Presentation of Research Paper "Real-time Screen-space Geometry Draping for 3D Digital Terrain Models" at 23rd International Conference on Information Visualisation (IV 2019) in Paris, France.

1. Real-time Screen-space Geometry Draping
for 3D Digital Terrain Models
Matthias Trapp, Jürgen Döllner
Hasso Plattner Institute, Faculty of Digital Engineering, University of Potsdam, Germany
23rd International Conference Information Visualisation
2 - 5 July 2019 ● University of Paris 13 ● Paris ● France

2. Motivation – What is Draping?
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Draping Process
Digital Surface/Elevation/Terrain Model: Draping Target (DT) DrapingcResult
Geometry: Draping Source (DS)
Projection of 2D/2.5D geometric features (lines, polygons) onto 2.5D/3D geometry.

3. Motivation – Draping Applications
In GIS, features = polygonal data representing:
§ Land coverage data
§ Glyphs
§ Transportation networks
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[Scheider & Klein, 2007]

4. Motivation – Draping Applications
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Golden Gate Bridge, Google Earth, version 7.1.2.2041
Street overlay does not match digital terrain model

5. Draping: Problem and Challenges
Problem: Different geometric representations:
§ Digital Surface Model (DSM)
§ Digital Elevation Model (DEM)
§ Digital Terrain Model (DTM)
Major challenges:
§ Terrain rendering can rely on Level-of-Detail mechanisms
§ Draping source or target geometry be dynamically
§ Sufficient run-time performance for interaction
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Mismatch in correspondence and tessellation level
Feature is matching digital elevation model
Feature is matching 3D terrain model

6. Categorization of Draping Approaches
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3D DTM Optimal Result Projective Texturing
Stencil-based Shadow Volumes Geometry Draping (Scene)

7. Texture-based Draping (TBD)
Traditional approach based on projective texturing:
§ 2D texture created from geometry via render-to-texture (possibly multi-resolution)
§ Using texture trees to organize texture data and for Level-of-Detail (LoD) rendering
§ Sufficient for large viewing-distances (flight simulation)
Problems:
§ High memory consumption
§ Pre-computation necessary
§ Not suitable for dynamic DS
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Dynamic texture tree [Kers]ng & Döllner, 2002]

8. Stencil-based Draping (SBD)
Feature projection based on Shadow Volumes:
§ GPU-based implementation of draping
§ Resulting appearance of DS are hard to control
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[Vaaraniemi et al., 2011]

9. Geometry-based Draping (GBD)
Scene-space approach:
§ CPU-based draping performed in pre-processing step
§ Common problems:
§ Preprocessing is LoD-dependent
§ Possible Z-figh]ng during rasteriza]on
Screen-space approach:
§ Draping based on projected scene geometry
§ Exis]ng approaches are precise but costly (CPU)
[Kers]ng & Döllner, 2002]
[Ohlarik & Cozzi, 2009]
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10. Screen-space Geometry Draping Pipeline
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11. G-Buffer Generation
Geometric information for projecting geometry in image-space:
§ G-Buffer contains: world-space normal and position (z-component only), color
§ Can also be used for deferred shading / stylization and generated in single-rendering pass
G-Buffer
Normal + Z Color
3D Geometry
Rasteriza]on
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12. Screen-space Geometry Projection
Approach:
§ Projection computed on a per-vertex basis
§ Binary search for corresponding surface point
§ Parallel implementation using vertex shader
Fitted Geometry Planar Geometry Normal + Z
N
VI
VO
fetch z-value
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13. ComposiUng and Display
§ Basically post-processing pass using screen-aligned quad
§ Integra]on of projected geometry using alpha-blending per close-up
§ Subsequent deferred shading/texturing/styliza]on is easily possible
Final Image Projected Geometry Color Texture
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14. Evaluation of Run-time Performance
Test Data (DT):
§ Regular grid (793 x 793)
§ 67 330 Ver]ces, 131 072 triangles (indexed)
Test Hardware:
§ NVIDIA GeForce GTX 970 GPU with 4096 MB VRAM
§ Intel Xeon CPU with 2:8 GHz and 12 GB RAM
Test Procedure :
§ DSs of three geometric complexi]es (indexed representa]on)
§ Viewport resolu]on: 1280 x 720
§ Average of 500 rendered frames
#VerKces #Triangels
Frames-per-
Second
2500 4800 310.6
4900 9522 226.3
10000 19602 152.1
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15. Extension: Adaptive Tessellation
Multi-pass hardware-accelerated tessellation of DS:
§ Computing geometric error/distance of primitives
§ Determine tessellation levels based on geometric error
§ Abort criteria for tessellation over geometric error
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Visualization of inner/outer
tessellation levels of OpenGL
tessellation shader API.
Pass 1 Pass 2 Pass 3
Pass 4 Pass 5 Pass 6

16. Ti
Vi0
Vi1
Vi2
Ni
EiA
dA
Extension: Adaptive Tessellation
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Possible GPU-based approach:
§ Compute distance variances per-edge for outer tessellation factor
§ Computer distance variances per-face for inner tessellation factor
Visualization of inner/outer
tessellation levels of OpenGL
tessellation shader API.

17. Extension: Overlapping Geometry
Possible GPU-based approach:
§ Represent geometry image using K-buffer
§ Extend projection algorithm to work with K-Buffer representation
§ Requires z-leveling of input geometry and reference surface normal
3D Terrain Model
Street (z-level: 0)
Street (z-level: 1)
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18. Wrap Up
Overview of draping approaches presented
Interactive GPU-based rendering technique:
§ Screen-space approach that supports dynamic draping sources and targets
§ Real-time rendering performance, space for improvements
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19. Thank you for your Attention
Contact Information:
§ Matthias Trapp
matthias.trapp@hpi.de
§ Jürgen Döllner
juergen.doellner@hpi.de
Computer Graphics System Group www.hpi3d.de