I've looked into it, and from what I can tell, the "3D was added late to the Saturn design" narrative is flawed.
It's commonly cited that VDP2 was added later to give it 3D support. But VDP2 doesn't do 3D at all, it's responsible for the SNES "mode 7" style background layers. If you remove VDP2 (and ignore the fact that VDP is responsible for video scanout) then the resulting console can still do both 3D just fine (Many 3D games leave VDP2 almost completely unused). 2D game would take a bit of a quality hit as they would have to render the background with hundreds of sprites.
If you instead removed VDP1, then all you have left are VDP2's 2D background layers. You don't have 3D and you can't put any sprites on the screen so it's basically useless at 2D games too.
As far as I can tell, the Saturn was always meant to have both VDP1 and VDP2. They were designed together to work in tandem. And I think the intention (from SEGA JP) was always for the design be a 2D powerhouse with some limited 3D capabilities, as we saw on the final design.
I'm not saying there wasn't arguments between SEGA JP and SEGA US. There seems to be plenty of evidence of that. But I don't think they munged the JP and US designs together at the last moment. And the PSX can't have had any influence on the argument, as the Saturn beat the PSX to market in Japan by 12 days.
This is typical of Sega arcade hardware of the era (Model 1 and Model 2); these systems have separate "geometry processor" and "rasterizer" boards, with onboard DSPs. If you squint, the Saturn is what someone might come up with as a cost-optimized version of that architecture.
Yeah. Especially if you rip out VDP2 and some of the more 2D orientated features of VDP1 it looks very much like an attempt at a cost-optimised Model 2.
And if you read the interview that ndiddy linked above (https://mdshock.com/2020/06/16/hideki-sato-discussing-the-se...), it might be accurate to say the Saturn design is what you get when an engineer with no background in 3D graphics (and unable to steal sufficient 3D graphics experience from the Sega Arcade division) attempts to create a cost-optimised Model 2.
I suspect the first pass was just a cost-optimised textured quad renderer, then Sato went back to his 2D graphics experience and extended it into a powerful 2D design.
It's commonly cited that VDP2 was added later to give it 3D support. But VDP2 doesn't do 3D at all, it's responsible for the SNES "mode 7" style background layers. If you remove VDP2 (and ignore the fact that VDP is responsible for video scanout) then the resulting console can still do both 3D just fine (Many 3D games leave VDP2 almost completely unused). 2D game would take a bit of a quality hit as they would have to render the background with hundreds of sprites.
If you instead removed VDP1, then all you have left are VDP2's 2D background layers. You don't have 3D and you can't put any sprites on the screen so it's basically useless at 2D games too.
As far as I can tell, the Saturn was always meant to have both VDP1 and VDP2. They were designed together to work in tandem. And I think the intention (from SEGA JP) was always for the design be a 2D powerhouse with some limited 3D capabilities, as we saw on the final design.
I'm not saying there wasn't arguments between SEGA JP and SEGA US. There seems to be plenty of evidence of that. But I don't think they munged the JP and US designs together at the last moment. And the PSX can't have had any influence on the argument, as the Saturn beat the PSX to market in Japan by 12 days.