Encoding data includes: encoding a residual of a first portion of an array of data to generate a first set of coefficients; decoding the first set of coefficients to generate a decoded representation of the first portion; computing an estimated covariance function for a residual of a second portion of the array of data based on a model that includes a gradient of a plurality of boundary data values located on a boundary of the decoded representation of the first portion; computing a set of transform basis functions from the estimated covariance function; and encoding the residual of the second portion using a first transform that uses the computed set of transform basis functions.