The technology described in this document relates to filters and functions that are based on exponential decay functions. In one aspect, the technology is embodied in a method that includes using a computing device to compute a first function as a combination of (i) an exponential decay function, a decay factor for which is chosen based on a Gaussian function, and (ii) at least a second function that is obtained by one or more convolution operations on the decay function. The first function provides an approximation of at least a portion of the Gaussian function.