Commodore Plus 4 - Page 224

212 Programming Graphics
Animation
Computer animation consists of rapidly displaying a series of graphics, each
changed somewhat from the previous one, to create the illusion of motion. There
are a number of methods for doing this. The more advanced can be accomplished
only through direct control of the graphics chip. Also, to attain the speeds
required to fool the human eye, machine language programming is usually
necessary.
Only a couple examples of straightforward methods using BASIC are pre
sented here. To get more involved with animation, study the concepts of directly
controlling the chip from machine language in the next section and experiment
with them.
Animation Using Characters Character animation can be extremely convinc
ing, even in BASIC. The pattern on the screen is defined by only 1 byte (a screen
code placed in the proper character cell), or possibly 2, when color changes are
occurring. So the pattern can be changed just by changing the value of that screen
code. This can be done very quickly.
For serious animation, direct access to the screen and color memories (with
POKE statements) is usually used. For information on these memories, see the
summary of memory maps later in this chapter. This example uses the CHAR
statement to place multicolor characters on the screen.
The animation consists of three positions (frames) of the two-character-wide
multicolor butterfly presented earlier. The first (frame 1) shows its wings wide
open. The second (frame 2) shows them partially closed. The last (frame 3) shows
them totally closed. To animate the butterfly, the frames must be shown in the
sequence 1,2,3,2,1,2,3,2,1, .... For the butterfly to appear to move, each
successive frame must be shown in a different (adjacent) pair of character cells
from the previous frame. So, we must erase the previous frame in addition to
showing the next frame.
Character Animation Example Program
In this example, type the small letters inside the quotes without shifting, and type
the capital letters inside the quotes shifted.
10 POKE1177,62
20 FORI=0TO727
30 POKE8192+1,PEEK(54272+1)
40 NEXT
50 POKE1177,63
60 POKE65299,(PEEK(65299)AND3)OR32
70 POKE65298,PEEK(65298)AND251
80 POKE65287,PEEK(65287)OR16