Getting rid of black triangle

[forthtremple]

Thanks for the great app. A problem I'm finding with it is there is a black triangle that appears occasionally. Do you have any ideas what is causing this?

[aeriksson]

Thanks! The holes are caused by an issue in the original marching cubes algorithm (which the code is using).

The marching cubes algorithm works by sampling the scalar field over a 3D grid, and rendering triangles between the points of the grid based on the sign of the scalar field at these points. However, this is a bit naive and will occasionally lead to ambiguities, which have to be resolved by sampling at additional points. When these ambiguities are resolved in the wrong way (which will happen in ~50% of cases), holes appear in the mesh. See http://undergraduate.csse.uwa.edu.au/units/CITS4241/Handouts/ppt/Lect07-Disambig.pdf or http://users.polytech.unice.fr/~lingrand/MarchingCubes/algo.html for details.

The issue can be resolved by switching to a more intricate algorithm (e.g. any of http://swiftcoder.wordpress.com/planets/isosurface-extraction/ or https://en.wikipedia.org/wiki/Marching_tetrahedra) or by identifying the ambiguous cases.