e-mail: my first name@dgp.toronto.edu
work phone: +1 (416) 946-8427
| Home | Publications (brief) | All Abstracts & Files | Academic Service | Completed Internships |
| I am a Ph.D candidate in the Dynamic Graphics Project lab at the University of Toronto. I completed a BASc in Honours Computer Engineering from the University of Waterloo in May 2005 and an MSc in Computer Science from the University of Toronto in October 2006. My research interests are realistic image synthesis, high-performance rendering using commodity graphics hardware and methods for real-time fluid simulation and animation. My supervisor is Dr. Eugene Fiume. I completed an internship at MSR Redmond in 2008 working on interactive illumination effects under the supervision of Dr. John Snyder. I will be conducting research at Disney Research Zurich as an intern during the summer of 2009 with Dr. Wojciech Jarosz. |
e-mail: my first name@dgp.toronto.edu work phone: +1 (416) 946-8427 |
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Fast Global Illumination on Dynamic Height FieldsDerek Nowrouzezahrai and John Snyder AbstractWe present a real-time method for rendering global illumination effects from large area and environmental lights on dynamic height fields. In contrast to previous work, our method handles inter-reflections (indirect lighting) and non-diffuse surfaces. To reduce sampling, we construct one multi-resolution pyramid for height variation to compute direct shadows, and another pyramid for each indirect bounce of incident radiance to compute interreflections. The basic principle is to sample the points blocking direct light, or shedding indirect light, from coarser levels of the pyramid the farther away they are from a given receiver point. We unify the representation of visibility and indirect radiance at discrete azimuthal directions (i.e., as a function of a single elevation angle) using the concept of a casting set of visible points along this direction whose contributions are collected in the basis of normalized Legendre polynomials. This analytic representation is compact, requires no precomputation, and allows efficient integration to produce the spherical visibility and indirect radiance signals. Sub-sampling visibility and indirect radiance, while shading with full-resolution surface normals, further increases performance without introducing noticeable artifacts. Our method renders 512x512 height fields (>500K triangles) at 36Hz. CitationDerek Nowrouzezahrai and John Snyder. Fast global illumination on dynamic height fields. Computer Graphics Forum: Eurographics Symposium on Rendering, June 2009. [BiBTeX] Links
Data-driven curvature for real-time line drawing of dynamic scenesEvangelos Kalogerakis, Derek Nowrouzezahrai, Patricio Simari, James Mccrae, Aaron Hertzmann, and Karan Singh AbstractThis paper presents a method for real-time line drawing of deforming objects. Object-space line drawing algorithms for many types of curves, including suggestive contours, highlights, ridges and valleys, rely on surface curvature and curvature derivatives. Unfortunately, these curvatures and their derivatives cannot be computed in real-time for animated, deforming objects. In a preprocessing step, our method learns the mapping from a low-dimensional set of animation parameters (e.g., joint angles) to surface curvatures for a deforming 3D mesh. The learned model can then accurately and efficiently predict curvatures and their derivatives, enabling real-time object-space rendering of suggestive contours and other such curves. This represents an order-of-magnitude speed-up over the fastest existing algorithm capable of estimating curvatures and their derivatives accurately enough for many different types of line drawings. The learned model can generalize to novel animation sequences, and is also very compact, typically requiring a few megabytes of storage at run-time. We demonstrate our method for various types of animated objects, including skeleton-based characters, cloth simulation and blend-shape facial animation, using a variety of non-photorealistic rendering styles.An important component of our system is the use of dimensionality reduction for differential mesh data. CitationEvangelos Kalogerakis, Derek Nowrouzezahrai, Patricio Simari, James Mccrae, Aaron Hertzmann, and Karan Singh. Data-driven curvature for real-time line drawing of dynamic scenes. ACM Transactions on Graphics, 28(1):1–13, May 2009. [BiBTeX] Links
