Do not share the indices among your triangles. Doing that way you “deform” the normals at the edges of the cube causing that odd lightning at each face. Try these positions and indices instead:

Positions=” 1 2 1 1 1 1 2 2 1 2 1 1 2 2 0 1 1 0 1 2 1 1 1 1 2 2 0 1 2 0 2 2 1 1 2 1 2 2 1 2 1 1 2 2 0 2 1 0 2 1 1 1 1 1 2 1 0 1 1 0 2 2 0 2 1 0 1 2 0 1 1 0″

TriangleIndices=”0 1 2 1 3 2 4 5 6 5 7 6 8 9 10 9 11 10 12 13 14 13 15 14 16 17 18 17 19 18 20 21 22 21 23 22″

… and a lightning coming from a position like this: Direction=”-3,-5,-7″

… and a lightning coming from a position like this:

Thanks for your post which helps me a lot in learning WPF 3D drawings.

One glitch is that the triangle pairs
6 2 0 2 0 4
in MeshGeometry3D.TriangleIndices should be
6 2 0 6 0 4

Marvelous post for starter anyway!