Geometry Nodes: How to correctly rotate collection instances on complex geometry according to normals/tangent?

Question

As a proof of concept (and to familiarize myself with Geometry Nodes), I am trying to make a (semi-)procedural date palm tree with Blender 3.3. I use one Bézier Curve with a Geometry Node setup to create the trunk. Within the trunk Geometry Node setup, I add instances of branches (multiple Bézier Curves in a collection) and populate the branches with leaves (multiple Meshes in a collection). Currently, I have troubles to rotate the leaf instances properly.

I found an answer to a similar question here, which does not work for my case due to the facts highlighted in bold in the introduction paragraph above.

Until now, I tried the following two approaches (the results of which are shown below):

1. Converting my branch Bézier Curve via the Curve to Mesh geometry node to a Mesh and than create the instance points via Distribute Points on Faces geometry node. This led to a very organic leaf distribution, but the rotation (even using the Normal information) was all over the place and did not behave predictably/controllable.
2. Directly instancing the leaves on the curve. This resulted in a more predictable rotation behavior, but looked artificial and the rotation was still not correct.

Issues

The following issues occur:

Issue 1

No matter how I try to rotate the leaves, the end of the branches do not look not correct (red circles in image above). As the rotation/length of leaves of palm trees change of the length of the branch, I would need a way to adjust the rotation/scale of the instances in relation to the relative position on the branch.

Issue 2

The z-direction of the leaves is aligned with the global z-direction in most of the cases (setting it to relative/local did change nothing). I would like to recreate the natural case, where the leaf orientation would follow more the branch orientation.

Questions

1. What would be a solution to adjust the rotation/scale of the instances in relation to the relative position on a Bézier Curve?
2. Is my approach of creating all the instances within the geometry node setup of the trunk unfavorable? If so, what is a better approach?

Geometry Node Setup

Important: The Geometry Node Setup for Approach 1 (Curve -> Mesh -> Distribute Points on Faces) is described.

Screenshot of the Geometry Nodes Workspace: All the assets which should be instanced are placed in a dedicated collection (Bézier Curves as well as Meshes). The Geometry Node Setup will be applied on the Bézier Curve PalmTreeGenerator.

For reference, the current PalmTreeGenerator Geometry Node Setup looks like this: Relevant for the problem are the following Groups: TopBulge (Emitter of Branches), Branches (Instancing of Branches on Emitter) and Leaves (Instancing of Leaves on Branch Instance).

The TopBulge Group loads an Icosphere from a Collection. To adjust its orientation to the trunk, I had to add an extra rotation. However, removing/changing the rotation has no effect on the overall problem; it still occurs.

The Branches Group picks one random instance of the Bézier Curves in the Collection. In order that the branches are not instanced in one spot, I need to use the z-rotation returned by the Distribute Points on Faces Geometry Node. If I would use all rotations, the branches would be skewed. The two Euler Rotate Geometry Nodes are remnants of my experiments to align the rotation to normals. For this post, I reset all the parameters to zero (problem becomes easier to see). All the Geometry Nodes on the right-hand side are used to convert the Bézier Curve into a mesh. It is done twice to have more control over the mesh (especially starting point of leaves and number of faces) used for the leaf instancing.

The Leaves Group instances the leaves on the branches. It is done twice (left/right side of branch). Here I manually set a rotation that looked best. Trying here to work with Normal information via Align Euler to Vector made the end result worse.

The blend file is available at my OpenGameart page under CC-BY-4.0.

Answer 1

In order to correctly position the objects subsequently instantiated on a mesh and to get the desired rotation/scaling, you need the data of the original curve, which you use in combination with Mesh to Curve to create the geometry.

You need two things here:

• The tangents of the curve, because you want to know in which direction the leaves should be rotated.
• The factor of the curve, because you want to scale the leaves smaller towards the end.

This is done in the group "Branches" before creating the mesh:

Here I store the two required values with Store Named Attribute, which are preserved in the geometry and can be used later.

The next step happens in the group where you instantiate the leaves.

The way you do this depends on what exactly you want to achieve.

I have built an example for you here. Of course it could be done differently (or easier), but I think this example gives you a good start:

Here I first create a Rotation with the previously saved tangent, so that the leaves are correctly aligned along the mesh.

The second rotation aligns the leaves so that they point in opposite directions.

Next, I use the previously saved value of the curve factor to create a smooth transition in combination with Float Curve.

One serves as an additional rotation to rotate the leaves more towards the end, and the other value serves as a scale to make the leaves smaller towards the end.

Depending on how you do all this exactly and which values you modify, the result could look something like this:

_{(Blender 3.3+)}