First pass size and quantizers

[len0x]

I wonder if XviD may have some kind of rate cotrol problem if quantizer limitation are used: say, I did first pass and aim at 68% of first pass size. This means that I can set max quantizers to 3 without having an oversized encode. However sometimes there is an oversize (5-20Mb). Is there a logical explanation to this?

[Didée]

Originally posted by len0x
say, I did first pass and aim at 68% of first pass size. This means that I can set max quantizers to 3 without having an oversized encode
I'm curious about your proof on that statement?

XviD might very well decide to use quants >3 for very demanding sections. If you forbid, then it'll take time to compensate for the forced oversize. If the demanding section is more near the clip's end, then perhaps there just isn't enough time to fully compensate.
Try such an "oversized" case with limiting @ 4, and the problem should vanish.

[len0x]

Originally posted by Didée
If you forbid, then it'll take time to compensate for the forced oversize. If the demanding section is more near the clip's end, then perhaps there just isn't enough time to fully compensate.


That was an explanation I was looking for. Of course setting max quant as 4 will help, but I was curious about the reason of oversize.
Now that I think of it - quants > 3 are for less demanding scenes actually. And considering that movies are usually ending with credits that might require less bitrate, oversize can be easily explainable in this case...

[Manao]

It may happen on some clips that q3 takes more than 68 % of the quant 2 size( where did you take that figure from ? ).

Anyway, there's no point in limiting the maximum quantizer, imho.