Color recovery from gray image based on analysis of wavelet packet sub-bands

This paper proposes a colorization method that uses wavelet packet sub-bands to embed color components. The proposed method, firstly, involves a color-to-gray process, in which an input RGB image is converted into Y, Cb, and Cr images, and a wavelet packet transform applied to Y image to divide it into 16 sub-bands. The Cb and Cr images are then embedded into two sub-bands that include minimum information on the Y image. Once the inverse wavelet packet transform is carried out, a new gray image with texture is obtained, where the color information appears as texture patterns that are changed according to the Cb and Cr components. Secondly, a gray-to-color process is performed. The printed textured-gray image is scanned and divided into 16 sub-bands using a wavelet packet transform to extract the Cb and Cr components, and an inverse wavelet packet transform is used to reconstruct the Y image. At this time, the original information is lost in the color-to-gray process. Nonetheless, the details of the reconstructed Y image are almost the same as those in the original Y image because it uses sub-bands with minimum information to embed the Cb and Cr components. The RGB image is then reconstructed by combining the Y image with the Cb and Cr images. In addition, to recover color saturations more accurately, gray patches for compensating the characteristics of printers and scanners are used. As a result, the proposed method can improve both the boundary details and the color saturations in recovered color images.