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56 SPRITES
by
Jonathan Rodda
COMMODORE POWER/PLAY MAGAZINE
April/May, 1986
p.97
Computer programmers are, by
nature, a bunch of showoffs. In
fact, sometimes the only way to get
them going is to tell them that
something is impossible. Then watch
out-- they will eat their disk
libraries before they throw in the
towel.
Take sprites, for example.
Everyone knows that you cannot have
more than eight sprites on the screen
at a time. End of argument. Even
programmers cannot change that law.
But with almost every rule, there
is an exception. 56 Sprites is an
example of just such an exception.
One of the most powerful features
of the Commodore 64 is the raster
interrupt. By using raster
interrupts, for example, you can
display several background colors on
the screen, or display graphics and
text on the same screen.
Or, as 56 SPRITES demonstrates, you
can have more than the usual eight
sprites on the screen.
But quick-- what exactly is a
"raster interrupt"? Forget the
interrupt-- what is a "raster"?
Everything centers around how
pictures are displayed on the the
monitor. The picture tube consists
of an electron gun on one end and a
screen on the other. The screen is
filled with rows of phosphor dots
that glow when they are struck by an
electron. These dots are called
"pixels"; a horizontal row of pixels
is called a raster.
When the picture signal is fed to
the electron gun, it scans each
raster from left to right, activating
the pixels as determined by the
signal. After one raster has been
scanned, the beam drops to the next
line.
The picture is redrawn sixty times
per second. This drawing process,
however, can be stopped for a moment
with a raster interrupt. During the
time that the drawing process is
halted, another operation can be
performed. Then the drawing process
continues where it left off.
Because pixels continue to glow for
a moment after being excited by an
electron, images generated during
the raster interrupt can appear to
exist simultaneously on the screen
when in fact you are seeing their
afterimages.
For example, the seven rows of
eight sprites you will see when you
run 56 SPRITES seem to occur all at
once on the screen.
Actually, there is only one row of
sprites which is redrawn in a
different part of the screen at every
screen update (once every 60
seconds). Because of the speed of
the update and the tendency of pixels
to hold an afterimage, there seem
to be 56 sprites on the screen.
To learn more about the ins and
outs of raster interrupts, refer to
Rodda's article in the Commodore
magazine cited above.
NOTE: Because of the nature of this
program, it can not be run from the
Loadstar environment. To run 56
Sprites, exit Loadstar, type
LOAD"56 SPRITES",8 <RETURN> and
RUN <RETURN>.
FILES REQUIRED:
56 SPRITES
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