Running Rhino 4.

The example shows three short lines in the z=0 plane, and two surfaces above them. The two surfaces look identical, but one "simple surface" is merely a triangle. The other "surface" coincides with the triangle almost exactly (edges are the same, maximum deviation about 10^-15, same Direction.) but has a more complicated structure.

When one Projects the short lines to the two surfaces, the resulting Projected lines have opposite Directions.

What is the rule for Direction of Projected curves?projection direction example.3dm (447.0 KB)