Some intuitively clear properties of continuous images have no straightforward analogy in the domain of digital images Metric properites of digital images Distance is an important example. The distance between two pixels in a digital image is a significant quantitative measure.
Pixel adjacency is another important concept in digital images.
Region is a contiguous set. The edge direction is perpendicular to the gradient direction which points in the direction of image function growth. The direction of the crack edge is that of increasing brightness, and is a multiple of 90 degrees, while its magnitude is the absolute difference between the brightness of the relevant pair of pixels. Topological properties of images are invariant to rubber sheet transformations . Stretching does not change contiguity of the object parts and does not change the number of holes in regions.