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Graphics Lunch - Spring, 2002

Graphics Lunch is a forum for informal and formal discussions over lunch for those interested in graphics and visualization issues at Maryland. It also serves as a forum for talks from visitors to our lab about their recent research in graphics and visualization. Students and faculty can use this venue to practise and prepare for their conference papers, discuss recent and upcoming papers and conferences, or inform others about graphics and visualization news. Meetings are held on Mondays from 12:00pm to 1:30pm in the CFAR Seminar Room (AVW 4424) .

May 20, 2002 Progressive Lossless Compression of Arbitrary Simplicial Complexes

Presented By Thomas Baby, University of Maryland, College Park

Comments Graphics Seminar Series

Abstract
Progressive Lossless Compression of Arbitrary Simplicial Complexes
By Pierre-Marie Gandoin and Olivier Devillers
Efficient algorithms for compressing geometric data have been widely developed in the recent years, but they are mainly designed for closed polyhedral surfaces which are manifold or nearly manifold. We propose here a progressive geometry compression scheme which can handle manifold models as well as triangle soups and 3D tetrahedral meshes. The method is lossless when the decompression is complete which is extremely important in some domains such as medical or finite element.
While most existing methods enumerate the vertices of the mesh in an order depending on the connectivity, we use a kd-tree technique [Devillers and Gandoin 2000] which does not depend on the connectivity. Then we compute a compatible sequence of meshes which can be encoded using edge expansion [Hoppe et al. 1993] and vertex split [Popovic and Hoppe 1997].
The main contributions of this paper are: the idea of using the kd-tree encoding of the geometry to drive the construction of a sequence of meshes, an improved coding of the edge expansion and vertex split since the vertices to split are implicitly defined, a prediction scheme which reduces the code for simplices incident to the split vertex, and a new generalization of the edge expansion operation to tetrahedral meshes.
May 13, 2002 Geometry Images

Presented By Chang Ha Lee, University of Maryland, College Park

Comments Graphics Seminar Series

Abstract
Geometry Images
by Xianfeng Gu, Steven J. Gortler, and Hugues Hoppe
Surface geometry is often modeled with irregular triangle meshes. The process of remeshing refers to approximating such geometry using a mesh with (semi)-regular connectivity, which has advantages for many graphics applications. However, current techniques for remeshing arbitrary surfaces create only semi-regular meshes. The original mesh is typically decomposed into a set of disk-like charts, onto which the geometry is parametrized and sampled. In this paper, we propose to remesh an arbitrary surface onto a completely regular structure we call a geometry image. It captures geometry as a simple 2D array of quantized points. Surface signals like normals and colors are stored in similar 2D arrays using the same implicit surface parametrization - texture coordinates are absent. To create a geometry image, we cut an arbitrary mesh along a network of edge paths, and parametrize the resulting single chart onto a square. Geometry images can be encoded using traditional image compression algorithms, such as wavelet-based coders.
April 17, 2002 Behind the Scenes at Pixar

Presented By Tony DeRose, Pixar Animation Studios

Comments Distinguished Seminar Series on Vision in honor of Prof. Azriel Rosenfeld

Abstract
For more than a decade Pixar has been a leader in the area of computer animation. In this talk I'll highlight: a number of the technical challenges that we've addressed in the past, some of the challenges that face us today, and some of the challenges that will likely face us in the coming decade.
March 18, 2002 A Real-time Seamless Tiled Display System for 3D Graphics

Presented By Zhiyun Li, University of Maryland, College Park

Comments Graphics Seminar Series

Abstract
We outline our seamless tiled display system for interactive 3D graphics applications that is low-cost, easy to calibrate, scalable, and portable. Our system achieves geometric alignment in software by pre-warping the 3D space in contrast with the current systems that usually achieve this by 2D image pre-warping. Our system accomplishes this through real-time image capture from a digital camcorder, image segmentation, and derivation of the 3D warping matrices for each 3D graphics pipeline that feeds a projector. Our prototype system demonstrates our results on a 2 x 2 tiled array of projectors.
March 6, 2002 Immersion and Tele-immersion in the Office of the Future

