Re: crazy idea for encoding

Ph. Marek wrote:
> Hello everybody!
> 
> This is my first time posting to the list so please be gentle with answers :-)

OK :)

What you have outlined would indeed provide significant compression if 
it could effectively be implemented. The problem is the implementation. 
Getting structure data from moving images is actually remarkable difficult.

Consider that we have bi-nocular vision or a known camera trajectory. 
Also assume that the points we observe are approximately stationary 
between frames. With such assumtions it is relatively trivial to derrive 
from perspective geometry the position in 3D space relative to the 
oberver a given point given two or more frames.

The problem comes when trying to track multiple points (e.g. the corners 
of a wall) in that it is remarkably difficult to say which points in 
image 'A' correspond to points in image 'B' (it is known as the 
'correspondance problem') consider the problems:

  - Points in image A might disappear off the edges in image B
  - Our 'wall-corner-detector' might fail and 'lose' one of the points 
in image B. How do we know which point in image A isn't in B?
  - Spurious points might show up in either image, how do we discount them?

Many techniques (such as the 'epipolar constraint' method) have been 
devised to reduce the search space in such problems but generally the 
problem is very difficult.

A pragmatic person might argue that we are not aiming for an accurate 3D 
model of the scene, only one which looks adequately like it when 
rendered from the observers vantage. Certainly such an algorithm could 
be created but one then has to worry about corner cases. The ones that 
leap to mind are:

  - Glass objects -- How do you handle transparency?
  - Moving objects -- In order to get a 3d model you must assume 
stationary scent and a known camera trajectory (or at least a scene 
which remains stationary with respect to itself). Removing this 
condition effectively increases your search space exponentially.

This is just a brain dump of things which should be considered if the 
coded were to be able to deal with 'real-life' videos.

-- 
Rich Wareham

"All good mathematicians are lazy but, unfortunately for most of us,
  not all lazy people are good mathematicians."
                                                    -- Anon.



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