Fourier Shell Correlation

The Fourier Shell Correlation (FSC) between two volumes is a measure of their similarity. It assigns a coefficient between -1 and 1 to each chosen frequency range.

FSC computations are used in different areas of Dynamo

1. During adaptive bandpassing, the FSC of the two half averages is used to measure the resolution attained at each iteration.
2. During a regular project, the FSC between the freshly computed iteration average and the one generated in the previous iteration is also computed. This information is used to measure the convergence.

The command line that computes the FSC is dfsc. In its default settings, the FSC for two volumes with a sidelength of N pixels would be a vector with a length of "N"/2 pixels.

1. compute A as fourier transform of real space volume a
2. compute B as fourier transform of real space volume a

1. for each i=1:N/2

   compute a discretized shell n(i)

   a fourier pixel with a distance k from the center is in n(i) if i-1<=|k|<i

   locate elements of A and B inside shell n(i)

   A_i = A|n(i) ; a vector with as many entries as non-zero elements in n(i)

   B_i = B|n(i) ;

   compute cross correlation of

   FSC(i) = A_i * B_i / |A_i| |B_i|

With this definition, the first element in a FSC would measure the similarity of the two first shells A_1,B_1.