[294] ....planetary imaging to his colleagues in
1968, when he turned his attention to a series of grants from the
National Institutes of Health to study applications of digital
image processing to microscopy and medical diagnosis. Robert
Selzer of JPL had applied the techniques to x-ray enhancement. For
Nathan, with a background in x-ray crystallography, this was a
natural step. Unfortunately, by 1973 the government canceled all
fundamental research grants in the field and Nathan found himself
without support and nearly without a JPL
position62.
Nathan managed to hold on for a few more
years at JPL on other projects until, in the late 1970s, he
thought of a way to increase the speed of the then
computer-time-hungry image-processing programs. With Mariner Mars
1971 it became possible to send images faster than they could be
processed. Since then, the ratio between transmission time and
processing time has gone way up in favor of transmit time. In
general, it does not really matter, since instant images are not
now a requirement, but for users of image processing other than
planetary scientists, additional speed is attractive. Also, as the
number of images has skyrocketed from Mariner Mars 1964's 22 to
literally tens of thousands in the Voyager and Galileo projects,
time to process the images is of interest even to the most
patient. The problem is that as the number of pixels has
increased, the number of individual computations also increases. A
1,000 by 1,000 pixel image weighted 35 by 35 times requires 1.225
billion multiplications63! If these are done in sequence, the amount of
processing time would be formidable.
To solve this problem, Nathan suggested
putting 35 sets of 35 multipliers in parallel on very large-scale
integration (VLSI) chips. By doing that, the amount of
calculations is reduced by 1,225 to 1. Recently, he has begun
design of a set of VLSI chips that will speed up the geometry or
reprojection operations64. Basically, the weighting algorithm is encapsulated
in a single chip as a unit of hardware, rather than as software.
Logic in hardware executes faster than logic in software because
all 1,225 multipliers are operating simultaneously in parallel
rather than one at a time serially as in a central processor.
Nathan's chips have been plugged into Digital Equipment
Corporation VAX 11/780 computers. When the computer is executing
an image-processing program and reaches the point where it wants
to do the algorithm on the chip, the computer "calls" the chip
just as though it were calling a software subroutine.