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From: lou@bearcat.rutgers.edu (Lou Steinberg)
Newsgroups: comp.graphics
Subject: Re: Grey Scale Slope Content Index
Message-ID: <704@bearcat.rutgers.edu>
Date: 3 Mar 88 17:34:44 GMT
References: <1988Feb29.223957.20454@lsuc.uucp>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 60
To: jimomura@lsuc.UUCP


In article <1988Feb29.223957.20454@lsuc.uucp> jimomura@lsuc.uucp (Jim Omura) writes:

>      I don't read 'comp.graphics' regularly so please comment via
> Net mail.
> 
> [...] With the advances in display technology we can now consider
> using digital technology in normal consumer photographic
> applications, in effect, replacing the traditional family photo
> albums with computers.  The current Amigas and Atari ST's are
> right on the border.  We are faced with the problem of evaluating
> equipment on for their ability to represent continuous tone
> images with visually satisfying detail.
> [and] goes on to propose a measure of this ability]

One major problem with your proposed measure of quality is that it
does not measure the ability to render large areas of smoothly
changing brightness.  For instance, you say

>     Taking the ST again, and using the lower resolution 320 *
>200 * 16 colors, we find that horizontally, we get 4 pixels in
>1/100th the width (3 and a bit of the 4th showing).  Of the 8
>grey levels, therefore, only 4 are useful in this small space.

This does reflect in some way the ability to show fine detail, but it
implies that a system with the same spatial resolution and only 4 grey
levels would be no worse at displaying pictures.  If you imagine a
picture that has a large area shading gradually from white to black,
you will see that those second 4 grey levels are a real help.  Of
course you can dither among the smaller number of grey levels, but
that effectively costs you spatial resolution.  

In fact, it is hard to see how you could get one single measure that
applies equally to pictures with fine detail or texture and those with
large areas of smooth shading.  Perhaps what you want is a graph,
rather than a single number.  The horizontal axis might measure
spatial resolution and the vertical axis might give the brightness
resolution for regions of that size.  E.g. for the ST described above,
for the horizontal direction, for a spatial resolution of 1/320 we get
a brightness range of 8, while for a spatial resolution of 4/320 we
get (8-1)*4+1 = 29 levels.  (For grey level all that matters is the
total brightness of the 4 pixels, i.e. the number of different totals
we can get by adding up the brightness of each of the 4 pixels, where
each pixel can be considered to be in the range 0-7.  In general for p
pixels each of which can have one of b brightnesses, we can get
(b-1)*p+1 different totals.  This assumes the palette for each pixel
is the same.  Hmm.. interesting idea: if alternate pixels, say, used
different palettes we could get a better brightness resolution for all
spatial resolutions above 1 pixel, using only a few more bits of
memory (to hold the second palette).)

Of course, only certain regions of this graph would be of interest for
any given application, depending on the picture to be displayed and
also on the visual angle subtended per pixel.  (There is no point
in displaying more resolution than the human visual system is capable
of perceiving, but that depends on the visual angle a region subtends
and not the fraction of the full picture it takes up.)
-- 
					Lou Steinberg

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