Hardware-Assisted
CSG Rendering
Fabiano Romeiro,
Harvard University
Luiz Velho,
IMPA
Luiz Henrique de Figueiredo
IMPA
Theme
• CSG (Constructive Solid Geometry):
Hierarchical modeling of complex objects
> Generated by combinations of simple primitives
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Motivation
• Interactivity is important:
– Makes design easier
– Handle complex objects
• However, it is difficult:
– Scalability
Previous Work
• Initial Approaches (CPU)
– Boundary representations
– Depth-layering
[Goldfeather et al.]
• Recent Approaches (GPU)
– Stencil Buffer
[Wiegand], [Rappoport], [Stewart]
– Depth-peeling
Blister, [Hable and Rossignac]
Our Approach
• Current Algorithms are bandwidth limited
(i.e., depth layering, depth peeling, etc)
• GOAL:
– Make better use of GPU processing power
– Shift bottleneck:
Bandwidth
GPU Instruction throughput
Overview
• Perform spatial subdivision on the CPU
until CSG expression is locally simple enough
to render on the GPU.
• Locally Simple
Surface within a cell is:
– Single primitive
– Boolean operation of two primitives
Algorithm
1. Subdivision Step
•
Create Hierarchical Spatial Decomposition
based on the CSG expression
2. Ray Tracing Step
•
Traverse Subdivision Structure and
Render each Cell
Subdivision Step
• Subdivision Structure
– Modified octree (sequenced kd-tree)
Illustrative Subdivision Example in 2D
PINK CELLS: boolean operation of 2 primitives
GREEN CELLS: single primitive
GREY CELLS: Do not intersect CSG surface
YELLOW CELLS: project to less than a
threshold of pixels and are ignored
Real Subdivision Example
Visual Trade-Off
• Quality x Speed:
Increasing threshold gives better performance but
introduces visible artifacts
1 pixel
* OBS: No make-up!
20 pixels
Ray-Tracing on the GPU
• Draw faces of Object’s
Proxy Bounding Volume
• Use Vertex and Pixel shaders
to trace rays on the GPU
Ray-Tracing Step
• Octree Traversal
– Recursive front-to-back order
• At leaf cells
– Take action according to cell classification:
1.
Bind Apropriate
Pixel Shader
2.
3.
Pass
Parameters
to Shader
Render
Faces of
Cell
CSG Ray-Tracing
• Two Types of Cells:
a) Single primitive shader
b) Boolean operation of 2 primitives
o
o
o
o
Results
• Measurements performed in 2 configurations:
– C1: Pentium-M 1.4GHz, Geforce FX Go5200
– C2: Athlon 64 3800+, Geforce 6800 GT
• To evaluate performance we measured:
– S: Subdivision time (CPU)
– TR: Traversal and rendering time (GPU)
– Ts: Setup time (Bandwidth)
Results
WIDGET
CK200
CK500
~ 1.4x
0% of rendering time
Model
S
CK1000
8% of rendering time
TS
TR
C1
C2
C1
C2
C1
C2
WIDGET
17
12
9
5
2262
59
40x
CK200
223
163
84
42
6331
346
18x
CK500
1232
888
432
213
16046
1514
11x
CK1000
3705
2658
1265
626
34975
4324
8x
1%
of rendering
time
3%
4% of
rendering time
14% of rendering time
12%
Demo
Conclusions
• Performance comparable to best current methods
• For models with ~ 500 primitives
– GPU instruction throughput limited
• Next generation GPUs
– Expect to surpass other methods on complex models