Patent ID: 7688993

Claim:
A computer implemented method of processing a two color image represented as Y(m,n) for watermarking wherein m<M, n<N, said method comprising: designating a plurality of pixels at odd row and odd column coordinates within said two color image as candidate pixels; identifying as suitable for storing watermark data by inversion all of said candidate pixels for which, one pixel directly above said candidate pixel and one pixel directly below said candidate pixel have different colors; and one pixel immediately to the left of said candidate pixel and one pixel immediately to the right of said candidate pixel have different colors, by forming a two dimensional array HL oe (i,j)=Y(2i1, 2j+1)⊕Y(2i+2, 2j+1), having one entry for each of said candidate pixels; forming a two dimensional array HL ee (i,j)=Y(2i, 2j+1)⊕Y(2i+1, 2j+1), having one entry for each of said candidate pixels; forming a two dimensional array LH ee (i,j)=Y(2i+1, 2j)⊕Y(2i+1, 2j+1), having one entry for each of said candidate pixels; forming a two dimensional array LH eo (i,j)=Y(2i+1, 2j+1)⊕Y(2i+1, 2j2), having one entry for each of said candidate pixels; assessing values of (i,j) for which (HL ee (i,j)⊕HL oe (i, j))^(LH ee (i, j)⊕LH eo (i, j))=1 to identify invertible candidate pixels at locations (2i+1, 2j+1) in said image, wherein i, j, m, n, M, N are positive integers and 2i+2<M, 2j+2<N.