Improving the Compression Efficiency of Displacement using Morton-ordered Micro-Image in Video-based Dynamic Mesh Coding

For immersive media services such as volumetric video, 3-dimensional (3D) data such as point cloud or mesh is commonly used. The size of such data is significantly larger than that of 2-dimensional (2D) video, and its characteristics are also different from those of conventional video. Recently, MPEG standardized video-based point cloud (V-PCC) and is now developing for video-based dynamic mesh coding (V-DMC), which changes connectivity information of vertices over time. Encoding of mesh parts is divided into generation of decimated mesh and compression of displacement for mesh reconstruction. The displacement data is being compressed using a conventional video CODEC in the form of YUV. However, in YUV of displacement, there is no spatial coherence which is commonly observed in general video. Therefore, currently adopted intra-prediction based video encoding in V-DMC, where prediction is performed in unit of blocks within one frame, is not able to exploit the characteristic of displacement data and brings low compression. This paper proposes a conversion technique for displacement data to increase compression efficiency. Displacement YUVs in a group-of-frames (GOF) are changed into a single micro image (MI) to secure better coherence. In order to maximize compression efficiency, pixels in MI are reordered in a Morton-order. This Morton-ordered MI (MoMI) performs decomposition of frequencies, and ensures a smooth area composed of zero pixels as much as possible. Experimental results show that the proposed MoMI gives better spatial coherency and improves the compression efficiency up to 22.64%