Understanding the geometry of octahedron. An octahedron is formed by 8 identical faces of equilateral triangle. Basically it is composed of 2 pyramids. To create it in Revit, I made two extrusion from a base square, one up and one down. The upper extrusion was then cut by a void sweep from the base path with an upper triangle profile. Similarly the lower extrusion was cut by another void sweep below the base square.
Parameters and Math