1. A signal processing apparatus comprising: circuitry configured to learn a plurality of base image signals of which coefficients become sparse, for each of a plurality of features of each of the plurality of base image signals, such that the learned plurality of base image signals are represented by a linear operation of the plurality of base image signals, wherein the plurality of features comprise a combination of one or more frequency bands indicating resolution of one or more learning blocks, each block of the one or more learning blocks comprising one or more of the plurality of base image signals, and depth direction position of one or more objects within the plurality of base image signals.