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

Playing Ping Pong with Light: Directional Emission of White Light

Over the last decades, light-emitting diodes (LED) have replaced common light bulbs in almost every application, from flashlights in smartphones to automotive headlights. Illuminating nightly streets requires LEDs to emit a light spectrum that is perceived as pure white by the human eye. The power associated with such a white light spectrum is not only distributed over the contributing wavelengths but also over the angles of vision. For many applications, the usable light rays are required to exit the LED in forward direction, namely under small angles to the perpendicular. In this work, we demonstrate that a specifically designed multi-layer thin film on top of a white LED increases the power of pure white light emitted in forward direction. Therefore, the deduced multi-objective optimization problem is reformulated via a real-valued physics-guided objective function that represents the hierarchical structure of our engineering problem. Variants of Bayesian optimization are employed to maximize this non-deterministic objective function based on ray tracing simulations. Eventually, the investigation of optical properties of suitable multi-layer thin films allowed to identify the mechanism behind the increased directionality of white light: angle and wavelength selective filtering causes the multi-layer thin film to play ping pong with rays of light

Efficient Many-Light Rendering of Scenes with Participating Media

We present several approaches based on virtual lights that aim at capturing the light transport without compromising quality, and while preserving the elegance and efficiency of many-light rendering. By reformulating the integration scheme, we obtain two numerically efficient techniques; one tailored specifically for interactive, high-quality lighting on surfaces, and one for handling scenes with participating media

Bi-directional Photogoniometer for the Assessment of the Luminous Properties of Fenestration Systems

Most energy saving applications of advanced fenestration systems (solar blinds, novel types of glazing and daylight redirecting devices) require a precise knowledge of their directional light transmission features. These photometric properties can be described by a Bi-directional Transmission Distribution Function (BTDF) whose experimental assessment requires appropriate equipment. A novel bi-directional transmission photogoniometer, based on digital imaging techniques, was designed and set up for that purpose. The apparatus takes advantage of a modern video image capturing device (CCD digital camera) as well as of powerful image analysis software (pattern recognition) to considerably reduce the scanning time of a BTDF measurement, in comparison to existing devices that use a conventional approach (mobile photometer). A detailed calibration and validation procedure was used to obtain optimal experimental accuracy for the device during the assessment of BTDF data. It included a spectral, a photometric and a geometrical calibration of the digital video system, as well as several additional corrections, leading to an overall relative accuracy better than 11% for BTDF data. A special effort was made to improve the user-friendliness of BTDF measurement by facilitating the data acquisition and treatment (definition of a data acquisition and electronic data format) and by offering different possibilities of BTDF visualisation (hemispherical representation, axonometric view of photometric solids, C-planes). Overall, the photometric equipment was used to assess the BTDFs of more than 20 novel fenestration products of the industrial partner of the project (Baumann-Hüppe Storen AG). The experimental data produced was successfully used by the company to optimise the visual and energy saving performance of their products, which confirms the adequacy of the novel bi-directional photogoniometer for practical building applications

Hierarchical Variance Reduction Techniques for Monte Carlo Rendering

Ever since the first three-dimensional computer graphics appeared half a century ago, the goal has been to model and simulate how light interacts with materials and objects to form an image. The ultimate goal is photorealistic rendering, where the created images reach a level of accuracy that makes them indistinguishable from photographs of the real world. There are many applications ñ visualization of products and architectural designs yet to be built, special effects, computer-generated films, virtual reality, and video games, to name a few. However, the problem has proven tremendously complex; the illumination at any point is described by a recursive integral to which a closed-form solution seldom exists. Instead, computer simulation and Monte Carlo methods are commonly used to statistically estimate the result. This introduces undesirable noise, or variance, and a large body of research has been devoted to finding ways to reduce the variance. I continue along this line of research, and present several novel techniques for variance reduction in Monte Carlo rendering, as well as a few related tools. The research in this dissertation focuses on using importance sampling to pick a small set of well-distributed point samples. As the primary contribution, I have developed the first methods to explicitly draw samples from the product of distant high-frequency lighting and complex reflectance functions. By sampling the product, low noise results can be achieved using a very small number of samples, which is important to minimize the rendering times. Several different hierarchical representations are explored to allow efficient product sampling. In the first publication, the key idea is to work in a compressed wavelet basis, which allows fast evaluation of the product. Many of the initial restrictions of this technique were removed in follow-up work, allowing higher-resolution uncompressed lighting and avoiding precomputation of reflectance functions. My second main contribution is to present one of the first techniques to take the triple product of lighting, visibility and reflectance into account to further reduce the variance in Monte Carlo rendering. For this purpose, control variates are combined with importance sampling to solve the problem in a novel way. A large part of the technique also focuses on analysis and approximation of the visibility function. To further refine the above techniques, several useful tools are introduced. These include a fast, low-distortion map to represent (hemi)spherical functions, a method to create high-quality quasi-random points, and an optimizing compiler for analyzing shaders using interval arithmetic. The latter automatically extracts bounds for importance sampling of arbitrary shaders, as opposed to using a priori known reflectance functions. In summary, the work presented here takes the field of computer graphics one step further towards making photorealistic rendering practical for a wide range of uses. By introducing several novel Monte Carlo methods, more sophisticated lighting and materials can be used without increasing the computation times. The research is aimed at domain-specific solutions to the rendering problem, but I believe that much of the new theory is applicable in other parts of computer graphics, as well as in other fields

Toward a Perceptually-relevant Theory of Appearance

Two approaches are commonly employed in Computer Graphics to design and adjust the appearance of objects in a scene. A full 3D environment may be created, through geometrical, material and lighting modeling, then rendered using a simulation of light transport; appearance is then controlled in ways similar to photography. A radically different approach consists in providing 2D digital drawing tools to an artist, whom with enough talent and time will be able to create images of objects having the desired appearance; this is obviously strongly similar to what traditional artists do, with the computer being a mere modern drawing tool.In this document, I present research projects that have investigated a third approach, whereby pictorial elements of appearance are explicitly manipulated by an artist. On the one side, such an alternative approach offers a direct control over appearance, with novel applications in vector drawing, scientific illustration, special effects and video games. On the other side, it provides an modern method for putting our current knowledge of the perception of appearance to the test, as well as to suggest new models for human vision along the way