[Ffmpeg-devel] [BUG] Error: 2pass curve failed to converge

[Sven Over]

Am Friday 04 August 2006 00:45 schrieb Corey Hickey:
> > [mpeg4 @ 0xc840d0]Error: 2pass curve failed to converge
[...]
> Can anybody fix this? The problem gets reported somewhat regularly on
> the mplayer/mencoder user lists. If I could fix it myself I would, but
> the bug is out of my league...
>
> I don't want to advocate reverting r5422, since doing so would only mask
> the bug, but functionally it would still be an improvement over what has
> been happening.

Hello.

I yesterday encountered the "2pass curve failed to converge" problem 
yesterday, when recoding DVB recorded material with a 528x768 resolution to a 
PAL-DVD compliant 720x768 resolution. Since I have plenty of space on the DVD 
and the original material has already poor quality (video bitrates around 
2000kbit/s are quite usual), I want to do whatever I can in order not to 
degrade the quality even more. Therefore I use 2pass encoding and increase 
the bitrate for the recoding process, depending on how much space I have on 
the DVD. It turns out that sometimes I can increase the video bitrate by 70% 
or so.

In this scenario, I find it quite natural, that ffmpeg cannot spend all the 
bits I offer. There is simply not enough information in the original video 
material that it can be encoded in such high bitrates. I guess reencoding 
with a 70% higher bitrate is a ridiculous idea itself. Anyway, in this case I 
don't want ffmpeg to exit with an error. Instead it should just use as many 
bits as needed, and so the resulting file will be smaller than calculated, 
but is of optimal quality.

In order to have it like this, I changed the relevant part in ratecontrol.c 
(compare 'r5422') like so:

if(fabs(expected_bits/all_available_bits - 1.0) > 0.01 ){
  if (expected_bits<all_available_bits){
    const double percent_too_low=(1.-(expected_bits/all_available_bits))*100.;
    av_log(s->avctx, AV_LOG_INFO, "INFO: 2pass curve failed to converge 
(expected_bits %.1f%% too low)\n",
           percent_too_low );
  }else{
    const double 
percent_too_high=((expected_bits/all_available_bits)-1.)*100.;
    av_log(s->avctx, AV_LOG_ERROR, "ERROR: 2pass curve failed to converge 
(expected_bits %.1f%% too high)\n",
           percent_too_high);
    return -1;
  }
}

This way, the "2pass curve failed" is only an INFO, when the outcome will be 
too small. It stays an ERROR, when ffmpeg is unable to fit the video into the 
requested number of bits. Too gain some experience, the log messages also 
contain also the magnitude of the mismatch in percent.

To come back to my application, I think I might also use 1pass encoding 
instead of 2pass, whenever I am increasing the bitrate more than, say, 30%. 
With so much extra-bitrate, the results should be optimal in the first pass.

Bye,
  Sven.

-- 
Sven Over
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