Level of Detail

1
Level of Detail

• CS184-Sp05 Section

2
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

3
Switching

• Popping problem
• Discrete Geometry LODs
• No blending. Bad popping.

4
Switching

• Blend LODs
• Render LOD1 opaquely first, then LOD2 w/
  increasing alpha val.
• High overhead during transitions.

5
Switching

• Alpha LODs
• Only 1 LOD per object.
• As metric for selection increases, transparency
  is increased.
• No popping.
• When object completely disappears, dont need to
  render it at all.
• Only at this point is speedup attained.

6
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

7
Switching

• Geomorph LODs
• Interpolate between 2 LODs
• Remember vertex connectivity between levels
• E.g., animating the edge collapse in CLODs would
  be a geomorph

8
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).

9
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
• Cant just store in a list like w/ CLODs

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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

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Basic Alg.
1) Create PM Mn lt- Mn-1 lt- M0
2) Create forest
M0
v1
v2
v3
Say our v.d. metric says to split v1 first. Can
we do that? (Hint No)
12
Billboards

• So far, weve 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

13
LOD Selection

• Back to meshes
• Very application oriented
• Range-Based
• Projected-Area Based