1. A method implemented by at least one computer processing device, the method comprising: extracting a plurality of features from an image; combining at least two of the plurality of features to generate a model comprising distributions for the at least two of the plurality of features across groups of pixels in the image, wherein the at least two of the plurality of features include one or more mid-level features; and categorizing a material of an object in the image based, at least in part, on the distributions in the model, wherein an individual mid-level feature is a texture-based feature that is not reflectance-based.