[XviD-devel] Re: h263/mpeg quant

[Michael Niedermayer]

Hi

On Thursday 27 March 2003 08:27, Milan Cutka wrote:
> Hi,
>
> please would it be possible to select h263 and mpeg quantization not
> just when initializing libavcodec, but even during encoding? I've
> implemented controls for this feature to ffvfw GUI (it was inspired by
> XviD, it's VFW frontend can switch quant matrices depending on current
> quantizer) but without realy testing it and today one user reported it
> as a bug.
NO!

this is not allowed in ISO-MPEG4 if i understand it correctly, see:
------------- 6.2.1 Start codes
...
The Visual Object Sequence Header, the Visual Object Header and the Video 
Object Layer Header may be repeated in a single visual bitstream. Repeating 
these headers enables random access into the visual bitstream and recovery of 
these headers when the original headers are corrupted by errors. This header 
repetition is used only when visual_object_type in the Visual Object Header 
indicates that visual object type is video. (i.e. visual_object_type== video 
ID ) All of the data elements in the Visual Object Sequence Header, the 
Visual Object Header and the Video Object Layer Header repeated in a visual 
bitstream shall have the same value as in the original headers, except that 
first_half_vbv_occupancy and latter_half_vbv_occupancy may be changed to 
specify the VBV occupancy just before the removal of the first VOP following 
the repeated Video Object Layer Header.
...
-------------
Note, yes the quantizer type & matrix is stored in the VOL header

iam allso CC-ing this to xvid-dev as u said xvid-vfw can switch quant matrixes 
...

btw, does anyone know how much PSNR is gained by using this h263/mpeg 
quantizer(matrix) switching?

[...]

Michael