Howdy all,

There is an open-source program that creates an image sa the output files (this comes from my earlier question about drawing chromosomes. I suggested to the primary developer that perhaps it would be useful to add testing to see if "back-end" changes ended up impacting the final product. He said "go for it". So I'm now, err, trying to do that!

So two basic questions:

1. Is there a general way to compare two binary files and see if they are identical?
2. Even better, is there a way to calculate the "distance" between two images of the same size?

So far, this is what I came up with:

1. encode both files (with something like ROT), write the encodings to a file, and do a diff on the encoded files
2. walk over each image, pixel by pixel and calculate the Euclidean distance between corresponding pixels on the two images

In case #2 isn't clear, I would use something like this:

# two image objects
my $image1 = new <some image object>;
my $image2 = new <some image object>;

# ensure images are same size
if ($image1->size != $image2->size) {
    die "Images different sizes\n";
    }

# locals
my $position = 0;
my $cumulative_distance = 0;
my %bad_pixels;
my $threshold = 100;

# loop over all pixels
while ($position < $image1->size) {

    # get current pixels
    my $pixel1 = $image1->getpixel($position);
    my $pixel2 = $image2->getpixel($position);

    my $distance = 0;

    # calculate distance for each colour
    $distance += ($pixel1->red    - $pixel2->red   )^2;
    $distance += ($pixel1->yellow - $pixel2->yellow)^2;
    $distance += ($pixel1->blue   - $pixel2->blue  )^2;

    $distance = $distance^0.5;

    # if distance very large, add to bad pixel-list
    if ($distance > $threshold) {
         $bad_pixels{$position} = $distance;
         }

    $cumulative_distance += $distance;
    $position++;

    }

my $average_distance = $cumulative_distance / $position;
my $bad_pixels = scalar(keys(%bad_pixels));

print "Total Distance: $cumulative_distance\n",
      "Avg   Distance: $average_distance\n",
      "Deviant Pixels: $bad_pixels\n";
[download]

Of course that all requires some image library that lets me walk through pixel-by-pixel and extract the colour values.

Any other (easier?) approaches I could go for? Also, any comments/criticisms of what I've come up with are always appreciated.

-Tats

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