A system includes a structure module, graininess module, mottle module, and standard deviation (STD) module. The structure module is to identify a structure metric based on an input digital image, and remove a structured nonuniformity from the input digital image to produce a second input digital image. The graininess module is to identify a graininess metric based on the second input digital image. The mottle module is to identify a mottle metric based on the second input digital image. The STD module is to identify a STD metric that is to correspond to overall smoothness of the input digital image.