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From: uselton@nas.nasa.gov (Samuel P. Uselton)
Newsgroups: comp.graphics
Subject: Re: Psycho Graphics
Message-ID: <1991Feb14.181142.21951@nas.nasa.gov>
Date: 14 Feb 91 18:11:42 GMT
References: <91Feb14.110238est.7256@neat.cs.toronto.edu>
Sender: Sam Uselton
Organization: NAS Program, NASA Ames Research Center, Moffett Field, CA
Lines: 78

In article <91Feb14.110238est.7256@neat.cs.toronto.edu> mgreen@cs.toronto.edu (Marc Green) writes:
>All this talk of "Psycho Graphics" shows that many computer graphics
>people badly need a basic perception course. Why? Because there is
>very little correlation between physics and perception.
>
I agree that studying perception is useful for people making images,
including computer generated images.  "little correlation" seems a little
strongly worded.  Yes (preception of) colors shift based on the background.
Yes sizes seem to vary based on color.  Yes, yes, yes.  But, it seems to me
that understanding the physics of making the images, in order to make images
without artifacts of the creation method, is needed.

>Perception is psychological while images are physical. The
>relationship between the two complicated and not deterministic. For
>example, it is easy to make physically identical lights look different
>and physically different lights look identical. The number of quanta
>in a light and the wavelength play a surprisingly small role in the
>way a light is perceived. The perception of motion, depth, size, etc.
>likewise cannot be readily predicted from the physical properties of
>an image alone. The mapping from images to perception cannot be
>predicted without knowing a great deal about the visual system.
>
Even worse, perception is also subjective (by which I mean it varies from
viewer to viewer even with the same stimulus).

>The basic fact to remember is that our perceptions are manufactured in
>our heads and depend as much or more on the way our visual systems are
>wired together than by the retinal image. 

Different images are more likely to produce different perceptions.  Identical
images at least have a CHANCE of producing the same perception.  Influencing
the CONTENT of the perception does indeed require a great understanding of
visual systems.  And psychology.  And viewing context.  And...
(That's why movie making is an art.  :-) ) 

>It is wasted effort to spend
>time worrying about complicated camera models and the like. 

This statement I take issue with.  If we can generate images from mathematical
models by use of a computer which are identical to images of real objects as
recorded by a camera and played back appropriately, we have accomplished
several things.  We have experimentally verified our understanding of the
physics producing the image.  We have a means for producing images of objects
that may be difficult or impossible to actually photograph.  We have a means
to present the stimulus to the observer in EXACTLY the same way.  Do you
claim that the perception of images will differ (for a particular observer)
even if the stimulus is the same?  If the observer has no information about
whether the image is "real"?

>For
>example, fancy models of motion blur simply are unnecessary. 

Whether something is unnecessary depends on the application.  They may be 
unnecessary FOR WHAT YOU DO and still be a reasonable thing for others to
work on.

>The real
>question is not how to make computer graphics look like cameras, but
>why the blur improves motion perception in the first place. 

There is not ONE real question.  Being able to make the blur, in various ways,
may in fact allow further study of the perception issues!
Please don't be so quick to condemn work, just because it is different than
what you choose to do.

>In fact,
>quite a bit of psychophysical work has been done in this area if
>anybody took the time to look for it in Vision Research, Perception,
>or any other other journals concerned with human vision.
>
>Marc Green

I'm just beginning to nibble at some of this material.  I agree it is 
worthwhile.  I'm just uncomfortable seeing and hearing condemnations of
other areas (close to home).

Sam Uselton		uselton@nas.nasa.gov		ex-prof
employed by CSC		working for NASA (Ames)		speaking for myself