Flip Faces by Instance in Geometry Nodes

Question

I have a set of Instances, some of which I want to mirror randomly. This works fine, but when I mirror the instances, their normals also reverse.

This seems like an easy fix with the Flip Faces node but that only seems to operate on the face domain and I can not figure out how to access any instance level information on that domain.

So how to I flip the faces of my geometry based on the instance index?

Answer 1

Actually, you should not have to worry about the orientation of the faces if you set up the setup as follows:

Instead of changing the points of the geometry here, I change the whole instance instead, which basically preserves the normals.

However, if you then use the node Realize Instances, the normals are flipped for some objects.

To avoid this problem, create a boolean value on the instance beforehand, which makes it clear that this instance should be flipped.

In this example I create this value via the Dot Product.

_{(Blender 3.2+)}