Twist in Loft

Hey all

Just a general quesiton, this is something that happens every now and again when working, but with no real understanding of why. i just quickly made this is to just ask the question.

Its just a loft along the catenary between two circles. But it creates these twists

I guess its about the direction as it crosses over axis. i can fix it, and know how edit the lists to get rid of the twists. but i was just wondering what the reason was why it does this.

Thanks

Attachments:

twist in loft.jpg, 479 KB

Replies to This Discussion

Permalink Reply by Danny Boyes on March 9, 2011 at 5:09am

It happens in Rhino too. If you select a series of curves to loft but select some curves at a point closer to the other end the loft will twist. What you are effectively doing is defining the direction of the curve for the loft. In Rhino to rectify this you can use the Alighn Curves.. button at the preview stage to untwist it.

In GH there is no user input to tell GH which direction you want so it takes the curves original direction. To rectify this manually it normally involves flipping each curve that is incorrect. However, you could automate this with the Flip Component which will use an optional guide curve to set the direction of all the others. From your list of Curves to Loft use the List Item Component to get the first curve and plug this into the G input and the whole list into C. Thereby forcing Grasshopper to compare each curve in the list with the first one, when it gets a false reading it will flip that curve.

Permalink Reply by Nick Tyrer on March 9, 2011 at 5:34am

Thanks for explaining danny,

ive just tried your example, and i cant get it to work on the example i showed. can you just check to see if this is correct.

Attachments:

twist in loft.ghx, 244 KB

Permalink Reply by Danny Boyes on March 9, 2011 at 7:19am

Hi Nick,

Everything you did was right, but I don't think that the method works for a Radial loft. But I also think that there's something amiss with the Catenary Component as I see it. It should start at A and end at B however in this case every curve between Pi/2 and 3Pi/4 is reversed. B-A.

There is another alternative to trying to flip the curves (In this case) create one Catenary Curve and Revolve it with the revolution Component. See Example attached.

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