Concept drawing for an immersive display consisting of a 360 degree display surface horizontally and 120 degree vertically.Top View
A 3D view with the same component colour coding is shown below, all the components are to scale.
The natural image projection is a spherical projection, the following test pattern is used in the above (interior surface) and will be used to illustrate warping and edge blending in what follows. Note this is a spherical projection of a polar map showing lines of latitude and longitude.
In order to see the type of image distortion required one can place a virtual camera in the position of each projector and with identical frustums (taking account of throw distance, aspect, offset, etc). The following is one of the cameras, the white lines frame each 90 section of the sphere (and the equator) and outline the region that each projector is required to cover. While the projected image can extend all the way to the top and bottom of the frame, some extra image is required on the left and right hand sides for edge-blending.
Each 90 degree section needs to be edge-blended with the sections illuminated by the neighbouring projectors. The edge blend mask is as follows, it is derived by calculating the warped version of a spherical projection with vertical edge-blend zones at +-45 degree intervals. The image projected is multiplied by this mask, the exact transition across the edge-blend zone needs to take into account the gamma for each r,g,b channel.
The final projected image with edge blending that will give rise to the test pattern of figure 2 is as follows. Note that a 16:9 aspect (HD) projector has been used, that aspect ratio is a better fit to spherical warping than 4:3.
The design is compatible with stereoscopic projection since the usual problematic areas at the upper and lower pole are not projected.
In the current proposed configuration each projector would be 1920x1080 and on average (about the equator) this would result is about 4300 pixels around the sphere. This is about 3mm square pixels viewed from 2m, or about 5 arc minutes. This is roughly the same as a 2m wide flat screen illuminated with a XGA projector and viewed from 2.3m away. The resolution increases in the upper half of the sphere and decreases in the lower half.
The exact dimensions of the sphere are not important, the entire system scales with sphere radius.
Going to 6 projectors swings the advantage back to 4:3 aspect projectors.
Spherical map: Angkor, Cambodia.|
Courtesy Sarah Kenderdine, Museum Victoria
Each of the 4 projected images with the edge blending added are shown on the right.
A three projector solution is proposed as a trade-off between the final resolution on the dome and the overall system complexity. Following based upon the InFocus IN5108, an SXGA+ (1400x1050) projector that supports vertical lens shift (symmetric frustums simplify geometry correction and edge-blending). Throw is 1.8:1, aspect ratio 3x4 (projectors mounted sideways). Lower cost XGA (1024x768) projectors could be substituted for a lower resolution effect.! Estimated average of 10 pixels per degree. Three projectors can be achieved with a single head graphics card and a Matrox triple-head-2-go unit.
Improved coverage at the north pole can be achieved by raising the projectors above the midline and compensating by either using the horizontal lens shift and/or rotating the projectors downwards. This is at the expense of less coverage at the south pole.
Mouse HMD PDF poster