FSO Fourier Shell Occupancy

Calculate Fourier Shell Occupancy - FSO cure - via directional FSC measurements Reference: JL. Vilas, H.D Tagare 2021

The following outputs are generated:

1. FSO curve
2. Global resolution from FSO and FSC
3. 3DFSC
4. Anisotropic filter

The Fourier Shell Occupancy Curve can be obtained from a set of directional FSC (see below) To do that, two half maps are used to determine the Global FSC at threshold 0.143. Then, the ratio between the number of directions with resolution higher (better) than the Global resolution and the total number of measured directions is calculated at different frequencies (resolutions). Note that this ratio is between 0 (resolution of all directions is worse than the global FSC) resolution than the global FSC) and 1 (all directions present better resolution than the FSC) at a given resolution. In the particular case, FSO curve takes the value of 0.5 (FSO=0.5), then half of the directions are better, and the other half are worse than the FSC, this situation occurs at the resoltuion of hte map. It means the FSO = 0.5 at the FSC resolution. A map is isotropic if all directional resolution are similar, and anisotropic is there are significant resolution values along different directions. Thus, when the FSO presents a sharp cliff, a step-like function, the map will be isotropic. In contrast, when the OFSC shows a slope the map will be anisotropic. The lesser slope the higher resolution isotropy.