Rhino3D Grasshopper: Handle Paths with List / Cull

You can extract Grasshopper Data Tree Paths just like simple List Items. It’s slightly more work though – Param Viewer and Tree Branch are key.

For a good introduction on Grasshopper’s Data Trees read this page from TU Delft.

New to Grasshopper? I suggest you read this article in the first place.

Need more learning resources? Check this out.

Disclaimer

As you will see in my screenshots most components carry a title above them displaying their name. This is because I use a plugin called Bifocals.

Also you’ll see my Grasshopper tab titles abbreviated because there’s not enough horizontal space. I will mention the titles in full in my script though.

As with everything Grasshopper, the technique I show you is no way my own discovery. I basically got the idea from Arturo Tedeschi who mentioned it in a webinar.

Let’s Start

As shown in this article you can pull paths using Tree Branch – which seems simple enough.

Of course, this works only with a fixed tree structure because you have to feed specific path names into the Tree Branch input.

If you need to extract or hide paths with a given index – like you do with ordinary list items – you have to put some more components together to get the job done.

Let’s look at a situation that comes up with Kangaroo:

Let’s say we want to retrieve the resulting mesh for further work. A panel shows that we have mixed output:

A Param Viewer component tells us exactly how many paths and items we’ve got here. OK – but how do we get hold of the first path {0}?

Again: A Tree Branch fed with a panelled {0} input would do – but this time we want to use a List Item component.

But of course, List and Cull components are made for lists, not for trees. So we have to use a kind of trick.

As you see a Param Viewer has been connected to our mixed output:

Since our plan is to extract the mesh – which is in {0} – get a List Item and connect it to Param Viewer’s output:

List Item’s default output is 0, meaning that – if our plan succeeds – it will pull our mesh. That’s fine, but there’s some more to do.

Pick a Tree Branch component and connect it to Kangaroo Solver’s output:

Tree Branch wants to know which path it’s supposed to extract for you (P?) – this is the place where we put in a path name, described in this article.

In this revised and more universal approach we connect P with our List Item’s output i:

That’s it – Using Param Viewer before and Tree Branch after List Item basically helped to extract a data tree path just like a normal list item.

I haven’t tested it altogether – but it seems that this way you can apply all of Grasshopper’s list an cull items on paths.

For a good introduction on Grasshopper’s Data Trees read this page from TU Delft.

New to Grasshopper? I suggest you read this article in the first place.

Need more learning resources? Check this out.

© Horst Sondermann 2019 // All Rights reserved