Shifting modelines/scanlines horizontally

[OxC0FFEE]

In the above demo, at about 4:30 in, the screen goes wavy.

 

I know it’s possible to shift the display list up and down easily enough by changing the address of the start of screen memory, but I don’t know how they achieved the sine-wavey effect where they move the scanlines/modelines horizontally. Does anyone know what trick was used to shift them sideways?

 

 

[+Stephen]

That's cool.  Without looking at the code in emulator, I would guess they are doing some pretty cool cycle-exact timing and repeatedly banging the DMA register.  Purely a guess.

[Irgendwer]

Just a guess, but it would be quite easy just to have PMGs looking like the screen and modify the HPOS register. The black columns introduced later (PMG data cleared) support this estimation.

[emkay]

The "ANTIC" demo is a "GTIA" demo

 

You see the upper part gets rather small vertically. 

For a short while , the screen seems to be switched to 32 bytes width (possibly it's just overlay of  Players to the left and the right of the screen)

Later the PMg (GTIA stuff) , is overlaid , using the shapes of the PMg.

Then the screen gets the 40 bytes width. 

And for the height of the effect screen, there is a "kernel" running. 

Using one offset for the PMg, you can re-use any Player or Missile on every scanline. 

So they automatically do the same step /wave on the screen. 

 

It's all smoke and mirrors, but nice smoke and mirrors

[Rybags]

PMGs for sure.

You can hit DMACTL to enable/disable the screen but the cycles you can possibly land on are too course - like about 32 of the possible 80 you'd need thanks to DMA and refresh losses.  Plus initial modelines of hires modes allow no such control practically all the onscreen cycles are lost.

 

What would have made that demo better is if there was some text or graphics doing the sine curve as well although in all likelihood that actual screen is totally static.

If you were really tricky you could do 320 pixel movement by using alternate character sets though of course the PMG/colour changes would still be constrained to 160 pixels.

[phaeron]

It is indeed an PMG cheat:

 

 

The only part that is playfield is the 40 column text, since only the playfield can output hires.

 

Even mucking around with DMACTL with tricky timing won't achieve an effect like this since the display start/stop points are hardcoded in ANTIC -- you can't shift them on a cycle or color clock basis by flipping stuff on and off. The best you can do is select between the preset narrow/normal/wide borders.

 

I'm surprised that all four players were used for this, given that it probably would have been easier to set up a wide playfield and mask the sides with players instead.

[MaPa]

Or it could be done with regular (not hi-res) graphics mode with same color as hi-res, wide screen and scrolling. Probably wouldn't go as far left as in the video (didn't chceck how clipped the screen is).

[Irgendwer]

Actually the effect could be a lot more convincing, if a narrow playfield with matching horizontally scrolled content and PMG borders would be used...

 

Edit: But that's what has been said above before...

 

Edited May 4, 2021 by Irgendwer

[Rybags]

There's no text or graphics in the H-scrolled region so really they could have just used Graphics 3 and gotten away with it using HScrol per scanline changes.  As such you get the single scanline resolution illusion as part of the effect.

[emkay]

15 hours ago, phaeron said:

 

 

I'm surprised that all four players were used for this, given that it probably would have been easier to set up a wide playfield and mask the sides with players instead.

Erm. No?

The logics were simple, using the players directly side a side. 

Doing the black lines would have been a lot more complicated.