1. A method of selecting transform matrices for use in a transcoder for down-sampling a DCT image of size M×N to a down-sampled DCT image of size I×J directly in a DCT domain, the method comprising: selecting a spatial domain down-sampling method; applying the selected spatial domain down-sampling method to the DCT image to produce a down-sampled DCT reference image; and selecting transform matrices that minimize an optimization problem, wherein the optimization problem is a function of the error between a down-sampled image obtained using the transform matrices and the down-sampled DCT reference image, and the optimization problem is also a function of the computational complexity of applying the transform matrices, the transform matrices comprising a row transform matrix and a column transform matrix.