Am 15.02.2016 um 18:10 schrieb Mike Horvath:

> I am trying to take the difference of one large instance of the rock and
> a smaller version of the same object, like this.
...
> But instead of a large rock with a smaller rock-shaped hole in it, both
> rocks simply disappear. Is there an issue when taking the difference of
> one mesh object from another? Can mesh objects not be used in CSG?

The meshes probably lack an inside_vector.

The proper syntax is

    mesh { ... inside_vector DIRECTION }
    mesh2 { ... inside_vector DIRECTION }

where DIRECTION is any vector other than <0,0,0>. The value usually
doesn't matter, but if you experience artifacts, changing the value
might be worth trying (you want to avoid directions that are
almost-but-not-quite parallel to any of the mesh's surfaces). What it
does is that to test whether a given point is inside the mesh POV-Ray
will trace a ray from there in the specified direction, and check
whether it intersects the mesh an even or odd number of times.

The inside_vector is necessary for all CSG operations except "union", as
well as for the "inside()" function.

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