Virtual Reality Hemispheric Dome - nimh-nif/SCNI_Toolbar GitHub Wiki
Background
One research interest of the SCNI is how the brain processes information during natural vision. Since the real world is dynamic, 3-dimensional, and all around us, an immersive virtual reality (VR) environment is a better simulation of the environment that the primate brain evolved in than traditional vision experiments provide (i.e. small, foveal, brief presentations of static 2D images). Since commercially available head-mounted displays designed for humans are difficult to modify for the smaller inter-pupillary distance of monkeys, we instead chose to use a VR dome in order to create an immersive experience for our subjects.
We designed our dome based on the designs of Australian 3D programmer Paul Bourke. This design has the benefit of being relatively compact and scalable to fit in our recording booths, and avoids many of the issues associated with fish-eye lens projection. The basic principle is to project images through a standard lens onto a spherical mirror placed inside the dome, which reflects onto the inner dome surface. The typical design places the front surface mirror at the feet of the viewer. However, since front surface mirrors are very delicate and our subjects can be very messy, we chose to invert the design: the mirror is instead suspended above the subject, and the truncated portion of the dome is on the upper half instead of the lower half.
Parts
1. Custom fiberglass hemispheric dome
We ordered a 6'4" diameter hemispheric fiberglass dome from Architectural Fiberglass Inc. (Model #A-139), which ships in 8 segments and must be assembled and finished. The four segments that made up the upper half of the dome were truncated at the line corresponding to +45 degrees elevation, per our customization request. On arrival, the segments were bolted together (3/8" - 2" grade 5 bolts), the interior surface was caulked, plastered and painted with silver paint developed specifically for polarized 3D projection (Goo Systems). Note that the finishing and painting requires some specialist equipment (for paint spraying and proper ventilation, etc.), and we therefore outsourced this step to a professional with experience in automotive body paint.
2. Front-surface Spherical Mirror
We ordered a 60cm diameter first surface half dome projection mirror from Acril Convex Mirrors based in Australia (although we've since learned that GoDome based in Texas supply a similar product). The mirror surface is coated in a very thin polymer to protect the delicate mirror, and the mirror should not be touched and cleaned only with air (no physical contact).
3. Optoma HD37 Projector
There are several criteria to consider when selecting a suitable projector for dome projection. These are outlined in more detail on Paul Bourke's website here. We wanted a projector with a minimum of 1920x1080 native resolution at 60Hz, and the capability of presenting stereoscopic 3D using passive polarizing filters. The HD37 from Optoma fits all of the criteria since it has a VESA sync port for synchronizing stereoscopic 3D hardware (see below).
4. Volfoni Smart Crystal Pro, 3D polarization modulator
In order to keep active electronics away from the neural recording electrodes, we prefer to use passive stereoscopic 3D-presentation methods. A polarization modulator is a device that is placed in front of the projector lens and rapidly alternates the orientation of polarization. A subject wearing polarizing glasses will only see each frame in one eye at a time, but the alternation between the two eyes occurs fast enough for this to be imperceptible. We chose the Smart Crystal Pro from Volfoni as representing teh best value for money, but alternatives are available from companies such as DepthQ, whose standard polarizer is offered by VPixx for use with their ProPixx projector system.