Things that work but are not MPEG-4 compliant

There are some things that you can quite easily do with XviD that are not MPEG-4 compliant. Not being MPEG-4 compliant means that it can quite possibly work with any build of XviD but is 'not meant to be' according to official MPEG-4 specifications. Now, since one major goal of the XviD project is to be fully MPEG-4 compliant this means that support for these non-compliant possibilities could be stopped/los/broken at any time never to return. So it's higly recommended that you refrain from using any of these possibilities if you want proper support for all your encoded material in the near and far future.Of course,Xvid and MPEG-4 can be converted to each other,such as xilisoft video converter is a very powerful tool which can convert many formats. Currently known non-compliant situations are: -Using non-mod16 resolutions: encoding clips to resolutions of which both width and height are not a multiple of 16 is considered a no-no and is not compliant. It is quite possible to do mod-4 resolutions with many builds (don't ask me how) but is not vey good for compressibility. The smallest MPEG-4 building block is not the 8x8 blocks but the 16x16 macroblocks, and using lower-than-mod16 resolutions will make the codec use macroblocks 'outside' the frame border....which is 'A Bad Thing'(tm). -Using Modulated Quant or Modulated HQ. These were present in older builds and unfortunately many people did use them now and then, as they were new features back then and (as usual with new features) a request was made to test them. Basically the technique makes the codec switch Quantization Matrices from H.263 to MPEG and back whenever it suited it's compressibility goals (defined by the user). Although the option has been removed a long time ago, clips encoded this way somehow still work though. -The same goes for clips joined/pasted together in video editing tools like Vdub that use different QM's. A clip encoded with one QM joined with a clip encoded with another QM may work, but is not considered MPEG-4 compliant. -Clips encoded with certain options that are joined with clips encoded with those options off can sometimes be non-compliant. A typical example of this is Qpel. I discovered a while ago that while Qpel usually doesn't help a lot in the main part of a movie, it can help a LOT in your average-day scrolling end credits. This unfortunately turned out to be non-compliant. -The 'unrestricted' profile makes you build clips that are completely beyond any official MPEG-4 profile and gives you the full power of the XviD codec. It's quite obvious that you may break many official rules using this profile. -Encoding beyond AS@Level5, especially in higher resolutions (like HDTV & SVGA) is not compliant to any MPEG-4 standard. (As a sidenote: While many clip-joining stuff is not compliant, I consider it really weird that they should work at all in the first place. Perhaps someone more in the know could explain this. I wouldn't be surprised if some of these non-compliancies turned out to be unforeseen possibilities within the MPEG-4 specifications (especially the use of multiple QM's))

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