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From: sher@rochester.UUCP
Newsgroups: net.ai
Subject: Top down vs Bottom up?
Message-ID: <13220@rochester.UUCP>
Date: Sat, 16-Nov-85 23:12:29 EST
Article-I.D.: rocheste.13220
Posted: Sat Nov 16 23:12:29 1985
Date-Received: Mon, 18-Nov-85 06:52:21 EST
Sender: sher@rochester.UUCP
Organization: U. of Rochester, CS Dept.
Lines: 65


In a recent discussion of my work with a famous and capable vision researcher
the issue of bottom up vs top down processing came up.  I was
surprized to find myself saying that I don't think the issue was
relevant to my research.  To make this article intelligible to others
perhaps I should define terms.  

Bottom up processing is taking a set of observed data and applying
a serries of not individually intelligent transformations to the data
to get an interesting result.  An example of this is taking an 
an image, applying a first derivative to it, and then doing shape from
shading relaxation to derive a field of surface orientations on the
image.  

Top down processing is a process of making hypotheses about the high
level structure of an image and verifying or eliminating the
hypotheses based on data in the image.  LL1 parsing is a simplistic
form of top down processing.  

I think that I have got these definitions correct but I may have
screwed them up somehow.  Feel free to correct me by mail or news
posting.  

My research is on applying a variety of models for image structure any
of which can apply to the image and several can apply in different
parts of the image.  I would then use the information in these models
and information on the reliability of these models to arrive at an
interpretation of the image.  An example of two models are one that
assumes that surfaces have uniform reflectance within a region while
another model may have textured reflectance within a region.  An image
may have regions with uniform reflectance and regions with textured
reflectance within them.  

Now I have the context built I can discuss the issues.  It has been a
bone of contention in the vision community whether different aspects
of visual processing are bottom up or top down.  There are two issues
in fact: 
1)	how does the brain do it?
2)	what is the best way to do it?

I believe that in low to intermediate level vision the bottom uppers
dominate but in high level vision the top downers are dominant.  

I would like to put foward a contention:  I feel that the issue of
bottom up to top down has the same relationship to vision research as
the issue of control structure has to theorem proving.

This means that much research can be done without even considering the
issue of top down to bottom up.  Any research on the mathematical
relationship between an image and its structure can be done without
the issue of bottom up to top down coming up.  

I would like to see some discussion of this.  I wouldn't mind seeing
how this relates to peoples research.  If I confused people simply
tell me and I will try to explain myself more fully.  

-David Sher
sher@rochester
seismo!rochester!sher


-- 
-David Sher
sher@rochester
seismo!rochester!sher