US Patent 8970910 Visible and invisible watermarking of printed images via 2

As set forth herein, computer-implemented methods and systems facilitate watermarking documents and/or images using 2nd generation stochastic halftoning. The watermark is used to spatially vary the gray level at which a frequency modulation-to-amplitude modulation transition occurs. The encoding algorithm uses as inputs a contone image and a watermark. The visibility of the watermark is controlled by the magnitude of the difference between the AM-to-FM transition threshold values.