Shadowing Dynamic Scenes with Arbitrary BRDFsDerek Nowrouzezahrai, Evangelos Kalogerakis, and Eugene Fiume AbstractWe present a real-time relighting and shadowing method for dynamic scenes with varying lighting, view and BRDFs. Our approach is based on a compact representation of reflectance data that allows for changing the BRDF at run-time and a data-driven method for accurately synthesizing self-shadows on articulated and deformable geometries. Unlike previous self-shadowing approaches, we do not rely on local blocking heuristics. We do not fit a model to the BRDF-weighted visibility, but rather only to the visibility that changes during animation. In this manner, our model is more compact than previous techniques and requires less computation both during fitting and at run-time. Our reflectance product operators can re-integrate arbitrary low-frequency view-dependent BRDF effects on-the-fly and are compatible with all previous dynamic visibility generation techniques as well as our own data-driven visibility model. We apply our reflectance product operators to three different visibility generation models, and our data-driven model can achieve framerates well over 300Hz. CitationDerek Nowrouzezahrai, Evangelos Kalogerakis, and Eugene Fiume. Shadowing dynamic scenes with arbitrary brdfs. Computer Graphics Forum: Eurographics Conference, 28:249–258(10), April 2009. [BiBTeX] Links
Multi-objective shape segmentation and labelingPatricio Simari, Derek Nowrouzezahrai, Evangelos Kalogerakis, and Karan Singh AbstractShape segmentations designed for different applications show significant variation in the composition of their parts. In this paper, we introduce the segmentation and labeling of shape based on the optimization of multiple objectives that capture application-specific segmentation criteria. We present a number of efficient objective functions that capture useful shape adjectives (compact, flat, narrow, perpendicular, etc.) Segmentation descriptions within our framework combine multiple such objective functions with optional labels to define each part. The optimization problem is simplified by proposing weighted Voronoi partitioning as a compact and continuous parameterization of spatially embedded shape segmentations. Separation of spatially close but geodesically distant parts is made possible using multi-dimensional scaling (MDS) prior to Voronoi partitioning. Optimization begins with an initial segmentation found using the centroids of a k-means clustering of surface elements. This partition is automatically labeled to optimize heterogeneous part objectives and the Voronoi centers and their weights optimized using Generalized Pattern Search (GPS). We illustrate our framework using several diverse segmentation applications: consistent segmentations with semantic labels, bounding volume hierarchies for path tracing, and automatic rig and clothing transfer between animation characters. CitationPatricio Simari, Derek Nowrouzezahrai, Evangelos Kalogerakis, and Karan Singh. Multi-objective shape segmentation and labeling. In Computer Graphics Forum: Eurographics Symposium on Geometry Processing, Aire-la-Ville, Switzerland, Switzerland, March 2009. Eurographics Association. [BiBTeX] Links
Extracting lines of curvature from noisy point cloudsEvangelos Kalogerakis, Derek Nowrouzezahrai, Patricio Simari, and Karan Singh AbstractWe present a robust framework for extracting lines of curvature from point clouds. First, we show a novel approach to denoising the input point cloud using robust statistical estimates of surface normal and curvature which automatically rejects outliers and corrects points by energy minimization. Then the lines of curvature are constructed on the point cloud with controllable density. Our approach is applicable to surfaces of arbitrary genus, with or without boundaries, and is statistically robust to noise and outliers while preserving sharp surface features. We show our approach to be effective over a range of synthetic and real-world input datasets with varying amounts of noise and outliers. The extraction of curvature information can benefit many applications in CAD, computer vision and graphics for point cloud shape analysis, recognition and segmentation. Here, we show the possibility of using the lines of curvature for feature-preserving mesh construction directly from noisy point clouds. CitationEvangelos Kalogerakis, Derek Nowrouzezahrai, Patricio Simari, and Karan Singh. Extracting lines of curvature from noisy point clouds. Computer Aided Design, 41(4):282–292, January 2009. [BiBTeX] Links
