A digital watermark is called
robust with respect to transformations if the embedded information may be detected reliably from the marked signal, even if degraded by severe or multiple transformations. Typical image degradations are
JPEG compression, rotation, cropping, additive noise, and
quantization. For video content, temporal modifications and
MPEG compression often are added to this list. A digital watermark is called
imperceptible if the watermarked content is perceptually equivalent to the original, unwatermarked content. In general, it is easy to create either robust watermarks imperceptible watermarks, but the creation of both robust imperceptible watermarks has proven to be quite challenging. Digital watermarking techniques may be classified in several ways.
Robustness A digital watermark is called
fragile if it fails to be detectable after the slightest modification. Fragile watermarks are commonly used for tamper detection (integrity proof). Modifications to an original work that clearly are noticeable, commonly are not referred to as watermarks, but as generalized
barcodes. A digital watermark is called
semi-fragile if it resists benign transformations, but fails detection after malignant transformations. Semi-fragile watermarks commonly are used to detect malignant transformations. A digital watermark is called
robust if it resists a designated class of transformations. Robust watermarks may be used in copy protection applications to carry copy and no access control information.
Perceptibility A digital watermark is called
imperceptible if the original cover signal and the marked signal are perceptually indistinguishable. A digital watermark is called
perceptible if its presence in the marked signal is noticeable (e.g. digital on-screen graphics like a network logo, content bug, codes, opaque images). On videos and images, some are made transparent/translucent for convenience for consumers due to the fact that they block portion of the view; therefore degrading it. This should not be confused with
perceptual, that is, watermarking which uses the limitations of human perception to be imperceptible.
Capacity The length of the embedded message determines two different main classes of digital watermarking schemes: • The message is conceptually zero-bit long and the system is designed in order to detect the presence or the absence of the watermark in the marked object. This kind of watermarking scheme is usually referred to as
zero-bit or
presence watermarking schemes. • The message is an n-bit-long stream \left(m=m_1\ldots m_n,\; n\in\N\right., with \left.n=|m|\right) or M=\{0,1\}^n and is modulated in the watermark. These kinds of schemes usually are referred to as multiple-bit watermarking or non-zero-bit watermarking schemes.
Embedding method A digital 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 digital watermarking method is said 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 digital watermarking method is referred to as
amplitude modulation if the marked signal is embedded by additive modification which is similar to spread spectrum method, but is particularly embedded in the spatial domain. == Evaluation and benchmarking ==