1. A computer-implemented method, comprising: obtaining, by a system of one or more computers, a first digital image; determining, by the system, values for a set of features of the first digital image, wherein determining the values for the set of features of the first digital image includes: receiving an input coordinate of a pixel in the first digital image; and determining the values of a rotational invariant form of two-dimensional S-Transform (RIST) at the input coordinate; generating, by the system, a sample of training data that associates the determined values for the set of features in the first digital image with an observed condition of a subject of the first digital image; determining, by the system and based on the sample of training data and other samples of training data, a model that is configured to estimate a condition of a subject of a digital image; and using the model to estimate a condition of a subject of a second digital image that is different from the first digital image.