Abstract: In a digital image processing system, a dedicated inverse discrete cosine transform (IDCT) processor comprising a controller and an array of accumulators is provided for performing n.times.n inverse discrete cosine transforms. The controller controls the computation of the output vector using a forward mapping procedure. The controller causes k unique kernel values of the reconstruction kernel of each non-zero transform domain coefficient to be selectively accumulated by the array of accumulators, where k equals at most (n.sup.2 +2n)/8. The array of accumulators comprises accumulator blocks sharing an input and a control line designed to exploit the symmetry characteristics of the reconstruction kernels. Each accumulator block is designed to perform a limited number of distinct operations. The accumulator blocks are logically grouped. P symmetry selection bits and less than an operation configuration selection bits, where 2.sup.q equals n.sup.

Abstract: In a digital image processing system, a CPU and a memory is provided to an image signal processing subsystem for computing the output vector of an inverse discrete cosine transform. The inverse discrete cosine transform is represented as a linear system and the output vector is computed using a forward mapping procedure where system matrix columns scaled by the non-zero quantized corresponding transform domain coefficient selected from the input vector are successively accumulated into the output vector. Dequantizations and scalings are performed as a combined single step by looking up the kernel values of the scaled reconstruction kernels from lookup tables corresponding to the selected transform domain coefficients' positions in the input vector. The lookup tables are highly optimized exploiting the symmetry characteristics of the reconstruction kernels, the inherent properties of quantization and the statistical attributes of the quantized transform domain coefficients.