# Fin and OffsetNormal fail on ruled surfaces

Both Fin and OffsetNormal commands fail when used on isocurves of a degree-1-surface.

In this example, the surface is a hyperbolic paraboloid with degree 1 in both directions, all isocurves are straight lines.

• OffsetNormal results in straight line, which has correct offset directions and distances at the ends but obviously comes closer to the surface towards the middle.
• Fin results in a HP-surface (degree 1/1), using the above offset curve as the new edge. The orientation is correctly perpendicular to the original surface, but the height of the fin surface is not constant.

If I’m not totally mistaken, the result should rather look like the green curve with all offset vectors of the same length. As soon as the original curve is NOT of degree 1 both commands seem to be working fine…

Fabian

Hi Fabian - make some lines normal to the surface along the isocurve - you’ll find that they match a Fin surface of the same depth - that base isocurve is in fact a line…

-Pascal

Hi Pascal,

That is actually what I did in the above example: The green arrows in the image are equally spaced along the isocurve, they are all normal to the surface, and all of them have the same length. Obviously both OffsetNormal and Fin create a result that does NOT keep equal distance along the offset.

If I got Euclid right, the offset of a straight line cannot be a straight line again, unless all offset vectors are parallel. Thus, the surface defining the offset vectors needs to be planar…

Attached the Rhino-model: FinFail.3dm (155.5 KB)

Hi Fabian - OK, I see what you mean- thanks for sending the surface. Hm. If you insert a bunch of knots across the surface you’ll see the Fin more closely follows the correct shape:

https://mcneel.myjetbrains.com/youtrack/issue/RH-42324

-Pascal

RH-42324 is fixed in the latest BETA