Section 2 - Commodore Computers

158 COMPUTE! May. 1982. 1982, Issue 24
Extra Colors
Extra Colors
For Atari
Through
Artifacting
Bill Mohn
Danville, CA
How can you getup to Four colors in GRAPH ICS
How can you get up to four colors in GRAPHICS
8' 8? The technique is called "artifacting"; it ii makes
use of an otherwise unintended characteristic of a
use of an otherwise uninte nded characte ristic or a
color display tube.
The highest resolution mode for the Atari is
GRAPHICS 8. This niode mode includes 51,200 pixels
(picture elements) arranged in a rectangular area
extending 320 units horizontallyy aand 160 units
vertically. Unlike modes 2 through 7, GRAPHICS
8 normally all ows onl y one color at two brightnesses.
8 normally allows only one color at two brightnesses.
The background hue and luminance is determined
by Color Register 2. "SETCOLOR 2,hue,lum"
establishes Color Register 2. If left unset, it defaults
establishes Color Register 2. If left unset, it defaults
to dark blue.
Colorr Register I, 1. which is set by SETCOLOR
SETCOI.OR
1 l,hue,lum is used onlyy to determine the luminance
off graphics points. The hue off graphics points is
determined by Color Register 2. A point is put on
the screen by BASIC's BASIC'S PLOT or ORA DRAWTO commands.
To be effective, a COLOR I 1 must preceed
the first Iirst PLOT.
In order to understand artif;,cting, artifacting. look closely
at a color television screen. Youu will see closely
spaced vertical stripes of red, , green and blue phosphor
elements. If these are ill uminated equally, a
phor elements. If these are illuminated equally, a
white while picture is produced. Unequal illumination
produces a ll of the other colors.
produces all of the other colors.
GRAPHICS 7 Versus 8
A pixel in GRAPHICS 8 is the size of one half off
one set of three color phosphor elements. IfI I' Color
Register 1I luminance is relatively high and a graphics
point is plotted ploued at an odd valued horizontal
coordinate, only the blue phosphor will be li ghted.
coordinate, only the blue phosphor will be lighted.
If I f a shape is drawn, taking care to use only odd
values for X, the entire shape will be blue.
On the othe r hand, ir onl y even values of X
On the other hand, if only even values of X
are chosen, the red and green phosphors will be
selected. The proportion of these will depend on
selected . The proportion of these will d e pend on
the e background hue (Color Register 2) aand the
resulting "even-only" figure can range from red
through brown to green. Ifboth even aand odd
points are plotted, the background hue will result
at a luminance specified ed by Color Registerr I. 1. Of
course if neither even nor odd points are plotted, ploued,
the result will be background hue and luminance.
An interesting side effect occurs when an "even
fi gure" overlaps a n "odd fi gure" - the area of
figure" overlaps an "odd figure" — the area of
overlap will be clearly visible.
GRAPHICS 7 has pixels four times as large as
G RAPH ICS 7 has pixels four tim es as large as
GRAPHICS 8. That is, they are twice as wide aand
twice as high. Since artifacting requires skipping
half the points in the horizontal direction, the
resolution in that direction with GRAPHICS 8
approximately equals that of GRAPHICS 7. However,
the vertical resolution of GRAPHICS 8 is
twice that of GRAPHICS 7.
The program here is a simple pie demons! monstration
off these principles. It first uses GRAPHICS 8 to
draw three overlappingg disks, thee upper r left using
odd points only, the upper right using g even points
only, and the lower using all points. Note that the
lower disk does have better horizontal resolution
than the otherr two. Afterr a deJa delay, the program
m
plots three similar ligures figures in GRAPI1ICS H ICS 7. Youu
will see that these disks have horizontal resulution
resolution
equal to and vertical resolution worse than the
previous upper disks.
previous upper disks.
You u may explore the variety of colors possible
with artifacting by changing the two SKTCOI.OR
with artifacring by cha ngin g the twO SETCOLOR
stateme nts. Statements 4000 through 4 120 may be
statements. Statements 4000 through ■! 120 may be
inserted to cause all 960 possible SETCOLOR
combinations to be displayed. This will step through
all combinations ns as long as START is depressed.
Notes On The Sample Program
1000-1200 Make an array of Y Y-values corresponding to
X-values for circles.
1300-1500 Setup for the first display in GRAPHICS 8.
1600-1900
Draw upper-left figure using odd points only.
2000-2300
Draw upper-right figure using even pointsonly.
only,
3000-3300
Draw lower figure using even and odd points.
3400
A delay loop to allow viewing the first display.
5000-5600
Setup and draw first figure in GRAPHICS 7.
5700-7300
Draw second and third figures.
Figures.
9000-9100
Anotherr delay loop followed by a return to repeat.
4000-4120 Three nested loops sstepping through all possible
hues and luminances.
4060 The ""arrow" is entered by typing ESC, CTRL,
CLEAR
4090 This loops until either r START, SELECT, or
OPTION is d epressed.
180
280
300
480
500
600
700
RBI
RBI
REM
REM
REM
REM
REM
OPTION is depressed.
"tr&k "it "it it it i: ■& Scm
*
* DEMONSTRATION OF
t
*
* ARTIFACTING IN GRAF-HICS 8 *
t
*
*
*
BY BILL MOHN
*
*
180 REM *****************************
200 REM 300 REM ID1ONSTRATION ~ 4B0 REM * ~TIFICTII{; IN GRAPHICS a *
500 REM 600 REM 8Y BILL totI 700 REM * *