Recursive fast computation of the two-dimensional discrete cosine transform

An efficient algorithm is presented for computing the two-dimensional discrete cosine transform (2-D DCT) whose size is a power of a prime. Based on a generalised 2-D to one-dimensional (1-D) index mapping scheme, the proposed algorithm decomposes the 2-D DCT outputs into three parts. The first part forms a 2D DCT of a smaller size. The remaining outputs are further decomposed into two parts, depending on the summation of their indices. The latter two parts can be reformulated as a set of circular correlation (CC) or skew-circular correlation (SCC) matrix-vector products by utilising the previously addressed maximum coset decomposition. Such a decomposition procedure can be repetitively carried out for the 2-D DCT of the first part, resulting in a sequence of CC and SCC matrix-vector products of various sizes. Employing fast algorithms for the computation of these CC/SCC operations can substantially reduce the numbers of multiplications compared with those of the conventional row-column decomposition approach. In the special case where the data size is a power of two, the proposed algorithm can be further simplified, calling for computations comparable with those of previous works.