Watermark embedding and detection of a motion image signal
View Patent ↗Methods and arrangements are disclosed for embedding and detecting a watermark in a cinema movie, such that the watermark can be detected in a copy made by a handheld video camera. The watermark embedder divides each image frame into two areas. A watermark bit ‘+1’ is embedded in a frame by increasing the luminance of the first part and decreasing the luminance of the second part. A watermark bit ‘−1’ is embedded by decreasing the luminance of the first part and increasing the luminance of the second part. It is achieved with the invention that the embedded watermark survives ‘de-flicker’ operations that are often used to remove flicker caused by the different frame rates of cinema projection equipment and consumer camcorders.
1. A method of embedding a watermark in a motion image signal, the motion image signal comprising a sequence of images, the method comprising:
representing the watermark by a sequence of watermark samples each having a value;
dividing an image of the sequence of images into at least a first and a second image area;
modifying the image in accordance with a watermark sample from the sequence of watermark samples, the modification comprising adding the watermark sample value to the luminance value of pixels of the first image area and subtracting the watermark sample value from the luminance value of pixels of the second image area to oppositely modify the mean luminances of the first and second image areas in accordance with the watermark sample; and
for each of the sequence of watermark samples, repeating the dividing an image of the sequence of images and modifying of the image in accordance with a watermark sample from the sequence of watermark samples such that each watermark sample oppositely modifies the mean luminances of the first and second image areas of a different image of the sequence of images until the entire watermark is embedded.
2. The method of claim 1 , wherein the image modifying comprises modifying a series of consecutive images in accordance with the same watermark sample.
3. The method of claim 1 , wherein the first and second image areas are the upper and lower of an image halves, respectively.
4. The method of claim 1 , wherein the first and second image areas are the left and right of an image halves, respectively.
5. An arrangement for embedding a watermark in a motion image signal, the motion image signal comprising a sequence of images, the arrangement comprising:
means for representing the watermark by a sequence of watermark samples each having a value;
means for dividing an image of the sequence of images into at least a first and a second image area; and
image modifying means being arranged for modifying the image in accordance with a watermark sample from the sequence of watermark samples, the modification comprising adding the watermark sample value to the luminance value of pixels of the first image area and subtracting the watermark sample value from the luminance value of pixels of the second image area to oppositely modify the mean luminances of the first and second image areas in accordance with the watermark sample,
wherein, the modifying means is further arranged to repeat the dividing an image of the sequence of images and modifying of the image for each of the sequence of watermark samples in accordance with a watermark sample from the sequence of watermark samples such that each watermark sample oppositely modifies the mean luminances of the first and second image areas of a different image of the sequence of images until the entire watermark is embedded.