Presented By Henry Fuchs, University of North Carolina at Chapel Hill

Comments Distinguished Seminar Series on Vision in honor of Prof. Azriel Rosenfeld

Abstract
We envision an Office of the Future in which images are displayed on walls and other surfaces to provide an immersive environment with a sense of common presence among local and distant participants and their shared work objects.
Much of the tele-immersion portion of our research has been part of a four-site National Tele-Immersion Initiative (NTII) lead by Jaron Lanier, chief scientist of the principal funder, Advanced Network & Services. In the current, primitive implementation, we at NTII use clusters of seven digital cameras to acquire the changing 3D surface of each remote partner. These live 3D images are merged with (presently, pre-acquired) 3D scans of each remote office and the common work objects and all these are displayed in head-tracked stereo on walls of the local office.
UNC's Office of the Future project and tele-collaboration has also been part of the 5-site (USA) NSF Science and Technology Center in Graphics and Scientific Visualization. Related efforts include panoramic acquisition by clusters of cameras, new image-based rendering methods, and wide-area displays ("video walls") built with numerous casually-placed ceiling-mounted projectors that are automatically calibrated by multiple cameras. We hope these efforts will improve today's ubiquitous personal computers so they will no longer be so restricted by their desktop monitors, but will emerge to integrate smoothly with their users' 3D physical work environments.
March 4, 2002 Point Matching

Presented By Chang Ha Lee, University of Maryland, College Park

Comments Graphics Seminar Series

Abstract
"Geometric Hashing: A General and Efficient Model-Based Recognition Scheme"

by Yehezkel Lamdan and Haim J. Wolfson

A general method for model-based object recognition in occluded scene is presented. It is based on geometric hashing. The method stands out for its efficiency. We describe the general framework of the method and illustrate its applications for various recognition problems both in 3-D and 2-D. Special attention is given to the recognition of 3-D objects in occluded scenes from 2-D gray scale images. New experimental results are included for this important case.
"Comparing Images Using the Hausdorff Distance"

by Daniel P. Huttenlocher, Gregory A. Klanderman, and William J. Rucklidge

The Hausdorff distance measures the extent to which each point of a model set lies near some point of an image set and vice versa. Thus this distance can be used to determine the degree of resemblance between two objects that are superimposed on one another. In this paper we provide efficient algorithms for computing the Hausdorff distance between all possible relative positions of a binary image and a model. We focus primarily on the case in which the model is only allowed to translate with respect to the image. Then we consider how to extend the techniques to rigid motion (translation and rotation). The Hausdorff distance computation differs from many other shape comparison methods in that no correspondence between the model and the image is derived. The method is quite tolerant of small position errors as occur with edge detectors and other feature extraction methods. Moreover, we show how the method extends naturally to the problem of comparing a portion of a model against an image.
February 25, 2002 Flash Animation

Presented By Zhiyun Li, University of Maryland, College Park

Comments Graphics Seminar Series

Abstract
1. Basic steps to create a flash animation.
2. Advanced animation effects including color effect, shape change,
curve path and multiple layers.
3. Video I/O.
4. Tips for easy operation.
5. Illustration of every steps by a simple demo.

February 18, 2002 Anti-aliased Hemicubes for Performance Improvement in Radiosity Solutions

Presented By Sharat Chandran, Visiting Professor, University of Maryland at College Park

Comments Graphics Seminar Series

Abstract
Several important and fascinating aspects of realistic images are captured by the radiosity method. In this work we use an alternate form of the classical hemicube that reduces aliasing problems inherent in the original method without giving up the computational advantages of the hemicube.
Unlike other methods, we explicitly consider the effect of the relative order of partial visibility in a hemicube cell when recording form factors. This enables us to compute form factors accurately (even) in progressive refinement radiosity with adaptive substructuring. Our empirical results with progressive refinement radiosity show superior mesh density where fine details are required, (such as in soft shadows), as well as in areas that produce singularities (such as when inter element distances tend to zero). Our method is contrasted with hierarchical radiosity which uses raycasting for form factor and visibility computations. On a low end Intel Linux platform, and for a comparable image quality, our method takes substantially less time.

