The I?S system at UCL has 12 512 x 512 pixel 8 bit refresh
memories. Each of these memory planes also has zoom, scroll, and
split screen registers. These determine which pixel is to be sent to
the display. For instance, if each pixel is read twice, and each line
twice, this has the effect of magnifying the image (or zooming) by 2
times. If the 100th pixel of the 100th line is the first pixel read in a
cycle, the image is effectively shifted up and to the left (scroll). The
split screen register allows the display to be segmented and different
refresh memories displayed simultaneously. Each of these facilities
has been utilised in the design of the digital system described here.
The I?S System 575 software is basically a series of
computer programs run on the VAX host computer and called from
the Command Interpreter. A subroutine library is provided, so it is
a relatively simple procedure to program the system to perform new
functions.
Design Implementation
The digital stereoplotting system described here has been
developed in parallel with a system on the Kern DSR1 analytical
plotter at UCL using photographic images of simulated SPOT data.
The system has three componenets. Firstly a digital stereo
comparator program is used to record sub-pixel image coordinates of
control points. The exterior orientations of the images are then
calculated using ground control point coordinates, and conventional
methods. The method of orientation of SPOT imagery is described by
Dowman and Gugan (1985). Following which a stereo model can be
formed on the I?S display monitor.
This paper will describe the digital stereoplotting instrument
design which is analogous to the hardware features of an analytical
plotter, rather than the orientation software.
Real-Time Model Formation
A stereo model is formed in an analytical plotter by using
the collinearity equations to find xy photo coordinates relating to a
defined XYZ ground point. Hand/foot wheel encoder readings (XYZ)
are transformed to photo coordinates by inner, relative and absolute
orientation parameters determined during the orientation phase. The
centres of the binocular trains are then mechanically positioned at
these xy photo points. With digital imagery, the floating mark is
digitally incorporated into the imagery, removing the necessity of
high precision optical/mechanical componenets (and also conforming
perfectly to Abbe's Comparator Principle.
To maintain the stereo model, the optical system (floating
mark) or the images must move relative to the defined ground point.
This is achieved in the I?S system by scrolling the images and
keeping the floating mark position fixed. One advantage of this
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