Figure 1.

HI3D workflow. The input asymmetric density map (a) that is arbitrarily positioned and orientated is automatically centered and vertically aligned (b). The aligned 3D map is resampled in the cylindrical coordinate to generate the cylindrical projection (c) to mathematically convert a helical structure in the original 3D map into a “2D crystal” image. The auto-correlation function of the cylindrical projection would generate a 2D lattice (d) that visually resembles the diffraction spots of a crystal. The unit cell vector (red arrow in d) with the shortest distance to the equator would correspond to the helical twist (x-coordinate) and rise (y-coordinate).