fingerprint

Definition: Technology for marking a digital image file in a way that is invisible and which survives all normal image manipulations but which can still be retrieved by suitable software: the marker carries an identifier for the copyright holder. * The favoured way is to superimpose a signal onto the waveform of the image, i.e. to apply the marker in the image's frequency domain. Editing changes usually take place in the spatial domain i.e. on the image as seen and have little effect overall effect on the frequency domain. Spare bits or channels in graphic file formats may also be used.
Finding the stolen goods doesn't nail the robber
Fingerprinting is a strategy to inhibit copyright infringement of digital images. However its weakness is that all it does is help establish the identify of the rightful owner. But first infringement must be discovered or suspected before it can be proved. Only then can one even consider how to enforce any restrictions or secure remedy.

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