3dfx's Visit to AnandTech - Voodoo4/5 in Action

From a feature perspective, FSAA is just as easy to enable as 32-bit color in a game, and the difference in image quality can be quite noticeable, but what kind of performance hit are we talking about when it is enabled?

Unfortunately, basic mathematics applies to the performance hit you can expect from enabling FSAA.  Let’s take the Voodoo5 5500 AGP that was demonstrated for us as an example. 

According to 3dfx, depending on the final clock speed of the VSA-100 chip itself, the Voodoo5 5500 AGP will feature a fill rate of anywhere between 667 – 733 Megapixels/s.  For the sake of comparison, a Voodoo3 3500 (the fastest Voodoo3 available) has a fill rate of 183 Megapixels/s and a GeForce 256 SDR/DDR has a 480 Megapixels/s fill rate.  So right out of the box, the Voodoo5 5500 should be faster than any currently available card, but that’s without FSAA enabled.

Enabling 2 sample FSAA forces the VSA-100 to render each frame twice thus taking two samples, effectively cutting the fill rate in half.  So the 667 – 733 MP/s range has now been reduced to 333 – 366 MP/s.  Even at 2 sample FSAA, there are still some “jaggies,” which are noticeable but there is a definite improvement over having no FSAA at all. 

Can you guess what happens when you enable 4 sample FSAA?

Enabling 4 sample FSAA forces the VSA-100 to render each frame four times, thus taking four samples, which effectively reduces the fill rate to 25%.  That impressive 667 – 733 MP/s range is now at 166 – 183 MP/s, about the speed of a Voodoo3 3000 – 3500. 

For you Voodoo3 owners, you should be able to run at your current performance level but with 4 sample FSAA enabled.  Or if you want more performance, you could always shift down to 2 sample FSAA and get a nice performance boost over the Voodoo3 3500 while improving on the image quality, although not getting rid of all of the “jaggies.” 

Even the Voodoo5 6000 outfitted with four VSA-100 chips is reduced from its 1.33 – 1.47 Gigapixels/s fill rate down to a 333 – 366 Megapixels/s fill rate when using 4 sample FSAA.  There is no real way around this performance hit, but it’s something you should definitely be aware of when you look at the incredible fill rates boasted by the Voodoo5 line.

There is a definite difference between 2 and 4 sample FSAA, but it isn’t as big as the difference that exists between no FSAA and enabling the feature. 

Because of this performance hit, 3dfx isn’t enabling FSAA (or any T-Buffer effects for that matter) on the Voodoo4.  With the fill rate on the Voodoo4 4500 at 333 – 367 MP/s, 2 or 4 sample FSAA would degrade performance to the point where 3dfx would be going against their “fill rate is king” policy of the past.