Miloš Hašan

Ph.D. Thesis

Matrix Sampling for Global Illumination Miloš Hašan (advised by Kavita Bala) Cornell, August 2009 Global illumination is the problem of rendering images by simulating the light transport in a scene, also considering the inter-reflection of light between surfaces. One general approach to global illumination that gained popularity during the last decade is the many-light formulation, whose idea is to approximate global illumination by many automatically generated virtual point lights. In this thesis, we address two fundamental issues that arise with the many-light formulation: scalability and generality. We present a new view of the many-light approach, by treating it as a large matrix of light-surface contributions. Our insight is that there is usually a significant amount of structure and redundancy in the matrix; this suggests that only a tiny subset of the elements might be needed for accurate reconstruction. First, we present a scalable rendering algorithm that exploits this insight by sampling a small subset of matrix rows and columns to reconstruct the image. This algorithm is very flexible in terms of the material and light types it can handle, and achieves high-quality rendering of complex scenes in several seconds on consumer-level graphics hardware. Furthermore, we extend this approach to render whole animations, by considering a 3D tensor of light-surface contributions over time. This allows us to further decrease the necessary number of samples by exploiting temporal coherence. We also address a long-standing limitation of all previous many-light approaches that leads to fundamentally incorrect results in scenes with glossy materials, by introducing a new virtual light type that does not have this limitation. Finally, we describe an algorithm that computes a wavelet-compressed approximation to the lighting matrix, which allows for interactive light placement in a scene with global illumination. [link]

