[MEncoder-users] Backup policies

[Michael Rozdoba]

Reimar Döffinger wrote:
> Hello,
> On Sat, Nov 17, 2007 at 08:26:11PM +0000, Michael Rozdoba wrote:
> [...]
>> My immediate intuitive reaction is to say what if one has a video with 
>> 30 minutes of a static scene followed by 30 minutes of noise? 2pass 
>> surely allocates most bits to the 2nd half while CRF, whatever it does, 
>> can't allocate different bit distributions in awareness of the 
>> distinction between 1st & 2nd halves (*).
> 
> Of course it does "allocate different bit distributions", it since it
> trivially reaches the desired quality in the first part so it uses very
> few bits, whereas in the second part it must use lots to get the desired
> quality.

Which is why I said 'in awareness of...'; it allocates them 
independently, which isn't a problem given no bitrate restrictions. The 
intuitive feeling however, which is probably wrong, is that awareness of 
  the bit distribution needs across the entire video (from a first pass) 
should be of benefit in influencing the bit distributing in a subsequent 
pass. It seems this isn't needed.

> Actually I don't know why 2pass should provide better quality than CRF
> (except that it can be a pain, if not almost impossible to get the desired
> file size with CRF).
> Well, actually there is one case I can think of: If you know that e.g. the next frame
> will be a keyframe due to its immense complexity, then you can encode
> the current frame with less bits with hardly a _visible_ loss of quality
> - which is one information you don't know with one-pass CRF.

Exactly, and one reason why it is necessary to look upto one gop ahead 
but probably no further.

-- 
Michael Rozdoba