[FFmpeg-trac] #9563(ffmpeg:new): Conversion between lossless formats not lossless

[FFmpeg]

#9563: Conversion between lossless formats not lossless
-------------------------------------+-------------------------------------
             Reporter:  Martijn van  |                     Type:  defect
  Beurden                            |
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:  git-
             Keywords:  wavpack      |  master
  bits_per_raw_sample lossless       |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug:
 When converting a WAV file containing 32-bit integer PCM to wavpack, the
 resulting wavpack contains 24-bit integer samples.

 How to reproduce:
 {{{
 % ffmpeg -i input.wav output.wv
 ffmpeg version N-104950-gc47896536c Copyright (c) 2000-2021 the FFmpeg
 developers
   built with gcc 10 (Debian 10.2.1-6)
   configuration: --enable-libx264 --enable-libfdk-aac --enable-libmp3lame
 --enable-gpl --enable-nonfree --enable-libopus
   libavutil      57. 11.100 / 57. 11.100
   libavcodec     59. 14.100 / 59. 14.100
   libavformat    59. 10.100 / 59. 10.100
   libavdevice    59.  0.101 / 59.  0.101
   libavfilter     8. 20.100 /  8. 20.100
   libswscale      6.  1.101 /  6.  1.101
   libswresample   4.  0.100 /  4.  0.100
   libpostproc    56.  0.100 / 56.  0.100
 Input #0, wav, from 'input.wav':
   Duration: 00:01:04.00, bitrate: 2822 kb/s
   Stream #0:0: Audio: pcm_s32le ([1][0][0][0] / 0x0001), 44100 Hz, stereo,
 s32, 2822 kb/s
 Stream mapping:
   Stream #0:0 -> #0:0 (pcm_s32le (native) -> wavpack (native))
 Press [q] to stop, [?] for help
 Output #0, wv, to 'output.wv':
   Metadata:
     encoder         : Lavf59.10.100
   Stream #0:0: Audio: wavpack, 44100 Hz, stereo, s32p, 128 kb/s
     Metadata:
       encoder         : Lavc59.14.100 wavpack
 size=   10720kB time=00:01:04.00 bitrate=1372.2kbits/s speed=86.9x
 video:0kB audio:10720kB subtitle:0kB other streams:0kB global headers:0kB
 muxing overhead: 0.000847%

 }}}

 Resulting wavpack file:
 {{{
 % ffprobe output.wv
 ffprobe version N-104950-gc47896536c Copyright (c) 2007-2021 the FFmpeg
 developers
   built with gcc 10 (Debian 10.2.1-6)
   configuration: --enable-libx264 --enable-libfdk-aac --enable-libmp3lame
 --enable-gpl --enable-nonfree --enable-libopus
   libavutil      57. 11.100 / 57. 11.100
   libavcodec     59. 14.100 / 59. 14.100
   libavformat    59. 10.100 / 59. 10.100
   libavdevice    59.  0.101 / 59.  0.101
   libavfilter     8. 20.100 /  8. 20.100
   libswscale      6.  1.101 /  6.  1.101
   libswresample   4.  0.100 /  4.  0.100
   libpostproc    56.  0.100 / 56.  0.100
 Input #0, wv, from 'output.wv':
   Metadata:
     encoder         : Lavf59.10.100
   Duration: 00:01:04.00, start: 0.000000, bitrate: 1372 kb/s
   Stream #0:0: Audio: wavpack, 44100 Hz, stereo, s32p (24 bit)
 }}}

 I have tried to trace the origin of the bug in the hope of submitting a
 patch, but I don't know enough about ffmpeg internals to fix this alone.

 In fftools/ffmpeg.c, the following can be found

 {{{
         if (ost->bits_per_raw_sample)
             enc_ctx->bits_per_raw_sample = ost->bits_per_raw_sample;
         else if (dec_ctx && ost->filter->graph->is_meta)
             enc_ctx->bits_per_raw_sample =
 FFMIN(dec_ctx->bits_per_raw_sample,
 av_pix_fmt_desc_get(enc_ctx->pix_fmt)->comp[0].depth);
 }}}


 If I remove ost->filter->graph->is_meta from the if, the conversion is
 lossless again. I think ost->filter->graph->is_meta is only initialized,
 not yet set at this point in the code. I'd like to fix this, but I'm not
 sure what to do. I think simply removing ost->filter->graph->is_meta as a
 condition is probably too simple.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9563>
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