1. A method comprising: receiving input defining an adjustment to be applied to a differential representation of a source image; calculating the differential representation of the source image; producing a structural representation of the source image, the structural representation corresponding to multiple types of contrast in the source image; modifying the differential representation based on the structural representation and the input defining the adjustment; and generating a modified image from the modified differential representation by solving a differential equation; wherein the calculating, the producing, the modifying, and the generating are performed by one or more computers.