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Efficient Methods for Volumetric Illumination Linköping Studies in Science and Technology Dissertations No. 1406 Efficient Methods for Volumetric Illumination Frida Gyllensvärd Department of Science and Technology Linköping University Norrköping 2011 Efficient Methods for Volumetric Illumination Frida Gyllensvärd Cover Image: An illuminated CT volume. The global transport of light is calculated using local piecewise integration. © Eurographics Association 2008. Copyright © 2011 Frida Gyllensvärd, unless otherwise noted. Printed by LiU-Tryck, Linköping 2011 Linköping Studies in Science and Technology Dissertations No. 1406 ISBN 978-91-7393-041-3 ISSN 0345-7524 Abstract Modern imaging modalities can generate three-dimensional datasets with a very high detail level. To transfer all the information to the user in an efficient way there is a need for three-dimensional visualization. In order to enhance the diagnostic capabilities the utilized methods must supply the user with fast renderings that are easy to interpret correctly. It can thus be a challenge to visualize a three-dimensional dataset in a way that allows the user to perceive depth and shapes. A number of stereoscopic solutions are available on the market but it is in many situations more practical and less expensive to use ordinary two-dimensional displays. Incorporation of advanced illumination can, however, improve the perception of depth in a rendering of a volume. Cast shadows provide the user with clues of distances and object hierarchy. Simulating realistic light conditions is, however, complex and it can be difficult to reach interactive frame rates. Approximations and clever implementations are consequently required. This thesis presents efficient methods for calculation of illumination with the objective of providing the user with high spatial and shape perception. Two main types of light conditions, a single point light source and omni-directional illumination, are considered. Global transport of light is efficiently estimated using local piecewise integration which allows a graceful speed up compared to brute force techniques. Ambient light conditions are calculated by integrating the incident light along rays within a local neighborhood around each point in the volume. Furthermore, an approach that allows the user to highlight different tissues, using luminous materials, is also available in this thesis. A multiresolution data structure is employed in all the presented methods in order to support evaluation of illumination for large scale data at interactive frame rates. iii Acknowledgements First of all, I would like to thank my supervisors Anders Ynnerman and Anders Persson for being supportive and inspiring. I would also like to show my appreciation to Claes Lundström for valuable assistance during my last year as a PhD student. A special thanks to my co-author Patric Ljung for fruitful collaboration and helpful discussions. Thanks also to my co-author Tan Khoa Nguyen. I would also like to acknowl- edge Matthew Cooper for assistance in proof-reading of manuscripts. Furthermore, I wish to express appreciation to Center for Medical Image Science and Visualization (CMIV) for supplying me with interesting data to visualize. Thanks also to former and present colleagues at Media and Information Technology (MIT) and CMIV. I would also like to thank my family for all their love and encouragement. I am very grateful to my parents and sisters for being so supportive. Most of all I wish to thank my husband Björn and our little sunshine Elsa. You two truly fill my life with light! ♦ This work has been supported by the Swedish Research Council, grant 621-2004-3829 and 621-2008-4257, the Linnaeus Center CADICS and the Strategic Research Center MOVIII, founded by the Swedish Foundation for Strategic Research, SSF. v Contents 1 Introduction 3 1.1 Medical Visualization .............................. 4 1.2 From Data Acquisition to User Interaction .................. 5 1.2.1 Generation of Medical Data ...................... 5 1.2.2 Voxel Classification ........................... 7 1.2.3 Direct Volume Rendering ........................ 8 1.3 Challenges in Volumetric Illumination ..................... 9 1.4 Contributions .................................. 11 2 Aspects of Volumetric Lighting 13 2.1 Light Interactions ................................ 14 2.2 The Volume Rendering Integral ........................ 16 2.2.1 Local Estimation of g(s) ........................ 17 2.2.2 Numerical Approximation ....................... 19 2.2.3 GPU Raycasting ............................ 19 2.3 Global Illumination Approximations ...................... 20 2.3.1 Volumetric Shadows and Scattering Effects .............. 21 2.3.2 Ambient Occlusion ........................... 