Direct Techniques for Optimal Sub-Pixel Motion Accuracy Estimation and Position Prediction

Direct techniques for the optimal resolution estimation and position prediction of subpel motion vectors (MVs) based on integer-pel MVs are investigated in this paper. Although it is common to determine the optimal MV position by fitting a local error surface using integer-pel MVs, the characteristics of the error surface have not been thoroughly studied in the past. Here, we use an approximate condition number of the Hessian matrix of the error surface to characterize its shape in a local region. By exploiting this shape information, we propose a block-based subpel MV resolution estimation method that allows each block to choose its optimal subpel MV resolution for the optimal rate-distortion (R-D) performance adaptively. Furthermore, we propose two MV position prediction schemes for ill and well-conditioned error surfaces, respectively. All proposed techniques are direct methods, where no iteration is required. Experimental results are given to show the R-D performance of the proposed subpel MV resolution estimation and position prediction schemes.