The result of the first iteration is a UNIX command-line program. The command-line parameters are the input file and output file; if one of these is missing, the program prompts the user for these data. Upon successful opening of the input file, the user is given the maximum number of characters allowed and asked to input a message to hide in the output image. After changing the necessary pixel values, the output image file is written. The input and output files are identical in size and the minute change in appearance is not noticeable unless the actual pixel values are compared.
Figure 2. message.pgm. Note the lack of perceptible difference between this image and fig1.pgm (Figure 1).
