  The colorful appearance of a physical painting is determined by the
distribution of paint pigments across the canvas, which we model as a per-pixel
mixture of a small number of pigments with multispectral absorption and
scattering coefficients. We present an algorithm to efficiently recover this
structure from an RGB image, yielding a plausible set of pigments and a low RGB
reconstruction error. We show that under certain circumstances we are able to
recover pigments that are close to ground truth, while in all cases our results
are always plausible. Using our decomposition, we repose standard digital image
editing operations as operations in pigment space rather than RGB, with
interestingly novel results. We demonstrate tonal adjustments, selection
masking, cut-copy-paste, recoloring, palette summarization, and edge
enhancement.
