Volume 31 (2012)

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https://diglib.eg.org/handle/10.2312/169

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Browsing Volume 31 (2012) by Subject "Color"

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    Accurate Fitting of Measured Reflectances Using a Shifted Gamma Micro-facet Distribution
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Bagher, Mohammad Mahdi; Soler, Cyril; Holzschuch, Nicolas; Fredo Durand and Diego Gutierrez
    Material models are essential to the production of photo-realistic images. Measured BRDFs provide accurate representation with complex visual appearance, but have larger storage cost. Analytical BRDFs such as Cook- Torrance provide a compact representation but fail to represent the effects we observe with measured appearance. Accurately fitting an analytical BRDF to measured data remains a challenging problem. In this paper we introduce the SGD micro-facet distribution for Cook-Torrance BRDF. This distribution accurately models the behavior of most materials. As a consequence, we accurately represent all measured BRDFs using a single lobe. Our fitting procedure is stable and robust, and does not require manual tweaking of the parameters.
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    Example-Based Fractured Appearance
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Glondu, Loeiz; Muguercia, Lien; Marchal, Maud; Bosch, Carles; Rushmeier, Holly; Dumont, Georges; Drettakis, George; Fredo Durand and Diego Gutierrez
    A common weathering effect is the appearance of cracks due to material fractures. Previous exemplar-based aging and weathering methods have either reused images or sought to replicate observed patterns exactly. We introduce a new approach to exemplar-based modeling that creates weathered patterns on synthetic objects by matching the statistics of fracture patterns in a photograph. We present a user study to determine which statistics are correlated to visual similarity and how they are perceived by the user. We then describe a revised physically-based fracture model capable of producing a wide range of crack patterns at interactive rates. We demonstrate how a Bayesian optimization method can determine the parameters of this model so it can produce a pattern with the same key statistics as an exemplar. Finally, we present results using our approach and various exemplars to produce a variety of fracture effects in synthetic renderings of complex environments. The speed of the fracture simulation allows interactive previews of the fractured results and its application on large scale environments.
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    ISHair: Importance Sampling for Hair Scattering
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Ou, Jiawei; Xie, Feng; Krishnamachari, Parashar; Pellacini, Fabio; Fredo Durand and Diego Gutierrez
    We present an importance sampling method for the bidirectional scattering distribution function (bsdf) of hair. Our method is based on the multi-lobe hair scattering model presented by Sadeghi et al. [SPJT10]. We reduce noise by drawing samples from a distribution that approximates the bsdf well. Our algorithm is efficient and easy to implement, since the sampling process requires only the evaluation of a few analytic functions, with no significant memory overhead or need for precomputation. We tested our method in a research raytracer and a production renderer based on micropolygon rasterization. We show significant improvements for rendering direct illumination using multiple importance sampling and for rendering indirect illumination using path tracing.
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    Parameterization-Aware MIP-Mapping
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Manson, Josiah; Schaefer, Scott; Fredo Durand and Diego Gutierrez
    We present a method of generating mipmaps that takes into account the distortions due to the parameterization of a surface. Existing algorithms for generating mipmaps assume that the texture is isometrically mapped to the surface and ignore the actual surface parameterization. Our method correctly downsamples warped textures by assigning texels weights proportional to their area on a surface. We also provide a least-squares approach to filtering over these warped domains that takes into account the postfilter used by the GPU. Our method improves texture filtering for most models but only modifies mipmap generation, requires no modification of art assets or rasterization algorithms, and does not affect run-time performance.
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    Pre-convolved Radiance Caching
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Scherzer, Daniel; Nguyen, Chuong; Ritschel, Tobias; Seidel, Hans-Peter; Fredo Durand and Diego Gutierrez
    The incident indirect light over a range of image pixels is often coherent. Two common approaches to exploit this inter-pixel coherence to improve rendering performance are Irradiance Caching and Radiance Caching. Both compute incident indirect light only for a small subset of pixels (the cache), and later interpolate between pixels. Irradiance Caching uses scalar values that can be interpolated efficiently, but cannot account for shading variations caused by normal and reflectance variation between cache items. Radiance Caching maintains directional information, e. g., to allow highlights between cache items, but at the cost of storing and evaluating a Spherical Harmonics (SH) function per pixel. The arithmetic and bandwidth cost for this evaluation is linear in the number of coefficients and can be substantial. In this paper, we propose a method to replace it by an efficient per-cache item pre-filtering based on MIP maps - such as previously done for environment maps - leading to a single constant-time lookup per pixel. Additionally, per-cache item geometry statistics stored in distance-MIP maps are used to improve the quality of each pixel's lookup. Our approximate interactive global illumination approach is an order of magnitude faster than Radiance Caching with Phong BRDFs and can be combined with Monte Carlo-raytracing, Point-based Global Illumination or Instant Radiosity.
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    A Statistical Method for SVBRDF Approximation from Video Sequences in General Lighting Conditions
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Palma, Gianpaolo; Callieri, Marco; Dellepiane, Matteo; Scopigno, Roberto; Fredo Durand and Diego Gutierrez
    We present a statistical method for the estimation of the Spatially Varying Bidirectional Reflectance Distribution Function (SVBRDF) of an object with complex geometry, starting from video sequences acquired with fixed but general lighting conditions. The aim of this work is to define a method that simplifies the acquisition phase of the object surface appearance and allows to reconstruct an approximated SVBRDF. The final output is suitable to be used with a 3D model of the object to obtain accurate and photo-realistic renderings. The method is composed by three steps: the approximation of the environment map of the acquisition scene, using the same object as a probe; the estimation of the diffuse color of the object; the estimation of the specular components of the main materials of the object, by using a Phong model. All the steps are based on statistical analysis of the color samples projected by the video sequences on the surface of the object. Although the method presents some limitations, the trade-off between the easiness of acquisition and the obtained results makes it useful for practical applications.