  Prior to encoding color images for RGB full-color, Bayer color filter array
(CFA), and digital time delay integration (DTDI) CFA images, performing chroma
subsampling on their converted chroma images is necessary and important.
  In this paper, we propose an effective general gradient descent-based chroma
subsampling method for the above three kinds of color images, achieving
substantial quality and quality-bitrate tradeoff improvement of the
reconstructed color images when compared with the related methods. First, a
bilinear interpolation based 2$\times$2 $t$ ($\in \{RGB, Bayer, DTDI\}$) color
block-distortion function is proposed at the server side, and then in real
domain, we prove that our general 2$\times$2 $t$ color block-distortion
function is a convex function. Furthermore, a general closed form is derived to
determine the initially subsampled chroma pair for each 2$\times$2 chroma
block. Finally, an effective iterative method is developed to improve the
initially subsampled $(U, V)$-pair. Based on the Kodak and IMAX datasets, the
comprehensive experimental results demonstrated that on the newly released
versatile video coding (VVC) platform VTM-8.0, for the above three kinds of
color images, our chroma subsampling method clearly outperforms the existing
chroma subsampling methods.
