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 Discrete Geometry LODs –No blending. Bad popping.

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

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.

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 …

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 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). v1 v2 v3 v1 v5 v3 v4 v5 v3 v4 v6 v7

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. 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 Alg. v1 v2 v3 v1 v5 v3 v4 v5 v3 v4 v6 v7 1) Create PM: M n <- M n-1 <- … M 0 2) Create forest M0M0 v1v2v3 Say our v.d. metric says to split v1 first. Can we do that? (Hint: No)

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