No flicker in Demon Attack?

[godzillajoe]

So when you get to that stage when the demons split and dive bomb on you, how are there 2 of them moving independently with no flicker? What fancy trick is this?

[+SpiceWare]

If you turn on Stella's debug color mode you'll see that it's very basic reuse of the players (sprites).

 

[RickR]

Shouldn't they flicker when they cross paths vertically? Maybe they do, just too fast to notice?

[+SpiceWare]

they don't cross paths vertically.

[nanochess]

So when you get to that stage when the demons split and dive bomb on you, how are there 2 of them moving independently with no flicker? What fancy trick is this?

 

There are 2 players (or sprites) of eight pixels, these can be in same line and them can have some repetition pattern (two or three times)

 

So essentially there is a chance you can see six sprites inline without flicker but choosing only from two drawings (Space Invaders uses this)

 

Apart from this you have the two missiles and the ball (essentially three individual pixels that can be used as vertical/diagonal lines) and the playfield (kinda of 40 "fat" pixel resolution per line)

 

Every Atari game uses efficiently these limited resources many times with a superb result.

Edited December 24, 2013 by nanochess

[godzillajoe]

Wow, I was a bit pissed when I posted this. Thanks Christmas booze! For some reason I thought reusing the same sprite would cause flicker like in Pac-Man or is this using the sprite copy trick like you have in Combat when there are 3 jets? But I thought those all moved together and the small demons seem to move independently. Or is it just clever use of that method.

[+SpiceWare]

You can reuse sprites vertically without flicker, just like in Air-Sea Battle.

 

 

The reason Pac Man uses flicker is the sprites often appear in the same horizontal section. It can be done more intelligently than in Pac-Man so the sprites only flicker when they're in the same section, as shown by Ms. Pac-Man and Jr. Pac Man.