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May 20, 2002	Progressive Lossless Compression of Arbitrary Simplicial Complexes
Presented By	Thomas Baby, University of Maryland, College Park
Comments	Graphics Seminar Series
Abstract	Progressive Lossless Compression of Arbitrary Simplicial Complexes By Pierre-Marie Gandoin and Olivier Devillers Efficient algorithms for compressing geometric data have been widely developed in the recent years, but they are mainly designed for closed polyhedral surfaces which are manifold or nearly manifold. We propose here a progressive geometry compression scheme which can handle manifold models as well as triangle soups and 3D tetrahedral meshes. The method is lossless when the decompression is complete which is extremely important in some domains such as medical or finite element. While most existing methods enumerate the vertices of the mesh in an order depending on the connectivity, we use a kd-tree technique [Devillers and Gandoin 2000] which does not depend on the connectivity. Then we compute a compatible sequence of meshes which can be encoded using edge expansion [Hoppe et al. 1993] and vertex split [Popovic and Hoppe 1997]. The main contributions of this paper are: the idea of using the kd-tree encoding of the geometry to drive the construction of a sequence of meshes, an improved coding of the edge expansion and vertex split since the vertices to split are implicitly defined, a prediction scheme which reduces the code for simplices incident to the split vertex, and a new generalization of the edge expansion operation to tetrahedral meshes.
May 13, 2002	Geometry Images
Presented By	Chang Ha Lee, University of Maryland, College Park
Comments	Graphics Seminar Series
Abstract	Geometry Images by Xianfeng Gu, Steven J. Gortler, and Hugues Hoppe Surface geometry is often modeled with irregular triangle meshes. The process of remeshing refers to approximating such geometry using a mesh with (semi)-regular connectivity, which has advantages for many graphics applications. However, current techniques for remeshing arbitrary surfaces create only semi-regular meshes. The original mesh is typically decomposed into a set of disk-like charts, onto which the geometry is parametrized and sampled. In this paper, we propose to remesh an arbitrary surface onto a completely regular structure we call a geometry image. It captures geometry as a simple 2D array of quantized points. Surface signals like normals and colors are stored in similar 2D arrays using the same implicit surface parametrization - texture coordinates are absent. To create a geometry image, we cut an arbitrary mesh along a network of edge paths, and parametrize the resulting single chart onto a square. Geometry images can be encoded using traditional image compression algorithms, such as wavelet-based coders.
April 17, 2002	Behind the Scenes at Pixar
Presented By	Tony DeRose, Pixar Animation Studios
Comments	Distinguished Seminar Series on Vision in honor of Prof. Azriel Rosenfeld
Abstract	For more than a decade Pixar has been a leader in the area of computer animation. In this talk I'll highlight: a number of the technical challenges that we've addressed in the past, some of the challenges that face us today, and some of the challenges that will likely face us in the coming decade.
March 18, 2002	A Real-time Seamless Tiled Display System for 3D Graphics
Presented By	Zhiyun Li, University of Maryland, College Park
Comments	Graphics Seminar Series
Abstract	We outline our seamless tiled display system for interactive 3D graphics applications that is low-cost, easy to calibrate, scalable, and portable. Our system achieves geometric alignment in software by pre-warping the 3D space in contrast with the current systems that usually achieve this by 2D image pre-warping. Our system accomplishes this through real-time image capture from a digital camcorder, image segmentation, and derivation of the 3D warping matrices for each 3D graphics pipeline that feeds a projector. Our prototype system demonstrates our results on a 2 x 2 tiled array of projectors.
March 6, 2002	Immersion and Tele-immersion in the Office of the Future
Presented By	Henry Fuchs, University of North Carolina at Chapel Hill
Comments	Distinguished Seminar Series on Vision in honor of Prof. Azriel Rosenfeld
