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foone @Foone
, 16 tweets, 3 min read Read on Twitter
Nice, a set of identical 3DFX Voodoo 2s, for SLI use!…
fun fact: 3dfx had SLI for using multiple cards at once, and NVIDIA has SLI for the same use, and NVIDIA bought the technology of 3DFX, right?

But it's a DIFFERENT SLI! it works in a completely different way.
it doesn't even stand for the same thing.
For 3dfx, SLI was "Scan-Line Interleave". The way it worked is that each card rendered at half-height and the two pictures were mixed together, using alternate lines.
NVIDIA's SLI is Scalable Link Interface. Instead of interleaving lines, it works in one of a few ways:
Split Frame Rendering, Alternate Frame Rendering, and SLI Antialiasing.
Split Frame Rendering is the simplest: It just splits the screen among cards (2-4).
So card 1 renders the top half, card 2 renders the bottom half.
Alternate Frame Rendering has each GPU render the full frame, but they're split in up in TIME.
So GPU 1 is rendering frame 1, and at the same time GPU 2 is rendering frame 2.
Then it starts showing frame 1 (then 2) while frames 3 and 4 are being rendered
SLI Antialiasing is interesting because it doesn't improve performance, but image quality.
Instead of splitting up rendering, it's double-rendered but with different anti-aliasing patterns, then merged in the final step.
So instead of the framerate going up, the framerate stays the same (or goes down a little) but the image quality goes up.
AMD, BTW, has their own version of this sort of multi-GPU rendering technique, called AMD CrossFireX. It works similarly, although there's less technical details I can find.
One difference from the old 3dfx SLI to the modern Nvidia SLI and AMD CrossFireX:
the 3dfx cards needed that little bridge connector to combine the two cards.

For Nvidia SLI & CrossFireX, bridge connectors help, but aren't required in all cases.
it's because PCI-e is a LOT faster than PCI and the two cards can just talk to each other over the bus at acceptable speeds, most of the time.
Higher-end configurations still benefit from the bridge connector, though.
a final fun fact: nVidia's Split Frame Rendering mode is smarter than just doing 50/50 split. It actually analyzes the scene and figures out how to balance the processing load.
The idea is that you might have a game that looks like this:
Most of the top half of the screen is just a skybox, and would render very fast. All the slow stuff is in the middle and bottom. A naive "even" split would do terribly on this scene.
So in this case you'd get something like 70/30 split instead of 50/50.
One difference between nVidia's SLI and AMD's CrossFireX is that for SLI you have to have matching GPUs, but AMD allows mixing different models (from the same line)
The example given on wikipedia is that you can use a Radeon 5830 with a Radeon 5870
Presumably this is just done by using something like the complex-splitting nVidia SLI uses: if you have two fast cards, they share 50/50.
If you have a fast card and a slow card, they share 60/40 or 70/30 or whatever is needed to maximize performance.
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