Video data has become the largest source of data consumed globally. Due to the rapid growth of video applications and boosting demands for higher quality video services, video data volume has been increasing explosively worldwide, which has been the most severe challenge for multimedia computing, transmission and storage. Video coding by compressing videos into a much smaller size is one of the key solutions; however, its development has become saturated to some extent while the compression ratio continuously grows in the last three decades. Machine leaning algorithms, especially those employing deep learning, which are capable of discovering knowledge from unstructured massive data and providing data-driven predictions, provide new opportunities for further upgrading video coding technologies. In this article, we present a review on machine learning based video encoding optimization, aiming to provide researchers with a strong foundation and inspire future developments for data-driven video coding. Firstly, we analyze the representations and redundancies of video data. Secondly, we review the development of video coding standards and key requirements. Subsequently, we present a systemic survey on the recent advances and challenges associated with the machine learning based video coding optimizations from three key aspects, including high efficiency, low complexity and high visual quality. Their workflows, representative schemes, performances, advantages and disadvantages are analyzed in detail. Finally, the challenges and opportunities are identified, which may provide the academic and industrial communities with groundwork and potential directions for future research. (C) 2019 Published by Elsevier Inc.
