Arithmetic edge coding for arbitrarily shaped sub-block motion prediction in depth video compression.

Depth map compression is important for compact representation of 3D visual data in "texture-plus-depth" format, where texture and depth maps of multiple closely spaced viewpoints are encoded and transmitted. A decoder can then freely synthesize any chosen intermediate view via depth-image-based rendering (DIBR) using neighboring coded texture and depth maps as anchors. In this work, we leverage on the observation that "pixels of similar depth have similar motion" to efficiently encode depth video. Specifically, we divide a depth block containing two zones of distinct values (e. g., foreground and background) into two sub-blocks along the dividing edge before performing separate motion prediction. While doing such arbitrarily shaped sub-block motion prediction can lead to very small prediction residuals (resulting in few bits required to code them), it incurs an overhead to losslessly encode dividing edges for sub-block identification. To minimize this overhead, we first devise an edge prediction scheme based on linear regression to predict the next edge direction in a contiguous contour. From the predicted edge direction, we assign probabilities to each possible edge direction using the von Mises distribution, which are subsequently inputted to a conditional arithmetic codec for entropy coding. Experimental results show an average overall bitrate reduction of up to 30% over classical H.264 implementation.