[Ffmpeg-devel] Discrete levels on image quality

Michael Niedermayer michaelni
Tue Jan 30 14:09:50 CET 2007


On Tue, Jan 30, 2007 at 08:55:48AM +0000, jean.mab at bluewin.ch wrote:
> Hi,
> I want to modify the quality compression of an image while using a 
> mpeg4 video compression.
> Here is what I have done:
>     c->flags=CODEC_FLAG_QSCALE;// enable quality compression
>    image->quality=100;
> I found that it's possible to change the quality between  1 (good) 
> and FF_LAMBDA_MAX (bad) where FF_LAMBDA_MAX =256*128-1.
> After some experiments, it seems that there are 29 steps between 
> these values making changing the quality. These steps are:

this greatly depends on the other options you use for encoding, try
to enable some high quality options like trellis quantization or
rate distorted macroblock decission or qprd or ...

> Does somebody explain me a little more about these steps?
> I thought that image->quality changes the quality factor of the 
> jpeg compression but it doesn't seem to be that.
> Does somebody know on which level does this quality act?

it changes the factor used in rate distortion optimization and directly
or indirectly QP depening on other parameters

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Many that live deserve death. And some that die deserve life. Can you give
it to them? Then do not be too eager to deal out death in judgement. For
even the very wise cannot see all ends. -- Gandalf
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