Houdini expressions are functions available to help you calculate values in the parameters of the nodes. You may have seen $HIPNAME which returns the name of the current hip file. Or the centroid one that I use most often for procedural modeling, $CEX $CEY $CEZ, which returns the centroid vector component.

This is an explanation of the math behind how the Height Ramp in the Height Field Mask by Feature node in Houdini is calculated.

This post explains a specific node, “set_density” and how the math works in calculating the density attribute. I’ll try to explain the math behind it and how the node works.