Department of Computer Science and Technology

Technical reports

Personal projected displays

Mark S. D. Ashdown

March 2004, 150 pages

This technical report is based on a dissertation submitted September 2003 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Churchill College.

DOI: 10.48456/tr-585


Since the inception of the personal computer, the interface presented to users has been defined by the monitor screen, keyboard, and mouse, and by the framework of the desktop metaphor. It is very different from a physical desktop which has a large horizontal surface, allows paper documents to be arranged, browsed, and annotated, and is controlled via continuous movements with both hands. The desktop metaphor will not scale to such a large display; the continuing profusion of paper, which is used as much as ever, attests to its unsurpassed affordances as a medium for manipulating documents; and despite its proven manual and cognitive benefits, two-handed input is still not used in computer interfaces.

I present a system called the Escritoire that uses a novel configuration of overlapping projectors to create a large desk display that fills the area of a conventional desk and also has a high resolution region in front of the user for precise work. The projectors need not be positioned exactly—the projected imagery is warped using standard 3D video hardware to compensate for rough projector positioning and oblique projection. Calibration involves computing planar homographies between the 2D co-ordinate spaces of the warped textures, projector framebuffers, desk, and input devices.

The video hardware can easily perform the necessary warping and achieves 30 frames per second for the dual-projector display. Oblique projection has proved to be a solution to the problem of occlusion common to front-projection systems. The combination of an electromagnetic digitizer and an ultrasonic pen allows simultaneous input with two hands. The pen for the non-dominant hand is simpler and coarser than that for the dominant hand, reflecting the differing roles of the hands in bimanual manipulation. I give a new algorithm for calibrating a pen, that uses piecewise linear interpolation between control points. I also give an algorithm to calibrate a wall display at distance using a device whose position and orientation are tracked in three dimensions.

The Escritoire software is divided into a client that exploits the video hardware and handles the input devices, and a server that processes events and stores all of the system state. Multiple clients can connect to a single server to support collaboration. Sheets of virtual paper on the Escritoire can be put in piles which can be browsed and reordered. As with physical paper this allows items to be arranged quickly and informally, avoiding the premature work required to add an item to a hierarchical file system. Another interface feature is pen traces, which allow remote users to gesture to each other. I report the results of tests with individuals and with pairs collaborating remotely. Collaborating participants found an audio channel and the shared desk surface much more useful than a video channel showing their faces.

The Escritoire is constructed from commodity components, and unlike multi-projector display walls its cost is feasible for an individual user and it fits into a normal office setting. It demonstrates a hardware configuration, calibration algorithm, graphics warping process, set of interface features, and distributed architecture that can make personal projected displays a reality.

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BibTeX record

  author =	 {Ashdown, Mark S. D.},
  title = 	 {{Personal projected displays}},
  year = 	 2004,
  month = 	 mar,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  doi = 	 {10.48456/tr-585},
  number = 	 {UCAM-CL-TR-585}