[Libav-user] What define the quality of a decoded video

[... ...]

Hi,
Thanks for all your responses, it makes things clearer. So if I understand, I can't encode video without losing quality (the same with decoding, but I don't actually have problems with decoding). But I can't skip decoding  for processing reasons. So how to define the encoding tweaks which will minimize the lost of quality ? Is there a function like AVOutputFormat* defineBestDecodeSettings(AVPacket *pkt, enum AVQuality flags); ?

Sincerely,Guillaume Bersac237bis rue de Pessac33000 Bordeaux06.65.30.89.43


> Date: Sun, 23 Jun 2013 23:49:17 +0300
> From: alexcohn at netvision.net.il
> To: libav-user at ffmpeg.org
> Subject: Re: [Libav-user] What define the quality of a decoded video
> 
> On Fri, Jun 21, 2013 at 5:45 PM, Guillaume Bersac <bersac_1 at hotmail.fr> wrote:
> > HI,
> >
> > I have a basic .avi video. I decode it, reencode it using the same type of
> > encoder without modifying the decoded data and finally I save it on one file
> > of my computer. The saved video has a good framerate but it had lost a lot
> > of quality in the process (it's now with a lot of pixel). I kwon that the
> > image has been decoded without losing quality. The lost of quality appeared
> > during the decoding and saving process.
> > What define the quality of a decoded video ?
> >
> > Thanks,
> > Guillaume Bersac
> > 237bis rue de Pessac
> > 33000 Bordeaux
> > 06.65.30.89.43
> 
> The quality was lost because of re-encoding. Video encoders involve
> irreversible transformations, e.g. quantization. Applying this kind of
> transformation several times may easily result in bad visual.
> 
> Luckily, you don't need to go through this process if your goal is to
> store some video file in a different format. ffmpeg accepts "-vcodec
> copy" parameter, which will bypass decoding and re-encoding, and keep
> the quality of the original. Same can be easily achieved
> programmatically using libavformat and libavcodec, if you are writing
> your program (implied by your posting to the libav-user list, and not
> to ffmpeg-user list).
> 
> Note however that not all container formats are compatible with all
> codecs, so in some use cases "-vcodec copy" is invalid.
> 
> Sincerely,
> Alex
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