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Even the prior two games had an extremely tasteful use of textures and geometry that just felt right and realistic enough to not break immersion despite the hardware limitations. This era of gaming rightfully and correctly recognized that any form of "realism" is ultimately an impression rather than an objective truth.

3dfx did it spatially - each card rendered half the scanlines and then interleaved them - hence Scan Line Interleave (SLI). The problem is that this assumes all the work is going into pushing polygons onto the framebuffer. This assumption broke fairly shortly after 3dfx started doing this and it's an assumption that only makes sense for 3dfx's obsession with fill rate.

Nvidia wound up doing basically everything temporally: one GPU renders one frame, the next GPU renders the next. The reason for this is that "modern" (as in, 2004-era, so OpenGL 2.x / DX9) games do a lot of intermediate work that has to be available during final raster on both cards. They might render a bunch of depth buffers from various light positions to construct shadow maps. Or they might accumulate all raster operations into a bunch of color framebuffers and then compute lighting with the information in those buffers. Or you might just have a post-process gaussian blur that requires mixing pixels from across both GPUs.

Theoretically, none of this is a problem - you just send the pixels the other GPU needs. In practice even the fastest Nvidia SLI bridge only ran at something like 3.25GB/s and that's not even enough bandwidth to deliver a final 4K 120FPS image, much less transfer every intermediate result from every rendering step to both GPUs. The math gets worse when you start adding more GPUs. It's far easier to send a whole frame to each card, but this adds latency and stuttering[0] because you have to run the game two, three, or even FOUR frames ahead instead of just one.

That being said, there was a way to deliver multi-card rendering on Nvidia hardware, it's just that Nvidia had no interest in shipping Nvlink support in a consumer card because they wanted AI customers to pay more for it. So instead they washed their hands of it and made multi-card rendering an explicit API thing that nobody implements because <1% of people are going to spend $10k on a rig for slightly better framerates. AFAIK if you DO use multi-card rendering this way, you're doing all the synchronization across the main PCIe bus (which is actually more bandwidth than SLI ever had).

[0] Which, funnily enough, is the same problem with DLSS Framegen. Nvidia really, really likes building things that make the FPS number go up without actually delivering a better experience.