computationally intensive and according to some documents from the JVT the only reason why there is any quality/compression gain at all is that the motion vector for skiped macroblocks is the GMC vector where without GMC it would be (0,0) now simply changing that to the predicted median vector should make the tiny gain GMC has dissapear
• a single set of fixed vlc tables, why? its just silly, several fixed sets or variable ones stored in a global header would mean very little additional complexity IMHO at least
• short header format (=every MPEG4 decoder must support simple H.263)
• different motion vector prediction in bframes, why? using the same as in P frames would be simpler and very likely higher quality
• no intra macroblocks in b frames
• macroblocks in B frames must be skiped if they where skiped in the last P frame, thanks to this idiotic idea encoders must check all b frames before they can mark a MB in a P frame as skiped
• no macroblocks with 4mv and dquant at the same time, requires special handling in the encoder and no advantage at all, h.263 demonstrates how to do it correctly
• all the “flash” animation multi object stuff in MPEG4
• dquant is limited to +-2 so encoders must somehow convert the ideal QP values from lumimasking/… to ones which dont change too much between MBs
• QP is limited to either even or odd values in b frames, yeah poor encoder must decide which it wants, cant have both, hmm and according to the comments in ffmpeg more code is needed for working around all these arbitarray mpeg4 QP limitations about then the actual QP selection needs

The 8×8 Inverse discrete cosine transform used in most video codecs converts the very sparse and thus easily compressible dct coefficient matrixes into the 8×8 blocks vissible or the 8×8 difference relative to some area from the previous frame
The IDCT in these codecs is not exactly specified, instead the MPEG and ITU commitees only require the used IDCT to be approximately equal to a idealized and very slow reference IDCT due to that, decoders and encoders use various different IDCTs, if the actually used encoder and decoder happen to use 2 different enough IDCTs and the video material, encoding parameters, moon phase and so on match then the tiny +-1 differences between the IDCTs will accumulate and turn the pale face of a corpse pink or green or add stripes or worse …
some examples of such artifacts:

LLM/IJG int	simpleidct
libmpeg2 idct	xvids idct

LLM/IJG int	simpleidct
libmpeg2 idct	xvids idct

Its also very important to keep in mind that these artifacts are caused by the difference between the encoder and decoder IDCT and not the difference to some idealized IDCT only on the decoder side, so using the reference IDCT on the decoder side will often not help at all, and sometimes might make the artifacts worse
now maybe you are curious which IDCT was used in the encoders for the 2 examples above, well iam curious too about that as theres no 100% certain way to find out, allthough i would guess the first used the idct from the msmpeg4v3 codec and the second the one from xvid
Which prameters can be tuned on the encoder side to reduce these artifacts? First use a smaller keyframe interval, then avoid qp=1, avoid qpel and use lots of b frames or use H.264 which doesnt suffer from this mess as it has a exactly specified idct approximation

Comparing the pp and spp filters from MPlayer/ffmpeg with the standard lena 256×256 image, the unfiltered one is a simple 10% quality jpeg saved by gimp

Original	unfiltered
pp=hb/vb/dr/fq=8	pp=hb/vb/dr/fq=16	pp=hb/vb/dr/fq=32
spp=6:8	spp=6:16	spp=6:32
uspp=6:8	uspp=6:16	uspp=6:32