histo_vols - Compute 2D intensity histogram of 2 volumes  

1997 Ulrik Kjems, DSP/IMM, Technical University of Denmark, uk@imm.dtu.dk
Use :    
   histo_vols [-i signal_level] [-d] volume1 volume2 [histogramfile]

Converts volume1 and volume2 to byteformat, and generates a 2D grey-level
co-occurrence histogram from the grayscale intensities in the two volumes.
The histogram is 256x256, x-axis is volume1 intensity, y-axis is volume2
intensity. If 'histogramfile' is specified the histogram is saved as a 2D
volume file, data type LONG.

At the same time, several voxel similarity measures are computed. The
program computes:

 mean square          (a-b)^2
 scaled mean square   (a-b*k)^2  k=sqrt(Var(a)/Var(b))
 entropy              sum p(a,b) log p(a,b)
 mutual information   sum p(a,b) log (p(a,b)/p(a)p(b))

where p(a,b) is the histogram normalized so that the sum over a and b is 1.
If -d is specified, the program writes out the computed similarity measures
in a single row of text so that sequential calls may be concatenated into a
matrix.

histo_vols -h produces above information