Sticking to the original question: the Opalvision card sits in an A2000-style video slot like a Microway flickerfixer or Video Toaster (it should also work in the A3000/A4000(T)). It simply passes through the original Amiga graphics modes for Workbench, etc. at 15kHz so you need to use a 1084 or similar Amiga monitor for display (or an external ff/sd with svga monitor). The real benefit of it is the 24 bit framebuffer that can be used to show images at much higher color depth than is possible in HAM6 mode (there are programs such as Viewtek that take advantage of the Opal board). There's also an animation program (opalanimate) that can be used as well. The card doesn't do rtg, scandoubling, or flickerfixing but it's not bad if you want to use it for the uses mentioned. It's kind of a poor man's AGA on the A2000.
