A Comparison of Techniques for Estimating Block Motion in Image Sequence Coding

Block motion estimation for motion compensation is a key component of a wide variety of video coding schemes. Applications range from real-time low bit rate teleconferencing codecs to moderate bit rate methods currently being proposed for the coding of video on CD-ROM's. The criteria used for evaluating the performance of block motion estimation algorithms vary significantly with the application. While low computational complexity is critical for real-time codec applications, it is only a secondary consideration for video coding on CD-ROM's where the encoding is performed off-line. Similarly, while consistency of motion estimates is not an important performance criteria in some applications, it is important in applications using motion interpolation to reduce the temporal frame rate. This paper compares five motion estimation algorithms in terms of three performance criteria: energy of the displaced frame difference (DFD), computational complexity, and entropy of the motion field. The energy of the DFD is a measure of the bit rate needed to reconstruct the current frame after motion compensation. The entropy of the motion field is a measure of the bit rate needed to send motion vectors. In addition, it is usually reasonable to assume that most moving objects in an image produce smooth variations in the motion field across the pixels representing those objects. Since smoothly varying motion fields have low entropy, this assumption leads to an interpretation of the entropy of the motion field as a measure of the consistency of the motion estimate. This interpretation is supported by simulation results.