Returns the (face, si, ti) coordinates of the center of the cell. Note
that although (si,ti) coordinates span the range [0,2**31] in general,
the cell center coordinates are always in the range [1,2**31-1] and
therefore can be represented using a signed 32-bit integer.
Returns the (face, si, ti) coordinates of the center of the cell. Note that although (si,ti) coordinates span the range [0,2**31] in general, the cell center coordinates are always in the range [1,2**31-1] and therefore can be represented using a signed 32-bit integer.