Scan Line Interleave

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Scheme of operation

Scan Line Interleave ( SLI ) is a method used in 3D computer graphics to distribute the load of computing work over several graphics chips . This process was originally developed by 3dfx in 1996 to increase the performance of graphics cards from the Voodoo series (especially in early 1998 for the Voodoo 2). Scan Line Interleave is the product name and the name of the actual process.

functionality

VGA loop cable from a Voodoo 1 card
Two single Voodoo2 cards connected with an SLI cable
Graphics card from Quantum3D with two Voodoo 2 in SLI

Usually two graphics cards or two graphics processors are connected to one another, one card or chip calculates all even screen lines (scan lines), the other all odd ones. Theoretically, this doubles the performance, but both cards or both chips must be supplied with geometry and texture information. This slows down the calculation, on the other hand, after the half-line image has been rendered, only half of the data has to be written into the frame or back buffer or read into the RAMDAC . The Voodoo 2 systems with Pentium II processor at the time achieved an increase in performance of around 40 to 60% - the resolution could be increased from 800 × 600 to 1024 × 768 with the same frame rate.

In the solution with two separate 3D accelerators, these are connected via a small ribbon cable, the RAMDAC of the first graphics card (to which the VGA monitor is connected) combines the two images and sends them to the monitor.

Since the Voodoo 2 could only calculate 3D graphics, a third 2D graphics card must still be available for normal desktop operation, which is connected to the first 3D graphics card with a VGA loop cable. In 2D operation, the 3D graphics card only loops through the signal.

With the Voodoo 5 series, 3dfx has also installed several SLI-compatible graphics chips ( VSA-100 ) on one graphics card.

Graphics chips with SLI support

The following graphics chips support SLI:

  • Voodoo Graphics (there were only cards from Quantum3D that could be connected later, all others were limited to single cards)
  • Voodoo 2 (with this graphics chipset, SLI could be used by private users for the first time)
  • Voodoo 3 (but there were only a few prototypes of Quantum3D that contained four chips on one PCB )
  • VSA-100 (there were cards with up to eight graphics chips on one PCB and a total of 512 MB video RAM)

With the Voodoo 2, two cards from the same manufacturer were always required at the beginning (with the exception of the Wicked3D driver). The Voodoo 2 from different manufacturers also worked together with later reference drivers. The only requirement for this was equipping with the same amount of VRAM . With Voodoo Graphics and VSA-100, SLI was only possible with graphics cards that were already equipped with two or more SLI-compatible graphics chips.

SLI from NVIDIA

The Scalable Link Interface Technology (SLI) introduced by Nvidia in 2004 , which also allows paired operation of 3D graphics cards for the purpose of load distribution, is not based on the scan line interleave method, even if the use of the suggests the same abbreviation.