I just saw a video of it. Impressive. Were the enemy ships hand drawn 3D plastered on sprites? Or was there some actual realtime 3D rendered by the CPU? The boss ship I saw looked like realtime 3D.
There's no realtime 3D. This boss fight [1] looks super impressive, but the boss structure is FMV, your ship and bullets and stuff is all sprites overlaid on the FMV. (and the FMV is decoded to sprites too)
(Stated confidently to ensure a correction if I'm wrong)
I think there is realtime 3D used for the player and enemy "sprites" (really 3D objects), including the end of stage bosses. They are small and extremely simple, as this was the maximum the Genesis/Megadrive's 68k CPU could handle. The final boss fight is the exception.
Silpheed was amazing back in the day. It was one of the first titles that really showcased what CD drives could add to games in terms of cinematic experience. The live orchestral music gave it an epic feel and the mecha-design of the SA-77 is beautiful and modern even by today's standards.
I'd be very surprised if player and enemy graphics are realtime 3D. Those look very much like typical sprites where you use different sprites for different angles.
The player ship rotates, but it only is 'rendered' at a handful of angles. This is a very common technique, but here they've used an art style to make it look like realtime 3d.
It's like stun runner on Lynx [1]. The Arcade stun runner was polygon based; but the Lynx doesn't have a chance, so there's just a ton of sprite options for rotation and a lot of work to make it look the part [2].
I'm sure they are 3d models and are rendered using the CPU. You can check by entering the secret Options Mode and selecting the Polygon Test, which will allow you to freely rotate your ship and the enemies, including the end of level bosses (except the final one). Screenshot: https://www.vizzed.com/vizzedboard/retro/user_screenshots/sa...
Fabien seems to be unaware of this, by the way.
This is also why the game play is so bland. The number of 3d objects and their complexity was severely constrained and did not allow for varied or more sophisticated game elements.
(Stated confidently to ensure a correction if I'm wrong)
[1] https://www.youtube.com/watch?v=GMO_tdlUvnc