[vlc-devel] The quantization parameter QP in x264.c

Moad Toutou moadtoufik at yahoo.fr
Fri Jul 27 14:13:42 CEST 2012

Thank you for your reply. In fact, i'm more interested in the  decoding video process, the coding is not my priority. for example: playing a video. I would like to evaluate the quantization parameter QP priodically
for example:
I start playing a video "toto.flv" with VLC and even when playing the file with vlc, we'll show on the console the quantization  paramaitre QP.

In X264.c file, there is a large structure:

static const char * const ppsz_sout_options [] = {
     "8x8dct", "asm", "aud", "bframes", "bime", "bpyramid"
     "b-adapt", "b-bias", "b-rdo", "cabac", "chroma-me", "chroma-qp-offset",
     "cplxblur", "crf", "dct-decimate", "deadzone-inter", "deadzone-intra",
     "deblock", "direct", "direct-8x8", "fast-pskip"
     "interlaced", "ipratio", "keyint counter", "level",
     "I", "merange", "min-keyint", "mixed-refs", "mvrange", "mvrange-thread",
     "nf", "non-deterministic", "nr", "partitions", "pass", "pbratio"
     "pre-scenecut", "psnr", "qblur", "qp", "qcomp", "qpstep", "qpmax"
     "qpmin", "quiet", "ratetol", "ref", "scenecut"
     "sps-id", "SSIM", "stats", "subme", "lattice"
     "verbose", "vbv-bufsize", "vbv-init", "vbv-maxrate", "weightb", "weightp"
     "aq-mode", "aq-strength", "psy-rd", "psycho", "profile", "lookahead", "slices"
     "slice-max-size", "slice-max-mbs", "intra-refresh", "mbtree", "hrd"
     "tune", "preset", "opengop", "bluray-compat", NULL

in this structure, there is the variable "qp", but it is not calculated.  do you have an explanation?

I find in the file OMX_Image.h, all data structures that handle the compression process and understood QP. but, these structures are not used in the code of vlc.

For example, in the file OMX_Image.h, there is a variable "nQFactor", but this one is not called in any files ".C" do you have an explanation?

you find attached "x264.c","omxil.c" and "OMX_Image.h" files


--- En date de : Mer 25.7.12, Ilkka Ollakka <ileoo at videolan.org> a écrit :

> De: Ilkka Ollakka <ileoo at videolan.org>
> Objet: Re: [vlc-devel] The quantization parameter QP in x264.c
> À: vlc-devel at videolan.org
> Date: Mercredi 25 juillet 2012, 8h52
> On Tue, Jul 24, 2012 at 03:53:51PM
> +0100, Moad Toutou wrote:
> > Hello,
> Hi,
> > I just installed the codec vlc compilation mode. I
> documented the
> > process of video compression and I found that video
> compression goes
> > through several stages. The step of quantization step
> is the most
> > impotent because this is when we decide the rate of
> compression. The
> > quantization step manipulates a parameter called: The
> quantization
> > parameter. I want to extract and learn which part of
> the code is
> > implemented ?
> You should check x264 sourcecode, which actually implements
> the
> encoding, vlc side is just code to utilize libx264 inside
> vlc.
> > For a given video, I would like to display
> (fprintf("Qp=%d",qp)) the
> > corresponding value of qp.  I looked at the file
> x264.c, I do not see
> > where is calculated the  quantization paramaitre
> QP
> > I would be grateful to give me ideas on this problem.
> > Thank you in advance.
> -- 
> Ilkka Ollakka
> If life is a stage, I want some better lighting.
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