Method and apparatus for discerning image distortion by reference to encoded marker signals

DETAILED DESCRIPTION Watermarking is a useful way of conveying hidden auxiliary data with an image.
But if the image is distorted in an unknown way (e.
g.
, scaled in size, or angularly rotated), it may be impossible to read the hidden data from the image.
One way to handle this problem is to compare the image with an undistorted version, permitting the distortion to be discerned by pattern matching or related techniques.
However, the undistorted image is commonly unknown or not available.
In accordance with a preferred embodiment of the present invention, this problem is overcome by encoding certain marker signals in the image at the time of watermarking.
By reference to such signals, retrieval of the auxiliary data is possible even if the image has been scaled or rotated.
In one particular implementation, the marker signals (termed a "subliminal graticule") take the form of an additional signal that is added to the image.
This signal--when represented in the spatial frequency domain--appears as a pattern of spots.
If the image is scaled or rotated, the pattern changes in known ways.
By examining the change in the pattern, the distortion of the image can be inferred--even without access to the original image.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.