David Luebke 1 04/22/23

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

My favorite ter

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LOD In A Nutshell

Create levels of detail (LODs) of each object in a preprocess:

50 Vertices 500 Vertices 2000 Vertices

Model courtesy of InfoGraphica

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LOD In A Nutshell

Distant objects use coarser LODs:

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Creating LODs

How to create LODs of a polygonal object? 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

What criteria should we try to preserve in the simplified object? A: Its visual appearance

Silhouettes matter (so what do we do?) Large flat regions can be simplified Watch for distortion of texture/color/normals

We often measure fidelity with geometric criteria: Volume swept out by displaced surface Distance from old surface to new surface

Can use this to estimate silhouette distortion in screenspace

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Creating LODs

How to generate a version of the object with fewer polygons? Four basic mechanisms:

Sample-and-reconstruct Decimation Vertex-merging Adaptive subdivision

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Creating LODs: Mechanism

Sample and reconstruct 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

Decimation Iteratively remove faces or vertices, retriangulating

hole created in current surface during the process (draw it)

Triangulation: well understood problem One common algorithm: Loop splitting

Pick which face/vertex to remove based on important criteria

Curvature, size of associated triangles

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Creating LODs: Mechanism

Vertex merging 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

Adaptive subdivision 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: Curvature (1 / maximum edge angle) Size of polygons (edge length)

Filter out degenerate polygons

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Rossignac-Borrel

Resolution of grid determines degree of simplification Coarse grid lots of simplification Fine grid little simplification

Representing degenerate triangles Edges use OpenGL line primitive Points use OpenGL point primitive

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Rossignac-Borrel

Low and Tan, 1997 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

Pros Fast, very fast Robust (topology-insensitive)

Cons 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

Rossignac-Borrel examples:

10,108 polys 1,383 polys 474 polys 46 polys

Courtesy IBM

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The End