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

Perceptually Validated Cross-Renderer Analytical BRDF Parameter Remapping

Material appearance of rendered objects depends on the underlying BRDF implementation used by rendering software packages. A lack of standards to exchange material parameters and data (between tools) means that artists in digital 3D prototyping and design, manually match the appearance of materials to a reference image. Since their effect on rendered output is often non-uniform and counter intuitive, selecting appropriate parameterisations for BRDF models is far from straightforward. We present a novel BRDF remapping technique, that automatically computes a mapping (BRDF Difference Probe) to match the appearance of a source material model to a target one. Through quantitative analysis, four user studies and psychometric scaling experiments, we validate our remapping framework and demonstrate that it yields a visually faithful remapping among analytical BRDFs. Most notably, our results show that even when the characteristics of the models are substantially different, such as in the case of a phenomenological model and a physically-based one, our remapped renderings are indistinguishable from the original source model

Matrix Determination of Reflectivity of Hidden Object via Indirect Photography

Indirect photography is a recently demonstrated technique which expands on the principles of dual photography and allows for the imaging of hidden objects. A camera and light source are collocated with neither having line-of-sight access to the hidden object. Light from the source, a laser, is reflected off a visible non-specular surface onto the hidden object, where it is reflected back to the initial non-specular surface and collected by the camera. This process may be repeated numerous times for various laser spot positions to yield slightly different camera images due to a variation in the illumination of the object. These images can then be used to construct an “indirect” image of the hidden object. This work provides an alternative method of processing the camera images by modeling this system as a set of transport and reflectance matrices. This approach reduces the required size of the visible scattering surface. Matrix formulation and those parameters shown in simulation to improve indirect image quality as measured by a modified MTF, including the method of matrix inversion, number and pattern of laser spots, are discussed

Practical photon mapping in hardware

Photon mapping is a popular global illumination algorithm that can reproduce a wide range of visual effects including indirect illumination, color bleeding and caustics on complex diffuse, glossy, and specular surfaces modeled using arbitrary geometric primitives. However, the large amount of computation and tremendous amount of memory bandwidth, terabytes per second, required makes photon mapping prohibitively expensive for interactive applications. In this dissertation I present three techniques that work together to reduce the bandwidth requirements of photon mapping by over an order of magnitude. These are combined in a hardware architecture that can provide interactive performance on moderately-sized indirectly-illuminated scenes using a pre-computed photon map. 1. The computations of the naive photon map algorithm are efficiently reordered, generating exactly the same image, but with an order of magnitude less bandwidth due to an easily cacheable sequence of memory accesses. 2. The irradiance caching algorithm is modified to allow fine-grain parallel execution by removing the sequential dependency between pixels. The bandwidth requirements of scenes with diffuse surfaces and low geometric complexity is reduced by an additional 40% or more. 3. Generating final gather rays in proportion to both the incident radiance and the reflectance functions requires fewer final gather rays for images of the same quality. Combined Importance Sampling is simple to implement, cheap to compute, compatible with query reordering, and can reduce bandwidth requirements by an order of magnitude. Functional simulation of a practical and scalable hardware architecture based on these three techniques shows that an implementation that would fit within a host workstation will achieve interactive rates. This architecture is therefore a candidate for the next generation of graphics hardware

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

Shader Programming: An Introduction Using the Effect Framework

Current commodity graphics cards offer programmability through vertex shaders and pixel shaders to create special effects by deformation, lighting, texturing, etc. The Effect framework introduced by Microsoft allows to store shader program code, settings, and a limited graphical user interface within a single .fx text file. This supports a division of labor between programmers writing the code and designers using the GUI elements to control settings. Furthermore, the Effect framework proves to be ideal for experimenting with shader programming — be it for learning purposes or for rapid prototyping. In this tutorial, we employ the Effect framework for an exploratory, hands-on approach, introducing first principles only as needed, not in advance. Simple shader programs are used to review basic 3D techniques such as homogeneous coordinates and the Phong shading model. Then we turn to basic deformation effects employing vertex shaders and the use of texture maps as decals or reflected environments inside pixel shaders. To create bump mapping and related effects, tangent space coordinates and normal maps are introduced. Finally, we treat more complex effects such as anisotropic specular highlights. Keywords: Pixel shader, Vertex shader, HLSL, Effect framewor