1. A variable data image watermarking method comprising: receiving input image data that define an input image in terms of RGB data defining a red (R) color separation, a green (G) color separation, and a blue (B) color separation; performing a color transform on said RGB data to a CMYK image defining a cyan (C), magenta (M), yellow (Y) and black (K) separation, to define a constant K image that corresponds in appearance to said input image and that includes a constant K black color separation K′; receiving variable infrared (IR) mark data defining an IR mark to be embedded in the constant K image; defining a watermarked black separation K″ by embedding a sparse two-dimensional pattern of black pixels into said constant K black separation K′, wherein said sparse two-dimensional pattern of black pixels corresponds to said received variable IR mark data, correcting local neighborhood values in the watermarked black separation K″, excluding the black pixels corresponding to said received variable IR mark, to maintain local average black levels; said color transformed cyan, magenta, and yellow color separations C, M, Y and said watermarked black separation K″ defining a watermarked image; and, generating a watermarked document by printing said watermarked image on paper using a multicolorant printer.