[Houdini] Introduction to Procedural Modeling – Voronoi Fracture and UV Mesh Unfold


In this article, I will explain how to place cobblestones on a plate polygon using Houdini.

If you use a square plate polygon, it will become a square cobblestone floor like this.


A cobblestone floor created from a square plate polygon


If you use a star shaped plate polygon, it will become a star shaped cobblestone floor like this.


A cobblestone floor created from a star shaped plate polygon

What You Can Learn from This Article

・How to use Houdini to place cobblestones on a floor

・How to unfold a UV mesh using Houdini

The Previous Article

Directions to Create a Cobblestone Floor

Prepare a Plate Polygon (Grid Node)

First, we need to prepare a plate polygon which we want to add cobblestones to.

I will use a Grid node (square plate polygon) for this tutorial.


Create a Grid node


A square mesh created with Grid node


Split the Plate (Voronoi Fracture)

Next, we will split the plate using an algorithm called Voronoi tessellation.

I will not explain the details of Voronoi tessellation in this article, but dividing a plate as in the image below is called Voronoi tessellation.


Plate divided by Voronoi tessellation

Prepare a Center Point for the Split (Scatter Node)

In order to divide a plate, we need to use a point cloud as a center point for the split.

We will use Scatter node to create a point cloud.


Attach Scatter to Grid


Point cloud created with the Scatter node


Using a Point Cloud for Voronoi Fracture (Voronoi Fracture Node)

Next, we will use a Voronoi Fracture node to split the plate.

Attach the plate polygon (Grid node) to the left of the Voronoi Fracture node, and the point cloud (Scatter node) to the right side of the Voronoi Fracture node.

Voronoi Fractureノードの作成

Create a Voronoi Fracture node


Voronoi fracture and the plate


The plate has been divided, but it does not represent cobblestones yet.

Additional Info: Display Wires

Wires can be displayed by pressing the Shift + W keys while selecting in Scene view.

By pressing it once more, they will turn invisible.

Rounding the Stones (Resample Node)


Rounding the stones


Attach a Resample node to the Voronoi Fracture, and resample the divided parts.


Attach Resample node


Resample Result

Apply the resample result again to round up the stones.


Attach Resample node


Resample Apply Result


Thicken the Stone (PolyExtrude node)

Attach the PolyExtrude node to the Resample node in order to thicken the stones.


Attach PolyExtrude node


PolyExtrude apply result


Unfold UV Mesh (UV Texture Node)

Lastly, unfold the UV mesh.

UV Textureノードを接続

Attach UV Texture node

Attach the UV Quick Shade node and display the UV mesh as in the image below.

UV Quick ShadeでUVを確認

Check the UV with the UV Quick Shade

This completes the process to make a cobblestone floor.


OBJ File Export (ROP Geometry Node)

Use the ROP Geometry node to export as an OBJ file.

Enter the obj file path for the Output File, and click the Save to Disk button to export as an obj file.

ROP Geometryノードを利用してOBJファイルをエクスポート

Use the ROP Geometry node and export as an OBJ file

If you import the OBJ file into a Unity scene, it will look like the image shown below.


Cobblestone floor in Unity

Organize Materials

When imported to Unity, the material of the OBJ file is divided into two.

We can prevent this by setting up a Group within Houdini.


Multi materials exist


If you check the Voronoi Fracture node from previous steps, the group settings are active so we need to deactivate them.

Uncheck the Interior Group and Exterior Group option.

Voronoi Fractureノードの設定を変更

Change the Voronoi Fracture node settings


If you export as an OBJ file with these options, only one default material will exist.


Only one material exists

How to Upload a Prefab from Unity to STYLY

In order to learn how to upload your created prefab to STYLY, refer to the article below.

Extra: Create a Round Floor

We used a Grid node to create a square cobblestone floor, but if you replace the Grid node with a Circle node, the result will be a circular cobblestone floor.


Round cobblestone floor

The normal direction of the Circle node is opposite of that of the Grid node, so I used the Reverse node to flip the direction.


Attach the Reverse node to the Circle node


Extra 2: Create a Star-Shaped Floor

I will explain how to create a star-shaped floor as seen in the image below.


Star-shaped floor

How to Create a Star-Shaped Floor

You can create a star shape by using the Circle node to create a regular decagon with smaller vertices.


Creating a star-shaped node


Circle Node Settings (Create a Regular Decagon)

Change the Division of the Circle node to 10, and the shape will be transformed into a regular decagon.


Change Circle node settings





Group by Range Node Settings (Select Even Vertices)

Apply group1 to the even vertices.

CircleノードにGroup by Rangeノードを接続

Attach Group by Range node to the Circle node




Transform Node Settings (Making the Selected Even Vertices Smaller)

Use the Transform node and change the even vertices to a 0.5 scale.


Use the Transform node to change the scale




Completed Star-Shaped Floor

By turning the UV Texture node Display flag on, a star-shaped cobblestone floor as in the image below will appear.


Turn the Display flag on