Kun Xu, Wei-Lun Sun, Zhao Dong, Dan-Yong Zhao, Run-Dong Wu, Shi-Min Hu
Tsinghua University; Cornell University
Spherical Gaussians (SGs) in Graphics; Other Spherical Functions used in PRT; Directional Statistics;Anisotropic Appearance
Bui Tuong Phong;
University of Utah
The quality of computer generated images of three-dimensional scenes depends on the shading technique used to paint the objects on the cathode-ray tube screen. The shading algorithm itself depends in part on the method for modeling the object, which also determines the hidden surface algorithm. The various methods of object modeling, shading, and hidden surface removal are thus strongly interconnected. Several shading techniques corresponding to different methods of object modeling and the related hidden surface algorithms are presented here. Human visual perception and the fundamental laws of optics are considered in the development of a shading rule that provides better quality and increased realism in generated images.
James F. Blinn
University of Utah
In the production of computer generated pictures of three dimensional objects, one stage of the calculation is the determination of the intensity of a given object once its visibility has been established. This is typically done by modelling the surface as a perfect diffuser, sometimes with a specular component added for the simulation of hilights. This paper presents a more accurate function for the generation of hilights which is based on some experimental measurements of how light reflects from real surfaces. It differs from previous models in that the intensity of the hilight changes with the direction of the light source. Also the position and shape of the hilights is somewhat different from that generated by simpler models. Finally, the hilight function generates different results when simulating metallic vs. nonmetallic surfaces. Many of the effects so generated are somewhat subtle and are apparent only during movie sequences. Some representative still frames from such movies are included.
Pat Hanrahan; Wolfgang Krueger;
Princeton University; German National Research Center for Computer Science
The reflection of light from most materials consists of two major terms: the specular and the diffuse. Specular reflection may be modeled from first principles by considering a rough surface consisting of perfect reflectors, or micro-facets. Diffuse reflection is generally considered to result from multiple scattering either from a rough surface or from within a layer near the surface. Accounting for diffuse reflection by Lambert’s Cosine Law, as is universally done in computer graphics, is not a physical theory based on first principles. This paper presents a model for subsurface scattering in layered surfaces in terms of one-dimensional linear transport theory. We derive explicit formulas for backscattering and transmission that can be directly incorporated in most rendering systems, and a general Monte Carlo method that is easily added to a ray tracer. This model is particularly appropriate for common layered materials appearing in nature, such as biological tissues (e.g. skin, leaves, etc.) or inorganic materials (e.g. snow, sand, paint, varnished or dusty surfaces). As an application of the model, we simulate the appearance of a face and a cluster of leaves from experimental data describing their layer properties.
Gladimir V. G. Baranoski, Aravind Krishnaswamy
University of Waterloo; Adobe Systems Inc.
Recent research efforts in image synthesis have been directed toward the rendering of believable and predictable images of biological materials. This course addresses an important topic in this area, namely the predictive simulation of skin's appearance. The modeling approaches, algorithms and data examined during this course can be also applied to the rendering of other organic materials such as hair and ocular tissues.
Craig Donner, Henrik Wann Jensen
University of California, San Diego
This paper introduces a shading model for light diffusion in multi-layered translucent materials. Previous work on diffusion in translucent materials has assumed smooth semi-infinite homogeneous materials and solved for the scattering of light using a dipole diffusion approximation. This approximation breaks down in the case of thin translucent slabs and multi-layered materials. We present a new efficient technique based on multiple dipoles to account for diffusion in thin slabs. We enhance this multipole theory to account for mismatching indices of refraction at the top and bottom of of translucent slabs, and to model the effects of rough surfaces. To model multiple layers, we extend this single slab theory by convolving the diffusion profiles of the individual slabs. We account for multiple scattering between slabs by using a variant of Kubelka-Munk theory in frequency space. Our results demonstrate diffusion of light in thin slabs and multi-layered materials such as paint, paper, and human skin.
Craig Donner, Henrik Wann Jensen
University of California, San Diego
We have developed an analytical spectral shading model for human skin. Our model accounts for both subsurface and surface scattering. To simulate the interaction of light with human skin, we have narrowed the number of necessary parameters down to just four, controlling the amount of oil, melanin, and hemoglobin, which makes it possible to match specific skin types. Using these physicallybased parameters we generate custom spectral diffusion profiles for a two-layer skin model (shown in the top left figure) that account for subsurface scattering within the skin. We use the diffusion profiles in combination with a Torrance-Sparrow model for surface scattering to simulate the reflectance of the specific skin type.
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