I suspect that a common technique for putting more objects on the screen
must be based on the multiple copies feature of Atari 2600 sprites.
Think about Activision Plaque Attack enemies (or a lot of other shooters),
where you have groups of three aliens:
X X X
They must be done whith a sprite in three copies/close mode
(NUSIZ=xxxxx011), as they are identical and move together on the same
pattern.
You say: ok, but what happens when one or two aliens are destroyed?
Well, if the rightmost alien is hit, set that sprite to two copies/close
mode (NUSIZ=xxxxx001):
X X
If the leftmost alien is hit... set the sprite to two copies/close and
shift it 16 color clocks to the right!
-->X X
And if only the center alien must die?? Set the sprite to two copies/med
(NUSIZ=xxxxx010)!
X X
When only one alien survives, just set the sprite to one copy only
(NUSIZ=xxxxx000) and shift it to the appropriate position.
There might be another way to do this: the sprite is always in three copies
mode, but its pattern is blanked just before drawing the missing alien.
That seems much more difficult to me, as it involves cycle counting, and in
the same scanline you might have to update the background and take care of
the other sprite and missiles.
And what about explosions? If you want to use the same sprite for a group
of aliens and their explosions, I think that a pattern change "on the fly"
is needed, as said above. Or maybe you might use the other sprite to draw
the exploding alien. Or just let them disappear... :-)
Please note that these are only my thoughts, I haven't checked the code of
any game.
Ciao,
P.
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