  In digital painting software, layers organize paintings. However, layers are
not explicitly represented, transmitted, or published with the final digital
painting. We propose a technique to decompose a digital painting into layers.
In our decomposition, each layer represents a coat of paint of a single paint
color applied with varying opacity throughout the image. Our decomposition is
based on the painting's RGB-space geometry. In RGB-space, a geometric structure
is revealed due to the linear nature of the standard Porter-Duff "over" pixel
compositing operation. The vertices of the convex hull of pixels in RGB-space
suggest paint colors. Users choose the degree of simplification to perform on
the convex hull, as well as a layer order for the colors. We solve a
constrained optimization problem to find maximally translucent, spatially
coherent opacity for each layer, such that the composition of the layers
reproduces the original image. We demonstrate the utility of the resulting
decompositions for re-editing.
