Efficient LDI Representation (TPCG 2008)

Matthias Trapp , Jürgen Döllner Hasso-Plattner-Institute, University of Potsdam, Germany 9 th June 2008 University of Manchester Efficient Representation of Layered Depth Images for Real-time Volumetric Tests

Motivation: Volumetric Tests … determine if a 3D point is inside or outside a given volume Areas of application: Generalized clipping, Rendering with hybrid styles, 3D magic lenses,… Characteristics: Volume representation = LDI Test performed in shader program Applicable in real-time

LDI Representation of Solid Meshes

Depth-Peeling to 3D Textures Algorithm for LDI creation: Setup orthographic projection, adjust near/far planes Determine depth complexity of shape Create and initialize 3D texture (LDI) Depth-peel shape: Render-to-texture (into slice of 3D texture) Use linear depth buffer values GLSL fragment shader for 2 nd depth test (SM4)

Volumetric Parity Test (VPT) Algorithm “ to determine if a point is inside or outside a complex 3D volume represented by an LDI” Given: arbitrary 3D point: Requested: Boolean parity: Solution: Transformation into LDI texture space: Perform ray-marching through depth maps

Ray-Marching through LDI Texture Space Algorithm: Construct ray R : Sample form each slice: Compare depth values:

Limitations of Current Representation Memory Consumptions: 3D texture size: resolution 2 * d Usually sparsely utilized Depth-Peeling Performance: Depends on depth complexity of shape Ray-Marching Performance: Depends on number of texture samples So far: d samples per ray necessary Goals: Minimal texture size: Cropping, Compression Minimal peeling passes: Viewpoint Selection Minimal texture samples per test: Compression

Outline – Next Topics Preprocessing Pipeline: Overview Viewpoint Selection Bounding Box Calculation & Cropping LDI Compression Algorithm Performance Results Limitations & Future Work Conclusions

Preprocessing Pipeline: Overview 5 Step process involves GPU and CPU: Viewpoint selection (O DP ) Depth-peeling to 3D texture (LDI) Calculate axis-aligned bounding box (AABB LDI ) Perform cropping (LDI Crop ) Compress cropped texture (LDI Comp )

Viewpoint Selection Find viewpoint for LDI creation based on: Minimal depth complexity: d Maximal texture coverage: o Determine camera orientation Altering viewpoint on bounding sphere Determine and store in list : Depth complexity Altered camera orientation Texture coverage Sort list to obtain result

Bounding Box Calculation & Cropping

Compressing Depth Ranges Depth Ranges: Group depth values: d i = 0,…,d-1 DR j = (S j ,E j ), j = 0,…,d/2 Interval [S j , E j ], S j = d 2j , E j = d 2j+1 denotes inside of volume Lossless Compression Idea: Sequential align all depth ranges of a Ray R Fetch start ( S j ) and end ( S j ) with single instruction DR 0 DR 1

Compression Algorithm – Overview Two-phase process: Extract depth ranges from 3D texture into intermediate representation Pack depth ranges from intermediate representation into 2D texture

Compression - Extract Phase Create intermediate representation: Prepare data structure for pack phase Read-back LDI from GPU Create 4 arrays from LDI: Index Segments ( IS offset, IS ranges ) Range Segments ( RS start , RS end )

Compression - Pack Phase Pack arrays into 2D texture Texture format: Luminance-Alpha Float point precision: unclamped values Texture resolution: Store successively: IS offset , RS start in luminance channel IS ranges , RS end in alpha channel Issues: 2D texture format introduces unwrapping step in decompression Better: 1D texture, but resolution limited to 8192 pixel Possible Solution: Uniform Buffer (NVIDIA G80) + Geometry Shader

Performance Results 23,232 34,344 2,903 6,146 768 994 #Vertex 0.58 0.32 0.39 0.58 1.44 1.78 C ration 1,835,528 1,089,288 1,204,352 3,317,888 3,011,058 3,742,848 M comp 3,143,880 3,358,720 3,026,688 5,683,200 2,097,152 2,097,152 M crop 9.265 12.187 8.469 7.172 3.085 2.532 t view 25.875 73.078 2.156 25.766 0.187 0.187 t peel 0.125 0.078 0.09 0.219 0.297 0.437 t comp 0.2 0.313 0.234 0.203 0.078 0.077 t crop 12 18 16 12 6 2 d max 6 10 8 6 2 2 d min Knot Hebe Cow Potato Complex Sphere

Limitations & Future Work Limitations: Specific approach for depth ranges Too much instruction for decompression Small compression ratio 1:2-3 GPU & CPU (hybrid) approach Future Work: Toward a complete GPU implementation: (Bounding box calculation, cropping, compression) Main Goal: enable preprocessing in real-time New filtering concepts necessary to compensate undersampling / alaising artefacts

Conclusions Efficient LDI Representation trough: Automatic viewpoint selection Fast bounding box calculation & cropping Lossless LDI compression Decompression algorithm applicable in shader Compression Characteristics: Ratio 1:2-3 (common for lossless compression) Effective for non-convex meshes with high depth complexity Compression should be avoided for symmetric convex meshes Preprocessing, no real-time performance Future Work: Implementation fully GPU-based

Questions ? Contact: Matthias Trapp [email_address] Computer Graphics Systems Group Prof. Dr. Jürgen Döllner www.hpi.uni-potsdam.de/3d Researchgroup 3D-Geoinformation www.3dgi.de

Efficient LDI Representation (TPCG 2008)

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Efficient LDI Representation (TPCG 2008)