[XviD-devel] Request for ideas/comments about 2pass

[Edouard Gomez]

Christoph Lampert (chl at math.uni-bonn.de) wrote:

> Twopass sets the quants of  bframes directly and doesn't calculated it
> from surrounding P-frames??? That can't be good... 

Yes that's more or less what happens now. 

> I'm  sure B-frames  scale  differently than  Pframes,  so they  should
> either  get their  own  routine  or their  values  just calculated  by
> offset/ratio.

Yep, B Frames do not scale like pframes, but the code is making this
assumption in the way it computes the quantizer for bframes. 

Here's what  i understood from  the code (call this  reverse engineering
;-):

1/ some scaling (valid for all frame types)
2/ if(frame type == bframe) frame length *= avg_pframe_length/avg_bframe_length;
3/ apply old algorithm for pframes
4/ if(frame type == bframe) frame length *= avg_bframe_length/avg_pframe_length;
5/ compute quant from scaled length in 4 and original quant.

Well  it's  not that  simple  and  depends on  the  code  path (alt  cc,
over/under avg  frame size conditions etc  etc) but this  is teh overall
idea behind our current 2pass code.

I  think  the  adaptation  of  bframes  length is  the  culprit  in  our
algorithm. I  suspect it  to be the  one that  makes the bquants  are so
high. In the first pass, we had this formula:
(1) bquant = ((quant + ref_quant)*ratio/2) + offset (ratio and offset in %)

Our 2pass code uses:
(2) scaled_quant/scaled_length = pass1_quant/pass1_length;

And then the encoder loop applies (1) again. This is the most noticeable
bug, but i suspect the step 2/ in the algorithm hurts too. 

I'm currently trying  to apply (1)^-1 before returning  the quant to the
encoder. This way  we would apply (1) only once. I  keep you informed of
my progress.

-- 
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