Prior to encoding RGB full-color images or Bayer color filter array (CFA) images, chroma subsampling is a necessary and crucial step at the server side. In this paper, we first propose a flow diagram approach to analyze the coordinate-inconsistency (CI) problem and the upsampling process-inconsistency (UPI) problem existing in the traditional and state-of-the-art chroma subsampling methods under the current coding environment. In addition, we explain why the two problems degrade the quality of the reconstructed images. Next, we propose a reduced codebook and re-interpolation (RCRI) approach to solve the two problems for enhancing the quality of the reconstructed images. Based on the testing RGB full-color images and Bayer CFA images, the comprehensive experimental results demonstrated at least 1.4 dB and 2.4 dB quality improvement effects, respectively, of our RCRI approach against the CI and UPI problems for the traditional and stateof-the-art chroma subsampling methods.

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<div>Prior to encoding an input RGB full-color image I^RGB, at the server side, performing chroma subsampling on the converted chroma image is a necessary step. After receiving the decompressed subsampled chroma image and luma</div><div>image at the client side, performing chroma upsampling is also a necessary step for reconstructing the RGB full-color image. In this paper, we consider seven commonly used chroma subsampling methods, denoted by C_s, and four widely used chroma upsampling methods, denoted by C_u. For each combination c_s-c_u in C_sxC_u, we first utilize the moment balance law to analyze the coordinate displacement (CD) bias problem occurring in c_s. Next, for the combination c_s-c_u, we analyze the CD bias problem occurring in the transition from the server side to the client side. Then, we explain why the CD bias problem degrades the quality of the reconstructed RGB full-color images in the current coding system. To remedy this CD bias problem, a CD compensationbased (CDC-based) quality enhancement method is proposed to improve the quality of the reconstructed images. To the best of our knowledge, this is the first work in this research direction. Based on the IMAX, Kodak, SCI (screen content images), and Video datasets, the comprehensive experimental results have demonstrated that on the newly released versatile video coding (VVC) platform VTM-12.0, the proposed CDC-based quality enhancement method in our augmented coding system can achieve substantial quality improvement for 17 combinations in C_sxC_u.</div>