1. A method of spectral decomposition comprising computer-executed steps of: accessing an image comprising pixels, each pixel containing spectral data; creating a matrix X=AS, where A is a n s ×n d matrix of spectral definitions, where n s is the number of spectral components and n d is the number of dyes into which the image is decomposed, and where S is a n d ×l matrix of amounts of each dye at every pixel, where l is the number of pixels; constraining the matrix X for solution by an unsupervised matrix decomposition method having constraints consistent with properties of additive color model; and using the constrained matrix X to produce at least one image for at least one dye of the number of dyes.