Rhino3D Grasshopper: Mesh Slope Colorizer

In Rhino3D and Grasshopper, colorize a mesh surface dependant on its slope angle. Inspired by Luis Fraguarda’s O’Reilly Course, but the final solution I chose is by David Rutten.

You can find the O’Reilly online course here. I highly recommend watching it: Lots of useful information, fast, intense. And David Rutten’s explanation can be found here in the Grasshopper forum.

I also advise you to work through some literature like: Arturo Tedeschi, AAD Algorithms-Aided Design: Parametric Strategies using Grasshopper.

Let’s start. The idea is to produce a terrain-like surface and assign colors that refer to the surface’s slope. This works best with a mesh which we’ll derive from a NURBS surface.

Start in Rhino3D: NURBS Surface

This surface is meant to be transformed into a mesh, so let’s add some subdivision via Rebuild: 

To distort our plain surface switch on control point display:

Then select small groups of control points and move them up or down to create a “terrain”.

Grasshopper: NURBS to Mesh

Pick a Surface input parameter …

… and load your Rhino3D surface:

To keep NURBS and mesh surface apart, produce and offset a copy of the surface via Move:

Choose a distance that makes sense, in my case it’s 20 units:

Tu turn the copied surface into a mesh, pick the Mesh Surface component:

Connect the moved surface’s Geometry output to the Mesh Surface input. This component allows for a customized UV subdivision. Set appropriate values, in my example it’s 50 for both U and V:

Switch off all previews but for the NURBS and mesh surface. Here you are:

Colorize your Mesh

To colorize the mesh slopes we need access to the corresponding mesh parameters, e.g. the mesh normals. We get these via a Deconstruct Mesh component:

As you can see, this component produces a list of vector coordinates (N) which define the directions of the mesh normals: 

And of course, the higher the Z-value, the steeper the normal. And vice versa: The lower the Z-value, the steeper the slope. So it makes sense to use these Z-values for the coloring. How to extract them? Pick a Deconstruct Vector component and connect it to the N output:

Just for a better understanding what’s going on, I used a Bounds and Deconstruct Domain component to display the Z min and max values. In my example, the lowest Z-value is 0.20327 which defines the steepest slope in my terrain. The highest Z-value is 1, meaning here my terrain is absolutely horizontal.

As I said, this last step’s purpose was only to display the Z-value domain in this mesh. Now let’s move on with colorizing. A Gradient component can be used for our purpose:

Choose a preset with more than one colour, but not too many:

Then, to assign the gradient to the mesh, we need a Mesh Colours component:

Connect the Gradient’s t input to the Deconstruct Vector Z output. Now each Z value will have a gradient colour assigned dependant on its position on the range between our min and max Z-values as shown above.

Connect the Mesh Surface output to the according Mesh Colours input. To make things complete, connect the Gradient’s output to Mesh Colours’ C input.

Already you see that it works:

It may be necessary to tune the gradient to get a more explicit colouring:

That’s fine. Now, as we still have a live connection between NURBS and mesh, feel free to recreate your NURBS surface and observe the change of mesh colours:

That’s it.

Basic information on Grashopper can be found here.

Grasshopper-ARCHICAD-connection: See here for more information.

© 2019 / Horst Sondermann / All Rights reserved