Tetrahelix - Connecting points in a list according to an expression

Hi guys,

I am trying to create a parametric tetrahelix.

I have a spiral curve which I have divided into points. I want to draw a series of polylines between points on this curve according to their point index. For example, I want to draw a polyline between points 0, 1 and 8; points 1, 8 and 9; points 1, 2 and 9 etc. You can see from the jpeg this would create a series of triangles which ascend the spiral.

I have attempted to use an evaluate function and list item to group the point indexes I need. The expression for the evaluate function are ‘x+1’ and ‘x+8’ but I can’t understand the relationship between these two functions. When I try to put the point list into the expression it comes up with the error “1. Expression generated an error for (x): Operator + not defined for point and int”

Using a simple list item function I have achieved the first triangle in the tetrahelix. Does anyone know how I can evaluate the list to create the remaining triangles?

Please see attached images for clarity.

Thanks, Joe

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Permalink Reply by Damon on October 10, 2014 at 8:54am

I think that the data you create from the evaluate function should be a list of numbers representing the indices of the points to plug into the List Item component. So you shouldn't be plugging the points directly into the evaluate functions, but rather their list of indices. Something like this:

The warnings on the List Item components are because I've created lists of indices that are larger than the list, so when I set "Wrap" to false, it can't find those indices. That's OK, GH just stops when it can't find any more.

Permalink Reply by Joe on October 11, 2014 at 2:37am

Hi Damon, Thanks very much for the quick response, that's just what I was after.

Permalink Reply by Chris Hanley on October 10, 2014 at 12:21pm

This isn't exactly the same approach, but it creates the triangular surfaces along a helix using series, list item, and shift list components. Some scaling built in for the GH generated curve, but using cageedit to manipulate a referenced Rhino curve gets some intresting results. Last bit was using exoskeleton to make a frame and WB to smooth it out.

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