Help Issue - "loft" sentence


#1

G’day McNeel,

Is this correct?

“Two straight lines that are not parallel are not developable.”

Surely if they are planar straight lines (not parallel) then they can be lofted also.

regards,
Nick.


(Pascal Golay) #2

Hi Nick- I guess this really means non-parallel and non coplanar, is that what you mean? If the result is a surface with a twist it will not be developable, is the idea I think.

-Pascal


(David Cockey) #3

A correct statement would be:
Two straight lines which are not planar are not developable. If two straight lines are not planar they cannot be parallel.

A lofted surface can be created from two straight lines which are not planar but the surface will not be developable. Depending on how close the lines are to being planar the resulting surface may be close enough to developable for the intended purpose, and the Rhino UnRollSrf command may unroll it.


(Margaret Becker) #4

Added to bugtracker http://mcneel.myjetbrains.com/youtrack/issue/RH-22036 (not yet available for public view).


#5

Thank you, I’m sure I wasn’t going mad. We’re always having debates in the office about what is developable and what isn’t (I used to think all ruled surfaces were developable, but hyperboloids are clearly not) so I thought reading the help file would help clarify things. When I tried two non-parallel lines it appeared that lofting between them most certainly created a developable surface (when planar), hence the need for clarification from the team. It’s easy to say that Gaussian curvature needs to be zero, but putting that into practice is another matter. Perhaps some graphics of classic examples of developable surfaces and “false” developable surfaces (hyperboloids, some ruled surfaces, lofts between non-planar straight lines, etc.) wouldn’t go astray in the help file either.

Nick.