[x264-devel] Levels

[Loren Merritt]

Ideally, encoders would signal the lowest level that they can be sure the
stream complies with. Decoders would document their officially supported
profiles and levels, but try to play any stream anyway.

In practice, lots of hardware decoders refuse to play any stream whose 
signaled level exceeds the player's nominal level, without even looking at 
the stream's contents. If you encode something in x264 without specifying 
a level, x264 can't know in advance what level it complies with, and so 
defaults to 5.1.
Thus, any file that was encoded by people not thinking about hardware 
decoders won't play on many hardware decoders, while a simple change of 
metadata causes them to work.
So I propose a change: x264 will default to the lowest level that complies
with the video's resolution, framerate, and DPB, while ignoring VBV
constraints unless VBV ratecontrol is enabled, and ignoring all of the 
minor constraints. This will sometimes underestimate the bitrate and thus 
generate a stream that doesn't comply with its own signaled level, but it 
will also increase the chance that any random file will play on any random 
hardware player.
What do you all think, is working better in the real world sufficient 
reason to intentionally generate possibly noncompliant streams?

I'd like to say that the worst that can happen is some player will try to 
play a stream it really can't. But I'm afraid I'm underestimating the 
stupidity of hardware manufacterers. I don't know how they'll manage to 
break it; maybe some player will infer max bitrate from level, and read 
the file from disk only at that bitrate even if the disk is capable of 
faster and the stream requires it?


The alternative is to keep track of peak bitrate and other constraints, so 
the encoder can know after encoding a movie what level it actually 
complied with. But that would require changing the SPS after encoding the 
whole movie, which is incompatible with the API of every encoding 
application I've ever heard of, so I don't think I'll implement it.

--Loren Merritt