Ryan Overbeck

PhD Student

Computer Graphics Group
Computer Science Department
Columbia University

Office: 6LE4 CEPSR
rso2102@cs.columbia.edu
http://www.cs.columbia.edu/~rso2102

Advisor: Ravi Ramamoorthi

Publications

2008

Large Ray Packets for Real-time Whitted Ray Tracing

Overbeck, Ryan; Ramamoorthi, Ravi; Mark, William R..
To Appear in IEEE/EG Symposium on Interactive Ray Tracing 2008. [pdf]

Abstract: In this paper, we explore large ray packet algorithms for acceleration structure traversal and frustum culling in the context of Whitted ray tracing, and examine how these methods respond to varying ray packet size, scene complexity, and ray recursion complexity. We offer a new algorithm for acceleration structure traversal which is robust to degrading coherence and a new method for generating frustum bounds around reflection and refraction ray packets. We compare, adjust, and finally compose the most effective algorithms into a real-time Whitted ray tracer. With the aid of multi-core CPU technology, our system renders complex scenes with reflections, refractions, and/or point-light shadows anywhere from 4--20 FPS.

2007

A Real-time Beam Tracer with Application to Exact Soft Shadows

Overbeck, Ryan; Ramamoorthi, Ravi; Mark, William R..
EGSR 2007. [pdf][video]

Abstract: Efficiently calculating accurate soft shadows cast by area light sources remains a difficult problem. Ray tracing based approaches are subject to noise or banding, and most other accurate methods either scale poorly with scene geometry or place restrictions on geometry and/or light source size and shape. Beam tracing is one solution which has historically been considered too slow and complicated for most practical rendering applications. Beam tracing's performance has been hindered by complex geometry intersection tests, and a lack of good acceleration structures with efficient algorithms to traverse them. We introduce fast new algorithms for beam tracing, specifically for beam--triangle intersection and beam--kd-tree traversal. The result is a beam tracer capable of calculating precise primary visibility and point light shadows in real-time. Moreover, beam tracing provides full area elements instead of point samples, which allows us to maintain coherence through to secondary effects and utilize the GPU for high quality antialiasing and shading with minimal extra cost. More importantly, our analysis shows that beam tracing is particularly well suited to soft shadows from area lights, and we generate essentially exact noise-free soft shadows for complex scenes in seconds rather than minutes or hours.

2006

Real-Time BRDF Editing in Complex Lighting
	Ben-Artzi, Aner; Overbeck, Ryan; Ramamoorthi, Ravi. SIGGRAPH 2006. [pdf] [video] Abstract: Current systems for editing BRDFs typically allow users to adjust analytic parameters while visualizing the results in a simplified setting (e.g. unshadowed point light). This paper describes a realtime rendering system that enables interactive edits of BRDFs, as rendered in their final placement on objects in a static scene, lit by direct, complex illumination. All-frequency effects (ranging from near-mirror reflections and hard shadows to diffuse shading and soft shadows) are rendered using a precomputation-based approach. Inspired by real-time relighting methods, we create a linear system that fixes lighting and view to allow real-time BRDF manipulation. In order to linearize the imageÂ’s response to BRDF parameters, we develop an intermediate curve-based representation, which also reduces the rendering and precomputation operations to 1D while maintaining accuracy for a very general class of BRDFs. Our system can be used to edit complex analytic BRDFs (including anisotropic models), as well as measured reflectance data. We improve on the standard precomputed radiance transfer (PRT) rendering computation by introducing an incremental rendering algorithm that takes advantage of frame-to-frame coherence. We show that it is possible to render reference-quality images while only updating 10% of the data at each frame, sustaining frame-rates of 25-30fps.
Exploiting Temporal Coherence for Incremental All-frequency Relighting
	Overbeck, Ryan; Ben-Artzi, Aner; Ramamoorthi, Ravi; Grinspun, Eitan. EGSR 2006. [pdf] [video] [presentation: ppt, files] Abstract: Precomputed radiance transfer (PRT) enables all-frequency relighting with complex illumination, materials and shadows. To achieve real-time performance, PRT exploits angular coherence in the illumination, and spatial coherence in the scene. Temporal coherence of the lighting from frame to frame is an important, but unexplored additional form of coherence for PRT. In this paper, we develop incremental methods for approximating the differences in lighting between consecutive frames. We analyze the lighting wavelet decomposition over typical motion sequences, and observe differing degrees of temporal coherence across levels of the wavelet hierarchy. To address this, our algorithm treats each level separately, adapting to available coherence. The proposed method is orthogonal to other forms of coherence, and can be added to almost any all-frequency PRT algorithm with minimal implementation, computation or memory overhead. We demonstrate our technique within existing codes for nonlinear wavelet approximation, changing view with BRDF factorization, and clustered PCA. Exploiting temporal coherence of dynamic lighting yields a 3×–4× performance improvement, e.g., all-frequency effects are achieved with 30 wavelet coefficients for the lighting, about the same as low-frequency spherical harmonic methods. Distinctly, our algorithm smoothly converges to the exact result within a few frames of the lighting becoming static.
Exploiting Temporal Coherence for Pre-computation Based Rendering
	Overbeck, Ryan Master's Thesis. Tech Report #: CUCS-025-06. May, 2006. [pdf] Abstract: Precomputed radiance transfer (PRT) generates impressive images with complex illumination, materials and shadows with real-time interactivity. These methods separate the scene’s static and dynamic components allowing the static portion to be computed as a preprocess. In this work, we hold geometry static and allow either the lighting or BRDF to be dynamic. To achieve real-time performance, both static and dynamic components are compressed by exploiting spatial and angular coherence. Temporal coherence of the dynamic component from frame to frame is an important, but unexplored additional form of coherence. In this thesis, we explore temporal coherence of two forms of all-frequency PRT: BRDF material editing and lighting design. We develop incremental methods for approximating the differences in the dynamic component between consecutive frames. For BRDF editing, we find that a pure incremental approach allows quick convergence to an exact solution with smooth real-time response. For relighting, we observe vastly differing degrees of temporal coherence accross levels of the lighting’s wavelet hierarchy. To address this, we develop an algorithm that treats each level separately, adapting to available coherence. The proposed methods are othogonal to other forms of coherence, and can be added to almost any PRT algorithm with minimal implementation, computation, or memory overhead. We demonstrate our technique within existing codes for nonlinear wavelet approximation, changing view with BRDF factorization, and clustered PCA. Exploiting temporal coherence of dynamic lighting yields a 3×–4× performance improvement, e.g., all-frequency effects are achieved with 30 wavelet coefficients, about the same as low-frequency spherical harmonic methods. Distinctly, our algorithm smoothly converges to the exact result within a few frames of the lighting becoming static.