DAVE Hardware Setup

As discussed earlier, the CAVE technology provides the most immer-sive VR experience. A first CAVE in Braunschweig was developed in order to make that technology more affordable and thus more available by greatly reducing its cost. It is called DAVE, for Defini-tively Affordable Virtual Environment. Building on our knowledge from the first DAVE in Braunschweig, a second improved DAVE was

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constructed in Graz, also targeting continued research, education and eventually commercial applications.

The key hardware components of the current DAVE are eight render PCs and a master PC, whose graphics cards can be easily exchanged every few years, time interlaced stereo projectors that were modified to run in synchronization, a self made tracking system consisting of four infrared cameras connected to a PC and the screens held by a wooden frame.

Figure 2.19: Second DAVE setup: a four-sided CAVE.

Figure 2.20: A user in the DAVE.

Topology and Geometric Setup. We considered a few geometrical possibilities for the DAVE in Graz. A setup with both floor and ceiling projections requires a rear projections and thus a raised transparent floor. We decided against such a setup out of room and cost restric-tions. Also, without a ceiling projection it is easy to mount cameras for the tracking system. We eventually decided to once again use three rear projection side walls plus a floor projection from above.

A disadvantage is the restricted field of view, especially for content displayed above the user and for tall people standing upright.

Figure 2.21: The DAVE seen from behind the screen. The light beams were added for illustration pur-poses.

To maximize the DAVE size and still be able to use a part of the remaining room for a different setup, we use a different location of the projectors compared to the original CAVE and the first DAVE in Braunschweig, placing them just above the top center of the screens pointing outwards. They reflect their image on large coated mirrors back to the screen. Advantages are that the projectors are positioned rather close together, allowing short cables. They are attached to the wooden frame of the DAVE. Being up high, they are out of the way, do not receive a lot of dust and are not so much in risk of being touched while cleaning or during maintenance of the DAVE room.

Unfortunately, our projectors needed a lot of maintenance in the first years and demounting the projectors takes some time. With a new version being available, we upgraded the projector hardware which made problems less severe. The mirror for the floor projection is placed above the front screen, so that the users’ shadows are cast to the rear side where they are least noticed.

The DAVE is rotated by 45 degrees with respect to the room walls in

order to minimize the used space. Another design choice was to build a visually pleasing surrounding entrance, completely hiding the tech-nology and computers. While this is great for demonstrations, some parts of development and debugging are unnecessarily complicated.

Figure 2.22: First version of our stereo projector hardware, still with two projectors in separate housings.

Projector Issues. The projectors buffer images from unsynchro-nized video streams, thus not requiring expensive genlock graphics cards. The projectors are modified bydigitalImageto synchronize to a chosen master projector by slowing down the system clock of the slaves. The parameters have to be calibrated by the manufacturer and locking to the correct signal usually takes around four minutes.

This modification is also the reason why the communication to the projectors via the serial port is often corrupted.

A previous attempt to access the projectors via their LAN connections failed, as only some of the projectors reacted. Hardware and software was changed to use the serial port instead.

Another issue with the projectors is that they may loose some set-tings after a power cut, sometimes requiring the access of the system menu. This is only possible by pointing the remote control through the ventilation slots at the correct angle, only hitting one of the two projector units, typing a key combination as password with a fast timeout, while standing on a ladder. After a power cut, two hours must be scheduled for resetting the projectors. For the reproduction of colors, a lookup table is used. From time to time it may reset to a wrong default and the correct one must be reloaded. Other issues include lamps and contacts of connections. After a failed firmware update, an EPROM had to be replaced. A few times we had to send in a projector because of a broken transformer. Looking at the price for a projector or even the price for insured shipping, these experiences were rather cost intensive. Since mid, another updated version is available, developed in collaboration with the projector manufacturer.

We assume that most problems mentioned are solved in that product.

Figure 2.23: Debugging of stereo projector hardware in the second version. Only one lens and one lamp per screen are needed, but the housing still contains two projector mainboards.

Screen Material and Frame. Our first solution for gapless projection in the corners was a custom made welding of the material including a loop held by an aluminum pipe which is pulled by rubber straps (see figure below, left side). The front wall of our first screen was 1cm too wide, what we only noticed when everything was set up, so the screen needed to be replaced. To ease the setup and realize a more transportable version,digitalImagedeveloped a new frame with acrylic glass corner with a 45 degree phase (see figures below). Each projection screen is independent from the other ones. The setup of such a DAVE only takes one day.

projector light

projector light

projector light projector light

acrylic glass

observer observer

wooden frame wooden

frame

rubber strap pipe

Figure 2.24: Old design on the left, with a single large frame. New de-sign on the right, each side can easily be separated, e.g. for transport or maintenance.

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Figure 2.25: Left: With the old frame design, exchanging the screen ma-terial is a lot of effort. In this case, the front side was manufactured 1cm too wide and the whole screen had to be exchanged. Right: New frame design by digitalImage. Photo courtesy of Armin Zink.

Figure 2.26: An ipod touch is used for system control, realized with a web browser.

Screen Color and Material. The frontal floor projection screen is reflective and should match the color of the rear projection screen of the side walls to get a similar brightness and dark level. With our gray rear projection material this was not respected at first. A photometric calibration to a common range may mean to loose a lot of contrast and dynamic range. To solve this problem, we tested a few materials and replaced the floor material to match the rear projection screen as good as possible. Following Majumder et al. [MS05a], smooth intensity transitions allow to retain a higher brightness. However, the strongest problem is that the projection surfaces are not Lambertian, i.e. the intensity depends an the view angle to the surface. Even though the rear projection screens have a low gain factor of 0.8, they suffer from hotspot problems, especially for wide angle projection like with our 1:1 lens. To lessen this effect, large fresnel lenses could be mounted just behind the screen. Finally, a dynamic software attenuation controlled by head tracking could effectively hide the transitions. However, even with our setup, many visitors do not or hardly perceive these edges and must be stopped from walking into the screen.

