Garbled graphics in Prince of Persia when drinking the "weightless" potion on Level 7

[hkzlab]

Opening this, but expecting it to be an issue with RGB cards in general and not specifically with the A2DVI.

I was testing A2DVI on an Apple IIe clone (see https://codeberg.org/hkzlab/NE_Delta_Schematics) with Prince of Persia.
Once I got to level 7 and drank the "weightless' potion, the screen became severely garbled

I suspected an incompatibility with the clone, but see what happens with AppleWin using the RGB monitor as a video output option:

while, using the composite out as an emulation option, I get the following image:

The picture from the Delta's video output is fine (even though the Delta has peculiar video circuitry, so, in this case, I was getting a clean, ungarbled, but B/W picture).

Also, refer to this youtube playthrough https://www.youtube.com/watch?app=desktop&v=qFOykHnNJ70, time 40:55 for how it should look on a real machine.

EDIT: I can provide floppy images with a savegame at the beginning of level 7, if useful

EDIT 2: Renamed the potion to conform on how it is called in the original source code.

[hkzlab]

The "trick" PoP uses to generate this effect can be found here: https://github.com/jmechner/Prince-of-Persia-Apple-II/blob/master/01%20POP%20Source/Source/SUBS.S around line 408. The "vibes" variable lives at $318 and setting it to anything different than 0 will trigger the effect.

Apparently, the game uses DHGR + 40 columns to generate the effect.

[dbalsom]

i suspect the issue is here

A2DVI-Firmware/firmware/render/render.c

Line 317 in c37246b

if(current_softsw & SOFTSW_DGR)

render_dhgr is called solely based on the status of SOFTSW_DGR which PoP sets to change the phase of the color carrier. however this does not actually toggle dhgr completely on its own.

with the caveat that i know next to nothing about the Apple II and am making educated guesses, i think the fix would be to add the same logic you have on other matches and check SOFTSW_80COL too.

[hkzlab]

I tested the fix by @dbalsom in https://github.com/dbalsom/A2DVI-Firmware/tree/purple_fix, it generates the correct colors once the shift is adjusted from 2 pixels to 1.

Here is a video of the result.

81852e52c1919e4c.mp4

[dbalsom]

#32