1. A method of processing an input image that corresponds to a decompressed version of an image that was compressed based on quantization parameters with respective values, comprising: dividing the input 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 respective ones of the quantization parameter values, wherein the determining comprises processing each of multiple of the frequency domain vectors in a respective iteration in a sequence of iterations, wherein the processing comprises determining the preliminary estimates from ones of the values in the multiple frequency domain vectors, and terminating the processing of the frequency domain vectors in response to a determination that a count of the determined preliminary estimates exceeds a first threshold that is less than a count of all the transform coefficients; and estimating values of the quantization parameters from the preliminary estimates; wherein the dividing, the generating, the determining, and the estimating are performed by computer hardware.