Ok Poindexter, What's the Point?

Original Image

The point is, once we can tell how two images are different, we can do some interesting things based on those differences.

On the previous page I showed you some simple color swatches, and made big changes to them so you could see what Difference was doing.

But many of the most interesting uses of this process are for images that differ just a little.

Let's take a look at these two images. They're the same, or are they?

Original Diff Image

The Diff image* on the left looks black, which would mean the two images are identical. But if we adjust the levels to increase the contrast, we find a hidden message (picture on right).

I typed that message in white on the altered image above, and then I faded the layer transparency to 1%. It's impossible to see, and you can't bring it out in the altered image by tweaking levels. But when you Diff the images, the changes are there for all to see.

Fun with JPEG

JPEG is a lossy image compression format, which means when you save as JPEG, you're throwing out a little bit of information. Ever wondered what you're losing?

You can Diff an original image and a JPEGed version of that image to see what you are loosing when you save at a particular quality level. Here's the difference between my original, and a JPEG at 50%. The horror!

Original saved as JPEG Q5