The test images selected by an appropriate square window can be passed to the image matching algorithm. Before the image matching process, the test image need to be converted to gray scale, and should be divided by the average brightness of the image in order to eliminate the effect of the brightness of the test image in the process of image matching. Average brightness was defines as 2nd norm of the skin-colored area of the test image. Note that it is not the 2nd norm applied to the total area of the test image, since the value that we are looking for is not the average brightness of the test image,
but the average brightness of the skin colored parts only.