[FFmpeg-user] Basic Video Manipulation Qustions

Carl Zwanzig cpz at tuunq.com
Thu Oct 27 19:09:06 EEST 2022

[knocking off a quick answer]

On 10/27/2022 8:46 AM, John Van Ostrand wrote:
> 1. Why does it make sense to convert from a lossy format to raw? You're not
> gaining any more detail.

It doesn't, unless you need raw for some further processing (that's what 
happens in the ffmpeg pipeline- demux, decode, filter/transform, encode, 
mux, see https://ffmpeg.org/ffmpeg.html#Detailed-description).

> 2. Does re-rendering video over and over at the same resolution, frame
> rate, and bit rate cause degradation? For example, if I want to make
> several changes to the video, like colour correction, splicing out
> sections, transitions, titling, etc. should I be trying to do that all in
> one command?

It can, and yes or store the intermediate files in a lossless format (which 
could be raw frames in a container); I prefer intermediate files as the 
command lines can get complex and if I mess something up or don't like it, 
part of the work has already been done (as an ancient IT person, keeping 
backups of each step is assumed).

Or use some ffmpeg library based software that allows for you to effectively 
script/render the output.

> 3. I understand that -c copy is a great way to eliminate time and preserve
> detail but what's the best way to do this when converting from one codec to
> another, or when performing editing or other changes?

Copy does just that, it copies; changing the encoding/size/pixel-format/etc 
requires going through an uncompressed/raw state (the pipeline).

> 4. How important is it to keep standard resolutions, frame rates, and bit
> rates? Do hardware decoders do better with those? Do software decoders
> handle weird resolutions better?  Will strange decoding artifacts appear
> more often with non-standard parameters?

Importance is relative. If the final output must be played on a wide 
varieties of player, then stick to the standards and it's more likely to 
work. Bit rates- AFAIK there are no "standards" for compressed video & 
audio, only conventions or guidelines to use a certain max rate to get a 
given quality (and some media, like dvd/bluray, can only go so fast). I 
can't answer for how s/w decoders play with non-standard resolutions. And as 
I understand, some decoders get wonky the resolution isn't divisible by 8.

Hope that helps, it's a bit shy on technical rigor.



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