  Watermarking is a technique which consists in introducing a brand, the name
or the logo of the author, in an image in order to protect it against illegal
copy. The capacity of the existing watermark channel is often limited. We
propose in this paper a new robust method which consists in adding the
triangular matrix of the mark obtained after the Schur decomposition to the DCT
transform of the host image. The unitary matrix acts as secret key for the
extraction of the mark. Unlike most watermarking algorithms, the host image and
the mark have the same size. The results show that our method is robust against
attack techniques as : JPEG compression, colors reducing, adding noise,
filtering, cropping, low rotations, and histogram spreading.
