1. A device for performing a two-dimensional transform on a block of input data elements arrayed in N rows and M columns, comprising: row transform means for receiving and sequentially performing a one-dimensional transform on the N rows of said input data to generate N rows of row transform data; and column transform means for receiving said N rows of row transform data row-by-row from said row transform means, performing parallel calculations on said rows of row transform data as said rows of row transform data are received to carry out a one-dimensional transform on said rows of row transform data in a direction orthogonal to said rows, and after receiving the Nth row of said row transform data, completing said calculations to provide an N.times.M transform data matrix, said column transform means operating to perform said parallel calculations with respect to a first one of said rows of row transform data at the same time that said row transform means is operating to generate a second one of said rows of row transform data from said block of input data elements.