In an example, a method includes, by one or more processors, acquiring a dot pattern having a distribution of dots. A greyscale image comprising a plurality of pixels each associated with a greyscale value may be derived from the dot pattern. Deriving the greyscale image may comprise, for each pixel, determining the closest dot to the pixel and associating a distance of the closest dot from the pixel with the pixel, and assigning a greyscale value to each pixel, wherein the greyscale value is based on the distance associated with that pixel.