CS 284: CAGD
Lecture #14  Th 10/12, 2006.
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Preparation:
Labsik and Greiner: "Interpolatory Root3 Subdivision".
Topic: Interpolating Subdivision Schemes for Surfaces (cont.)
Discussion of the "Root3" paper:

What are the key ideas in the paper ?
 What is the topological subdivision stencil ?
 What is the formula for placing the new face vertex ?
 How many vertices are involved in computing the new point ?
 in the regular case ?
 in the irregular cases ?
 Treatment of boundary edges ?

How can such interpolated regions be stitched together ?
 What are soem of the NEW concepts that we have not yet discussed in class ?
 Forming cusps or creases by joining two patches along a common boundary curve ... (more below)
 Variable "depth" subdivision for adaptive refinement ... see: Leif Kobbelt: "Root3 Subdivision", Siggraph 2000.
 For a good topological subdivision step:
 there is always an approximating scheme to place new vertices
 and often there is also an interpolating scheme for vertex placement.
(see comparison list below).
Adaptive Refinement by Subdivision
Figures from: Leif Kobbelt: "Root3 Subdivision", Siggraph 2000.
The basic root3 scheme.
Typical mesh generated.
The "rotation" implied by a single subdivision step.
Adaptive refinement to different levels  may cause cracks.
Bad triangles or a large "transition zone" are generated to reach a desired deep level of subdivision.
Root3 scheme has no crack problem and generates good transition zones.
Nice adptive refinement meshes on "the bunny".
Maintaining smooth boundaries in the root3 scheme.
Forming cusps or creases by joining two patches along a common boundary curve.
The root3 scheme allows finer gradation of uniform subdivision than the Loop scheme.
How would you do adaptive refinement in your own quad scheme ?
Summary of Subdivision Schemes:
 DOO_SABIN
 quadratic, approximating; quadrilateral facets.  CATMULL_CLARK
 cubic, approximating; quadrilateral facets.
 LOOP
 quartic, approximating; triangular facets.
 Butterfly Scheme
 cubic, interpolating; triangular facets.
 ZORIN
 cubic, interpolating; triangular facets.
 Interpolating Root3 Subdivision
 cubic, interpolating; triangular facets.
 YourOwn QuadScheme
 cubic, interpolating; quadrilateral facets.  KOBBELT: Root3
 cubic, interpolating; triangular facets.
 HALSTEAD: Interpolating CatmullClark
 cubic, interpolating; quadrilateral facets.
When should you choose which subdivision scheme ??
Sharp Features in Subdivision Surfaces
Many objects have a combination of smooth, rounded surface elements and sharp edges and corners (examples ... ?).
Remember what we did with splines to obtain sharp kinks and corners (discuss ... !).
Now we need to do the same thing for subdivision curves and surfaces....
Key paper:
"Subdivision
Surfaces in Character Animation" DeRose et al.
PREVIEW:
Start with CatmullClark Subdivision
Introduce sharp features: creases, edges, corners...
Add smooth variable radius fillets and blends with
a mix of sharp and smooth subdivision steps:
 take s sharp steps and then all smooth steps afterwards.
Blend between a sharp and a smooth subdivision step.
ALSO:
Modelling of loose clothes.
Collision detection.
Smooth scalar fields for texturing.
...
Make Your Paper Selection & Start thinking about your Course Project
==> PROJECTPLANNING
Look at the Paper Selection List  Pick your top five candidates  Fill out questionaire (handout) by Tuesday 10/17.
Reading Assignment:
"Subdivision
Surfaces in Character Animation" DeRose et al.
Be prepared to answer the following questions:

What subdivision scheme are they using ?

What are the key additions to that scheme that they have made ?

Why do they prefer quadrilaterals ?

What is the basic approach to creating semisharp edges ?

What is the underlying model for cloth simulation ?

How is excessive wrinkling of the cloth avoided ?

What are virtual threads ?  and what is their purpose ?

What is the data structure to make collision detection efficient ?

How is made it possible to apply texture maps and procedural shaders ?

What was one of the general implementation requirements for the Pixar production
environment ?
Current Homework Assignment:  Two options to choose from:

Implement your proposed quad subdivision scheme as a team (in the language of your choice).
Hand in a report of your apporach (can be same or extended version of your first report)
and describe your findings, difficulties, insights.
Add a couple of pictures of test cases that show what your implementation can do.
OR:
 Compare several different subdivision schemes in SLIDE.
Also summarize your findings in a report with pictures of crucial test cases.
DUE: TUESDAY 10/17/2006, 2:10pm.
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