Transcript lecture11.ppt

CS 551/651:
Advanced Computer Graphics
Regulating Level of Detail
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Level of Detail
Problem: even after visibility, model may
contain too many polygons
 Idea: reduce complexity further by managing
level of detail (LOD)
 A.k.a. multiresolution modeling
 A.k.a. polygonal simplification
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My favorite ter
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LOD In A Nutshell
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Create levels of detail (LODs) of each object in
a preprocess:
50 Vertices
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500 Vertices
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2000 Vertices
Model courtesy of InfoGraphica
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LOD In A Nutshell
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Distant objects use coarser LODs:
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Creating LODs
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How to create LODs of a polygonal object?
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Where should we simplify the object and where
should we preserve detail?
What mechanism to generate a version of the
object with fewer polygons?
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Creating LODs
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What criteria should we try to preserve in the
simplified object?
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A: Its visual appearance
 Silhouettes
matter (so what do we do?)
 Large flat regions can be simplified
 Watch for distortion of texture/color/normals
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We often measure fidelity with geometric criteria:
 Volume
swept out by displaced surface
 Distance from old surface to new surface
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Can use this to estimate silhouette distortion in screenspace
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Creating LODs
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How to generate a version of the object with
fewer polygons?
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Four basic mechanisms:
 Sample-and-reconstruct
 Decimation
 Vertex-merging
 Adaptive
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subdivision
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Creating LODs: Mechanism
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Sample and reconstruct
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Scatter surface with sample points, then
recreate using fewer sample points than original
surface had vertices
Where to put the sample points?
One answer: scatter at random, then let them
repel each other
How to recreate surface from samples?
A: Good question! (interesting, hot topic)
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Creating LODs: Mechanism
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Decimation
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Iteratively remove faces or vertices, retriangulating
hole created in current surface during the process
(draw it)
Triangulation: well understood problem
 One
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common algorithm: Loop splitting
Pick which face/vertex to remove based on
important criteria
 Curvature,
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size of associated triangles
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Creating LODs: Mechanism
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Vertex merging
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Collapse multiple vertices together and remove
degenerate triangles
Edge collapse: Specific form of vertex merge
operating on exactly two vertices that share an
edge
 Removes
exactly two (adjacent) triangles
 Why might this be preferable? Why not?
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Creating LODs: Mechanism
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Adaptive subdivision
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Create a very simple base model that represents the
model
Selectively subdivide faces of base model until
fidelity criterion met (draw)
Big potential application: multiresolution modeling
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Algorithm 1:
Rossignac-Borrel
Rossignac and Borrel, 1992
 Apply a uniform 3D grid to the object
 Collapse all vertices in each grid cell to single
most important vertex, defined by:
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Curvature (1 / maximum edge angle)
Size of polygons (edge length)
Filter out degenerate polygons
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Rossignac-Borrel
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Resolution of grid determines degree of
simplification
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Coarse grid  lots of simplification
Fine grid  little simplification
Representing degenerate triangles
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Edges  use OpenGL line primitive
Points  use OpenGL point primitive
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Rossignac-Borrel
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Low and Tan, 1997
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Refinement of Rossignac-Borrel
 Use
cos(max edge angle/2) for curvature
 Floating-cell clustering
 Thick lines and dynamic shading
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Rossignac-Borrel
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Pros
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Fast, very fast
Robust (topology-insensitive)
Cons
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Difficult to specify simplification degree
Low fidelity (topology-insensitive)
Underlying grid creates sensitivity to model
orientation in Rossignac-Borrel
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Rossignac-Borrel
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Rossignac-Borrel examples:
10,108 polys 1,383 polys
Courtesy IBM
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474 polys
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46 polys
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The End
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