In the last decade digital watermarking techniques have been devised to answer the ever-growing need to protect the
intellectual property of digital still images, video sequences or audio from piracy attacks. Because of the proliferation of
watermarking algorithms and their applications some benchmarks have been created in order to help watermarkers
comparing their algorithms in terms of robustness against various attacks (i.e. Stirmark, Checkmark). However, no equal
attention has been devoted to the proposition of benchmarks tailored to assess the watermark perceptual transparency. In
this work, we study several watermarking techniques in terms of the mark invisibility through subjective experiments.
Moreover, we test the ability of several objective metrics, used in the literature mainly to evaluate distortions due to the
coding process, to be correlated with subjective scores. The conclusions drawn in the paper are supported by extensive
experimentations using both several watermarking techniques and objective metrics.