Fig. 1.

A digital image is an unusually tractable kind of matrix in that row number, column number, and subscript-to-subscript Euclidean distance all have physical interpretations. This example is a very small synthetic slice of the full-color image of the NLM Visible Female (“Eve”): a medial section of one of her central lower incisors, with its canal, in the jawbone. This is a real image, not a virtual one, and it is realistically noisy. Colors are those of the original tissues except that blue represents the latex used to fix movable structures (here, the teeth themselves) against the forces exerted by the microtome, the forces that are also responsible for the left-to-right smearing in some portions of the image. Original sections were horizontal at spacing $300\mathrm{\mu }$, photographed with pixel size also $300\mathrm{\mu }$ in order to yield cubical voxels. Image produced in W. D. K. Green’s Edgewarp software package. The original image is $5180\times 960\times 1664\times 3,$ about 24 gigabytes; the three thousand or so pixels of this extract are thus a very small selection (Color figure online)