Digital watermarking

Digital watermarking is the process of embedding auxiliary information into a digital signal -the cover signal. Depending on the context, the notion digital watermark either refers to the information that is embedded into the cover signal or to the difference between the marked signal and the cover signal. Digital watermarking is related to physical paper watermarking, in this case the auxiliary information being an unobtrusive ingrained logo. Watermarking is also closely related to steganography, the art of secret communication.

A digital watermark is called robust with respect to a class of transformations T if the embedded information can reliably be detected from the marked signal even if degraded by any transformation in T. Typical image degradations are JPEG compression, rotation, cropping, additive noise and quantization. For video content temporal modifications and MPEG compression are often added to this list. A watermark is called imperceptible if the cover signal and marked signal are indistinguishable with respect to an appropriate perceptual metric. In general it is easy to create robust watermarks or imperceptible watermarks, but the creation of robust and imperceptible watermarks has proven to be quite challenging . Robust imperceptible watermarks have been proposed as tool for the protection of digital content, for example as an embedded 'no-copy-allowed' flag in professional video content .

• Robustness
  • A watermark is called fragile if it fails to be detected after the slightest modification. Fragile watermarks are commonly used for tamper detection.
  • A watermark is called semi-fragile if it resist benign transformations but fails detection after malignant transformations. Semi-fragile watermarks are commonly used to detect malignant transformations.
  • A watermark is called robust if it resists a designated class of transformations. Robust watermarks are commonly used in copyright applications (to carry ownership or forensic information) and copy protection applications (to carry copy and access control information).
• Perceptibility
  • A watermark is called imperceptible if the original cover signal and the marked signal are (close to) perceptually indistinguishable.
  • A watermark is called perceptible if its presence in the marked signal is noticeable, but non-intrusive.
  • Modification to an original work that are clearly noticeable are commonly not referred to as watermarks, but referred to as generalized barcodes.
• Embedding method
  • A watermarking method is referred to as spread-spectrum if the marked signal is obtained by an additive modification. Spread-spectrum watermarks are known to be modestly robust, but also to have a low information capacity due to host interference.
  • A watermarking method is referred to be of quantization type if the marked signal is obtained by quantization. Quantization watermarks suffer from low robustness, but have a high information capacity due to rejection of host interference.
  • A watermarking method is referred to as amplitude modulation if the marked signal is embedded by additive modification method which it similar to spread spectrum method but this method is especially embedded in spatial domain.