respectively. In both cases, the moved part appears at the old as well as at the new location and the stationary object almost disappears. This technique is based on the assumption that XORing two similar grayvalues produces a low output, whereas two distinct inputs yield a high output. However, this is not always true, e.g. XORing 127 and 128 yields 255. These effects can be seen at the boundary of the stationary object, where the pixels have an intermediate graylevel and might, due to noise, differ slightly between two of the images. Hence, we can see a line with high values around the stationary object. A similar problem is that the output for the moved pen is much higher than the output for the moved piece of paper, although the contrast between their intensities and that of the background value is roughly the same. Because of these problems it is often better to use image subtraction or image division for change detection.