<title>Reversible digital images</title>

Abstract

A method has been developed to hide one image inside another with little loss in image quality. If the second image is a logo or watermark, then this method may be used to protect the ownership rights of the first image and to guarantee the authenticity of the image. The two images to be combined may be either black & white or color continuous tone images. A reversible image is created by incorporating the first image in the upper 4 bits and the second image in the lower 4 bits. When viewed normally, the reversible image appears to be the first image. To view the hidden image, the bits of the combined image are reversed, exchanging all of the lower and higher order bits. When viewed in the reversed mode, the image appears to be the second or hidden image. To maintain a high level of image quality for both images, two simultaneous error diffusion calculations are run to ensure that both views of the reversible image have the same visual appearance as the originals. Any alteration of one of the images locally destroys the other image at the site of the alterations. This provides a method to detect alterations of the original image.