Solving Radiance Transport as a Differential EquationDerek Nowrouzezahrai and Chris Gonterman AbstractWe introduce an alternative to Monte-Carlo techniques for solving radiance transport problems for participating media. We use a reformulation of the volume rendering equation from its standard integro-differential form to a purely differential form. We then leverage the large body of work in numerical methods for solving differential equations by framing and analyzing the problem as a differential equation. To our knowledge, this is the first application of such techniques in the area of photo-realistic rendering of volumes based on ray optics. CitationDerek Nowrouzezahrai and Chris Gonterman. Solving radiance transport as a differential equation. Technical Report CSRG-588, Department of Computer Science, University of Toronto, Toronto, Canada, December 2008. [BiBTeX] 
Fast Soft Self-Shadowing on Dynamic Height FieldsJohn Snyder and Derek Nowrouzezahrai AbstractWe present a new, real-time method for rendering soft shadows from large light sources or lighting environments on dynamic height fields. The method first computes a horizon map for a set of azimuthal directions. To reduce sampling, we compute a multi-resolution pyramid on the height field. Coarser pyramid levels are indexed as the distance from caster to receiver increases. For every receiver point and every azimuthal direction, a smooth function of blocking angle in terms of log distance is reconstructed from a height difference sample at each pyramid level. This function's maximum approximates the horizon angle. We then sum visibility at each receiver point over wedges determined by successive pairs of horizon angles. Each wedge represents a linear transition in blocking angle over its azimuthal extent. It is precomputed in the order-4 spherical harmonic (SH) basis, for a canonical azimuthal origin and fixed extent, resulting in a 2D table. The SH triple product of 16D vectors representing lighting, total visibility, and diffuse reflectance then yields the soft-shadowed result. Two types of light sources are considered; both are distant and low-frequency. Environmental lights require visibility sampling around the complete 360 degree azimuth, while key lights sample visibility within a partial swath. Restricting the swath concentrates samples where the light comes from (e.g. 3 azimuthal directions vs. 16-32 for a full swath) and obtains sharper shadows. Our GPU implementation handles height fields up to 1024x1024 in real-time. The computation is simple, local, and parallel, with performance independent of geometric content. CitationJohn Snyder and Derek Nowrouzezahrai. Fast soft self-shadowing on dynamic height fields. Computer Graphics Forum: Eurographics Symposium on Rendering, 27(4):1275–1283, June 2008. [BiBTeX] Links
Shadowed Relighting of Dynamic Geometry with 1D BRDFsDerek Nowrouzezahrai, Evangelos Kalogerakis, Patricio Simari, and Eugene Fiume AbstractWe present a method for synthesizing the dynamic self-occlusion of an articulating character in real-time (> 170Hz) while incorporating reflection effects from 1D BRDFs under dynamic lighting and view conditions. We introduce and derive a general operator form for convolving spherical harmonics (SH) occlusion vectors with arbitrary 1D BRDF kernels. This operator, coupled with a compact linear model for predicting SH occlusion over articulating meshes, segments the BRDF and visibility terms of the direct illumination integral. We illustrate our results on a thin-membrane translucency model and the normalized Phong BRDF. CitationDerek Nowrouzezahrai, Evangelos Kalogerakis, Patricio Simari, and Eugene Fiume. Shadowed relighting of dynamic geometry with 1d brdfs. Proceedings of Eurographics Short Papers, January 2008. [BiBTeX] Links
Video browsing by direct manipulationPierre Dragicevic, Gonzalo Ramos, Jacobo Bibliowicz, Derek Nowrouzezahrai, Ravin Balakrishnan, and Karan Singh AbstractWe present a method for browsing videos by directly dragging their content. This method brings the benefits of direct manipulation to an activity typically mediated by widgets. We support this new type of interactivity by: 1) automatically extracting motion data from videos; and 2) a new technique called relative flow dragging that lets users control video playback by moving objects of interest along their visual trajectory. We show that this method can outperform the traditional seeker bar in video browsing tasks that focus on visual content rather than time. CitationPierre Dragicevic, Gonzalo Ramos, Jacobo Bibliowicz, Derek Nowrouzezahrai, Ravin Balakrishnan, and Karan Singh. Video browsing by direct manipulation. In CHI '08: Proceeding of the twenty-sixth annual SIGCHI conference on Human factors in computing systems, pages 237–246, New York, NY, USA, 2008. ACM. [BiBTeX] Links
