Abstract
The Bayer and RGBW color filter array (CFA) raw images, denoted by <inline-formula> <tex-math notation="LaTeX">$I^{Bayer}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$I^{RGB}$ </tex-math></inline-formula>, respectively, have been widely used in consumer markets. In the demosaicking-first compression scheme, chroma subsampling is necessary prior to compressing <inline-formula> <tex-math notation="LaTeX">$I^{RGB}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$I^{RGBW}$ </tex-math></inline-formula>. Several linear interpolation-based chroma subsampling methods have been developed for <inline-formula> <tex-math notation="LaTeX">$I^{Bayer}$ </tex-math></inline-formula>, but no nonlinear interpolation-based chroma subsampling methods have targeted the above two CFA image types simultaneously. In this paper, we first propose a nonlinear interpolation-based, namely the cubic convolution interpolation-based (CCI-based), <inline-formula> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> block-distortion function for each <inline-formula> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> CFA block <inline-formula> <tex-math notation="LaTeX">$B^{CbCr}$ </tex-math></inline-formula>. Next, using the Cauchy-Schwarz inequality, we prove that the proposed block-distortion function is a convex function in the real domain, which further serves as the base of the initial subsampled chroma solution. Then, a CCI-based iterative method is proposed to improve the initial subsampled chroma solution. The results of comprehensive experimental tests using Bayer and RGBW CFA images created from the three RGB full-color datasets, namely IMAX, SCI (screen content images), and CI (classical images), demonstrate that on the Versatile Video Coding (VVC) platform VTM-11.0, the proposed method achieves substantial quality enhancement and quality-bitrate tradeoff merits of the reconstructed Bayer and RGBW CFA images compared with existing chroma subsampling methods.
Highlights
To save hardware cost, most color digital cameras employ a single-sensor technology with a Bayer color filter array (CFA) to capture real-world scenes
Instead of the COPY-based block-distortion functions used for only IBayer [18], [4], [19], in this paper, a new and more effective nonlinear interpolation-based, namely the cubic convolution interpolation-based (CCI-based), blockdistortion function is proposed for IBayer and IRGBW simultaneously
(4) Based on the ground truth Bayer and RGBW CFA images which are created from the three RGB full-color datasets, namely IMAX [12], SCI [30], and CI [44], the comprehensive experimental results show that on VTM-11.0 [34], our method achieves the best PSNR, SSIM [35], FSIM [42], visual effect, and BD-PSNR
Summary
Most color digital cameras employ a single-sensor technology with a Bayer color filter array (CFA) to capture real-world scenes. At the same time, compressing raw Bayer CFA images has the potential to improve compression performance relative to working with RGB full-color images. Several compression-first schemes were developed to compress raw Bayer CFA images directly, and these schemes include the geometric rotation approach [23], the integer-reversible spectral–spatial color domain approach [25], and the structure conversion approach [10] [7]. The Bayer CFA image IBayer, we consider the RGBW CFA image IRGBW B color pixels in the reconstructed RGBW CFA image Irec,RGBW can be obtained directly using Eq (2), while the W color pixels in Irec,RGBW can be obtained by averaging the reconstructed R, G, and B color values at the same location
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