For many reasons, it is desirable to enable data hiding within a digital image. Those reasons include image security, authentication, covert communication, rendering instructions, and providing additional useful information. Some existing digital watermarking methods do not survive the printing process as most watermarking methods are designed for use in a non-printed contone image. Existing methods designed for contone images are too fragile to be encoded into a print and be retrievable. Glyphs and other low frequency methods that can be used in a print setting often introduce undesirable textures or lower the spatial resolution of the image.
Many printing processes produce color by combining 4 or more colorants. The most common case is the use of cyan, magenta, yellow, and black colorants, referred to herein as C,M,Y,K. It is well known that these 4 colorants can be combined in many different ways to produce the same color, as perceived by the human visual system. For a given image or application, the specific method of combining C, M, Y, K is determined by the gray-component replacement (GCR) function. This function is, in general, a multidimensional mapping that relates the basic subtractive primaries, C, M, Y to the actual C, M, Y, K colorants that constitute the final printed image. A simple and common form of the GCR function is defined by a pair of functions: one that relates min(C, M, Y) to the amount of K to be added, and another that relates min(C, M, Y) to the amount of C, M, Y to be removed or replaced by the K. The two functions are dependent upon each other in a way that yields the desired color. Note that the quantity min(C, M, Y) is the amount in which all three colorants are present, and thus is qualitatively thought of as the “gray component” of the color. Functions of this quantity that dictate the amount of K to be added and the amount of CMY to be replaced are thus collectively termed “gray component replacement”.
In principle, a given image can be rendered with different GCR functions to yield visually similar or identical color renditions. Of these a single GCR function is generally chosen to render a given image or object within a page. The choice is based on a number of factors, including achievable print gamut, colorant area coverage limitations, smoothness and texture considerations, sensitivity to registration, colorant cost, etc.
What is needed in the art is a watermarking method designed for use in the printing process which does not degrade textures and which has a high information channel capacity.