# Directionally orient grid

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Hello and welcome to this post.

Synopsis:

In nature, we see a phenomenon called tropism (the way a leaf orients itself towards the sun). I need help building a rig that will simulate this effect. These are the objectives:

1. Create a source object acting as the reference point for tropism
2. Get the leaf to orient itself in relation to the source

It would be ideal to shade the front and back face of the leaf with a different texture. What is a neat VOP setup which would simulate this?

Here is a  basic HIP file to begin messing around with.

Edited by voxelfox

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Maybe It Helps
And Consider Using L-stem For those Stuff they already have Build inside(vectors,directions) Tropism
https://www.houdinikitchen.net/2019/12/23/l-systems-node/

https://handlespixels.wordpress.com/2019/01/24/texturing-in-houdini/

Edited by Librarian
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Hi Michael,

to orient the leaves set the normals to the cross product of the curve's tangents and the direction towards the sun. Set the up vector to the curves tangents, as well. For shading front and back sides just set a switch after a is front face-node.

leaves.hipnc

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Hello all,

Thank you for the responsive feedback.

Konstantin, I like your thinking. I'll take a look at implementing your techniques into the rig. You also present a good point for using the is front-face VOP.

Tesan, This is a splendid collection of resources thanks for digging around. I have built a full L-System this setup is a mock to simulate the principle features.

Any other feedback is welcome, this post could be useful for someone in the future.

All the best,

Michael

Edited by voxelfox
Spelling and comment
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if you are a fan of quaternions and matrices you can do a spherical interpolation (slerp) of 2 quaternions which represent the local orient of the leaf and the global orientation to the sun

orient_leaf.hiplc

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Hello Matt,

This is a phenomenal piece of work that you have whipped together. I can see value in taking time to study the processess documented in your scene. Could you offer some advice for one who would be looking to learn quaternion based orientaions? Any tips such as:

• resorces
• study methods
• application in a Houdini context

Thank you again for sharing your skills with others.

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