Abstract	We envision an Office of the Future in which images are displayed on walls and other surfaces to provide an immersive environment with a sense of common presence among local and distant participants and their shared work objects. Much of the tele-immersion portion of our research has been part of a four-site National Tele-Immersion Initiative (NTII) lead by Jaron Lanier, chief scientist of the principal funder, Advanced Network & Services. In the current, primitive implementation, we at NTII use clusters of seven digital cameras to acquire the changing 3D surface of each remote partner. These live 3D images are merged with (presently, pre-acquired) 3D scans of each remote office and the common work objects and all these are displayed in head-tracked stereo on walls of the local office. UNC's Office of the Future project and tele-collaboration has also been part of the 5-site (USA) NSF Science and Technology Center in Graphics and Scientific Visualization. Related efforts include panoramic acquisition by clusters of cameras, new image-based rendering methods, and wide-area displays ("video walls") built with numerous casually-placed ceiling-mounted projectors that are automatically calibrated by multiple cameras. We hope these efforts will improve today's ubiquitous personal computers so they will no longer be so restricted by their desktop monitors, but will emerge to integrate smoothly with their users' 3D physical work environments.
March 4, 2002	Point Matching
Presented By	Chang Ha Lee, University of Maryland, College Park
Comments	Graphics Seminar Series
Abstract	"Geometric Hashing: A General and Efficient Model-Based Recognition Scheme" by Yehezkel Lamdan and Haim J. Wolfson A general method for model-based object recognition in occluded scene is presented. It is based on geometric hashing. The method stands out for its efficiency. We describe the general framework of the method and illustrate its applications for various recognition problems both in 3-D and 2-D. Special attention is given to the recognition of 3-D objects in occluded scenes from 2-D gray scale images. New experimental results are included for this important case. "Comparing Images Using the Hausdorff Distance" by Daniel P. Huttenlocher, Gregory A. Klanderman, and William J. Rucklidge The Hausdorff distance measures the extent to which each point of a model set lies near some point of an image set and vice versa. Thus this distance can be used to determine the degree of resemblance between two objects that are superimposed on one another. In this paper we provide efficient algorithms for computing the Hausdorff distance between all possible relative positions of a binary image and a model. We focus primarily on the case in which the model is only allowed to translate with respect to the image. Then we consider how to extend the techniques to rigid motion (translation and rotation). The Hausdorff distance computation differs from many other shape comparison methods in that no correspondence between the model and the image is derived. The method is quite tolerant of small position errors as occur with edge detectors and other feature extraction methods. Moreover, we show how the method extends naturally to the problem of comparing a portion of a model against an image.
February 25, 2002	Flash Animation
Presented By	Zhiyun Li, University of Maryland, College Park
Comments	Graphics Seminar Series
Abstract	1. Basic steps to create a flash animation. 2. Advanced animation effects including color effect, shape change, curve path and multiple layers. 3. Video I/O. 4. Tips for easy operation. 5. Illustration of every steps by a simple demo.
February 18, 2002	Anti-aliased Hemicubes for Performance Improvement in Radiosity Solutions
Presented By	Sharat Chandran, Visiting Professor, University of Maryland at College Park
Comments	Graphics Seminar Series
Abstract	Several important and fascinating aspects of realistic images are captured by the radiosity method. In this work we use an alternate form of the classical hemicube that reduces aliasing problems inherent in the original method without giving up the computational advantages of the hemicube. Unlike other methods, we explicitly consider the effect of the relative order of partial visibility in a hemicube cell when recording form factors. This enables us to compute form factors accurately (even) in progressive refinement radiosity with adaptive substructuring. Our empirical results with progressive refinement radiosity show superior mesh density where fine details are required, (such as in soft shadows), as well as in areas that produce singularities (such as when inter element distances tend to zero). Our method is contrasted with hierarchical radiosity which uses raycasting for form factor and visibility computations. On a low end Intel Linux platform, and for a comparable image quality, our method takes substantially less time.