I am amazed at how the Atari programmer pull off creating the 2600 Galaxian
with minimal flicker. In Galaxian, there can be up to eight aliens per
row. From what I see in visual observation, it appears that the entire
marching force of aliens uses the player1 sprite while all flying aliens are
drawn using the player2 sprite. The only noticeable flicker is when two
flying aliens are overlap the same horizontal row. If this was how Galaxian
was done, that programmer/designer must have written some VERY tight code.
Steve
From: Glenn Saunders <cybpunks@xxxxxxxxxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Thursday, May 18, 2000 6:57 PM
Subject: Re: [stella] Stella Programmer's Guide Overhaul Project
> At 06:02 PM 5/18/2000 -0700, you wrote:
> >So, it just makes three copies of the players for each
> >row and spaces them so that they alternate, or does it
> >draw one invader and, as it switches to the other
> >player, move the sprite over so it gets drawn again?
>
> The 2600 handles sprite cloning in hardware automatically.
> Therefore the program only has to set the shapes, color, and then run a
> positioning routine for two sprites and the 2600 does the rest. Because
> the spacing is set to wide, you can use one of the other players to fill
> the gaps between the copies of the other.
>
> P1 P2 P1 P2 P1 P2
>
> Not to be too much of a pitchman, but this technique is explained in more
> visual (albeit no code samples) terms on Stella at 20: Volume 2. Not too
> many people have bought the tape vs. the CD so I thought I'd point out its
> usefulness to potential 2600 programmers, since I'd imagine that group
> would get the most out of it vs. the regular classic gamer.
>
> The video also shows other tricks like the Video Chess kernel, and the
> moving "6 char" for Dragster, which is an elaboration of what was done in
> Space Invaders.
>
> BTW, I've heard that it was Blackjack that first rewrote sprite shapes
> inbetween copies. Has anyone disassembled that game? It seems like card
> and board games aren't games that people often disassemble and analyze.
It
> doesn't build a megasprite, but appears to use the other sprite as a way
to
> paint underneath the card icons using sprite priority.
>
> >So, if I can run with this ball, to draw the other
> >asteroids in progressively smaller sizes, because it
> >can clone the images but not change sizes, it draws
> >the different sized asteroids on seperate frames,
> >right?
>
> I think the asteroids only fly in two different speeds, and then there is
> an option for how diagonal the small ones fly. It's a pretty complicated
> kernel, I'm sure, because of how the sprites have to break off and fly
> around, which complicates things, which is why Asteroids is one of if not
> the first 8K game. I'm sure the flickering could have been minimized by a
> more intelligent kernel, but it's still a good one as it is.
>
> Let me give you an example of a kernel idea that probably hasn't been done
> that much, that I just thought of.
>
> Assuming you can have 6 strips of sprites running down the screen (ala
> Space Invaders) and you can change their shape between copies (like
> Dragster) then you could create vertical independence between the
> 'sprites', with the simulation of six (or more, as long as they don't run
> into eachother vertically, in which case they'd have to be the same color)
> sprites or flickered. You just couldn't move an individual sprite
> horizontally, they'd have to move as a group. If you wanted the illusion
> of the sprite moving into the other "lane" you'd have to shift the data
> over where part is written to sprite one and part to sprite one copy two,
> and so on.
>
> Most games run sprites along horizontal rails so that they can get two
> groups of sprites, 1, two, or 3 copies a piece, and slide them back and
> forth. Megamania and Oystron are good examples of that. Other games like
> Journey Escape do regular sprite clones and then scroll those down. But
by
> rewriting the sprite shapes, you could then not only control the vertical
> position of the sprite, but all of its clones by simply deciding where and
> when to write the other shapes into the other copies.
>
> Do any games use this sort of kernel??
>
> Of course, taken to its extremes, you get the Suicide Mission kernel,
which
> just lets you draw arbitrarily on the screen (at the cost of flicker and
> spread out pixels) or Stellasketch. At that point you basically are
> superimposing software sprites onto a matrix of hardware sprites. But
it's
> really cool that these things are possible.
>
>
> Glenn Saunders - Producer - Cyberpunks Entertainment
> Personal homepage: http://www.geocities.com/Hollywood/1698
> Cyberpunks Entertainment: http://cyberpunks.uni.cc
>
>
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