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Item A Rendering Algorithm for Discrete Volume Density Objects(Blackwell Science Ltd and the Eurographics Association, 1993) Blasi, Philippe; Le Saec, Bertrand; Schlick, ChristopheWe present a new algorithm for simulating the effect of light travelling through volume objects. Such objects (haze, fog, clouds.) are usually modelized by voxel grids which define their density distribution in a discrete tridimensional space. The method we propose is a two-pass Monte-Carlo ray-tracing algorithm that does not make any restrictive assumptions neither about the characteristics of the objects (both arbitrary density distributions and phase functions are allowed) nor about the physical phenomena included in the rendering process (multiple scattering is accounted for). The driving idea of the algorithm is to use the phase function for Monte-Carlo sampling, in order to modify the direction of the ray during scattering.Item A Survey of Shading and Reflectance Models(Blackwell Science Ltd and the Eurographics Association, 1994) Schlick, ChristopheSince the beginning of computer graphics, three decades ago, a large number of models intended to describe the behaviour of light on a given point of a surface have been proposed. Almost every author uses his own terminology and/or notation. To understand clearly the similarities and the differences between existing models, reformulating them with a unified notation is essential. This has been done by Hall in 1986. This paper is a new survey of shading and reflectance models, including the most recent models. Moreover, after the lengthy enumeration, some original models are proposed, which attempt to include interesting features of previous disjointed work into new formulations.Item An Inexpensive BRDF Model for Physically-based Rendering(Blackwell Science Ltd and the Eurographics Association, 1994) Schlick, ChristopheA new BRDF model is presented which can be viewed as an kind of intermediary model between empirism and theory. Main results of physics are observed (energy conservation, reciprocity rule, microfacet theory) and numerous phenomena involved in light reflection are accounted for, in a physically plausible way (incoherent and coherent reflection, spectrum modifications, anisotropy, self-shadowing, multiple surface and subsurface reflection, differences between homogeneous and heterogeneous materials). The model has been especially intended for computer graphics applications and therefore includes two main features: simplicity (a small number of intuitively understandable parameters controls the model) and efficiency (the formulation provides adequation to Monte-Carlo rendering techniques and/or hardware implementations).