Let’s make a simple experiment: there is a green square and four gradient stripes in the upper picture. Let’s move the stripes towards the center (the middle picture). As the stripes move closer to the center, you will see dirt near to their “supposed” intersection. Look at the lower picture. The mud circle around the cross has become evident. Although in reality there is no dirt at all. The color around the cross is pure white. Thus, you can see an illusion of image contrast in the picture.