Linux Powers Four-Wall 3-D Display

Replacing a visualization supercomputing platform driving a four-wall immersive display system with a PC commodity cluster.
System Implementation

A variety of software is used to deal with the active stereo and scene synchronization needs of the cluster. The commodity cluster was built upon a standard installation of Red Hat Linux 7.2, and the kernel was then patched to include the Real-Time Application Interface (RTAI), which allows for low latency and task completion timing.

SoftGenLock and RTAI are used in concert to provide a software active stereo solution. The RTAI kernel module detects the vertical refresh of the monitor and changes a pointer in the video card memory that tells the video card what to draw on the screen. A double-sized buffer is provided to the application by specifying a virtual frame buffer in XFree86, which is twice the size of the actual frame buffer being drawn to the screen. For example, if displaying at a resolution of 1024 × 768 in active stereo, the virtual desktop would need to be at a resolution of 2048 × 768. The RTAI kernel module splits the frame buffer in half and alternatingly displays the scene in 1024 × 768 pieces. An application draws in stereo by rendering to the right and left sides of the X frame buffer for the right and left eye.

Genlock/data lock is achieved by synchronizing the machines in the cluster over the parallel ports. The RTAI kernel module writes to one pin on the parallel port and reads from another to make sure that the other machines in the cluster have completed a frame. The master tells all the other nodes in the cluster when to draw, and the other nodes in the cluster report back when they are ready for a new frame. Data lock is achieved by making sure that the slave machines in the cluster are on the correct eye when the parallel port is in a certain state.

SoftGenLock does not synchronize applications between the nodes of a cluster; it provides data lock and stereo. It is the responsibility of the application to synchronize the viewing frustum and animations in the application. Lastly, since SoftGenLock only uses VGA registers, it potentially can work with any graphics card.

When installing a PC cluster, a lot of issues must be dealt with before setup. Here are the important points and some recommendations on how to set up the system properly:

  • Be sure to have adequate HVAC, power and network access where the cluster will be set up.

  • Set up the cluster and display system in separate rooms, because noise levels from fans, drives, etc., may be distracting.

  • Prepare to deal with a lot of issues associated with laying down the cables. The time taken at this stage to label and check the cabling for proper size and lengths will save time later. This will alleviate problems with signal degradation and simply losing a cable in the “nest”.

  • Create a master power switch to turn off all units at once.

We constructed and installed a three-wall cluster in less than two weeks. This cluster was configured to use active stereo and has the ability to demonstrate swap locking and data locking.

Testing and Evaluation

We tested the PC commodity cluster by implementing a simple visualization software system. We utilized an interoperable software architecture, VR Juggler, which provides a set of programming abstractions for interfacing with a variety of display, tracking and computing systems and a variety of interaction devices. The software, which is used for visualizing terrain information, has a variety of display modes including stereo. It can be recompiled to work on different computing architectures and reconfigured at execution time by including different default device configuration files.

The application integrated smoothly with the cluster system and performed better than expected. The quality of the displays and stereo viewing was comparable to the same software running on three walls using an SGI Onyx 2 with IR2 graphics. However, the cluster was able to visualize the data with better performance. Figure 3 shows the cluster software running in the four-wall display system on the three side walls.

Figure 3. A Test Application Driving a Four-Wall Display

Summary and Conclusions

The use of a graphics PC cluster is now becoming a viable low-cost alternative to the use of single large visualization supercomputers. The PCs and related video hardware are fairly cheap, computationally powerful and flexible. There also is an abundance of software available for them, such as VR Juggler, which allow for applications to interface with a variety of display systems and interaction devices easily. Our cluster experiment explored the feasibility of phasing out and replacing existing expensive single large computing hardware. The results show we can make this kind of transition in the near future, and we believe our experiences will be motivation for others to follow suit.

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