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The Hemicube.

The 'hemicube' form factor calculation method involves placing the
center of a cube at a point on a surface, and using the upper half of
the cube (the 'hemicube' which is visible above the surface) as a
projection body as defined by the 'Nusselt analog'.

Each face of the hemicube is subdivided into a set of small, usually
square ('discrete') areas, each of which has a pre-computed form
factor value.  When a surface is projected onto the hemicube, the sum
of the form factor values of the discrete areas of the hemicube faces
which are covered by the projection of the surface is the form factor
between the point on the first surface (about which the cube is placed)
and the second surface (the one which was projected).

The speed and accuracy of this method of form factor calculation can
be affected by changing the size and number of discrete areas on the
faces of the hemicube.