DHRG on the IIgs vs IIe/IIc

[Tempest]

I keep hearing that the Apple IIgs doesn't display Double Hi-Res Graphics 100% correctly when in Apple IIe mode. Unfortunately I can't seem to find any pictures to do comparisons or any that show what the differences are. Does anyone know what the problem is and what to look for? It's been awhile since I've had my IIgs hooked up, but IIRC the DHRG games looked fine (maybe a little blurry but that could just be my crappy monitor). At this point I'm just curious and would like to see the difference for myself. I'll dig out my IIgs if I must, but I was hoping someone had already done this. :)

[D Finnigan]

This is probably related to the Alternate Display Mode setting in the text
control panel which affects text screen 2.

As I recall, some games such as Pac-Man would display as a screen full of
the digit '2' until the Alternate Display mode was activated.

--
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[Tempest]

No that's not it. I know about the Alternate Display Mode thing like you describe. This was something that only affected DHRG. I think it was something to do with the pixel width or something of that nature.

[Daniel Kruszyna]

It depends on your definition of "correctly", but the IIgs does not
display any of the 8 bit graphics modes exactly like a //e does.

For DHGR, the difference is that the 4 bit sliding window used to decode
NTSC colors has some additional circuitry to detect when the content of
the window is changing. When this situation occurs, the previous pixel's
color is repeated until the value of the window stabilizes.

How does it look? Well, it's hard to see since there are still
artifacts -- just different ones. Look for transitions in color from
left to right on the screen. You'll see that the first color stretches a
little farther right than expected, possibly covering up some fringe
colors that a //e would have produced.

You'll have to use a real IIgs to see it as no emulator I know of
accurately reproduces this behavior.

-- Daniel

[Tempest]

I got around to hooking up my IIgs and comparing it side by side with my IIe. There's a difference alright, and it's (mostly) GOOD! I had forgotten how bad the vertical lines in the IIe composite monitor were until I saw the same image in RGB where all the colors were solid. There's some odd stuff going on with the text/graphic fringing, but overall I like the results. The graphics may be a tad blurrier on the RGB monitor, but it's really hard to tell and it's only on the very fine details like the status bar in Airheart (even then it's negligible). The different color fringing on the text is actually a slight improvement in my book, but then your mileage may vary.

I need to test out the games that had that weird red pixel problem still and see if I can recreate it. But beyond that it looks like my IIe is going into storage and my IIgs is coming back out.