Commodore Plus 4 - Page 227

Graphics Programming in BASIC 215
250 Restore K§ SHIFT keys.
260 End of program execution.
270 Put two space characters in the position of the previous frame for the
rightmost butterfly (saved in Yl).
280 Set color to multicolor blue and put up the current frame for the
rightmost butterfly.
290 Put two space characters in the position of the previous frame for the
leftmost butterfly.
300 Set color to multicolor green and put up the current frame for the
leftmost butterfly.
310 Save the current position in Yl. Change Y to plot the butterflies one
character line higher on the next frame. But if they fly off the screen,
restart them at the bottom.
320 End of subroutine.
330-380 Data for the three frames of the butterfly.
Animation in Multicolor Graphics When using a bit-mapped screen for ani
mation, you can actually have the moving objects appear to move in front of a
stationary background. Usually this is done in machine language because consid
erable speed is required to avoid flicker. But, an example program can be done in
BASIC using the SSHAPE and GSHAPE statements.
The example uses two frames of a pogo stick jumper. First, the background
information that will be under the jumper is saved (SSHAPE). Then, he is plotted
to the screen (GSHAPE). To prepare for the next frame, the jumper is erased (by
replacing the background saved before), and the process starts again.
The SSHAPE statement allows you to save a portion of the screen in a string
variable. Because a single string can be at most 255 bytes long, only a limited
amount of the screen can be saved at one time. If you try to save too large a
portion, a STRING TOO LONG ERROR results. SSHAPE saves only the
pattern of bits in the area, not the color information.
The GSHAPE statement places such a string anywhere on the screen. The
colors used are the current colors. If they have changed since the shape was saved,
the new ones are used. The last parameter of GSHAPE lets you decide how the
shape is to be placed on the screen:
0 = shape replaces background
1 = the inverted shape replaces background