Convex hull per instance and attached group

Question

I have a tree hierarchy "trunk > branch > twig". Each branch is a curve instance on trunk, each twig is a curve instance on branch. The whole thing is created with geometry nodes following this scheme (simplified):

on the left primary curve ("trunk") with 3 secondary curves instanced on it ("branch") with each 4 tertiary curves instanced on it ("twig")

I would like to get a convex hull for each branch and its "attached" twigs. Is there a way to "group" one curve instance and the other curve instances "attached to it" for the convex hull to operate on? Right now I only succeed in getting the convex hull for the whole tree or for each curve instance.

here an example of what I'm trying to achieve

• red: manually made branch+twigs convex hull example
• green: branch (curve instance on trunk)
• blue: twig (curve instance on branch)

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Thanks to below answer by @Nathan, I could capture and store instance index, and select geometry where this value is equal to 1.

first "capture attribute" "index" on "instance
then store named attribute
finally separate geometry where the attribute is equal to "1"

The problem of this is that "separate geometry" produces two outputs (selection and inverted), while I would like to perform a convex hull on each of the groups.

Answer 1

Use collection membership to group the objects however you'd like. Instance that collection in geometry nodes and realize instances to do anything you want to it, including generating a convex hull:

In light of edits to your question:

If we want to organize instances inside of a geometry nodes modifier, we can capture the index of the instancing point, which the instances will inherit. After realizing instances, we can use that index as a sort of group for our instances. Here's an example, using what you're showing as a base:

I'm making a "trunk", capturing the index of the instancing point onto branches (and onto anything those branches instance.) I'm also capturing the index of the instancing point from the branches onto the twigs. Two separate attributes.

Later on, to reference these and create convex hulls, I can use these indices. In one arm, I'm making one convex hull of all branches and twigs with an index equal to 1. In another arm, I'm making a convex hull of all branches and twigs with an index equal to 2. In a third arm, I'm demonstrating how I can do this further down the line: I'm making a convex hull of a twig, with trunk index equal to 0 (indicating the branch) and branch index equal to 1. Math/multiply, operating on 1/0, true/false values, is the equivalent of a Boolean AND operation.

Rather than operating directly on the curves, I'm giving them some radius so that you can actually see the convex hull of the twig (otherwise, the convex hull of a line is no bigger than that line.)