Being bored myself of my usual checkerboard routines, by using their principles I wanted to have a try at something quite different.
So here’s a simple BASIC program for the Commodore 64 that shows a big moving “hello”.
Download: Moving Hello (C64 BASIC)
How does the program work? Well, consider the letter H. Moving it vertically is a matter of moving two vertical bars and one horizontal bar.
The principle is modifying the shape of programmable characters on the fly, so that by just poking a very few locations in RAM memory the VIC-2 chip takes care of a lot of pixels on the screen.
Now, here’s the characters pattern used by the program:
Let’s consider again letter H. As you can see, characters from A to R make up the two vertical bars, while characters from S to X make up the horizontal bar. But why all of these characters? Well, for each bar we have to provide space for all its possible positions.
The vertical movement is by five characters, that’s why for instance we have letters from S to X for the horizontal bar of letter H.
The very same principle is used for the other letters. This way, we can get this big “hello” word moving by only changing four memory locations in RAM per frame. Lines 2 and 3 take care of this (they contain the animation loop).