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Image Space Based Visualization of Unsteady
Flow on Surfaces
Robert LarameeBruno JobardHelwig Hauser
Presenter: Bob Armstrong24 January 2007
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What's This About?
New Algorithm: ISBV Generates dense representations of
arbitrary fluid flow on computational fluid dynamics (CFD) surfaces
Applied to unsteady flow on boundary surfaces of complex meshes from CFDs of more than 250K polygons, dynamic meshes, medical data
Seeing is believing
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Visualization of flow on the surface of an intake manifold. The ideal intake manifold distributes flow evenly to the piston valves.
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Visualization of flow at the complex surface of a cooling jacket – a composite of over 250,000 polygons
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The Baldwin-Lomax model is a classical algebraic turbulence model which is suitable for high-speed flows with thin attached boundary-layers, typically present in aerospace and turbomachinery applications.
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What's the Problem? Traditional direct visualization of
unsteady flow is computationally expensive No real-time production Animation/Simulation is expensive
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Their Approach
Build off of Past Work LEA IBFV
Reduce complexity by generating 3D vector fields onto 2D image space
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Past Work
Foundation for the ISBV Algorithm Both produce dense representations of
unsteady, 2D vector fields LEA:
Lagrangian-Eulerian Advection IBFV:
Image Based Flow Visualization
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Lagrangian-Eulerian Advection
Produces animations with high spatio-temporal correlation
All data stored in 2D arrays Each frame depicts instantaneous flow
structure Generates animations at interactive
frame rates
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Image Based Flow Visualization
Simulation of 2D fluid flow Each frame of a flow animation is a
blend between a warped version of the previous image and a number of background images.
Relies on graphics hardware
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IBFV Method
image k distort render blend
k = k + 1
Note Distortion Phase: There is nothing to stop the image from being advected outside of the
geometrical boundary.
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Definition: CFD
Computational Fluid Dynamics Branch of Fluid Mechanics Relies on Numerical Methods &
Algorithms
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More CFD
How does one treat a continuous fluid in a discretized fashion on a computer? Widely used: Discretize the spatial
domain into small cells that form a grid or mesh
Apply suitable algorithm to solve the equations of motion.
www.cfd-online.com
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Definition: Advection
Advection is transport of a conserved scalar quantity in a vector field.
A good example is the transport of pollutants or silt in a river: the motion of the water carries these impurities downstream.
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Different Advections
Velocity Vorticity Vorticity mapped to helicity
More information here:http://www.vrvis.at/scivis/laramee/isa-streamsurface/
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A hybrid stream surface-texture advection visualization showing tumble motion inside a gas engine cylinder.
Texture is advected according to the velocity field and color is mapped to velocity magnitude.
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The same stream surface geometry.
Texture is advected according to the vorticity field and color is mapped to vorticity magnitude. The relationship to the velocity field can be explored in a novel fashion.
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The same stream surface geometry.
Texture is advected according to the vorticity field and color is mapped to helicity. Mapping color to helicity indicates candidate vortex core regions.
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Traditional Visualization Methods
Maps one or more 2D textures to a 3D geometrical surface
Textured geometry rendered to an image space
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ISBV Algorithm
Project surface geometry to image space
Apply texturing Texture properties are advected in
image space
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ISBV Method (1)
Associate the 3D flow data with polygons at boundary surface
Project the surface and vector field onto the image plane
Identify geometric discontinuities Advect texture properties according to
the vector field in image space
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ISBV Method (2)
Inject and blend noise Apply additional blending along the
geometric discontinuities previously identified
Overlay all optional visualization cues such as showing a semi-transparent representation of the surface with shading
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Flow DiagramVector FieldProjection
Edge Detection
ComputeAdvection Mesh
Edge Blending
Noise Blending
Image OverlayApplication
Image Advection
DynamicCasek = k+1
StaticCase
k = k+1
k = time as a frame #
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Vector Edge Projection
Project vector field to the image plane Velocity vectors stored at the polygon
vertices Encode velocity vectors as color values
at the mesh vertices
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Color & Velocity Component
Color assignment done as a scaling operation
Each color component assigned velocity
h r =v x−vminx
vmax x−vminx
hg =v y−vmin y
ymax y−vmin y
hb = v z−vminz
vmax z−vminz
hrgbv xvz
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Why Use Velocity Image Fields?
Advection computation, noise blending simpler in image space
Vector field and polygon mesh decoupled
Hardware-accelerated occlusion culling Supported on graphics hardware
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Velocity Vectors Decoded vectors used to compute
advection mesh, then projected upon the image plane
v x = hr ⋅ vmax x−vminx vminx
v y = hg ⋅ vmax y−vminy vmin y
vz = hb ⋅ vmax z−vminz vmin z
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Timing of Projection of Vectors
Project vectors to image plane after velocity image construction Don't have to project all vectors Use original 3D vectors for the velocity
mask
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Example Velocity Image Field
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Edge Detection
Edge detection solves the problem of artificial flow continuity because of artifacts from 3D to 2D image space projection
Selectively detects only edges affecting flow texture properties
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Problem with Sharp Edges
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Geometric Edge Boundaries
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Edge Blending
During this phase of the algorithm Introduce discontinuities in the texture
aligned with discontinuities from the surface
Essentially, ensure that edges are accounted for visually
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Geometric Edge Detection
Disabled Enabled
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Compute Advection Mesh
Distort regular, rectilinear mesh according to velocity vectors stored at mesh grid intersections
p k −1 = p k− vp p k−1 ; t t
p is a path line, k is a frame
Above ensures that distortion does not extend beyond geometric boundaries.
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Image Advection & Texture Clipping
Coarse ResolutionAdvection Mesh, With Artifacts
Texture Clipping applied
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Noise Blending
Use IBVF method for noise injection and blending
F p ; k = i=0k −1 1− iG P k −1 ; k− 1
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Blending
SampleNoiseImage
SampleBlendedImage
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No Edge Blending
With Edge
Blending
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Image Overlay Application
Optional step Overlay enhances visualization with
colorization, shading, etc.
OverlayApplied
Compositeof All
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Vector Projection
The projection of the vectors to the image plane is done after velocity image construction for 2 reasons: not all of the vectors have to be
projected, thus saving computation time can use the original 3D vectors for the
velocity mask
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Performance
Data Set
Ring 10K128 x 128 18 (5)256 x 256 9 (3)512 x 512 3 (1)
Intake Manifold 48K128 x 128 22 (2)256 x 256 14 (2)512 x 512 6 (1)
79K128 x 128 17 (2)256 x 256 10 (2)512 x 512 4 (1)
Intake Port 221K128 x 128 17 (0.5)256 x 256 7 (0.5)512 x 512 2 (0.3)
# of Polygons
Advection Mesh Resolution
Frames Per Sec
Combustion Chamber
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Performance Discussed
Authors looking for speedup; specifically, increased frame rates up to 20 fps
Performance tied to image resolution & polygon count (unsteady state flow)
Don't have any good comparative figures
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79K polygons
dynamic geometry
dynamic topology
time-lapsed pics
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221K Polygonal Intake Port
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Intake Port Mesh Close-Up