Looks like the author's using parenthetical array subscripts, which *might* be confusing IDL since parentheses are usually reserved for function calls and pulling fields out of structures (e.g., my_structure_variable.(i) returns the ith field in the structure variable) . I took the liberty of updating the code so it now follows the current convention (see below). I also tested it with one of the images that comes with ENVI (bhtmref.img) and it worked--but you are limited to inputted a 2D array so I only passed along the first band of data. Here are some lines of code you can type at the command prompt to test the program (make sure it's compiled first): input_file = 'c:\rsi\idl63\products\envi43\data\bhtmref.img' envi_open_file, input_file, r_fid=input_fid envi_file_query, input_fid, dims=dims ;retrieve first band of data from the file data = envi_get_data(fid=input_fid, dims=dims, pos=0) ;call entropy procedure, pass in 2D image array entropy_value = entropy_func(data) ============================================================================= f unction entropy_func, arrayin compile_opt idl2 ;-------------------------------------------------------------------- ; Modified from the procedure to return the entropy value ; Calculates the line-by-line image entropy of the image stored in ; 'arrayin'. Prints the entropy value to screen. If the array is of ; integer type, a 12 bit image is assumed. If it is of byte type, ; an 8 bit image is assumed. ; Usage: entropy_value = entropy(image_array) ; David O'Brien, 5/30/97 -- problem fixed 8/1/97 ;--------------------------------------------------------------------- ; Get array dimensions and type dimensions = size(arrayin) xsize = dimensions[1] ysize = dimensions[2] type = dimensions[3] ; Fix array to integer type array = fix(arrayin) ; Find correct size for probability array and create it if type eq 1 then array_size = 256*2-1 if type eq 2 then array_size = 4096*2-1 if (type ne 1) and (type ne 2) then print, 'Not a byte or integer array' prob_array = fltarr(array_size) ; Loop over image array elements and count occurences of each possible ; pixel to pixel difference value. Store these values in prob_array for j = 0, ysize-1 do $ for i = 0, xsize-2 do begin diff = array[i+1,j] - array[i,j] if diff lt [array_size+1]/2 and diff gt -[array_size+1]/2 then begin prob_array[diff+[array_size-1]/2] = prob_array[diff+[array_size-1]/2] + 1 endif endfor ; Convert values in prob_array to probabilities and compute entropy n = total(prob_array) entrop = 0 for i = 0, array_size-1 do begin prob_array[i] = prob_array[i]/n ; Base 2 log of x is Ln(x)/Ln(2). Take Ln of array element ; here and divide final sum by Ln(2) if prob_array[i] ne 0 then begin entrop = entrop - prob_array[i]*alog(prob_array[i]) endif endfor entrop = entrop/alog(2) return, entrop end