MF = circmaskf(F,X0,Y0,R0) applies a circular mask (with NaNs), of 
center [X0,Y0] and radius R0 (given in physical units), to the fields F. 
 
 If no output argument, the result is displayed by showf. 
 
 Example: 
   v = loadvec('*.vc7'); 
   showf(circmaskf(v,[0,0],1)); 
 

truncf, rotatef, shiftf, extractf, IMRESIZE.