How the Change Detection Works

The change detection used here is relatively simple:

  1. Each picture is divided into lots of small dots, called "pixels", arranged in a grid pattern. Each pixel contains a number that says how bright or dark that particular pixel was.Here is a sample picture with a section enlarged so that you can see the individual pixel squares:
  2. We subtract one picture from the other by subtracting the "brightness value" (also known as radiometric intensity) of corresponding pixels from the two images and take the absolute value. This produces an image that has bright values where the images were different and dark values where they were nearly the same. Here is a sample "raw" difference image:

    Note that this image contains lots of dark, but non-black pixels, indicating small changes in brightness.
  3. We decide how large a difference is important for us to see by setting a "threshold". Pixels that are not bright enough to meet our threshold are set to black. The remaining pixels are colored purple to help them stand out better. Here is a sample image after a threshold of 25% was used:
    If we set the threshold value lower, then smaller changes qualify. Here is the same change image after a threshold of 5% was used:
  4. Finally, just as we started off by subtracting the two images, we add together the original (primary) image and the change detection image. Since the black areas in the change image have brightness values of 0, they don't change anything on the original image. Only the areas in purple are highlighted in the resulting image. Finally, we add an image that contains a picture of a grid. The resulting picture for a 25% threshold is:

There are many different kinds of software capable of performing these manipulations of images. We used the IMAGEMAGICK package from http://www.imagemagick.org. to create our images. The programs that come with IMAGEMAGICK have a huge number of options. The specific set of commands we used was: