# How would I retrieve the 'inner surface' of an arbitrary mesh?

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In my project I am working on a 'subset' of cloth simulation in which I attempt to fit one mesh over another. My solution involves deforming a 'physical mesh' based on depth fields and using it to control the deformation of the complex, detailed mesh.

I have seen impressive mesh optimization methods, but I don't want to optimize the mesh so much as extract part of it. What I want is a way to approximate the 'inside surface' of a mesh, since in the 'real world' this is what would interact physically with mesh being deformed with.

Take the images below; the second mesh contains no overlapping polygons - the lapels, shoulder straps and buttons are gone - it is a single surface consisting of the points closest to the character.

[attachment=8440:jck.jpg]

(Checking for and removing overlapping polygons would be one way I suppose, but how to decide which are the 'outer' and which are the 'inner' bearing in mind the normals of the semantically inside polys won't necessarily emanate from the geometric centre of the mesh)

Does anyone know of an existing implementation that does something like this?

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I wonder if you could clarify your example? Is anything you've illustrated above the detailed mesh? This may not help find a solution, but it might.

Usually the meshes to be displayed are modeled optimally by an artist. The pieces to be removed as you described...there might be some automation but probably in practice it is done with the aid of a 3D modeling package, then compiled/preprocessed/exported from the modeling package into game- or simulation-ready media. So, therefore, one possibility is to have a human in the loop. I guess that's what you did above? I can imagine doing something where you do something like:

1) Draw all the meshes as shaded using additive blending...where there are overlapping meshes the image will appear brighter
2) 3D modeling human-in-the-loop user clicks one of the bright spots where it is clear there are overlapping meshes
3) Code ray traces through meshes until it hits the underlying character mesh. Sort the ray trace selection results and tag all meshes *except* the one closest to the character mesh as "delete me!". Perhaps color these differently and redraw just to visually indicate action by the user.
4) Repeat until all bright areas are handled.
5) Hit a button to delete all meshes tagged "delete me!"

So, human in the loop but reasonably straightforward to do. Full automation is going to be pretty challenging, but you might be able to imagine using the above process as a starting point. Next step could be to build some automation to tag the bright areas but keep a human-in-the-loop to validate or "undo" the automated tagging before deleting the meshes.

Graham

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