What's your video card? Check the compatibility matrix; some cards do this in EGA modes: https://gona.mactar.hu/DOS_TESTS/

Yep, I ran across a couple of cards that did that. It's not the fault of OSSC. I used OSSC on MDA, CGA, EGA and VGA cards. None had any issues with colors. When I had the issue you're showing, it showed on my Dell 27" monitor that has a direct VGA input. I tried right away on a CRT and it produced the same color artifacts.. turned out it was the graphics card. I forgot which card caused this, but I asked about the issue here and there's a software fix for it.

And here is a link to the topic, which has a link to the topic:

Commander Keen games have a red tint

It may help to change the clamp/ALC setting toward a negative value, with the assumption that this changes the black reference sampling point after sync pulse, and that the tint is due to border extending into that area, causing incorrect black reference and thus messed up color on rest of the line.

Yeah, that thread wbahnassi posted has the fix linked therein: wbcbz7's MS-DOS VGA TSR goodies

I had forgotten about that.

What is unique about Commander Keen and Catacomb 3D and other Apogee games is that they use the VGA border/overscan area (the space between active framebuffer and display blank) for an artistic effect, adjusting it from black to some other color: cyan in the case of Commander Keen games.

So it does then sound that the root problem is that e.g. OSSC assumes that the overscan/border area would be the "ground truth" or zero level of what is supposed to be black, and the active image values are compared against that. So hence a cyan border (0 red, 255 green, and 255 blue) causes the ADC conversion to crush green and blue color channels.

In Catacomb 3D the border color flashes when the player is hit. Maybe that might cause a varying color degradation on OSSC that changes when the border changes? Might be interesting to test.

Looking at KEENFIX.ASM in the link, it seems to do the fix at least by shrinking the VGA border area to zero, causing the border color to vanish. That is one way to resolve the issue.

Thanks very much for your help All, and apols on not returning sooner - I need to sort out my notification settings.

Video card is a FX 5900 - can't spot it in that table

I posted at the same time over at Videogamingperfection and it was also suggested to change the clamp/ALC-offset, I did this but for me on my setup I had to use a negative number (-8) and it has mostly fixed the issue I think with no other side effects: https://ibb.co/FqLkVGQj Thanks for the explanation on it and the link too. I don't notice any negatives so far in leaving an offset of -8 for other software. Nice to put all of this together and understand what has caused it in comparison to other EGA games.

clb wrote on 2025-04-08, 19:04:

In Catacomb 3D the border color flashes when the player is hit. Maybe that might cause a varying color degradation on OSSC that changes when the border changes? Might be interesting to test.

Here's a recording of Catacomb3D with offset at zero (red tint) but I think the rest of the screen colours get changed anyway: https://youtu.be/CM3IzRKyboU
Hovertank also seems to be effected in same way when taking a hit, the colours seem to fade so I think this one is easier to see the effect (sorry no recording)