RFC 599

Title: Modify edge use for B-Reps with analytic surfaces

Submitted by: Edward Clapp
Autodesk, Inc.

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Description of Problem

Consider a solid constructed as follows:

 
• Create a sphere centered at the origin with radius 10. 
• Create a sphere centered at the origin with radius 5. 
• Intersect the two with the XY plane. 
• Keep the portion above the XY plane. 
• Subtract the smaller sphere from the larger one.

Call the resulting solid a half-coconut.

A half-coconut cannot be represented in IGES using analytic surfaces. IGES says you have to use each edge once in each direction but that doesn't work in this case as one of the spheres has its normal pointing into void and the other has its pointing into matter. This forces the loop for one of the half-spheres to use an edge in the same direction as its use in the plane that joins them together. Edge-use is defined only in terms of the underlying curve and surface, so you can't use the orientation flag in the shell entity to clean things up.

The half-coconut is merely a simple example of a larger problem. More generally, solids may not be constructed using analytic surfaces where surfaces are used with normals pointing to both the solid and to void.

Proposed Solution

Change the description of how edges are used in shells and MSBOs. This is a minimal change to the specification with no modification to the syntax of the B-rep entities. It preserves upwards compatibility and probably represents how translators currently create these entities.

 
1. G.9, last sentence of paragraph 4 reads 

   Each edge is used once in each orientation and therefore should be  referenced exactly twice in an MSBO. 

      Replace with 

      Each edge must be used exactly twice in an MSBO. See the shell discussion  for further details. 

     

2. G9, last sentence of paragraph 9 reads 

      Each edge must be used exactly twice, once in each orientation, in the  shell. 

      Replace with 

      Each edge must be used exactly twice in a shell. If the orientation  flags of the faces using the edge are identical, each edge is used once  in each orientation. If the orientation flags are not identical, one of  the two edge orientations will not be used and the other will be used twice. 

     

3. G.48, last sentence of paragraph 2 reads: 

      An edge-use is only used once in the shell. 

      Replace with 

      An edge-use may be used zero, one, or two times in the shell. See the  shell discussion in G.9 for further details.