|
The number of display pixels required to present a holographic image can be reduced to a minimum determined by optical physics, but inevitably grows with the display size and angle of view The MIT holographic video system has recently been scaled up to provide images 85 mm high by 135 mm wide by ca. 200 mm deep, with an angle of view of 30=B0 and a vertical resolution of 144 lines. The minimum number ofpixels needed to support this display is 37.7 million (36 Mp), presented as 144 lines of 262 thousand (256 Kp). This large number of pixels presents considerable challenges for the interactive computation, communication and display of holographic video images. The presentation of such an image is made possible by optical parallelization of the display optics wherever possible. We use an 1 8-channel acousto-optical modulator, and a Fresnel-type moving mirror scanning system to overcome several practical limitations. The computation of such large holograms is speeded up by the elimination of as much information as possible from the hologram, notably its vertical parallax. Techniques of space discretization and table iookup reduce the computation time to a few seconds in most cases. Methods of "difftaction-specific' fringe computation and compression reduce the computation time even frirther. The use ofadaptive sub-sampling allows the lossless "compression" ofhologram data by a factor of two for speedy transmission and reconstruction at the display end of the channel. The general structure of the system includes an IBM Power Visualization System, which provides a 32-processor parallelized system for rapid compuation and compression of the holographic video pixels. A HiPPI link between the PVS and a specially-adapted digital image processing and frame store system (the Cheops system, designed for the Television of Tomorrow research program at MIT) allows transfer times of less than half a second. The output from eighteen synchronized 2-MB video memory circuits with arbitrarilylong line lengths are then converted to a 50-100 MHz signal suitable for the transducers for the acoustooptic modulators.
|
