Graphics in Super Cobra and Amidar

[frenchman]

Hello,

I am new to the Atari 2600 and I just recently received Super Cobra and Amidar. I was wondering why the graphics in these two games are different to other 2600 graphics, the graphics in these two games are like lines. Are those vector graphics?

[+save2600]

Hello,

I am new to the Atari 2600 and I just recently received Super Cobra and Amidar. I was wondering why the graphics in these two games are different to other 2600 graphics, the graphics in these two games are like lines. Are those vector graphics?

 

Welcome to Atari 2600 hi-res from the early 80's! Quite an achievement back then. And no, Atari can not do true vector. It's all raster for play on regular tele's.

[Kurt_Woloch]

OK, here's a rather long explanation...

 

The lines on these two games are due to the fact that both games have an asymmetrical playfield. What does this mean?

As you may know, the Atari 2600 had very primitive hardware. The display chip called TIA had no means of doing DMA, so all screen changes had to be done by the CPU in each scanline where something should be changed. Without those changes, the TIA would only be able to display vertical bars on its own, and not even do a proper frame refresh.

 

Now there's another problem with the background graphics. The TIA has only got memory for half a line of background graphics, which can either be repeated or mirrored (Examples for repeated playfields would be some sections of Vanguard, while we have a mirrored playfield on River Raid). However, you can get a full line of different graphics by rewriting the background graphics registers on the fly. But the CPU only has a limited amount of time per scanline (74 cycles, as far as I know), and the rewriting of these graphics takes away a lot of this time.

 

Another oddity of the Atari 2600 is that it's only got 2 players (the same as "sprites" in other systems, but with an unlimited height). Now, if you need more than 2 objects on screen, and they should have different vertical positions (which is the case in both games), you have to "reposition" one of the players, which unfortunately also takes rather much time since the CPU has to wait until the electron beam has reached about the correct vertical position and then do a write to a TIA register. This, however, conflicts with the ability to write a full display line. So as far as I know, it's impossible to write a full display line and to reposition a player on the same scanline. That's why many programmers resorted to display their asymmetrical playfield only on some lines and leave other lines blank. As you can see, Super Cobra displays its background only every other line.

 

On Amidar, the problem is even a bit more complicated. Here they use 2 different foreground colors to denote the lines the objects can go and the filled-out areas. But the TIA can only display one foreground and one background color per scanline, so they display those colors on alternating lines. If you look closely, you can see that the lines don't even strictly alternate... there are single blue scanlines followed by one or more red scanlines. I believe this because of the need to reposition players which they wouldn't have if they were constantly changing between red and blue lines.

 

That's why both games display a bunch of lines instead of a solid display for their background.

[frenchman]

Thanks for the great explanation.

I think they look great on the games too. Are there anymore VCS games with graphic lines like that?

[Red 5]

I think the first game to ever use this is Video Chess.

(I could be wrong. I was wrong three other times in my life...)

[Thorsten Günther]

Thanks for the great explanation.

I think they look great on the games too. Are there anymore VCS games with graphic lines like that?

 

Thrust+ and Demolition Herby spring to mind.

 

Thrust+ even uses this effect to reach a comparably smooth horizontal scrolling.

 

 

Thorsten

[shadow460]

Space Invaders uses it for the score. The technique is called "venetian blinds". Phaser Patrol uses the a combination of venetian blinds and flicker for its text display.

Doesn't Stellar Track also use venetian blinds?

Edited January 19, 2009 by shadow460

[supercat]

Doesn't Stellar Track also use venetian blinds?

 

Stellar Track appears to use the same "flicker-blinds" kernel as Basic Programming; according to Warren Robinett, that kernel was invented by David Crane.