[Ffmpeg-devel] [PATCH] fix (?) 2pass convergence errors

[Michael Niedermayer]

Hi

On Mon, Sep 04, 2006 at 12:35:21PM -0700, Corey Hickey wrote:
> Michael Niedermayer wrote:
> > may i suggest a slight modification to the convergence analysis ...
> > 
> > replace
> >     if(expected_bits > all_available_bits) rate_factor-= step;
> > by
> > int toobig=0
> >     ...
> >     if(expected_bits > all_available_bits){
> >         rate_factor-= step;
> >         toobig++;
> >     }
> > 
> > and then
> > 
> > assert(toobig<=40);
> 
> Ok. So far so good.
> 
> > <some debuging av_log which prints expected bitrate, wanted bitrate, avg-qp, ...>
> 
> I haven't figured out how to get the average qp. The attached code
> * sums up rce->qscale for each rcc->entry[i]
> * divides the result by rcc->num_entries and then by FF_QP2LAMBDA
> * does the same thing for rce->new_qscale
> * prints out both the results
> 
> According to several tests, the average of qscale more or less matches
> the average qp of the first pass and the average of new_qscale sort of
> matches the average qp of the second pass. Both of them get thrown off
> when lmin is below qmin. Is there a better way? I understand
> approximately 5% of the ratecontrol code so far, so I'm probably
> overlooking something easy.

its ok i guess ...


> 
> 
> > if(toobig==0){
> >     av_log(s->avctx, AV_LOG_INFO,
> >            "[lavc ratecontrol] Using all of requested bitrate is not "
> >            "possible for this video with these parameters. (try decreasing qmin)\n");
> 
> I thought about a recommendation like that earlier, but I decided
> against it because there are too many possible solutions to include in a
> small message.
> 
> * decrease qmin
> * decrease lmin if necessary
> * decrease vratetol if you have a short video
> * make sure lavc is getting the right framerate and number of frames
> * don't downscale the source video as much
> * don't denoise as much
> * don't worry about it; be happy with a smaller file
> * you can use fewer high-quality options if you want faster processing

ok s/(try decreasing qmin)/see DOCS\/foobar.txt/


> 
> For mencoder, at least, I'm loosely planning to add a small section to
> the docs with expanded versions of the above possibilities. I'm
> definitely open to suggestions, however; how to "make" lavc use the
> specified bitrate is a rather common subject on the user lists.
> 
> 
> Another thing: I just now changed the phrase:
> "Using all of requested bitrate is not possible"
> ...to...
> "Using all of requested bitrate is not necessary"
> 
> The former implies lavc is deficient in not doing what the user wants,
> which isn't the case.
> 
> > }else if(toobig==40){
> >     av_log(s->avctx, AV_LOG_ERROR,
> >            "[lavc ratecontrol] Error: bitrate too low for this video with "
> >            "these parameters.\n");
> >     return -1;
> > }else if(fabs(expected_bits/all_available_bits - 1.0) > 0.01){
> >     av_log(s->avctx, AV_LOG_ERROR, "[lavc ratecontrol] 2pass curve failed to converge\n");
> >     return -1;
> > }
> > 
> > 
> > PS: the step_mult stuff from x264 ratecontrol.c should be ported assuming
> > they (loren?) doesnt mind relicensing the few related lines under LGPL of
> > course ...
> 
> I hadn't realized how similar the two ratecontrol.c files were. :) Are
> you talking about simply:
> 
> 1. add this calculation:
> step_mult = all_available_bits / expected_bits;
> 
> 2. Use this for the corresponding loop control:
> for(step = 1E4 * step_mult; step > 1E-7 * step_mult; step *= 0.5){
> 
> 3. alter the if(toobig==40) you recommended because the for() will loop
> a different number of times.
> 
> Would I have to change anything else?

yes, the (1pass qscale) expected_bits calculation, in x264 its:

    expected_bits = 1;
    for(i=0; i<rcc->num_entries; i++)
        expected_bits += qscale2bits(&rcc->entry[i], get_qscale(h, &rcc->entry[i], 1.0, i));
    step_mult = all_available_bits / expected_bits;


[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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