Many headed video cards for AGP motherboards

Updated: 2005-03-22
Created: 2003-11

The requirements are more or less:

An important constraint is that there can only be one AGP slot per motherboard (slot, not card: there can be two AGP graphis cards, as long as one is integrated), and while there can be more than one PCI-Express slot per motherboard, most only have one.

But thanks to NVIDIA and their fashionable SLI products there are some with two x8 or x16 slots.

Dual x16 slot motherboard are designed for SLI, but SLI is not mandatory, this making it possible to put two full speed dual head graphics cards in without connecting them and obtaining a four head configuration with ordinary dual head graphics cards.

There are also motherboards with both a PCI-Express x16 slot and an AGP slot, even if it is an AGP slot but on the PCI, not an AGP, bus.

The other limitation is that there must be a nice Linux driver with 3D acceleration. Both ATI and NVIDIA seem to have fairly decent Linux drivers for their cards, and XiG does proprietary drivers for various other cards.

There are four levels of recent video card generations, that are usually labeled by the major release of DirectX (there is a similar set of OpenGL levels) that supports them:

Each generation tends to be a fair bit faster than the previous one when using just the features of the previous one. In Each generation there are usually three models, normal, LE/SE/VE/MX that is slower/cheaper and Pro/Ultra/XT that is faster/hotter/more expensive.

Note: I haven't really discussed performance much, as I assume that almost any recent card will do, at least for something as simple and small as a simulator's CAVE. But I have found some:

There are very few recent games that use pixel shaders at all, which take a lot of GPU power, to do sophisticated lighting effects on textures and shadows.

Multiple monitor support is a complicated issue, and even under Windows there are driver issues, especially with supporting 3D acceleration on all monitors. One review discusses some of the more common brands in this respect.

To get four screens the most straightforward possibilities (without considering any driver issues, that need to be checked in detail) are:

Single PC, one AGP card with four outputs
The Matrox G450 based card is probably too slow, and the Xentera GT4 is probably the fastest, but one would need to check the Linux driver support. Probably the standard ATI driver would work.
Single PC, one AGP card with three outputs
This means using the Matrox Parhelia(s), consumer version. The Parhelia Windows drivers can support 3D acceleration on all three displays
Then one can cover four displays by having one output cover both the front and upper window, pre-distorted so when it looks undistorted when viewed. Or else by using a slower PCI card to do independently the fourth display (e.g. the right side, in which almost nothing happens, as it is mostly occluded by the base of the boom).
Single PC, two cards (AGP/PCI or PCI/PCI) with two outputs each.
The primary difficulty is finding non-AGP video cards. I think it is instead fairly easy to support such a setup under X11 on Linux. I am pretty sure it is possible for 2D applications, less sure for 3D accelerated ones, in particular using DRI. It may be easier with the non-DRI driver from NVIDIA.
There are three types of PCI buses, PCI, PCI64 and PCI-X, and a fourth type has been announced PCI-Express, initially as a replacement for AGP, but also of PCI64 and PCI-X.
PCI is 32 bits wide and 33MHz, PCI64 64 bits wide and 66MHz (optionally) and PCI-X is twice as fast again as PCI64. There are PCI slots in every consumer PC, there are 1-2 PCI64 slots in many cheap (>=CA$300) server-level motherboards, and several motherboards already have PCI-X slots, but PCI-Express won't be around for a while yet.
PCI64 and PCI-X is usually put into server for storage bus cards (SCSI RAID etc.) or for Gigabit Ethernet. I discovered that one version of the Parhelia supports PCI64.
Two PCs, each with one AGP card with two outputs
Using two separate PCs might require some form of hardware or software lock, and these two ATI cards support both lock and stereo, and under Linux too, and are rather cheaper than the equivalent NVIDIA ones: