Fill in the Blanks Sushrut Pavanaskar History Solid

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 • Sur. A – Bk – Sur. B – Not always a crack! • Sur. A – Sur. X – Sur. B – Sur. X can be Sur. A/B • Bk – Sur. A (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 Surface. IDs 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