Ian HanschenI ported a USB DisplayLink driver to @polpo's PicoGUS card so I could use it to watch MS-DOS games make changes to the VGA palette in realtime. Specifically, I ported "libdlo" to use the TinyUSB stack in host mode, then spent way too much time optimizing it until it was sufficiently fast.
PypeBros@furan @polpo wait ... so #JazzJackRabbit does the exact opposite of what an #Amiga game would do ? use abundance of colour to simulate a raster interrupt effect ? and the raster is never modified in VRAM, instead the palette is shifted so that the highlight appears higher or lower 0_0
@PypeBros @polpo palette modification on amiga was just as prevalent and they kicked it up a notch by doing it on a scanline basis from the copper.
Amiga Graphics Archive - Specials - Copper
The Copper was a coprocessor of the Amiga's chipset that could be used to change video settings while the cpu was busy doing more important stuff. It was often used to change the color registers for different rows of an image, allowing the Amiga to show more colors on screen at once than it was actually capable. Being independent of the cpu made it possible to have these effects practically for free (i.e. without using up precious cpu time)
@furan @polpo the difference I see is that amiga had little palette slots and would extend the amount of colours on screen by changing the palette on the flight during the same image.
Here with Jazz's blue rasters, there are up to 64 palette slots dedicated to the background raster and the whole 64 colours are shifted (but none is added) between two frames to create the illusion of a parallax screen.
@furan @polpo I'm sure other games used per-frame palette tricks to emulate parallax, although rarely with so many colours. What is remarkable here is that they used it to mimmic a amiga effect that would consume only 1 palette slot on the amiga, while they could have gone for almost any pattern given the amount of palette slots involved.