This article is presented by:
Naoya Takao
Jianbo Shi
Simon Baker

Tele-Graffiti: A Camera-Projector Based Remote Sketching System
with Hand-Based User Interface and Automatic Session Summarization

Abstract
One way to build a remote sketching system is to use a video camera to image what each user draws at their site, transmit the video to the other sites, and display it there using an LCD projector. Such camera-projector based remote sketching systems date back to Paul Wellner’s (largely unimplemented) Xerox Double DigitalDesk. To make such a system usable, however, the users have to be able to move the paper on which they are drawing, they have to be able to interact with the system using a convenient interface, and sketching sessions must be stored in a compact format so that they can be replayed later. We have recently developed Tele-Graffiti, a remote sketching system with the following three features: (1) real-time paper tracking to allow the users to move their paper during system operation, (2) a hand based user interface, and (3) automatic session summarization and playback. In this paper, we describe the design, implementation, and performance of Tele-Graffiti. Keywords: Camera-projector based remote sketching systems, remote communication and collaboration, video compression and transmission, paper detection and tracking, hand-based user interfaces, automatic summarization, archiving, and playback.
Introduction
There are several ways of building a remote sketching system. One way is to use a tablet and a stylus to input the sketch, and a computer monitor to display the sketch at the remote site. Such systems have a number of disadvantages. Writing with a stylus on a glass tablet is unnatural compared to sketching with a regular pen and paper. Shading and other effects are harder to achieve. Changing color means using the computer to select a new color rather than using a different pen. Incorporating existing hard-copy documents such as a graded exam is impossible. Another way of building a remote sketching system is to use a video camera to image the sketch at one end, transmit the video to the other end, and display it there using an projector. See Figure 1 for a schematic diagram of such a system. The first such camera-projector based remote sketching system was Pierre Wellner’s Xerox “Double DigitalDesk” [Wellner, 1993]. Since 1993 systems combining video cameras and projectors have become more and more prevalent. Besides the Xerox “DigitalDesk”, other such systems include the University of North Carolina’s “Office of the Future” [Raskar et al., 1998], INRIA Grenoble’s “MagicBoard” [Hall et al., 1999], and Yoichi Sato’s “Augmented Desk” [Sato et al., 2000]. A related projector system is Wolfgang Krueger’s “Responsive Workbench” [Krueger et al., 1995], used in Stanford University’s “ResponsiveWorkbench” project [Agrawala et al., 1997] and in Georgia Tech’s “Perceptive Workbench” [Leibe et al., 2000]. Recently, cameras and projectors have also been combined to develop smart displays [Sukthankar et al., 2001] with automatic keystone correction, laser pointer control, automatic alignment of multiple displays, and shadow elimination. Although this list is by no means comprehensive, it clearly demonstrates the growing interest in such systems.

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