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Level of Detail. CS184-Sp05 Section. Level of Detail. Basic Idea Use simpler versions of an object as it makes less of a contribution Generation How to make different LODs Selection Chooses LOD based on some criteria Switching Change from one LOD to another. Switching. Popping problem

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Level of detail

Level of Detail

CS184-Sp05 Section


Level of detail1
Level of Detail

  • Basic Idea

    • Use simpler versions of an object as it makes less of a contribution

  • Generation

    • How to make different LODs

  • Selection

    • Chooses LOD based on some criteria

  • Switching

    • Change from one LOD to another


Switching
Switching

  • Popping problem

  • Discrete Geometry LODs

    • No blending. Bad popping.


Switching1
Switching

  • Blend LODs

    • Render LOD1 opaquely first, then LOD2 w/ increasing alpha val.

    • High overhead during transitions.


Switching2
Switching

  • Alpha LODs

    • Only 1 LOD per object.

    • As metric for selection increases, transparency is increased.

    • No popping.

    • When object completely disappears, don’t need to render it at all.

      • Only at this point is speedup attained.


Switching3
Switching

  • CLODs (Continuous Levels of Detail)

    • Huge set of LODs

      • Each level has 1 edge collapse (2 fewer polygons)

        • Edge collapse – vertex joined with another

        • Keep track of edge collapses to reverse (vertex split)

    • Can animate edge collapse for smooth transition

    • Problems:

      • Not all models look good.

      • Have to go through all prior edge collapses to get to a particular LOD


Switching4
Switching

  • Geomorph LODs

    • Interpolate between 2 LODs

      • Remember vertex connectivity between levels

    • E.g., animating the edge collapse in CLODs would be a geomorph


Progressive mesh

v6

v1

v1

v7

v4

v4

v2

v3

v3

v3

v5

v5

Progressive Mesh

  • CLOD as described earlier

    • Order to collapse edges is determined by some absolute cost metric

      • Always collapse in same order

    • Store edge, adjacent faces, and vertices of adjacent faces

      • When an edge collapses, the 2 endpoints disappear and a NEW vertex is placed somewhere (e.g. along the old edge).


View dependent progressive mesh
View-Dependent Progressive Mesh

  • View-dependent progressive mesh

    • Order in which we want to collapse edges is based on view

      • Will collapse in different orders for different views

      • Can’t just store in a list like w/ CLODs


Basic alg
Basic Alg.

  • First create the view-independent progressive mesh

  • At run-time, create a forest of LOD info

    • Start w/ the coarsest mesh

    • Try to split “best” vertex given view-dependent metric

      • If that split depends on splitting another vertex first, do that


Basic alg1

v6

v1

v1

v7

v4

v4

v2

v3

v3

v3

v5

v5

Basic Alg.

1) Create PM: Mn <- Mn-1 <- … M0

2) Create forest

M0

v1

v2

v3

Say our v.d. metric says to split v1 first.

Can we do that? (Hint: No)


Billboards
Billboards

  • So far, we’ve been working with meshes

  • Billboards

    • Render an image onto a polygon facing the viewer OR have several “cross-billboards”

    • Used in video games

    • Useful for simulating “special effects”

      • Lens flares

  • Switching between different LOD textures will create creases

    • Fix problem by using mipmaps


Lod selection
LOD Selection

  • Back to meshes

  • Very application oriented

    • Range-Based

    • Projected-Area Based


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