All you do is add the current path direction to the character's position each frame.Įnemy AI is probably just a matter of waiting a random or predetermined amount of time, then playing a path. You could make character positions 16 bit and make the direction vectors 16 bit if you need more accuracy (taking the top bits as their onscreen position). That way I could make a set of arbitrary paths and apply them to any of the characters as needed. Path B = 0 (down), 4 frames, 15 (down-down left), 6 frames, 14 (down left), 8 frames. Path A = 0 (down), 5 frames, 1 (down-down right), 5 frames, 2 down right), 10 frames.

Then path lists: direction, frames to continue in direction Table of 16 direction pairs (vectors): x per frame, y per frame I would probably write the path data like this: Probably there are a bunch of paths stored that get applied to the enemies. I don't know exactly what it does, but it looks like enemies mostly follow very fixed patterns, repeating over and over. There are standard 6502 division algorithms you can look up.Ģ. Doing a division once when the player game overs isn't any kind of performance concern, it only really matters when you're trying to do it multiple times per frame.

It seems more complicated to model than something like dragon warrior or mario.ģ) How does it compute these patterns that enemies follow before retreating?ġ. For those who don't know, it's basically an evolution of space invaders.Īnyways, it impresses me for a NES game I'm not sure how it does a few things (though some of these things that I wonder about aren't extant in only the NES version):ġ) How does it compute the division of shots_hit / shots_fired? It certainly isn't using a table, and you can even get over 100% because you can hit multiple enemies with a single shot.Ģ) How does it store stages? (Each stage seems to have multiple waves, each of which a) has a group of enemies (there are many kinds), b) follows a pattern to appear on the screen, c) and then retreats to the top of the screen.) It must use some kind of timer to judge when the next wave should appear, since they appear whether you shoot or not. Galaga was originally an arcade game, but a NES port was eventually made.

Galaxian arcade game sprite