Re: Dream Grafix & VOC

[toddpw@ugcs.caltech.edu (Todd P. Whitesel)]

johnt@meaddata.com (John Townsend) writes:

>I can't be the first person to have this problem, but I haven't seen it
>mentioned here before:  3200 mode graphics appear to be incompatible with
>the Video Overlay Card, at least in Dream Grafix and Dream Voir.  Each line
>appears to be stuck with the same pallete if the monitor is plugged into the
>VOC output, but looks just fine if it's plugged into the normal IIgs output.
>What's the deal here?

I posted about this in gory detail a long time ago. The condensed version is
that most 3200 software uses the motherboard video counters ($C02E/$C02F) to
synchronize their palette management, but on the VOC you simply can't access
those locations. If you sacrifice your overlay capability and plug the IIgs
output into the VOC's input, the VOC will lock onto the motherboard video and
the motherboard video counters will suddenly be correct and things will start
working.

Rewriting 3200 display routines to work 100% with the VOC involves converting
them to use scan-line interrupts somehow, because that _can_ be read from the
VOC. I wrote a very simple (and very buggy!) 3200 viewer that worked properly
both with and without a VOC a long time ago (trust me, you don't want to use
it, it sucked) and the source code has been used by John MacLean in The RGB
Exchange. The DreamWorld folks have a copy of the source code, but have not
gotten around to retooling their display routines to cope with it yet. I'm
not surprised, actually, and I don't think less of them for it.

Writing real programs that do this is not easy; my viewer simply locked out
interrupts and polled the interrupt-happened bit, and never went through the
messy interrupt firmware. Like I said, it was horrible and served only to
prove that it could be done. (The fact that I sorta bragged to Gogs at
AnimeCon '91 that I could have it working in a week or so did figure into it
a little, but that really only made it happen faster.)

Todd Whitesel
toddpw @ ugcs.caltech.edu