MASKULINITY

by Rick O'Neill



Right, hands up who knows what a masked sprite is. Hmm. Well, perhaps

I'd better explain. You see, the trouble with ordinary sprites is that

they tend to leave a trail of devastation behind them. Move them over

a background of any kind and they'll tend to muck it up completely.

Not so with masked sprites. These little devils cunningly keep a copy

of whatever they pass over and replace it when they move on. They also

have the advantage that you can see through them, where appropriate.



This is where Pitstop veteran Rick O'Neill's handiwork comes in. He's

done all the hard work for you, and has come up with a complete

masking package which even boasts its own demo.



As you might expect, getting it going isn't going to be easy. On the

face of it, this one looks like a straight "type in the Basic, save it

with SAVE "SPRITE" LINE 9800, and then type in the hex using the hex

loader and save that after the Basic" job. And indeed it is. Do so,

run it and you'll find yourself staring at the wonderful demo routine.

This, however, is just a taste of things to come. The next step is to

do your own thing with it, a much more tricky feat altogether.



The first step is to define your sprites, and store them in memory.

They're stored upside-down (it sez 'ere), a row at a time, with two

bytes for the sprite being followed by two bytes for the mask each

time (the mask shows which sections of the background are to be

blanked out and then restored each time the sprite is moved). The

program also needs to store a copy of the background screen, so plonk

one at any address that's a multiple of 256 by doing RANDOMIZE USR

33316 (or 33327 if you've already stored one). The best way of doing

all this is to make use of the relocation routine in the Basic

program, which should explain it all miles better than I ever could.



Then poke in the X and Y coordinates and the sprite number (check out

the Basic once again for details) and RANDOMIZE USR 32768 to set the

ball rolling (or alien, spaceship, elephant or whatever).

Alternatively, machine code fans can call Start Address + 4, with IX

holding the address of the sprite information. The format is

x-coordinate, y-coordinate, sprite number followed by three bytes used

for information about the last sprite printed.



And if you can work out what's going on there you probably ought to be

contributing to Pitstop yourself.