Hello, wondering if someone can help me out with this. A few months ago I bought one of these: https://www.ebay.com/itm/296108803958
It generally works great on my Enhanced 2E, however, Double HiRes doesn't look correct on it.. it's almost like it's just showing a standard HiRes interpretation of images. I've tried different slots and even updated to the latest AppleIIVGA firmware. Anyone have any idea why this may be, or is this expected behavior? Here's some pics of images from Total Replay and the game Airheart, both Double Hires:
This is more or less what you can expect from an RGB display interpreting a DHGR image. It treats the signal as 16 colors at 140 x 192 resolution. It's good for some things, but games designed for an NTSC composite decoder tend to suffer.
Apple IIgs RGB for comparison:
Air heart iigs.jpg
It looks like that VGA card is just doing a simple conversion from monochrome 560x192 to 16-color 140x192, which loses a lot of fidelity.
Looking at the source code, it may just be a limitation of the card.
The IIGS works hard to give one a NTSC-like experience. RGB systems like Video-7 and Chat Mauve simply map every four bits to a color, which is what these A2VGA-type systems do as well. I think the A2VGA hardware has the potential for more, with the right firmware.
That's too bad, I was really hoping this was just a total solution to being able to use a VGA/HDMI monitor. Thanks for the insight!
David Kuder has already implemented an improved variant, with an optional "interpolation mode" for double hires, which tries to emulate analog display effects: https://github.com/V2RetroComputing/analog
DHGR certainly looks better with his variant.
However, his firmware requires a different PCB, since he also made other improvements which required hardware changes. So his firmware only works with his adapted PCB design (or with the A2VGA PCBs by Ralle Palaveev, which were also popular in this forum: https://github.com/rallepalaveev/analog ).
And while we're at it: my firmware for the variant with the HDMI connector (https://github.com/ThorstenBr/A2DVI-Firmware) is in turn based on the improved rendering code by David, however, currently the interpolation mode for double hires isn't ported yet. (But A2DVI will also support it eventually.)