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

Henk D. Schoneveld belcampo at zonnet.nl
Tue Mar 8 19:22:24 CET 2016

On 08 Mar 2016, at 17:54, Sreenath BH <bhsreenath at gmail.com> wrote:

> On 3/8/16, Moritz Barsnick <barsnick at gmx.net> wrote:
>> 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
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> Hi
> Thanks for your comments. I understand that two codecs even with same
> presets and other similar settings can produce differing output.
> I wanted to know if there was a way to achieve the results I wanted
> with the codec we prefer to use (nvidia) because it have significant
> speed increase.
As with most things in life, the advantage at one side has a downside somewhere else.
> thanks,
> Sreenath
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