The orientation of Trim wrt to Edge is given by m_bRev3d flag on ON_BrepTrim. In a solid manifold brep model the two trims of an edge should have opposite values for m_bRev3d, because the material is always on the left of the trim. If a trim’s m_bRev3d flag is “false” then its mate trim’s flag has to be “true”. I am finding that it is not so in a simple model. I have uploaded a model difference.3dm. Is there some other way or factor to be considered to get trim’s orientation with respect to edge.Diffrence.3dm (99.7 KB)

# Orientations of Trims wrt to Edge

**chuck**(Chuck Welsh) #3

Hi MK1. It is true that the material is always to the left of the trim, but left is wrt the surface normal. An `ON_BrepFace`

has it’s `m_bRev`

flag which, when true, means that the “up” side of the face is in the opposite direction of the surface normal. So, if you have faces that share an edge in a manifold Brep, and those faces have different `m_bRev`

flags, then the two trims of the edge will have the same `m_bRev3d`

flag.