[XviD-devel] quick question about the quality switch

Christoph Lampert chl at math.uni-bonn.de
Thu May 5 15:53:11 CEST 2005


Hi Tom,

it might be too late for your paper, but this is the explanation of the 
quality switch: 

The setting indeed mainly relates to the motion estimation (ME). 
But, as you will find out when doing a little more research on 
XviD, we of course don't use use full search to find the MVs, because 
that would be much too slow. We use a selfmade and -tuned version 
of the "zonal algorithm" idea by Alexis Tourapis, which is kind of 
a mixture between PMVfast and EPZS. There is no "search window" that 
could be influenced. But there are many other parameters than can be 
switched on or off, which you will find in the XviD source at src/xvid.h 
and e.g. in vfw/codec.c you can check which are used when. 

The higher the quality level, the more of these features are switched
on, e.g. 
* refining to halfpel position instead of just fullpel 
* using 4 instead of just 1 motion vector per 16x16 macroblock
* using square patterns instead of diamond pattern for the ME
* use several starting positions for gradient descent instead of 
  just one 

To explain how these work in detail would not be a "brief description" 
anymore. 

gruel



On Wed, 4 May 2005, Tom Jacobs wrote:
> could someone give a a very brief desciption about the differences between
> -q 1 to 7? i need to add it to a paper and its in today :$ i have (well
> someone else in my lab who has left) profiled the encoder at different
> levels and now the reviewer of the paper wants a line or two saying the
> difference.   im guessing the levels relate to how good ME is?  xvid only
> uses full search to find MV though doesnt it? so does this just effect the
> search range used? are there any other differences?
> 
> many thanks
> 
> Tom
> 
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> XviD-devel at xvid.org
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> 
> 


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