Compact and efficient generation of radiance transfer for dynamically articulated charactersDerek Nowrouzezahrai, Patricio Simari, Evangelos Kalogerakis, Karan Singh, and Eugene Fiume AbstractWe present a data-driven technique for generating the precomputed radiance transfer vectors of an animated character as a function of its joint angles. We learn a linear model for generating real-time lighting effects on articulated characters while capturing soft self shadows caused by dynamic distant lighting. Indirect illumination can also be reproduced using our framework. Previous data-driven techniques have either restricted the type of lighting response (generating only ambient occlusion), the type of animated sequences (response functions to external forces) or have complicated runtime algorithms and incur non-trivial memory costs. We provide insights into the dimensionality reduction of the pose and coefficient spaces. Our model can be fit quickly as a preprocess, is very compact (~1MB) and runtime transfer vectors are generated using a simple algorithm in real-time (>100 Hz using a CPU-only implementation.) We can reproduce lighting effects on hundreds of trained poses using less memory than required to store a single mesh's PRT coefficients. Moreover, our model extrapolates to produce smooth, believable lighting results on novel poses and our method can be easily integrated into existing interactive content pipelines. CitationDerek Nowrouzezahrai, Patricio Simari, Evangelos Kalogerakis, Karan Singh, and Eugene Fiume. Compact and efficient generation of radiance transfer for dynamically articulated characters. In GRAPHITE '07: Proceedings of the 5th international conference on Computer graphics and interactive techniques in Australia and Southeast Asia, pages 147–154, New York, NY, USA, January 2007. ACM. [BiBTeX] Links
Eigentransport for efficient and accurate all-frequency relightingDerek Nowrouzezahrai, Patricio Simari, Evangelos Kalogerakis, and Eugene Fiume Winner: Best paper award!AbstractWe present a method for creating a geometry-dependent basis for diffuse precomputed radiance transfer. Unlike previous PRT bases, ours is derived from Principal Component Analysis of the sampled transport functions at each vertex, without relying on pre-projections to secondary bases, such as the Spherical Harmonics or Haar wavelets. It allows for efficient evaluation of shading, has low memory requirements and produces accurate results with few coefficients. We are able to capture all-frequency effects from both distant and near-field dynamic lighting in real-time and present a simple and efficient rotation scheme. Reconstruction of the final shading becomes a low-order dot product and is performed on the GPU. CitationDerek Nowrouzezahrai, Patricio Simari, Evangelos Kalogerakis, and Eugene Fiume. Eigentransport for efficient and accurate all-frequency relighting. In GRAPHITE '07: Proceedings of the 5th international conference on Computer graphics and interactive techniques in Australia and Southeast Asia, pages 163–169, New York, NY, USA, January 2007. ACM. [BiBTeX] Links
Image-Based Proxy Accumulation for Real-Time Soft Global IlluminationPeter-Pike Sloan, Naga K. Govindaraju, Derek Nowrouzezahrai, and John Snyder AbstractWe present a new, general, and real-time technique for soft global illumination in low-frequency environmental lighting. It accumulates over relatively few spherical proxies which approximate the light blocking and re-radiating effect of dynamic geometry. Soft shadows are computed by accumulating log visibility vectors for each sphere proxy as seen by each receiver point. Inter-reflections are computed by accumulating vectors representing the proxy's unshadowed radiance when illuminated by the environment. Both vectors capture low-frequency directional dependence using the spherical harmonic basis. We also present a new proxy accumulation strategy that splats each proxy to receiver pixels in image space to collect its shadowing and indirect lighting contribution. Our soft GI rendering pipeline unifies direct and indirect soft effects with a simple accumulation strategy that maps entirely to the GPU and outperforms previous vertex-based methods. CitationPeter-Pike Sloan, Naga K. Govindaraju, Derek Nowrouzezahrai, and John Snyder. Image-based proxy accumulation for real-time soft global illumination. Pacific Conference on Computer Graphics and Applications, 0:97–105, January 2007. [BiBTeX] Links
