Software sprites on a VIC-20

The VIC-20 has no hardware sprites, and the processor is to slow to draw a bitmap image. If you would like to have sprites floating around on the screen, how can you create them?

The VIC-20 lacks the ability to present a bitmap. Normally this can be solved by redefining the bitmap of the character set. One way to display this is by creating a bitmap by arranging characters like:

ABCDEFG
HIJKLMN
OPQRSTU

and we will get a 7x3 char= 7*8 x 3*8 = 56 x 21 pixels bitmap.

If we use 2 characters we can let a ”sprite” float in x OR y direction by changing the patterns in these characters. We can place the 2 characters like:

AB OR    A
                B

If we instead use 4 characters we can let the sprite float in directions x AND y by changing the patterns. We can place the 4 characters like:
AC
BD

The figure 1 and 2 demonstrate how a letter A can float like a sprite on a display that does not actually have sprites.

Figure 1: Original A in middle                                                    Figure 2: A moved 1 pixel in both x and y.

Link to a demo program I wrote in 6502 assembler:
Run able file: vic20sprites.prg (at least 8k expanded)
Source file: vic20sprites.asm

Project in zip-file: vic20_sprites_v3.9_Floats in x and y.zip

Link to development tool: CBM .prg Studio   Link to WinVice: WinVICE-2.4-x86

Figure 3: Screenshot when the program is running

Use keys the following keys to move the “A”:
A=up ; Z=down; N=left; M=right