Interactive Rendering of Meso-structure Surface Details using Semi-transparent 3D Textures
Vision, Modeling, Visualization
Erlangen, Germany
November 16-18, 2005
Jean-François Dufort, Luc Leblanc, Pierre Poulin
LIGUM, Université de Montréal
Goals and Motivation
Hardware rendering semi-transparent details Flexible
Arbitrary mesh and 3D texture Mesh animation Texture animation
Semi-transparency neglected in most applications Important rendering features
Color blending because of semi-transparency Filtering Displaced silhouettes
Motivation
Motivation
Previous Work :Bump and Parallax mapping
[Welsh 2004]
Previous Work : Displacement Mapping
[Wang et al. 2003]
Adaptive tesselation [Moule and McCool 2002]
View-dependent displacement map [Wang et al. 2003]
Ray-tracing in height field [Hirche et al. 2003]
Previous Work : 3D Textures
Using proxy geometry
[Meyer and Neyret 1998]
[Lensch et al. 2002]
Generalized displacement maps
[Wang et al. 2004]
Shells maps
[Porumbescu et al. 2005][Wang et al. 2004]
Previous Work :Volume Rendering
Data inside a grid Regular Tetrahedra [Kraus et al. 2004]
Ray marching
Plan
Algorithm overview Mesh processing Tetrahedra sorting Vertex shader: Ray construction Fragment shader: Opacity/color integration Results Conclusion and future work
Algorithm Overview
Mesh extrusionCPU
• Computed once
• Extrude a shell from the surface triangular mesh
• Divide each shell prism into three tetrahedra
Algorithm Overview
Mesh extrusionCPU Tetrahedra sorting
• Alpha blending involves sorting
• Sorting tetrahedra with the SXMPVO algorithm
Algorithm Overview
Mesh extrusionCPU
GPU
Tetrahedra sorting
Ray creation
• Vertex shader
• Defines an object-space ray within each tetrahedron
Algorithm Overview
Mesh extrusionCPU
GPU
Tetrahedra sorting
Ray creationRendering loop
Color integration
• Per pixel, maps object ray in
texture space
• 3D texture sampling by ray marching
• Accumulate color in frame buffer
Mesh Processing
We map tetrahedra on base mesh Defines a tetrahedral shell in which the 3D
texture is mapped
Mesh Processing
Mesh Processing
Mesh Processing
Affine transformation from object space to texture space
Ptex = Mobj->tex Pobj
Tetrahedra Sorting
Alpha blending requires sorting SXMPVO: graph of « behind » relations
[Cook et al. 2004]
GA
B CF
E
D
Tetrahedra Sorting
Adjacent faces : object space
A
B C
G
F
E
D
Tetrahedra Sorting
Non-adjacent faces using A-buffer : screen space
A
B C
G
F
E
D
Tetrahedra Sorting
Non-adjacent faces using A-buffer : screen space
A
B C
G
F
E
D
Tetrahedra Sorting
Depth-first traversal
A
B
D
E
CF
G
F
F
G
G
E
E
C
C
D
D
B
B
A
A
A
B CF
E
D
G
Ray Construction
Tetrahedra sent down the pipeline Each vertex has
View vector Plane equation of other three faces
Vertex shader computes ray intersection with faces
Ray Construction : 2D Example Red triangle rasterized
Vertices A and B, Planes F1 and F2
B
F1
F2
A
Ray Construction : 2D Example
Compute distance between A and other faces from viewpoint
AIA1
IA2
Ray Construction : 2D Example
Compute distance between B and other faces from viewpoint
BIB1
IB2
Ray Construction : 2D Example Rasterization interpolates distances for each pixel covered
We keep the closest valid point
B
A
Color/Opacity Integration
Fragment shader picks closest valid intersection point
Matrix Mobj->tex maps point in texture space Defines texture ray in texture space Ray marching
Color/Opacity Integration
3D normal map for shading
Front-to-back blending Alpha at sample
location is modified based on distance traveled by light in object space
Performances
Implemented 2 schemes Uniform sampling DDA grid traversal
Brute force uniform sampling worked best in our test cases (3D textures 512x512x32) Fixed number of iterations User defined, trade-off for quality
Main bottleneck : sorting on CPU
Results : Filtering
Results : Transparency
Results : Volumetric Shadows
Results : Simpler Opaque Case
Results : Real-time Video
Conclusion
We reached our goals Interactive rendering of arbitrary 3D textures using GPU Flexible: Semi-transparent and opaque Rendering effects (transparency, volumetric shadows) Allows animation of mesh and texture
However… Not fast enough for real time Improve performance with precomputation? Faster sampling scheme?
Future Work
Level-of-detail framework Geometric details filtering Improve shading effects
Absorption within 3D texture Light scattering
Thank you! Questions?