Robust statistical estimation of curvature on discretized surfacesEvangelos Kalogerakis, Patricio Simari, Derek Nowrouzezahrai, and Karan Singh AbstractA robust statistics approach to curvature estimation on discretely sampled surfaces, namely polygon meshes and point clouds, is presented. The method exhibits accuracy, stability and consistency even for noisy, non-uniformly sampled surfaces with irregular configurations. Within an M-estimation framework, the algorithm is able to reject noise and structured outliers by sampling normal variations in an adaptively reweighted neighborhood around each point. The algorithm can be used to reliably derive higher order differential attributes and even correct noisy surface normals while preserving the fine features of the normal and curvature field. The approach is compared with state-of-the-art curvature estimation methods and shown to improve accuracy by up to an order of magnitude across ground truth test surfaces under varying tessellation densities and types as well as increasing degrees of noise. Finally, the benefits of a robust statistical estimation of curvature are illustrated by applying it to the popular applications of mesh segmentation and suggestive contour rendering. CitationEvangelos Kalogerakis, Patricio Simari, Derek Nowrouzezahrai, and Karan Singh. Robust statistical estimation of curvature on discretized surfaces. In Computer Graphics Forum: Eurographics Symposium on Geometry Processing, pages 13–22, Aire-la-Ville, Switzerland, Switzerland, January 2007. Eurographics Association. [BiBTeX] Links
Vortex Based Smoke Simulation and ControlDerek Nowrouzezahrai AbstractThis thesis is an extension of our work published at the Symposium of Computer Animation with additional investigations on the performance benefits of a parallelized GPU implementation of the work, including a full re-implementation of the GPU framework. CitationDerek Nowrouzezahrai. Vortex Based Smoke Simulation and Control. Master's thesis, University of Toronto, Toronto, Canada, October 2006. [BiBTeX] Links
A controllable, fast and stable basis for vortex based smoke simulationAlexis Angelidis, Fabrice Neyret, Karan Singh, and Derek Nowrouzezahrai AbstractWe introduce a novel method for describing and controlling a 3D smoke simulation. Using harmonic analysis and principal component analysis, we define an underlying description of the fluid flow that is compact and meaningful to non-expert users. The motion of the smoke can be modified with high level tools, such as animated current curves, attractors and tornadoes. Our simulation is controllable, interactive and stable for arbitrarily long periods of time. The simulation's computational cost increases linearly in the number of motion samples and smoke particles. Our adaptive smoke particle representation conveniently incorporates the surface-like characteristics of real smoke. CitationAlexis Angelidis, Fabrice Neyret, Karan Singh, and Derek Nowrouzezahrai. A controllable, fast and stable basis for vortex based smoke simulation. In SCA '06: Proceedings of the 2006 ACM SIGGRAPH/Eurographics symposium on Computer animation, pages 25–32, Aire-la-Ville, Switzerland, Switzerland, January 2006. Eurographics Association. [BiBTeX] Links
GPU-accelerated ray casting of node-based implicitsChristian Lessig, Derek Nowrouzezahrai, and Karan Singh AbstractWe demonstrate that state-of-the-art GPUs are well suited for the visualization of node-based implicit surfaces that are the natural surface representation for Lagrangian simulations which are used for example for the simulation of fluids. CitationChristian Lessig, Derek Nowrouzezahrai, and Karan Singh. Gpu-accelerated ray casting of node-based implicits. In ACM SIGGRAPH 2006 Research posters, page 54, New York, NY, USA, January 2006. ACM. [BiBTeX] Links
High-Performance Double-Precision Cosine GenerationDerek Nowrouzezahrai, Brian Decker, and William Bishop AbstractThe trigonometric cosine function plays an important role in communication systems, digital signal processing systems, and graphical systems. This paper presents a technique for generating the cosine of an angle that is more computationally efficient than the CORDIC algorithm and its many variants for double-precision floating point calculations. A hardware design implementation of the cosine generator has been developed. Simulation results for an Altera Stratix II FPGA implementation indicate that the hardware design is both more efficient and more precise than previous published implementations. CitationDerek Nowrouzezahrai, Brian Decker, and William Bishop. High-performance double-precision cosine generation. Proceedings of the 2005 International Conference on Computer Design, June 2005. [BiBTeX] Links
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