Microcomputer Graphics

240 Microcomputer Graphics
the beginning of the table. Another area that usually is safe lies just
above the second page of high resolution graphics. Program A2.2 stores
a shape table there.
While the location of a shape table in memory is somewhat arbitrary,
the address of the beginning of the table must be made known to
the shape drawing commands. When either DRAW or XDRA Wis used,
the address of the shape table is found in locations 232 (low part of the
address) and 233 (high part of the address). It is the programmer's
responsibility to see that these locations contain the proper addresses.
In general, for a shape table stored at location A, location 232
should contain the value A - 2 5 6 * < INT > , while location 233
should contain I NT • These values may be set when the table is
loaded from disk. Several tables may be used within a program if the
contents of locations 232 and 233 are redefined at appropriate times.
Values of ROT, SCALE, and HCOLOR should be defined before
using either DRAW or XDRA W. (See the Applesoft Basic Programming
Reference Manual for a description of these commands.) You will
find that some distortion occurs when shapes are rotated or scaled.
Also, since most shapes contain vertical line segments, you will find that
colors are not easily used. White is the preferred color for shapes.
The following short program illustrates a use of shape tables to simulate
a radar screen. The shape table is identified in lines 20 and 30. It
consists of two shapes: a single plot-then-move-up vector, and a collection
of vectors that define an airplane. The shapes are illustrated in Fig-
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(a)
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(b)
Figure A2.2