[FFmpeg-user] ? About ffmpeg's prores implemention

[Philippe Laterrade]

Hi Valentin,

I made some tests very recently with several qcsale value, but never reached exactly the bit rate specs announced by Apple. Did anyone has already success to get those specs?

BR.
Philippe

 


-----Message d'origine-----
De : ffmpeg-user [mailto:ffmpeg-user-bounces at ffmpeg.org] De la part de Valentin Schweitzer
Envoyé : mercredi 2 septembre 2020 17:29
À : ffmpeg-user at ffmpeg.org
Objet : Re: [FFmpeg-user] ? About ffmpeg's prores implemention

On 31/08/2020 10:41, Phil Rhodes via ffmpeg-user wrote:

> I honestly can't remember most of what that does but the number after "-qscale:v" controls overall bitrate from 1 to 31. Reducing it increases bitrate (obviously!) and if you set it very low, it may create files that are wildly out of specification that Apple decoders may not like.

Hi,
I think i may be able to clear up the behavior of qscale
a little. According to https://trac.ffmpeg.org/wiki/Encode/MPEG-4
qscale behaves similar to the quantization parameter in h264.
That parameter is pretty well explained here:
https://www.pixeltools.com/rate_control_paper.html
but in short:
The quantization parameter basically tells an encoder
how much detail can be "thrown away" during encoding.
So if the parameter is lower, more detail is preserved,
if it's higher, more is thrown away, which leads to
a higher or lower bitrate respectively.

This is not the most detailed explanation,
but I hope it helps to clear up why a lower
number leads to a higher bitrate.

Hope that helps,
Valentin
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