08-16-2017, 10:01 PM
We analyze the feedback loop in Tele-Graffiti, a cameraprojectorbased remote sketching system which we recentlydeveloped. We derive the gain through the feedback loopand the final images that will be viewed by the users of thesystem. We then derive the optimal gain as the gain thatresults in the final viewed images being as close as possibleto the sum of the images actually drawn on the paper. Wealso propose a way of using the projector to compensate forinsufficient ambient light and describe how our analysis isaffected by: (1) automatic gain control (AGC) and (2) colorimagery. We end by proposing an algorithm to decomposethe final viewed images into estimates of the drawn images.
1 Introduction
Systems combining video cameras and LCD projectors arebecoming more and more prevalent. The first such systemwas Pierre Wellner s Xerox DigitalDesk [Wellner, 1993].Other such systems include the University of North Carolina s Office of the Future [Raskar et al., 1998], INRIAGrenoble s MagicBoard [Hall et al., 1999], and YoichiSato s Augmented Desk [Sato et al., 2000]. Recently,there has been a growing body of work in the computer visionliterature aimed at using video cameras to improve theuse of projectors as display devices [Raskar and Beardsley,2001, Sukthankar et al., 2001a, Sukthankar et al., 2001b].Recently we have developed Tele-Graffiti, a cameraprojectorbased remote sketching system [Takao et al.,2003]. Tele-Graffiti allows two people to communicate remotelyvia hand-drawn sketches. What one person drawsat one site is captured using a video camera, transmitted tothe other site, and displayed there using an LCD projector.See Figure 1(a) for a schematic diagram of two Tele-Graffitisites connect by a network and Figure 1(b) for an image ofa real Tele-Graffiti system. See the accompanying movie telegraffiti-uidemo.mpg for an example of two people interactingusing Tele-Graffiti. Also, see [Takao et al., 2002]for larger versions of the figures in this paper.The combination of projectors and cameras in the samesystem can either be beneficial or a hindrance, dependingon what you want to do. In hand-tracking systems for desktopuser-interfaces, the light projected by the projector canmake simple background subtraction algorithms inapplicable.Instead, several groups have used infra-red cameras forhand-tracking, a modality unaffected by the light radiatedby the projector [Sato et al., 2000, Leibe et al., 2000]. Onthe other hand, the projector can be used to project structuredlight, thereby allowing the estimation of 3D shape usingthe laser range finding principal. By carefully synchronizingthe camera and the projector, the structured lightcan even be time-multiplexed with the projected image tomake it imperceptible to humans [Raskar et al., 1998].A particular problem in certain applications is feedback.If the projector radiates light that is then imaged by the camera,and the resulting image then passed back to the projector,potentially after being processed in one way or another,there is the potential for feedback. It is possible that the projectedimage may get brighter and brighter until the camerasaturates resulting in an unusable system. Alternatively, thesystem might oscillate between two states. Finally, there isthe possibility of visual echoing [Takao et al., 2002]. Onthe other hand, feedback can be used to help the system, asis done in [Sukthankar et al., 2001a] for shadow removal.Camera-projector based communication systems are allsusceptible to feedback. When we first gave a demonstrationof Tele-Graffiti at ICCV [Takao et al., 2001], the systeminitially suffered from severe feedback, quickly saturated,and became unusable. At the time we added extralights as a quick fix . In this paper we study the cause ofthe feedback and propose a principled solution to it.In particular we present a steady-state analysis of thefeedback in Tele-Graffiti. Much of the analysis is applicableto other camera-projector systems. Starting with photometricmodels of the projector, the camera, and the paper usedto sketch on, we derive the gain of the system, the final statethat the system converges to, and the final image that willbe viewed by the users. Based on this result, we derive the optimal gain as the gain that results in the final viewedimage being as close as possible to the sum of the two original,non-feedback, images. Another common problem withLCD projector-based desktop applications is that the ambientlight is sometimes very weak compared to the projector.(This was the problem at the ICCV demo.) We propose away of using the projector to augment the ambient light inthe scene, and derive appropriate settings for this scheme.We also briefly discuss how our analysis is affected
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