Optical spacing for cross-staff beams

Normally, the human eye perceives the evenness of rhythmic spacing according to the distance between noteheads. However, for cross-staff beams we consider the distance between stems, rather than the noteheads, to be even/uneven.

Figure: Default spacing: The distance between noteheads is optimized.

Figure: Optical spacing for cross-staff beaming: the distance between stems is optimized.