Embedding and detection of watermark in a motion image signal
View Patent ↗A video watermarking scheme is disclosed, which is designed for the digital cinema format, as it will be used on large projector screens in theaters. The watermark is designed in such a way that it has minimal impact on the video quality, but is still detectable after capture with a handheld camera and conversion to, for instance, VHS, CD-Video or DVD format. The proposed watermarking system only exploits the temporal axis. This makes it invulnerable to geometrical distortions generally caused by such a way of capturing. The watermark is embedded by modulating a global property of the frames (e.g. the mean luminance) in accordance with the samples of the watermark. The embedding depth is preferably locally adapted within each frame to local statistics of the respective image. Watermark detection is performed by correlating the watermark sequence with extracted mean luminance values of a sequence of frames.
1. A method of detecting a watermark in a motion image signal comprising a corresponding sequence of images, the method comprising:
providing a reference watermark, via a watermark detector, as a sequence of reference watermark samples, each watermark sample forming a portion of the watermark but not the entire watermark;
forming a corresponding sequence of mean luminances by determining, for each image of the corresponding sequence of images, a mean luminance of the pixels constituting each image;
correlating the sequence of reference watermark samples with the corresponding sequence of the mean luminances;
testing a correlation value generated from the correlating to determine whether the correlation value exceeds a predetermined threshold; and
generating an output signal, in correspondence with the testing when the correlation value exceeds a predetermined threshold, indicating that the watermark is detected.
2. An apparatus for detecting a watermark in a motion image signal, the apparatus comprising:
means for providing a reference watermark as a sequence of reference watermark samples, each watermark sample forming a portion of the watermark but not the entire watemark;
means for forming a corresponding sequence of mean luminances by determining, for each image of a corresponding sequence of images, a mean luminance of the pixels constituting each image;
means for correlating the sequence of reference watermark samples with the corresponding sequence of the mean luminances;
means for testing a correlation value generated by the means for correlating to determine whether the correlation value exceeds a predetermined threshold; and
means for generating an output signal, in correspondence with the testing when the correlation value exceeds a predetermined threshold, indicating that the watermark is detected.