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Technical Details of the Powerwall

Rendering images for a high-resolution rear-projection system is a demanding task. For generating a seamless image spanning several 4K projectors requires a high-quality screen and an appropriately sized compute cluster.

The powerwall at VISUS consists of the following parts:


Projection Screen

The image is projected from the back on a laminated glass screen. It has dimensions of about 6 × 2.25 metres. The size of a single pixel is about 0.55 mm, which allows for discering fine details and reading text. The pixel size is roughly the same as on a standard desktop monitor. The system show a total of 44 million pixels for each eye.

Channel separation for stereoscopic display is done using INFITEC. This method uses interference filters and can be considered an advanced version of the widely known red-cyan glasses. The glasses allow only specific and for each eye different wavelength to pass. The one of the other eye are blocked. As the parts visible for each eye are spread over the whole colour spectrum users can see almost normal colours compared to the simple red-cyan glasses. An advantage of INFITEC channel separation - and active shutters for time multiplexing - over systems using linear polarisation is that the user can move his head freely.

Projectors

The projection system consists of ten JVC DLA SH4K projectors originally developed for use in cinemas. Each of the projectors is capable to display images with a resolution of 4,096 x 2,400 pixels.

The devices are installed in portrait mode in order to minimise the number of overlapping areas. Every two projectors form a stereo pair projecting on the same area. The image therefore consists of five columns with four blending areas resulting in a total resolution of 10,800 x 2,400 pixels.

Rendering Cluster

Each of the porjectors requires input from four graphics cards. A set of ten dedicated display nodes consisting of ten machines with two graphics processors each is used to provide this input. A single machine therefore can drive one projector.

As the intended use of the projection system is interactive computer graphics and visualisation with at least 30 frames per second, more compute power is required to render complex scenes that the display nodes can provide. Additional 64 rendering nodes are used for that. Each of these prepares a small part of the image which is then delivered via a high speed interconnect to the display nodes.