Detailed view of two of the six extended faces. The dark outer boundary with width c_m is the margin that is discarded after the reconstruction step, and the two dark squares in the corners of the equatorial face on the right are treated likewise. The remaining part of the extended faces has a gray outer boundary of width 2c_t. In conjunction with the gray squares in the corners of the equatorial face, this boundary forms the transition zone that contains the values shared with the neighboring extended faces.