A hybrid data-driven BSDF model to predict light transmission trough complex fenestration systems including high incident directions

The transmission and distribution of light through Complex Fenestration Systems (CFS) impacts visual comfort, solar gains and the overall energy performance of buildings. For most fenestration, scattering of light can be approximated as the optical property of a thin surface, the Bidirectional Scattering Distribution Function (BSDF). It is modelled in simulation software to replicate the optical behaviour of materials and surface finishes. Data-driven BSDF models are a generic means to model the irregular scattering by CFS employing measured or computed data-sets. While measurements are preferred by researchers aiming at realism, they are constraint by the measurement geometries of the employed instrumentation. Particularly for large samples prevailing in the field of building sciences, measurements of the BSDF for directions close to grazing are impacted by shadowing and edge effects. Reliable extrapolation techniques are not available due to the irregularity of the BSDF. Computational simulation is not limited by such constraints at the cost of lower realism. A hybrid approach is therefore proposed. The BSDF of a CFS is measured for incident elevation angles from 0° to 60°. For incident elevation angles from 0° to 85°, the BSDF of the sample is computed. The BSDF acquired by both techniques in the overlapping range of directions between 0° to 60° is compared and reveals good qualitative accordance. The variance of the direct-hemispherical reflection and transmission based on the two techniques is between 3% and 28%. A hybrid data-set is then generated, utilizing measurements where possible and simulations where instrumentation cannot provide reliable data. A data-driven model based on this data-set is implemented in simulation software. This hybrid model is tested by comparison with the geometrical model of the sample. The hybrid approach to BSDF modelling shall support the utilization of BSDF models based on measured data by selectively overcoming the lack of reliable measured or extrapolated data

BRDF Measurement Modelling using Wavelets for Efficient Path Tracing

International audiencePhysically based rendering needs numerical models from real measurements, or analytical models from material definitions, of the Bidirectional Reflectance Distribution Function (BRDF). However, measured BRDF data sets are too large and provide no functionalities to be practically used in Monte Carlo path tracing algorithms. In this paper, we present a wavelet-based generic BRDF model suitable for both physical analysis and path tracing. The model is based on the separation of spectral and geometrical aspect of the BRDF and allows a compact and efficient representation of isotropic, anisotropic and/or spectral BRDFs. After a brief survey of BRDF and wavelet theory, we present our software architecture for generic wavelet transform and how to use it to model BRDFs. Then, modelling results are presented on real and virtual BRDF measurements. Finally, we show how to exploit the multiresolution property of the wavelet encoding to reduce the variance by importance sampling in a path tracing algorithm

Image based surface reflectance remapping for consistent and tool independent material appearence

Physically-based rendering in Computer Graphics requires the knowledge of material properties other than 3D shapes, textures and colors, in order to solve the rendering equation. A number of material models have been developed, since no model is currently able to reproduce the full range of available materials. Although only few material models have been widely adopted in current rendering systems, the lack of standardisation causes several issues in the 3D modelling workflow, leading to a heavy tool dependency of material appearance. In industry, final decisions about products are often based on a virtual prototype, a crucial step for the production pipeline, usually developed by a collaborations among several departments, which exchange data. Unfortunately, exchanged data often tends to differ from the original, when imported into a different application. As a result, delivering consistent visual results requires time, labour and computational cost. This thesis begins with an examination of the current state of the art in material appearance representation and capture, in order to identify a suitable strategy to tackle material appearance consistency. Automatic solutions to this problem are suggested in this work, accounting for the constraints of real-world scenarios, where the only available information is a reference rendering and the renderer used to obtain it, with no access to the implementation of the shaders. In particular, two image-based frameworks are proposed, working under these constraints. The first one, validated by means of perceptual studies, is aimed to the remapping of BRDF parameters and useful when the parameters used for the reference rendering are available. The second one provides consistent material appearance across different renderers, even when the parameters used for the reference are unknown. It allows the selection of an arbitrary reference rendering tool, and manipulates the output of other renderers in order to be consistent with the reference

Etude statistique des chemins de premier retour aux nombres de Knudsen intermédiaires : De la simulation par méthode de Monte Carlo à l'utilisation de l'approximation de diffusion

En présence de diffusions multiples, les algorithmes de Monte-Carlo sont trop coûteux pour être employés dans les algorithmes de reconstruction d'images de géométries tridimensionnelles réalistes. Pour des trajectoires de premiers retours, l'approximation de diffusion est communément employée afin de représenter la statistique des chemins aux nombres de Knudsen tendant vers zéro. En formulant des problèmes équivalents sur des trajectoires de premiers passages, l'usage de l'approximation est étendue en un développement théorique. Cette nouvelle formulation assure un bon niveau de précision, sur une large plage de valeurs du nombre de Knudsen en ce qui concerne l'évaluation des moments de la distribution des longueurs des chemins de premier retour. La résolution numérique du modèle formulé est confrontée aux simulations numériques type Monte- Carlo sur des géométries mono-dimensionnelles et un cas tridimensionel ouvrant des perspectives vers une généralisation aux applications réelles. ABSTRACT : For multiple scattering, Monte-Carlo algorithms are computationally too demanding for use in image reconstruction of 3D realistic geometries. In the study of first return path, the diffusion approximation is commonly used to represent their statistical behaviour when the Knudsen number tends to zero. With the formulation of equivalent problems for first passage path, the use of the approximation is extended in a theoretical development. The new model provides a good level of accuracy, for a wide distribution of Knudsen numbers when evaluating the moments distribution of the first return paths length. Numerical application of the model is confronted to Monte-Carlo simulations on one dimension geometries and a simple three-dimension case opening perspectives for the generalization to practical applications

Volume xx (200y), Number z, pp. 1–11 BRDF Measurement Modelling Using Wavelets For Efficient Path Tracing Abstract

Physically based rendering needs numerical models from real measurements, or analytical models from material definitions, of the Bi-directional Reflectance Distribution Function (BRDF). However, measured BRDF data sets are too large and provide no functionalities to be practically used in Monte Carlo path tracing algorithms. In this paper, we present a wavelet based generic BRDF model suitable for both physical analysis and path tracing. The model is based on the separation of spectral and geometrical aspect of the BRDF and allows a compact and efficient representation of isotropic, anisotropic and/or spectral BRDFs. After a brief survey of BRDF and wavelet theory, we present our software architecture for generic wavelet transform and how to use it to model BRDFs. Then, modelling results are presented on real and virtual BRDF measurements. Finally, we show how to exploit the multi-resolution property of the wavelet encoding to reduce the variance by importance sampling in a path tracing algorithm. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realis