1. A method of processing an image, comprising: dividing the image into a population of image blocks; generating frequency domain vectors from respective ones of the image blocks, wherein each of the frequency domain vectors comprises a respective set of values corresponding to a set of transform coefficients; determining preliminary estimates of quantization parameter values from frequency distributions of the transform coefficient values in a set of the frequency domain vectors corresponding to a variable sample of the population, the variable sample being determined at least in part by at least one threshold; and estimating values of quantization parameters from the preliminary estimates.