29-11-2012, 11:55 AM
TECHNIQUES FOR DIGITAL WATERMARKING
DIGITAL WATERMARKING.pptx (Size: 315.68 KB / Downloads: 26)
Watermarking
Paper watermarking inspired.
Digital watermarking is the need of advance technology.
Definition
Gives copyright protection to digital products
Features of Digital Watermarking
Transparency:
The most fundamental requirement for any Watermarking method shall be such that it is transparent to the end user. The watermarked content should be consumable at the intended user device without giving annoyance to the user. Watermark only shows up at the watermark-detector device.
Security:
security implies that the watermark should be difficult to protect watermark information, without loss of generality, watermarking security can be regarded as the ability to assure secrecy and integrity of the watermark information, and resist malicious attacks
Ease of embedding and retrieval:
Ideally, Watermarking on digital media should be possible to be performed “on the fly”. The computation need for the selected algorithm should be minimum
Robustness :
Watermark robustness accounts for the capability of the watermark to survive signal manipulations
Effect on bandwidth:
Watermarking should be done in such a way that it doesn’t increase the bandwidth required for transmission. If Watermarking becomes a burden for the available bandwidth, the method will be rejected
Imperceptibility:
The imperceptibility refers to the perceptual transparency of the watermark. Ideally, no perceptible difference between the watermarked and original signal should exist
Capacity:
Watermarking capacity normally refers to the amount of information that can be embedded into a host signal. Generally speaking, capacity requirement always struggle against two other important requirements, that is, imperceptibility and robustness .A higher capacity is usually obtained at the expense of either robustness strength or imperceptibility, or both
Visible watermarking
Fragile Watermarks
They are designed with very low robustness. The slightest alteration of the image destroys the watermark.
Robust Watermarks
The embedded watermark should be such that it is invariant to various distortion and signal manipulaiton.
Least Significant Bit Coding (LSB)
LSB coding is one of the earliest methods. It can be applied to any form of watermarking. In this method the LSB of the carrier signal is substituted with the watermark. The bits are embedded in a sequence which acts as the key. In order to retrieve it back this sequence should be known. The watermark encoder first selects a subset of pixel values on which the watermark has to be embedded. It then embeds the information on the LSBs of the pixels from this subset. LSB coding is a very simple technique but the robustness of the watermark will be too low. With LSB coding almost always the watermark cannot be retrieved without a noise component.
Predictive Coding Schemes
Predictive coding scheme was proposed for gray scale images. In this method the correlation between adjacent pixels are exploited. A set of pixels where the watermark has to be embedded is chosen and alternate pixels are replaced by the difference between the adjacent pixels. This can be further improved by adding a constant to all the differences. A cipher key is created which enables the retrieval of the embedded watermark at the receiver. This is much more robust when compared to LSB coding.
Watermarking in DFT Domain
The DFT of an image is generally complex valued.
Most of the information about any typical image is contained in the phase.
Adding a watermark to the phase of the DFT improves the robustness
more immune to noise
DFT is RST invariant.
Discrete cosine transform (DCT) technique:
DCT expresses a sequence of finitely many data points in term of a sum of cosine function oscillating at a different frequency.
The 2-D DCT follows straightforward form of 1D DCT.
Higher value of transform coefficient are concentrated on the top left corner.
It divides image into distinct frequency.
Trade-off occurs.