4-Projector Display under Ubuntu 11.04

For my tabletop interaction research project, I need to create a large extended display with 4 projectors in a 2x2 configuration. I could not make all 4 projectors to display at the same time with the original 2 Nvidia GeForce 8600GT graphics cards. Turns out it is especially hard to do this, if not impossible. Also, the Nvidia cards are kind of old, so I switched to a new graphics card, ATI Radeon HD 6870 with two DVI ports, 2 Mini DisplayPorts and 1 HDMI port.

After connecting the projectors to the 2 DVI ports and 2 Mini DisplayPorts, all 4 projectors can display now, however the displayed areas are not well-aligned as before. Specifically, the two displays from the projectors connected to the DVI ports are smaller with black area around them.

The problem is with underscan. The Catalyst Control Center does have the underscan and overscan option for HDMI but it doesn't always show up. It is supposed to be under "Display Manager" -> <a particular projector/monitor> -> Adjustment, but the "Adjustment" tab does not always appear. For me, it appears for one projector and I changed the scaling to 0% underscan. This did expand the displayed area for that projector. But I couldn't do the same for the other projector.

Finally, what worked is using the command line to disable underscan:

sudo aticonfig --set-pcs-val=MCIL,DigitalHDTVDefaultUnderscan,0

DLP vs LCD Projectors

I'm using 4 DLP projectors (Dell 5100MP) for my tabletop display system and so far they are giving me a lot of troubles. First, it produces rainbow artifacts when the display is seen through a camera. Now, it produces significant flickering when projecting through an optical shutter. The LCD projector does not produce such artifacts. It seems like we should have gone for the LCD projectors at the very beginning. I think by understanding better how each of them works will allow us to make better decisions in the future and also for the current research.

LCD Projector
LCD (liquid crystal display) projectors contain three separate LCD glass panels, one each for the red, green, and blue components of the video signal. Each LCD panel contains thousands (or millions) of liquid crystals that can be aligned in either open, closed, or partially closed positions to allow light to pass through. The alignment is controlled by the electric field between the two transparent electrodes on the two sides of the liquid crystal molecules. The projector typically sends light through special dichroic filter / reflector assemblies called "dichroic mirror" that separates light to the three LCD panels. Wikipedia has a nice explanation and a diagram on how it works.

DLP Projector
DLP (Digital Light Processing) projectors use a DLP chip with reflective surface made up of millions of tiny mirrors. To define color, a color wheel is used which contains a red, green and blue filter. This wheel spins in the light path between the lamp and the DLP chip and alternates the color of the light hitting the chip. The mirrors tilt away form or into the lens path based upon how much each color is required for each pixel. In the most expensive DLP projectors, there are three separate DLP chips, one each for the red, green and blue channels. They are usually over $10,000 and do not use a color wheel.

Rainbow Artifacts and Flickering
The wheel based DLP projectors produce rainbow artifacts because of the sequential color updating from the wheel. As the color wheel spins, the image on the screen is either red, green or blue at any given instant in time. Since LCD projectors and 3-chip DLP projectors always deliver a constant red, green, and blue image simultaneously, they do not create rainbow artifacts.

Under a switching optical shutter, flickering occurs with the 1-chip DLP projector because the period of the rotating wheel does not match with the period of the optical shutter.

Source: http://www.projectorcentral.com/lcd_dlp_comparison.htm