Colorization algorithms, which hide color information in gray images and find them to recover color images, have recently been developed. In these methods, it is important to minimize the loss of original information while the color components are embedded and extracted. In this article, we propose a colorization method using a wavelet packet transform in order to embed color components with minimum loss of original information. In addition, the compensation processing of color saturation in the recovered color images is achieved. In the color-to-gray process, an input red-green-blue image is converted into Y, Cb, and Cr images, and a wavelet packet transform is applied to the Y image. After analyzing the amounts of total energy for each subband, color components are embedded into two subbands including a minimum amount of energy on the Y image. This makes it possible not only to hide color components in the Y image, but to recover the Y image with minimum loss of original information. In the gray-to-color process, the color saturation of the recovered color images is decreased by the printing and scanning process. To increase color saturation, the characteristic curve between printer and scanner, which can estimate the change of pixel values before and after the printing and scanning process, is used to compensate the pixel values of printed and scanned gray images. In addition, the scaling method of the Cb and Cr components is applied to the gray-to-color process. Through the experiments, it is shown that the proposed method improves both boundary details and color saturation in the recovered color images.
Kyung-Woo Ko, Oh-Seol Kwon, Chang-Hwan Son, Yeong-Ho Ha, "Color Embedding and Recovery Based on Wavelet Packet Transform" in Journal of Imaging Science and Technology, 2008, pp 10501-1 - 10501-10, https://doi.org/10.2352/J.ImagingSci.Technol.(2008)52:1(010501)