Publications

	Combining Global and Local Virtual Lights for Detailed Glossy Illumination Tomáš Davidovič, Jaroslav Křivánek, Miloš Hašan, Philipp Slusallek, Kavita Bala SIGGRAPH Asia 2010 Accurately rendering glossy materials in design applications, where previewing and interactivity are important, remains a major challenge. While many fast global illumination solutions have been proposed, all of them work under limiting assumptions on the materials and lighting in the scene. In the presence of many glossy (directionally scattering) materials, fast solutions either fail or degenerate to inefficient, brute-force simulations of the underlying light transport. In particular, many-light algorithms are able to provide fast approximations by clamping elements of the light transport matrix, but they eliminate the part of the transport that contributes to accurate glossy appearance. In this paper we introduce a solution that separately solves for the global (low-rank, dense) and local (high-rank, sparse) illumination components. For the low-rank component we introduce visibility clustering and approximation, while for the high-rank component we introduce a local light technique to correct for the missing illumination. Compared to competing techniques we achieve superior gloss rendering in minutes, making our technique suitable for applications such as industrial design and architecture, where material appearance is critical. [PDF] [Supplemental PDF] [PPTX]
	Physical Reproduction of Materials with Specified Subsurface Scattering Miloš Hašan, Martin Fuchs, Wojciech Matusik, Hanspeter Pfister, Szymon Rusinkiewicz SIGGRAPH 2010 We investigate a complete pipeline for measuring, modeling, and fabricating objects with specified subsurface scattering behaviors. The process starts with measuring the scattering properties of a given set of base materials, determining their radial reflection and transmission profiles. We describe a mathematical model that predicts the profiles of different stackings of base materials, at arbitrary thicknesses. In an inverse process, we can then specify a desired reflection profile and compute a layered composite material that best approximates it. Our algorithm efficiently searches the space of possible combinations of base materials, pruning unsatisfactory states imposed by physical constraints. We validate our process by producing both homogeneous and heterogeneous composites fabricated using a multi-material 3D printer. We demonstrate reproductions that have scattering properties approximating complex materials. [PDF] [slides]
	Virtual Spherical Lights for Many-Light Rendering of Glossy Scenes Miloš Hašan, Jaroslav Křivánek, Bruce Walter, Kavita Bala SIGGRAPH Asia 2009 In this paper, we aim to lift the accuracy limitations of many-light algorithms by introducing a new light type, the virtual spherical light (VSL). The illumination contribution of a VSL is computed over a non-zero solid angle, thus eliminating the illumination spikes that virtual point lights used in traditional many-light methods are notorious for. The VSL enables application of many-light approaches in scenes with glossy materials and complex illumination that could previously be rendered only by much slower algorithms. By combining VSLs with the matrix row-column sampling algorithm, we achieve high-quality images in one to four minutes, even in scenes where path tracing or photon mapping take hours to converge. [PDF] [PPTX] [HLSL shader]
	Automatic Bounding of Programmable Shaders for Efficient Global Illumination Edgar Velázquez-Armendáriz, Shuang Zhao, Miloš Hašan, Bruce Walter, Kavita Bala SIGGRAPH Asia 2009 This paper describes a technique to automatically adapt programmable shaders for use in physically-based rendering algorithms. Programmable shading provides great flexibility and power for creating rich local material detail, but only allows the material to be queried in one limited way: point sampling. Physically-based rendering algorithms simulate the complex global flow of light through an environment but rely on higher level information about the material properties, such as importance sampling and bounding, to intelligently solve high dimensional rendering integrals. We propose using a compiler to automatically generate interval versions of programmable shaders that can be used to provide the higher level query functions needed by physically-based rendering without the need for user intervention or expertise. We demonstrate the use of programmable shaders in two such algorithms, multidimensional lightcuts and photon mapping, for a wide range of scenes including complex geometry, materials and lighting. [PDF]
	Tensor Clustering for Rendering Many-Light Animations Miloš Hašan, Edgar Velázquez-Armendáriz, Fabio Pellacini, Kavita Bala EGSR 2008 Rendering animations of scenes with deformable objects, camera motion, and complex illumination, including indirect lighting and arbitrary shading, is a long-standing challenge. Prior work has shown that complex lighting can be accurately approximated by a large collection of point lights. In this formulation, rendering of animation sequences becomes the problem of efficiently shading many surface samples from many lights across several frames. This paper presents a tensor formulation of the animated many-light problem, where each element of the tensor expresses the contribution of one light to one pixel in one frame. We sparsely sample rows and columns of the tensor, and introduce a clustering algorithm to select a small number of representative lights to efficiently approximate the animation. Our algorithm achieves efficiency by reusing representatives across frames, while minimizing temporal flicker. We demonstrate our algorithm in a variety of scenes that include deformable objects, complex illumination and arbitrary shading and show that a surprisingly small number of representative lights is sufficient for high quality rendering. We believe out algorithm will find practical use in applications that require fast previews of complex animations. [PDF] [PPT] [video] [comparison video] [diff video]
	Matrix Row-Column Sampling for the Many-Light Problem Miloš Hašan, Fabio Pellacini, Kavita Bala SIGGRAPH 2007 Rendering complex scenes with indirect illumination, high dynamic range environment lighting, and many direct light sources remains a challenging problem. Prior work has shown that all these effects can be approximated by many point lights. This paper presents a scalable solution to the many-light problem suitable for a GPU implementation. We view the problem as a large matrix of sample-light interactions; the ideal final image is the sum of the matrix columns. We propose an algorithm for approximating this sum by sampling entire rows and columns of the matrix on the GPU using shadow mapping. The key observation is that the inherent structure of the transfer matrix can be revealed by sampling just a small number of rows and columns. Our prototype implementation can compute the light transfer within a few seconds for scenes with indirect and environment illumination, area lights, complex geometry and arbitrary shaders. We believe this approach can be very useful for rapid previewing in applications like cinematic and architectural lighting design. [PDF] [PPT]
	Direct-to-Indirect Transfer for Cinematic Relighting Miloš Hašan, Fabio Pellacini, Kavita Bala SIGGRAPH 2006 This paper presents an interactive GPU-based system for cinematic relighting with multiple-bounce indirect illumination from a fixed view-point. We use a deep frame-buffer containing a set of view samples, whose indirect illumination is recomputed from the direct illumination on a large set of gather samples, distributed around the scene. This direct-to-indirect transfer is a linear transform which is particularly large, given the size of the view and gather sets. This makes it hard to precompute, store and multiply with. We address this problem by representing the transform as a set of sparse matrices encoded in wavelet space. A hierarchical construction is used to impose a wavelet basis on the unstructured gather cloud, and an image-based approach is used to map the sparse matrix computations to the GPU. We precompute the transfer matrices using a hierarchical algorithm and a variation of photon mapping in less than three hours on one processor. We achieve high-quality indirect illumination at 10-20 frames per second for complex scenes with over 2 million polygons, with diffuse and glossy materials, and arbitrary direct lighting models (expressed using shaders). We compute per-pixel indirect illumination without the need of irradiance caching or other subsampling techniques. [PDF] [PPT] [video]

