Novel and Optimal Luma Modification-Based Chroma Downsampling for Bayer Color Filter Array Images

Abstract

The Bayer color filter array (CFA) pattern has been widely used in modern digital color cameras, and the captured image is called the Bayer CFA image I Bayer. Chroma downsampling is a necessary and important step in the coding system. In this paper, a novel and optimal luma modificationbased chroma downsampling (LMCD) method is proposed for I Bayer. For each converted 2 × 2 YUV block, the downsampled (U, V)-pair and the modified luma values by the proposed LMCD method can be optimally determined, and the optimal variant of our LMCD method associated with the two modified luma values has the best quality among all sixteen variants of LMCD. Based on the Kodak and IMAX datasets, on the High Efficiency Video Coding (HEVC) platform HM-16.18, the Versatile Video Coding (VVC) platform VTM-8.0, and the Joint Photographic Experts Group (JPEG) platform, the comprehensive experimental data showed the substantial quality and quality-bitrate tradeoff merits of the reconstructed images by our optimal LMCD method relative to the traditional and state-of-the-art chroma downsampling methods.

A Reduced Codebook and Re-Interpolation Approach for Enhancing Quality in Chroma Subsampling

“…B. Quality Enhancement Merit of Our RCRI Approach for 𝐼 𝐵𝑎𝑦𝑒𝑟 For 𝐼 𝐵𝑎𝑦𝑒𝑟 , this subsection presents the quality enhancement effects of our RCRI approach against the UPI problems and true CI problems for 4:2:0(L), 4:2:0(R), and the state-ofthe-art combinations [20], [7], [8], and [22] under the current coding environment.…”

“…4. Based on the subsampled chroma parameter-pair located at ( 1 2 , 1 2 ), the CSLM [4] and 𝐶𝑆𝐿 𝑀 𝐵𝑎𝑦𝑒𝑟 [8] methods apply the BILI-based chroma upsampling process to estimate the four chroma pairs of each 2×2 chroma block 𝐵 𝐶𝑏𝐶𝑟 for building up their own block-distortion functions. Therefore, the subsampled chroma position of the two methods is expressed as ( 12 , 1 2 ).…”

“…In particular, after analyzing all the sixteen (= 2 4 ) luma-selection cases, only two luma parameters, namely 𝑌 1 and 𝑌 2 , are modified such that the 2×2 Bayer CFA block-distortion could be minimized. Experimental data demonstrated that the 𝐶𝑆𝐿 𝑀 𝐵𝑎𝑦𝑒𝑟 -BILI combination [8] outperforms DM-COPY [20], GD-BILI [7], and modified 4:2:0(A)-TN [22].…”

“…For 𝐼 𝐵𝑎𝑦𝑒𝑟 , we mainly introduce the four state-of-the-art chroma subsampling methods [20], [7], [22], [8].…”

“…In [8], based on the BILI-based chroma upsampling process at the server side, combining chroma subsampling, luma modification, and the Bayer CFA pattern together, a 𝐶𝑆𝐿 𝑀 𝐵𝑎𝑦𝑒𝑟 -BILI combination was proposed. For each 2×2 YCbCr block 𝐵 𝑌 𝐶𝑏𝐶𝑟 , 𝐶𝑆𝐿 𝑀 𝐵𝑎𝑦𝑒𝑟 -BILI determined the best solution of the subsampled (𝐶𝑏, 𝐶𝑟)-pair and the modified luma values for 𝐵 𝑌 𝐶𝑏𝐶𝑟 simultaneously.…”