1. A non-transitory storage medium storing instructions executable by a processor to perform a method comprising: modeling local descriptors X extracted from an input image using a probabilistic model to generate an input image representation comprising probabilistic model component values for a set of probabilistic model components comprising components of a gradient vector ∇ λ ƒ λ (X) where ƒ λ (X) denotes a probability function parameterized by parameters λ; and during or after the generating, adjusting the input image representation, the adjusting comprising performing a signed power-based or signed logarithmic-based sparsity reduction operation on components z of the input image representation where the components z comprise one of: each probabilistic model component value, and each probabilistic model component value after adjustment by at least one other adjusting operation.