Subchapters:

The magic "where" function

Suppose you have an 512X512 image and you want to perform operations ONLY on the pixels, let's say, which have an intensity lower than 4 but non equal to 0. Let's have a look at the min and max of the image:

IDL> print , min(image) , max(image)
0 18

In a first step, let's create the table of the indices of the pixel on which we have to work.

      IDL> index = where(image lt 4 and image ne 0)
      IDL> help,index
            INDEX           LONG      = Array[39253]

The index variable is a vector containing all the subscripts of the pixels matching the condition. You access them in the image and modify them. For example, let's give to all those elements the value 1:

IDL> image[index] = 1

You can also just know how many pixels are over 14:

      IDL> print,n_elements(image)
                  262144
      IDL> print, n_elements(where(image gt 14))
                   52600

You can also check if any element meet the criterion before applying a command:

      IDL> toto = where(image gt 14,counts)
      IDL> print,counts
      IDL> if counts ge 1 then ...

You will see that the combination of where with the structure is a powerful way to extract sub_arrays verifying some criteria.