Figure 2.27: An LED strip is used to highlight the small step into the DAVE to avoid injuries. The floor projection is protected against dirt and scratches by felt slippers.

As the screens for the side walls are tightened also on the bottom, the floor of the DAVE is raised to about 10cm above ground. To make the step clearly visible, we highlight it with stripes in the background image and with an additional LED illumination. The lights are not directly visible from the inside and do not disturb the users. We ask users to wear felt slippers in order to keep the floor clean. This is inconvenient, especially for elderly people who might need to sit down to put on the shoes. A minor positive side effect is that visitors may be more aware of the sensitive equipment and may behave in a more cautious way.

Mirrors. One problem we encountered with the mirrors is their nonplanarity. After loosening the tight mounting in their aluminum frame, it got a lot better. Still, this is a concern because our current soft-ware calibration can only correct linear distortions (see section 4.2.3.1).

Figure 2.28: The linear calibration is not correct along the complete edge.

Here, the corners are aligned but towards the center of the edge, the ef-fects of nonplanarity of our large mirrors become visible. In addition, the screen is not exactly planar either.

Floor Projection Clipping. In the DAVE, none of the rear projected images shine on the same surface. Only the floor projection is a front projection and may also shine on the side and front walls. The angle is very steep and thus the light is considerably dimmer. We mechanically adjust the projector so that the image accurately aligns with the front wall. We use hardware blend masks for both sides, cropping the original image with a 4:3 aspect ratio to the square shaped floor.

Figure 2.29: A wooden frame is suspended from the ceiling in front of the projector (left). Black cardboards can be adjusted to block unwanted light (center). Here, only the projector for the floor is turned on, demonstrating that no unwanted direct light reaches the front projection (right image, left side) and some still shines on the side wall (right image, right side).

A further and sharper reduction of unwanted direct light may be achieved with an additional software mask. It can easily be generated with a slight modification of the tool that we use to compute an undis-torted background image for the operating system (see section 4.2.3.1).

The image can be displayed on top of the image content, attenuating unwanted light. This is already supported by our frameworks (see section 4.2.3.4), but as this effect is rarely noticeable, we have not implemented this yet.

Figure 2.30: Lightweight shutter glasses with retroreflective markers for head tracking.

Stereoscopic Shutter Glasses. When the DAVE was build, we used relatively heavy and very expensive shutter glasses. Around

much cheaper and lighter glasses were available. We developed an exchangeable mounting of the reflective markers, so that glasses can be cleaned or swapped easily.

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Cheap Home Theater 3D Projectors. An interesting recent change is caused by the home cinema market, the now rapidly growing stereo sector. In around, the price for time interlaced 3D TV sets and projectors dropped dramatically, because consumer devices were introduced, thanks to another recent 3D movie hype. While resolution and brightness do not match high end professional projectors yet, they are orders of magnitudes cheaper and lead to a vaster availability of stereo hardware in public. As they synchronize on the video signal, multi projector setups require genlock graphics hardware or a multi headed graphics card with several outputs.

For a setup like the DAVE in Graz, prices are compared between the existing setup and a fictional one using the new home theater projec-tors, with prices from 2011. For enhanced resolution, we consider a setup with two tiled projections per wall for the fictional setup.

Professional Home Theater Number of projectors one per side two per side

Total number of pixels 5.5M 7.5M

Total brightness 7,500 AL 24,000 AL

Total price for projectors 75,000 EUR 8,000 EUR Total price for graphics cards 2,000 EUR 8,000 EUR Figure 2.31: Price calculation comparing a DAVE setup with professional 3D projectors and with home theater projectors.

It is clearly visible, that for a much brighter system and slightly higher resolution, the price is much lower. Including also the price of updat-ing graphics cards three times over the followupdat-ing years, the costs are still halved.

Comparison to an HMD. We once had the occasion to borrow an HMD for a week. We directly compared it to our DAVE for an in-door architecture application. Especially, visual immersion and size perception were of interest. The HMD was an eMagin Z800 3DVisor with 800x600 pixels per eye, a horizontal field of view of about 33 degrees and a weight of 227g. In the short time we did not manage to get the included inertial tracker to work but used the DAVE tracking instead. While the stereoscopic effect was visible, the impression was far inferior to the DAVE. The exact reasons for this are not known.

Speculations by HMD experts are a combination of imperfect optics, a small field of view, a larger influence of tracking latency and the vergence-accommodation conflict.

Brain Computer Interface. In collaboration with the Laboratory of Brain-Computer Interfaces at the Institute for Knowledge Discovery, TU Graz, we connected a brain computer interface (BCI) to the DAVE.

For psychological experiments, BCI signals were evaluated and sim-ple commands send to the DAVE server via a network connection, allowing limited navigation trough a scene (see section 5.1.1.5 and section 6.3.10.1).

Figure 2.32: The brain computer interface (BCI) and a test subject in the DAVE. Electrodes are placed on the head to measure electroencephalo-gram (EEG) signals. Recognizing previously learned patterns, the subject is able to navigate through a 3D scene in a very limited way.

In document Visual Computing in Virtual Environments (sider 23-29)