3 - Vertical scrolling - I have no idea how it works and yes MUSHA uses it to a limited degree. I have seen games with horizontal line scrolling combined with the 8 pixel vertical scrolling to do a limited rotation effect, so I don't know about scrolling being locked to a horizontal line (but then again I am not an authority on how that was achieved). You can see this in quite a few later Genesis games like Gunstar Heroes, Adventures of Batman and Robin, Shinobi III (barely). Hell even Gynough/Wings of Wor used this for the BG(s) in a level and that was a fairly early game, come to remember.
What I mean is that on a H-sync interrupt, once you write a new value to the vertical or horizontal register of the VDP, that value takes place on the next scanline. So multiple writes would be useless as the last write would be the one taking effect for the next line. Charles MacDonald would know better than I on this one.
For the rotating effects - I believe Gunstar Heroes (rotating plane) and Contra (big rotating dancing mini Boss) use the equivalent effects as Cotton(PCE) first level boss. It's a tile swapping/building/shifting method, but the effect limits all tiles to 16colors for that layer. The larger the image and more complex effect, the more bandwidth to the VDP is required. The 68000 DMA to the VDP VRAM port is pretty fast(needed for the effect) - faster than the SNES DMA.
4 - Genesis multi-plane scrolling - This isn't in response to anything, but I'm pretty sure the Genesis can, for example, have layer 1 as the backmost BG layer, then layer 2 overlapping in front of it, and then layer 1 overlapping in front of layer 2, and then layer 2 overlapping in front of layer 1 again, etc etc etc just as long as layer 1 never overlaps itself horizontally and the same with layer 2. I think the 32X worked in a similar way. It imported the graphics from the Genesis and layed 32X graphics over that and then output. However I have seen 32X graphics be between Genesis layers which is pretty amazing considering the layering must be done in analog.
Yeah, you can set priorities between the BG layers. Thunder Force 4 does this for the first level(ocean level) on the clouds to make it look like there are more than 2 BG layers. The effect is pretty convincing until you try to scroll the fake BG layers vertically independently of the main BG - that's why all BG's move vertically in unison. The SNES has a lot of systems beat with up to 4 independent BG layers, but it handicapped with just 64k VRAM for all those layers.
Anyone know why'd they do this?
I'm not sure if this applies to this screen, but simply saying a system can do 64 colors does not imply just that. Infact, the genesis, turbo, snes can only do 16colors per 8x8 tile. The genesis has 4 16color palettes to chose from, the SNES has 8 16color palettes, and the PCE/TG/SGX has 16 16color palettes. Any pixel inside a tile can not access/use a color outside its 16color assigned palette. Technically, the genesis can display 64 colors, but in reality certain 60 or less color backgrounds are not possible on the genesis display with some sort of (limited) palette trick. This excluding shadow/highlight which is taxing to the sprite count/scanline limit or use of second layer BG.
Without some real technical skills it would be impossible to prove either way, but I think its safe to say they aren't real-time since there really wouldn't be any point in it.
I can say. I've looked at the spites and tiles of the ISO data track and all the polygons are pre-rendered - enemies and bosses. Kind of sad as these could have been nicely hand drawn art instead
