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Re: [Opendivx] wavelets



Well, wavelets get much of their superiority from the fact that they tend
not to show blocking effects (but they have a much worse ringing). If you
use a good dct implementation (with adaptive quantization matrices) and a
postprocessing filter you get exactly the same visual quality of wavelets
on static images. When you use motion prediction methods (and
differentials as a way to produce error images) wavelets no not have any
advantage over dct for inter-frame coding. In fact, the use of wavelets
is mainly for low bit rate coding, when compressing in the mid rate
(400-900 kbit/s for 24/25 frames/sec, film like sources) the visual
difference with dct is not perceptible.
It is better to:

- prefilter material to make the artifacts more acceptable, or to spread
the error over different frames (warning- it seems that this is patented
somehow) (approximate compression gain: 10%-15% depending on the material)
- use a sophisticated motion matching algorithm; it is better *not* to use
the block that minimize the RMS error, but to choose a block that does
follow the optical flow of the moving sequence (there is a nice library
from Intel that does it). Also, estimating the optical flow can be used
for prefiltering (direction-oriented smoothing using constraint-based
diffusion is used for this) (appr. gain: 15%)
- use a postfilter to reduce the error created by the quantization step
(there are several, good adaptive systems for that) (appr. gain: 20%)
- if the material is to be seen from CDs or from broadband sources, you
can allow a much larger VBR extension (appr. gain: 10%)
DCT+Postfilter is roughly equivalent to DCT; there is an advantage thanks
to hardware DCT implementation common in many graphic cards or thanks to
the highly optimized libraries available for dct (there are none for
wavelet).
cheers
						Carlo Daffara
						Conecta Telematica

On Thu, 1 Feb 2001, ridaas syncprodz.com wrote:

> i have very few knowledge about video coding since my proper job is more focused on transport-layer of mpeg standard (the so called mpeg2-TS).
> i am not sure if mpeg4 standard talk or not about wavelets, maybe is possible than more then one coding techinique is allowed.
> what i know is this:
> 
> -wavelet is based on subband coding, the wavelet transform scheme combine transform coding with subband coding
> 
> -wavelet is supposed to be very good for I-frame coding (same as jpeg coding) and for low-bitrate videos (12-48kbps) the I-frame coding takes about 40-70% of the bit used in the stream, this means that for very low bitrate you save a lot respect typical dct.
> 
> -wavelet is supposed to be more cpu intensive, i checked some article and it seems the coder (on a dsp implementation) has 50% less framerate.
> 
> -the problem with video and wavelet-based coding scheme is find a mechanism to exploit the temporal redundancies and here i found 3 proposals:
> 1) extensions of the 2-D wavelet-based scheme to 3-D subband coding (3-D SBC)
> 2) use of a multiresolutional motion compensation (MRMC) in the wavelet domain
> 3) so-called overlapped block motion compensation
> 
> now don't ask me details cause i am not in this things at all :(
> 
> good references can be found (where i found them, but there are lotsa of article cited there) in this article:
> "Very low bit-rate video coding using wavelet-based techniques" by D.Marpe H.Cycon (IEEE Transactions on circuits and system for video technology vol.9 feb 1999)
> 
> i will check this wavelet things and mpeg4 and divx code (if i can manage it) and will try to post my 2 cents when i will know more :)
> 
> federico
> ps: i thought divx was the microsoft hacked mpeg4 encoder btw..:)


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