A Qualitative and Quantitative Evaluation of 8 Clear Sky Models.

We provide a qualitative and quantitative evaluation of 8 clear sky models used in Computer Graphics. We compare the models with each other as well as with measurements and with a reference model from the physics community. After a short summary of the physics of the problem, we present the measurements and the reference model, and how we "invert" it to get the model parameters. We then give an overview of each CG model, and detail its scope, its algorithmic complexity, and its results using the same parameters as in the reference model. We also compare the models with a perceptual study. Our quantitative results confirm that the less simplifications and approximations are used to solve the physical equations, the more accurate are the results. We conclude with a discussion of the advantages and drawbacks of each model, and how to further improve their accuracy.

Size Determination of Porosity Inclusions in an Organic Solid Material by (1)H NMR Diffusion and SEM-FIB Experiments: The TATB Case.

An original approach has been presented to characterize the local geometry of pores containing protonated small molecule impurities in organic materials. It was here applied in TATB (1,3,5-triamino-2,4,6-trinitrobenzene) powder material to investigate the porosity able to enclose water molecules. The presence of such defects may have a significant impact on TATB-based compositions mechanical properties, efficiency, and shock sensitivity. Apparent self-diffusion coefficients measured on the low water signal residue were consistent with highly mobile species experiencing restricted diffusion in confined porosities. Applying the methodology commonly used for the characterization of porous systems, we could demonstrate for the first time that pores, with an average size in the 3-5 µm range, were already present in the native TATB powder before any formulation and that these pores were closed and filled with water/NH4Cl solutions. The presence of such macrocavities was further confirmed by analyzing the TATB 3D porous structure using the SEM-FIB dual-beam technique at the scale of particle fractions. Calculation of the pore volumes was performed to deduce pore size distributions assuming spherical cavities. A volume-weighted average size was calculated showing a satisfactory agreement with NMR results and a suitable complementarity of the two approaches.

A survey of nonlinear prefiltering methods for efficient and accurate surface shading.

Rendering a complex surface accurately and without aliasing requires the evaluation of an integral for each pixel, namely, a weighted average of the outgoing radiance over the pixel footprint on the surface. The outgoing radiance is itself given by a local illumination equation as a function of the incident radiance and of the surface properties. Computing all this numerically during rendering can be extremely costly. For efficiency, especially for real-time rendering, it is necessary to use precomputations. When the fine scale surface geometry, reflectance, and illumination properties are specified with maps on a coarse mesh (such as color maps, normal maps, horizon maps, or shadow maps), a frequently used simple idea is to prefilter each map linearly and separately. The averaged outgoing radiance, i.e., the average of the values given by the local illumination equation is then estimated by applying this equation to the averaged surface parameters. But this is really not accurate because this equation is nonlinear, due to self-occlusions, self-shadowing, nonlinear reflectance functions, etc. Some methods use more complex prefiltering algorithms to cope with these nonlinear effects. This paper is a survey of these methods. We start with a general presentation of the problem of prefiltering complex surfaces. We then present and classify the existing methods according to the approximations they make to tackle this difficult problem. Finally, an analysis of these methods allows us to highlight some generic tools to prefilter maps used in nonlinear functions, and to identify open issues to address the general problem.