Under submission

Interactive Editing of Path-Traced Multiple-Scattering Materials Miloš Hašan, Ravi Ramamoorthi Materials such as clothing or carpets, or complex assemblies of small leaves, flower petals or mosses, do not fit well into either BRDF or BSSRDF models. Their appearance is a complex combination of reflection, transmission, scattering, shadowing and inter-reflection. This complexity can be handled by simulating the full volumetric light transport within these materials by Monte Carlo algorithms. However, there is no easy way to construct the necessary distributions of local material properties that would lead to the desired global appearance. In this paper, we consider one way to alleviate the problem: an editing algorithm that enables a material designer to set the local (single-scattering) albedo coefficients interactively, and see an immediate update of the emergent appearance in the image. This is a difficult problem, since the function from materials to pixel values is neither linear nor low-order polynomial. We combine the following two ideas to achieve high-dimensional heterogeneous edits: precomputing the homogeneous mapping of albedo to intensity, and a large Jacobian matrix, which encodes the derivatives of each image pixel with respect to each albedo coefficient. Combining these two datasets leads to an interactive editing algorithm with a very good visual match to a fully path-traced ground truth. We also discuss an extension of this technique to modify the mean free path of the material. [available by e-mail]

Shadie: A Domain-Specific Language for Volume Visualization Miloš Hašan, John Wolfgang, George Chen, Hanspeter Pfister Despite abundant research on volume rendering, its usage among medical professionals is still limited. We believe this is partly due to limited flexibility of existing systems and the difficulty of customizing them without low-level software engineering expertise. In this paper we introduce Shadie, an experimental domain-specific language (DSL) and volume visualization framework. Shadie is built around the concept of self-contained shaders: high-level descriptions of the desired interactive visualization, written in a Python-like language that can be easily written by domain professionals (in our case, radiation physicists). Type inference and source-to-source translation to efficient CUDA code allow for interactive framerates. The main contribution of this work is the demonstration of the concept of a volume-rendering DSL in a practical prototype, developed and used in a real radiation oncology department. Our results of an informal user study demonstrate that Shadie can be effectively used by novice programmers to quickly create complex, interactive visualizations. [available by e-mail]

Other

	Interactive Cinematic Relighting with Global Illumination Fabio Pellacini, Miloš Hašan, Kavita Bala Chapter 9, GPU Gems 3
	Volume Rendering of Dosimetric Distribution and Biological Response from 3D/4D Treatment and Delivery Miloš Hašan, Hanspeter Pfister, George Chen, John Wolfgang Appears in the 2010 Annual Meeting of the American Association of Physics in Medicine (AAPM)
	Interactive 4D Visualization of Radiological Path Length Variation for Proton Treatment Port Selection Miloš Hašan, Hanspeter Pfister, George Chen, John Wolfgang Poster in the 2010 Annual Meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO)
	Real-time Hardware-accelerated Relighting with Approximate Indirect Illumination Miloš Hašan, Fabio Pellacini, Kavita Bala Cornell CIS Technical Report TR2005-1999 [PDF]
	An Efficient F-rep Visualization Framework Miloš Hašan, Alexander Pasko, Andrej Ferko Undergraduate thesis, Comenius University [available by e-mail]