Deleting faces that are not visible from certain direction

Question

I am dealing with a very large 3d scanned mesh that has a lot of unnecessary detail:

I am looking to create one coherent mesh that is only the outer surface of the city model, without the "inner" and "below surface" vertices. (Imagine it like a layer of fine dust settling on a city)

My first approach was to manually apply a selection from top view, but the process is tedious and selection is too inconsistent. A lot of people on here asked to delete the "interior parts" of meshes. However the proposed methods like Blender's "Select interior faces" are not really applicable to my case.

A member of the SideFx forum called "zarti" provided a clever way to delete inner parts of meshes in Houdini, that uses a ray node. I tried to alter this method to work on my model with Blender Geometry Nodes (Raycast Node). It seemed very simple but I was not familiar enough with the logic of the Blender Geometry Node system.

The basic idea is to cast rays onto the models from -z direction and delete every face that does not get hit.

Of course this method would be still too brute and would need some refinement, like some kind of angle deviation or "diffusion" to hit all the relevant faces.

In that respect I also tried methods of baking different maps (Diffuse/Shadow/AO) onto the vertex color map and select/delete vertices accordingly, but the results were very messy.

Maybe someone with more insight into the Blender Geometry Nodes System could help me to find an elegant solution for this problem.

Answer 1

Simple scenario

Filter Faces using the Normals

Get the position of the camera with Object Info.

Subtract this position from the positions of the individual faces.

If the dot product of the normals of the faces and this vector is greater than $0$, you know that the faces do not point in the direction of the camera.

Complex scenario

Filter hidden geometry

OK, you didn't want to share your mesh unfortunately, so I can only assume it works for you too, but you might be happy with the following solution:

I first apply the technique shown above, and filter out the faces whose normals don't point towards the camera.

Then I split the remaining mesh and convert it to curves. This way I get one closed curve per face.

If I now move their points very slightly towards the camera and send a raycast in the direction of the camera, I get the information which of the points of a curve are hidden by the geometry.

If all points of a curve are hidden, I filter out this curve. But if even one point of a curve is not hidden, I leave it untouched.

Finally, I simply check if for the points of the pre-filtered geometry there are still some in the geometry converted to curves.

If not, I know the face needs to be filtered as well because it is obviously completely hidden.

_{Suzanne with internal geometry (Solidify Modifier)}

Here is the node tree at a glance:

_{(Blender 3.2)}

Note, however, that you may create some weird smoothing of the normals by doing this, since you end up changing the topology of the mesh as well.

PS: I haven't tested this extensively now, but it should work fine in more complex scenarios. Please provide feedback.