the angle of the corner:  wall = 0  for straight, positive inside
the angle of the molding: slant = 0 for vertical, positive out from wall
                                 38 is the predominant angle between the molding and wall
the crosscut angle:       rake = 0  for square,
                          miter = 90 for square
                                  >90 for undercut
                                  <90 for overcut
the bevel angle:          bevel = 0 for square, positive for overcut
                          included = 90 for square
                                   < 90 for overcut

the ceiling-wall edge for the left wall   0,1,0
the molding lateral vector                sin(slope),0,cos(slope)

the ceiling-wall edge for the right wall  sin(wall),cos(wall),0
the molding lateral vector                sin(slope)cos(slope)-sin(slope)sin(wall),0,cos(slope)

L is unit normal vector for molding on left or on 1,0,0 or x=0 wall
R is unit normal vector for molding on right or cos(wall),-sin(wall),0 wall

       i          j  k
L =    0          1  0
      sin(slope)  0  cos(slope)

     i                    j                     k
R =  sin(wall)            cos(wall)             0
     sin(slope)cos(wall)  -sin(slope)sin(wall)  cos(slope)

the edge between the moldings is the vector cross product of L and R:

L X R =



  =  ( -cos(slope)sin(slope)sin(wall),
        cos(slope)sin(slope)(cos(wall)-1),
       -cos(slope)cos(slope)sin(wall) )


unit(L X R) = (L X R) / (cos**2(slope)*(sin**2(wall) + sin**2(slope)(1-cos(wall))**2)

the y component of unit(L x R) is cos(rake)
or arc tangent of the y component over
  the hypotenuse of the x and z component

  hypotenuse = sqrt( sin**2(slope)sin**2(wall) + cos**2(slope)sin**2(wall)) = sin(wall)

rake = atan( sin(slope)*(cos(wall)-1) / sin(wall) )

when wall is positive
rake is 0 for square and positive for top edge shorter than bottom edge
when wall is negative 
rake is 0 for square and positive top edge longer than bottom edge


cos(rake) = (1-cos(wall)) / (sin**2(wall)/sin**-2(slant)+(1-cos(wall)**2)**0.5


L dot R = cos**2(slant)*cos(wall) + sin**2(slant)
bevel is 0 for square and positive for back longer than front
  bevel = 0.5 acos(L dot R)