[XviD-devel] XviD options nearly all documented in mencoder... need you help!

Edouard Gomez ed.gomez at free.fr
Tue Sep 7 00:01:44 CEST 2004


Guillaume POIRIER (guigui-nospam at laposte.net) wrote:
> Aspect Ratio: par, par_width and par_height.
> par_width and par_height need an integer parameter, and par a string,
> but I don't know what they mean.
> 
> If any of you guys can help me here, I'd really appreciate it.

PAR means Pixel Aspect Ratio. Not to be confused with DAR
which is Display Aspect Ratio. PAR is the ratio of the width
and height of a single pixel ! So both ar related as follow:

             width
DAR = PAR * -------
             height


Ok so here is the par option description:
MPEG4 defines 5 standard Pixel Aspect Ratio and one extended
one, giving the opportunity to specify a specific pixel aspect
ratio. 5 standard modes can be specified using par=xxx, where
xx is one of these items:
 - vga11: it's the usual PAR for PC content. Pixels are a square
   unit.
 - pal43: PAL standard 4:3 PAR (see code for exact ratio).
   Pixels are rectangles
 - pal169: same here
 - ntsc43: same here
 - ntsc169: same here (don't forget to give the exact ratio)

You can specify your own pixel aspect ratio using, the "ext" one.
So for par=ext, two new options are accessible to specify the
width and height rationals for a pixel.
 - par_width: 0<x<=255
 - par_height: 0<y<=255

Eg: par=ext:par_width=56:par_height=13

You should insist about the fact what users usually know, is
the DAR not the PAR. It's quite disturbing to set a PAR to
pal43, and don't get a display ratio of 4:3 because the
f(DAR)=PAR relation depends also on the sampled image width
and height. Hope you get the idea, better using, autoaspect or
aspect for joe user, it's something he's more used to.

If my module updates go into the CVS, you can also add that if
an extended PAR is specified and matches a standard one, then
the standard will be used for the bitstream syntax, even if
the extended one is still valid syntax.

-- 
Edouard Gomez


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