Updating my code to handle this, rather than using the values in RAM, fixed some games; notably Phantasy Star and Space Harrier.
Thanks to Maxim's post, I fixed the Codemasters mapping for the Excellent Dizzy Collection:
A fun bit of hardware is the 3D glasses. These LCD shutter glasses could be used with certain games, and are controlled by writing to the memory range $FFF8..$FFFB. The least-significant bit controls which shutter is open and which is shut, and by alternating frames rapidly in time with the shutters you can display two views; one for the left eye, one for the right.
I've added four different 3D glasses mode. No effect doesn't do anything special, so you just get an unsightly flickery view. Frozen displays frames for one eye only, giving you a 2D view of the 3D game.
To recreate the 3D view on your PC, though, there are two methods. The first method is the standard red-green anaglyph view, for use with those red-green glasses:
Unfortunately, on top of the usual shimmery view you get from anaglyphs, this is made even worse by the fact that I don't remove the existing colour information from the frames. This looks nicer without glasses, but looks really quite horrible when viewed (as intended) through the glasses. For example, the red text is invisible through the green filter, but strongly visible through the red filter, making it shimmer.
By sacrificing the colour and only taking the luminance of the orignal frames, you get an anaglyph that remains stable when viewed.
Fortunately, there is an easy way to retain full colour information in a 3D image, which is to use a stereo pair.
The image on the left is to be viewed by your right eye, and the one on the right by your left eye. If you cross your eyes until the two images overlap, and concentrate on the middle one, you should be able to focus on a 3D image.
I tried adding sound emulation back in, but the timing has confused me once more. If you can help, I've posted the relevant thread here.