21 3 Improving Volumetric Illumination 25 3.1 Ambient Occlusion for Direct Volume Rendering ............... 25 3.2 Local Ambient Occlusion ............................ 26 3.2.1 Light Contribution ........................... 27 3.2.2 Absorption TF ............................. 29 3.3 Luminous Illumination Effects ......................... 29 3.3.1 Using Light as an Information Carrier ................ 31 3.4 Global Light Transport ............................. 32 3.4.1 Local Piecewise Integration ...................... 33 3.4.2 First Order Scattering Effects ..................... 37 3.5 Pipeline Processing ............................... 37 3.5.1 Flat Multiresolution Blocking ..................... 38 3.5.2 Multiresolution Illumination Estimations ............... 38 3.5.3 LAO Pipeline .............................. 40 vii viii 3.5.4 Pipeline for Illumination Estimations with Piecewise Integration .. 41 3.6 Sampling ..................................... 42 3.7 Illumination Quality .............................. 44 3.7.1 LAO Accuracy ............................. 44 3.7.2 Local Piecewise Integration Accuracy ................. 46 3.8 Performance ................................... 46 3.8.1 LAO Performance ............................ 46 3.8.2 Performance for Concurrent Volume Visualization .......... 47 3.8.3 Performance for Illumination Estimated with Piecewise Integration .......................... 48 4 Conclusions 51 4.1 Summary of Contributions ........................... 51 4.2 Approached Challenges ............................. 51 4.3 Future Research ................................. 54 Bibliography 57 I Efficient Ambient and Emissive Tissue Illumination using Local Occlusion in Multiresolution Volume Rendering 61 II Interactive Global Light Propagation in Direct Volume Rendering using Local Piecewise Integration 71 III Local Ambient Occlusion in Direct Volume Rendering 81 IV Concurrent Volume Visualization of Real-Time fMRI 95 List of Publications This thesis is based on the following papers, which will be referred to in the text by their Roman numerals. The papers are appended at the end of the thesis. I Efficient Ambient and Emissive Tissue Illumination using Local Occlusion in Multiresolution Volume Rendering Frida Hernell, Patric Ljung and Anders Ynnerman In Proceedings Eurographics/IEEE VGTC Symposium on Volume Graphics 2007, Prague, Czech Republic II Interactive Global Light Propagation in Direct Volume Rendering using Local Piecewise Integration Frida Hernell, Patric Ljung and Anders Ynnerman In Proceedings Eurographics/IEEE VGTC on Volume and Point-Based Graphics 2008, Los Angeles, California, USA III Local Ambient Occlusion in Direct Volume Rendering Frida Hernell, Patric Ljung and Anders Ynnerman IEEE Transactions on Visualization and Computer Graphics, Volume 16, Issue 4 (July- Aug), 2010 IV Concurrent Volume Visualization of Real-Time fMRI Tan Khoa Nguyen, A. Eklund, Henrik Ohlsson, Frida Hernell, Patric Ljung, Camilla Forsell, Mats Andersson, Hans Knutsson and Anders Ynnerman In Proceedings Eurographics/IEEE VGTC on Volume Graphics 2010, Norrköping, Sweden 1 2 Introduction 1 Vision is one of the body’s most important senses in order to create an awareness of the surrounding world. The ability to see is made possible by allowing the retina to intercept visible light and convert it to impulses that are sent to the brain for interpretation. The three-dimensional world can easily be misinterpreted since the retina only captures a two-dimensional image. Different cues are therefore necessary in order to perceive depth correctly. The most obvious cue is stereopsis which uses the binocular disparity from the eyes horizontal separation supplying the brain with clues of distances. In art it is desirable to trick the brain to believe that a painting has depth. In this case it is not possible to use stereopsis, so other cues are needed. A number of cues can be used such as, linear perspective, interposition and relative size. However, a very effective cue is caused by lighting [Lip82]. Cast shadows caused by a light source can help us to comprehend the spatial relationships between objects. Figure 1.1 demonstrates the importance of shadowing for determination of distance. In the left image the cone seems to be located behind the box and the cylinder, which is an illusion. As the shadows are shown, in the right image, it is suddenly more obvious that the cone is hovering above the ground and located along the same line as the other objects. Figure 1.1: The cast shadows (right image) are important in order to perceive the spatial relations between the three objects. When shadows are absent (left image) it becomes difficult to comprehend that the cone is hovering above the line that the box and the cylinder are located along. 3 4 1. INTRODUCTION Figure 1.2: Shading is very important
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