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Tessellation Shaders

Tessellation Shaders. Bezier Surface. similar to Bezier curve, except need two parameters u and v 16 control points instead of 4 parametric matrix equation see Dr. Bailey’s tessellation shader slides 27-35. Whole-Sphere Subdivision.

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Tessellation Shaders

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  1. Tessellation Shaders

  2. Bezier Surface • similar to Bezier curve, except • need two parameters u and v • 16 control points instead of 4 • parametric matrix equation • see Dr. Bailey’s tessellation shader slides 27-35

  3. Whole-Sphere Subdivision • a tessellation shader can easily be used to create a sphere given only its location and radius • a sphere can be parameterized using two angles • just use a quad

  4. Whole-Sphere Subdivision • the outer tessellation levels are useful here • low tessellation on the two outer edges at the poles • high tessellation on the two outer edges that are meridians • see Dr. Bailey’s tessellation shader slides 36-40

  5. Adapting to Screen Coverage • so far we have been using inner and outer tessellation levels that are passed in as uniform variables • this is somewhat inflexible • it would be useful if the tessellation shader could determine an “optimal” level of tessellation • one idea • adjust tessellation based on the area of the screen covered • TCS needs to be able to compute or guess what the final patch will look like on the screen for this to work

  6. Adapting to Screen Coverage • for the whole-sphere example this is straightforward • compute the extents of the three axes of the sphere in NDC or screen coordinates • use the extents to set the number of outer tessellation levels for the meridian edges • see Dr. Bailey’s tessellation shader slides 41-44

  7. Adapting to Screen Coverage

  8. Adapting to Screen Coverage

  9. Adapting to Screen Coverage

  10. Adapting to Screen Coverage

  11. PN Triangles • a method of tessellating objects made up of triangles where the per-vertex normal vectors are known (or can be calculated) • useful when your models are made up of triangles instead of smooth patches (like Bezier or B spline surfaces) • basic idea • use the vertices and normal vectors of the input triangle to compute a displacement field that produces a “Bezier triangle” • see Dr. Bailey’s tessellation shader slides 45-51 • see jdupuy’s blog posting

  12. Phong Tessellation • essentially a geometric version of Phong normal interpolation where the vertex positions are interpolated (instead of the normal vectors) http://perso.telecom-paristech.fr/~boubek/papers/PhongTessellation/

  13. Phong Tessellation • simpler than PN triangles and produces similar results • see jdupuy’s blog posting for a shader • see original paper for full details http://perso.telecom-paristech.fr/~boubek/papers/PhongTessellation/

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