Fill in the Blanks.
Sushrut Pavanaskar
History
• Solid modeling (e.g. Solidworks, Catia, Pro-E..)
• Use of NURBS, Trimmed
– Extreme control
– Well developed utilities
• HEAVY Complex models
• Need math to compute
– GPU: Dedicated Hardware
• Our research – evaluation parallelized
Blanks (Gaps/Cracks)
• NURBS model = many surfaces
• GPU evaluation is parallel
• Neighbors don’t know
– So they don’t match!
• CRACKS
• NOT acceptable
– MUST be removed
Anticipated Work
• Determination of cracks with IDs
– Contours, external edges need not be filled
•
•
•
•
Determination of edges, colors to be filled in
Actual computation of areas to fill
Filling in appropriate direction
Challenges
–
–
–
–
All this to be done on the GPU frag. Shader
No neighbor information available
No curvature information available
This has to be quick – does no value addition
Determination of a CRACK
• First pass over current view port for IDs
• Reads and keeps ID_Tex on the GPU itself
• “Cracks” seen clearly
Cracks and Neighborhood
• Cracks have nonmatching IDs
• But could be anything
Types
• SurA – Bk – SurB
– Not always a crack! 
• SurA – SurX – SurB
– SurX can be SurA/B 
• Bk – SurA (Silhouette)
– I need not care for them
Real Issue of “knowledge”
• Fragment shader knows nothing @ neighbors
– Just knows about that fragment, and IDs of neigh
• Fragment shader (GPU) cant color anything
– It can just permit rendering or not
• When rendering, no “knowledge” of surfaces
– So no filling there
• GPU  CPU data transfer takes time!
– So SurfaceIDs available only to fragment shader
Anticipated work
• Determination of cracks with IDs
– Contours, external edges need not be filled
•
•
•
•
Determination of edges, colors to be filled in
Actual computation of areas to fill
Filling in appropriate direction
Challenges
–
–
–
–
All this to be done on the GPU frag. Shader
No neighbor information available
No curvature information available
This has to be quick – does no value addition
Solution (as anticipated) - Edges
• Reading the edges
– More technical challenge than geometrical
– Read ACIS data structure
– Loop through, enquire, read and store colors
• Coloring for OPENGL – readily done
– kdcolor, kscolor, ka use with normals
• Sampled edge for 10 parametrically
equidistant points, stored as “edge-points”
Triangle Strip Surrounding the Edge
• Can know exact properties of edge,
– So draw edge quads, but render selectively
(Un)anticipated Work
• Model space   Screen space
– Wanted triangle strips on screen space
– But edges in model space
• Use OPENGL transformation + view matrix
– Invert, calculate quad dimensions, re-invert
• All drawing is done in model space
Dealing with Silhouette Edges
• Used out-normal of edge
• Dot prod with view vector
– Same sign => render
– Diff sign => don’t
• Still a preliminary test
– Sometimes “ab”-normals 
Preliminary Result