A channel is inserted into a sequence of frames for an image, by varying
one or more display characteristics of the resulting image across the
display. The watermark and other data channel may be employed to watermark
the video signal, and the watermark may correspond to the presence of the
channel or may be data in accordance with watermark information carried
within the channel. For most display devices, display variations are
minimized during the design process, but human viewers may still tolerate
and accept subtle variations in a displayed image. For exemplary
embodiments in accordance with the present invention, slightly changing
one or more display characteristics from center-to-side in accordance with
watermark information allows for watermarking of the image since viewers
may not be aware that display characteristics are changing. For example, a
tapering function may be used in a transmitter to vary one or more color
components of the video signal representing the image. A receiver includes
a watermark detector that has a priori information about the watermark.
The watermark detector examines specific regions of the displayed image
over time, and tests the regions for the watermark during time intervals
when the image remains relatively still. A histogram of each color
component of the region may be generated, and the watermark detected by
comparing the histograms for the watermarked and non-watermarked color
components. The watermark is detected if the histogram of the watermarked
color component varies in manner related to the tapering function.
Watermarking of the image in this manner is robust to subsequent signal
processing methods, such as scaling, cropping, rotation, and compression.
