Adaptive color-to-gray image mapping using directional transform

Several color-to-gray mapping methods were proposed to restore a color image from its corresponding grayscale image. These methods are based on embedding chrominance signals into high-frequency subbands of the luminance signal. In the conventional methods, only separable filters are used for the subband decomposition. In addition, subbands at fixed positions are always used to embed chrominance signals. However, in many images, diagonal components are included as well as horizontal/vertical ones. Therefore, qualities of chrominance-embedded grayscale and restored color images are sometimes degraded especially for images with rich textures. In this paper, we propose an improved color restoration method using a directional transform named hybrid wavelets and directional filter banks which considers directionality in images. Experimental results show our method improves the image qualities of the restored color images as well as the chrominance-embedded grayscale images.