Joint Chroma Subsampling and Distortion-Minimization-Based Luma Modification for RGB Color Images With Application

In this paper, we propose a novel and effective hybrid method, which joins the conventional chroma subsampling and the distortion-minimization-based luma modification together, to improve the quality of the reconstructed RGB fullcolor image. Assume the input RGB full-color image has been transformed to a YUV image, prior to compression. For each 2×2 UV block, one 4:2:0 subsampling is applied to determine the one subsampled U and V components, Us and Vs. Based on Us, Vs, and the corresponding 2×2 original RGB block, a main theorem is provided to determine the ideally modified 2×2 luma block in constant time such that the color peak signalto-noise ratio (CPSNR) quality distortion between the original 2×2 RGB block and the reconstructed 2×2 RGB block can be minimized in a globally optimal sense. Furthermore, the proposed hybrid method and the delivered theorem are adjusted to tackle the digital time delay integration images and the Bayer mosaic images whose Bayer CFA structure has been widely used in modern commercial digital cameras. Based on the IMAX, Kodak, and screen content test image sets, the experimental results demonstrate that in high efficiency video coding, the proposed hybrid method has substantial quality improvement, in terms of the CPSNR quality, visual effect, CPSNR-bitrate trade-off, and Bjøntegaard delta PSNR performance, of the reconstructed RGB images when compared with existing chroma subsampling schemes.

I. Introduction

Prior to compression, the input red-green-blue (RGB) full-color image is first transformed to a YUV image by (1), where (,,) and (,,) denote the original RGB triple value and the co-located YUV triple value, respectively, at pixel location , , in each RGB and YUV block-pair based on row-major order. Next, a chroma 4:2:0 subsampling scheme is adopted to subsample the one sampled U and V components, and , for each UV block. As shown in Fig. 1, the subsampled YUV image is then conveyed into the encoder for compression. Throughout this paper, we only discuss the chroma 4:2:0 subsampling, although our discussion is also applicable to chroma 4:2:2 subsampling. The commonly used seven chroma subsampling schemes are the 4:2:0(L), 4:2:0(R), 4:2:0(A), 4:2:0(DIRECT), 4:2:0(BRIGHT), 4:2:0(BRIGHT_MEAN) [13], and 4:2:0(MPEG-B) [16]. \begin{equation} \begin{bmatrix} Y_{i} \\ U_{i} \\ V_{i} \end{bmatrix} = \begin{bmatrix} 0.257 & 0.504 & 0.098 \\ -0.148 & -0.291 & 0.439 \\ 0.439 & -0.368 & -0.071 \end{bmatrix} \begin{bmatrix} R_{i}^{ori} \\ G_{i}^{ori} \\ B_{i}^{ori} \end{bmatrix} \!+\! \begin{bmatrix} 16 \\ 128 \\ 128 \end{bmatrix} \end{equation} Fig. 1.

The conventional chroma subsampling scheme.