I want to know when to draw the edges in a mesh.
I can get the normals of the two faces:
norm1=bpy.data.objects[1].data.polygons[0].normal
norm2=bpy.data.objects[1].data.polygons[1].normal
If the angle between them is >= 90 degrees, then the line is visible.
I can use
norm1.angle(norm2)
to get the angle.
But I obviously also need to draw the line if the edge is a perimeter line with reference to the camera.
For this I would need the direction the camera is looking.
What else do I need?