[FFmpeg-user] size and bitrate issues with nvidia accelerated transcoding

[Moritz Barsnick]

Hi Sreenath,

On Mon, Mar 07, 2016 at 22:28:32 +0530, Sreenath BH wrote:
> As can be seen the two outputs differ in the H264 profile(High vs
> Main), bitrate, the pixel format and of course the file sizes.

Different encoders can have different defaults. It appears that nvenc
defaults to "Main", while libx264 defaults to preset "medium" which
apparently implies "High" (but might depend on other things).

If you want the same profiles and levels, you will need to request them
explicitly.

The same thing goes for the quality: libx264 defaults to crf=23, I
don't know about nvenc. Furthermore, encoders can trade off quality (or
size) for conversion speed. So if you want the same size, you have to
specify "-b:v" explicitly. If you want the same quality, I guess you
may have to experiment with the settings available to find something
comparable.

And finally, don't expect the same results as soon as you have the same
profile, levels and bitrate (or quality). One encoder will be more
efficient than the other, or give different results in different scene
scenarios.

Does this clarify some things?

Moritz