A Fast Inverse Motion Compensation Algorithm for DCT-Domain Video Transcoder

The existing methods to perform motion compensation (MC) in discrete cosine transform (DCT) treat each 8 times 8 block as a fundamental unit and, therefore, involve the high cost of reconstructing prediction frames, especially when the half-pixel motion vectors (MVs) are involved. The proposed method operates on a block of variable size 16 N_y times 16 N_x, where N_x and N_y are the number of adjoining 16 times 16 macroblocks with common MV, along the horizontal and vertical directions, respectively. We demonstrate that a basic operation in reconstructing an 16 N_y times 16 N_x DCT-MC block, both with integer and half-pixel MV, can be represented as a multiplication by fixed matrices, and the computations can be greatly simplified through decomposition of DCT/ IDCT operations. Experimental results using cascaded DCT domain transcoder show substantial reduction in computations at quality close to pixel domain transcoding.