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Handling small images

Below a certain size it becomes difficult to embed and reliably recover a watermark from an image. Even the better watermarking methods have difficulty retaining watermarks in 100 by 100 pixel images that are lossily compressed. It is not clear that this problem can ever be solved by technical means, as it is stems from a simple lack of bandwidth. As the effective bandwidth of the picture is (roughly) proportional to its area, halving the dimensions of it reduces the available bandwidth by 4. Often small images are also color-reduced for efficient storage, further restricting the available bandwidth.

It should be noted that as image size drops, the concern about copyright violations also drops. Small text extracts from larger works are allowed for personal use under the copyright act [Cin95]. In music, samples of a couple of seconds or less, especially if they have been distorted in some way, are generally accepted as ``fair use''. Longer, more recognizable samples require copyright clearance .

Most of the literature on watermarks avoids mentioning this problem, because it seems obvious. However end users of watermarking are often not aware of the problems small images can cause, or if they are, are uncertain as to how large an image has to be before its watermark is secure. This makes it essential that a watermarking system has some mechanism for warning a user that a small-image watermark may not be robust.