Yes they have to follow the same set of rules, but those rules have variables and not all people are going to encode using the best set. As stated in the rules:
- - Constant Rate Factor (--crf) must be as follows: -
- -ªªªªªªªªªªªªªªªªªTªªªªªªªTªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªª¼ -
- - Compressibility - CRF - General Examples - -
- +ªªªªªªªªªªªªªªªªª+ªªªªªªª+ªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªª+ -
- - High - 19-20 - Scripted, Talk Shows, Poker, Animation - -
- - Medium - 21-22 - Documentary, Reality, Variety - -
- - Low - 23-24 - Sports, Awards, Live Events, Competitive- - -
- - - - Reality - -
- Lªªªªªªªªªªªªªªªªª+ªªªªªªª+ªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªªª- -
- - x264 parameters shall not vary within a release -
- - Zones (--zones) are forbidden. -
- - Any deviation in CRF from given examples must be specifically justified -
- in the NFO. Use discretion when deviating CRF by matching the -
- compressibility of the show to a corresponding CRF value. CRF values -
- below 19 and above 24 are never permitted. -
So, if the MP4s are compressed at the highter end at CRF 24, you are going to get a video with a lot of artifacting (blocking). Even setting the CRF at 23, can make a lot of difference in the quality of the video, just a small change in the CRF makes a big difference in compression and quality.
You can not make a comparrison between xvid and x264 without knowing how the video was encoded and what compressions were set to encode. Even then you have to figure in how the original was encoded. If the original was already compressed, say it was a compressed xvid and someone re-encoded to x264/MP4 and applied compression to it, then that will affect the quaility.
Even though all groups has to abide by a set of rules, due to the CRF variables it doesn’t mean that the quaility will be the same.