Digital libraries meet electronic commerce: on-screen intellectual property
Michael Lesk · 2002
Publishers wishing to distribute text online fear that customers will download their product and redistribute it illegally. Although constraining the users to access the data only through proprietary software that does not allow downloading helps, it still leaves the possibility that users could take screen dumps of the material to capture it. The technique described in the paper relies on the perceptual properties of the human eye, using two unreadable images interleaved quickly to create a readable image, which cannot be screen-dumped since the readability depends on averaging in the human eye. Our program flickers two images of the text each with an admixture of grey noise. Your eye sorts out the letters and reads them, not paying close attention to the grey background; but any screen dump captures the item at one instant including the noise. The text is also scrolled up and down slowly, which again your eye can track, but which would frustrate a program trying to average out the flickering.