The ability to merge a real video image (RVI) with a computer-generated image (CGI) enhances the usefulness of both. To go beyond ``cut and paste'' and chroma-keying, and merge the two images successfully, one must solve the problems of common viewing parameters, common visibility and common illumination. The results can be dubbed Computer Augmented Reality (CAR). We present in this paper techniques for approximating the common global illumination for RVIs and CGIs, assuming some elements of the scene geometry of the real world and common viewing parameters are known. Since the real image is a projection of the exact solution for the global illumination in the real world (done by nature), we approximate the global illumination of the merged image by making the RVI part of the solution to the common global illumination computation. The objects in the real scene are replaced by few boxes covering them; the image intensity of the RVI is used as the initial surface radiosity of the visible part of the boxes; the surface reflectance of the boxes is approximated by subtracting an estimate of the illuminant intensity based on the concept of ambient light; finally global illumination using a classic radiosity computation is used to render the surface of the CGIs with respect to their new environment and for calculating the amount of image intensity correction needed for surfaces of the real image. An example animation testing these techniques has been produced. Most of the geometric problems have been solved in a relatively ad hoc manner. The viewing parameters were extracted by interactively matching of the synthetic scene with the RVIs. The visibility is determined by the relative position of the ``blocks'' representing the real objects and the computer generated objects, and a moving computer generated light has been inserted. The results of the merging are encouraging, and would be effective for many applications.
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