Time-multiplexed autostereoscopic camera system

Presented at SPIE Symposium on Stereoscopic Displays and Applications VIII, San Jose, California, 11-13 February, 1997.

Published in Proc. SPIE, vol. 3012, pp. 72-83.

Abstract

A camera system has been developed at the University of Cambridge to provide live 3D video input to our time-multiplexed autostereoscopic display. The system is capable of taking video input from up to sixteen sources and multiplexing these into a single video output stream with a pixel rate an order of magnitude faster than the individual input streams. Testing of the system with eight cameras and a Cambridge autostereo display has produced excellent live autostereoscopic video. This paper describes the camera system and the challenges involved in electronically aligning and synchronising the multiple inputs.

The time-multiplexed autostereoscopic display developed at the University of Cambridge is based on a fast CRT with an additional active optical element to direct each image on the CRT to one of several abutting zones in front of the display. Originally demonstrated in 1991, the display has undergone considerable development over the past five years.

The input to the display is a simple extension to conventional video. Standard video, horizontal sync and vertical sync signals are provided at high speed. Adjacent views are placed in sequential fields. An additional azimuth sync (Z-sync) signal is required to indicate to the display which fields contain images for the left-most view.

The main image source for the display has been an off-the-shelf graphics card installed in an IBM PC clone. This has produced monochrome 3D video with eight views at VGA resolution or sixteen views at half VGA resolution. A colour version of the display, developed in 1994, can be driven with six views at half VGA resolution. Colour is obtaining using a colour-sequential method, and provides 24-bit colour rendition.

The display is in the process of being commercialised. Potential uses include visualisation, entertainment and remote manipulation. For the latter, live 3D video input is required. This requirement led to the development of the prototype autostereoscopic camera system described in this paper.

The basic operation of the camera system is to digitise multiple input video streams, one for each view direction, and to multiplex these into a single autostereoscopic video stream. A simple circuit board (the camera board) can digitise, process and buffer the video input from a single video source. Several of these are connected together via a backplane to another circuit board (the multiplexer board) which contains all the circuitry necessary for generating the output video signal and the synchronisation signals and for controlling the rest of the system. The backplane distributes data, control signals and power between the boards.

An image processing chip on the camera board allows arbitrary down-sizing and windowing of the video input. This provides pixel-level control of the alignment of each input. Inputs must be aligned precisely with one another to prevent eye strain. In practice a combination of coarse physical alignment and fine electronic alignment proved very effective.

The other major challenge that needed to be met in the design of the system was ensuring that all components were correctly synchronised. The video sources are interlaced, as is the autostereo display. Alternate fields on the autostereo display are odd and even, which means that the video stream to the display must consist of an odd field from source one, followed by an even field from source two, then odd three, even four, odd five, and so forth. This requires careful control of the buffers on the camera boards. Additionally it is necessary that video sources and autostereo display be kept in precise synchronisation. Thus the time taken to read in a field from one video source must be equal to the time taken to output one field for every view on the autostereo display.

For monochromatic operation, all video data is held on the individual camera boards. The multiplexer board pulls the appropriate fields off each camera board in turn and runs than across the backplane into a D/A converter. Running at 144 MHz pixel rate, this system is capable of eight views at a resolution of 648 x 576.

The same camera boards are used for colour operation in conjunction with a modified multiplexer board. Significant buffering is required on the colour multiplexer board to allow operation with parallel colour input from the camera boards and sequential colour output to the autostereo display. The colour system runs at 110 MHz pixel rate, providing six view output at 384 x 288 resolution (quarter PAL).

Subjectively, the camera system and display give a good 3D effect. This prototype is quite expensive and, to be commercially viable, several cost-reducing measures would need to be implemented. Nevertheless the system has proved the feasibility of time-multiplexed live 3D video.