[FFmpeg-devel] Please, clarify -r/-fps_mode usage for YUV encoding

[Rogozhkin, Dmitry V]

Hi,

Can someone, please, help to clarify ffmpeg behavior after one of the
recent changes [1]. This commit turns warning into error to prevent
usage of conflicting options. Error printed out is:
“One of -r/-fpsmax was specified together a non-CFR -vsync/-fps_mode.
This is contradictory.”

In our command lines [2] we were using -fps_mode passthrough to make
sure that there is no frame rate conversion/frame dropping which is
desirable for quality benchmarking use case. We did not use -r option
for any transcoding use cases. However, we did use -r for YUV encoding
use cases along with -fps_mode passthrough, i.e. command lines were
looking like:

ffmpeg <...>  \
  -f rawvideo -pix_fmt yuv420p -s:v ${w}x${h} -r $framerate -i $yuv \
  -c:v h264_qsv <...> -fps_mode passthrough -y $output

Above YUV encoding command line continues to work after [1] without
warnings or errors. My understanding is that in case of YUV -r
specifies a framerate of the input YUV file while for transcoding case
-r actually overrides framerate of the input bitstream and causes frame
rate conversion per se which clearly conflicts with intend of -fps_mode
passthrough which specifies that the frame rate is “passing through”
without conversion. Thus, using -r along with -fps_mode passthrough is
a valid for YUV encoding (since fps_mode will be the only frame rate
conversion being requested), but that’s not a valid combination for
transcoding use cases. Is this interpretation correct?

[1] 
https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/260f3918937463f1b90e1b77cba80b917f3cb8c1
[2] https://github.com/intel/media-delivery/blob/master/doc/quality.rst

Regards,
Dmitry.