Motion-Compensated Transform Coding

Interframe hybrid transform/dpcm coders encode television signals by taking a spatial transform of a block of picture elements in a frame and predictively coding the resulting coefficients using the corresponding coefficients of the spatial block at the same location in the previous frame. These coders can be made more efficient for scenes containing objects in translational motion by first estimating the translational displacement of objects and then using coefficients of a spatially displaced block in the previous frame for prediction. This paper presents simulation results for such motion-compensated transform coders using two algorithms for estimating displacements. The first algorithm, which is developed in a companion paper, recursively estimates the displacements from the previously transmitted transform coefficients, thereby eliminating the need to transmit the displacement estimates. The second algorithm, due to Limb and Murphy, estimates displacements by taking ratios of accummulated frame difference and spatial difference signals in a block. In this scheme, the displacement estimates are transmitted to the receiver. Computer simulations on two typical real-life sequences of frames show that motion-compensated coefficient prediction results in coder bit rates that are 20 to 40 percent tower than conventional interframe transform coders using “frame difference of coefficients.” Comparisons of bit rates for approximately the same picture quality show that the two methods of displacement estimation are quite similar in performance with a slight preference for the scheme with recursive displacement estimation.