Thanks. Been there, done that. Unfortunately rotation is calculated based purely on normals/object Z. Deform the geometry and those will change. The correct term probably is “spin”. That is - the instances will maintain rotation to normals, but the spin around normal is controlled by tangent to Z. To lock spin to geometry one needs a tangent to a property of the face (UV usually).
But it seems the DIY solution is somewhat possible: Extra Nodes for GeometryNodes - #2 by Strike_Digital if the required data can be accessed from Python.
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