sepia_img = img.dot(sepia_matrix.T) Since sepia matrix rows do not have unit sums, once the full sepia image is constructed, we need to rescale the result. sepia_img /= sepia_img.max() then extend ...

Linear Transformations in Hilbert Space: and their Applications to Analysis. By Prof. Marshall Harvey Stone. (American Mathematical Society Colloquium Publications, Vol. 15.) ...