Efficient motion-estimation algorithm for reduced frame-rate video transcoder

In multimedia applications, it is often necessary to adapt the bit rates of the coded video bit streams to the available bandwidths of various communication channels. Because different networks may have different bandwidths, a gateway can include a transcoder to adapt the video bit rates in order to provide video services to users on different networks. In transcoding, motion estimation is usually not performed in the transcoder because of its heavy computational complexity. To speed up the operation, a video transcoder usually reuses the decoded motion vectors (MVs) from the incoming bitstream. Previously, the bilinear interpolation and forward dominant vector selection methods were proposed to compose the missing MVs for reuse. We propose a new algorithm, called the activity dominant vector selection, which utilizes the quantized discrete cosine transform coefficients of residual blocks for composing a MV from the incoming ones. In addition, a motion-vector refinement algorithm called variable step-size search is presented. The performance can be improved while maintaining low computational complexity for a reduced frame-rate video transcoder.