**1.3 Classification of watermarking techniques**

Watermarking techniques can be divided into four categories according to the type of document to be watermarked as follows [1]: Text Watermarking, Image Watermarking, Audio Watermarking and Video Watermarking.

In the case of images from implementation point of view, watermarks can be applied in spatial domain and in frequency domain. In Spatial domain, pixels of one and two randomly selected subsets of an image are modified based on perceptual analysis of the original image. In Frequency domain, values of certain frequencies are altered from their original.

According to human perception, digital watermarks can be divided into three categories as follows [2]: Visible, Invisible-robust and Invisible-Fragile. Visible watermark is where the secondary translucent overlaid into the primary content and appears visible on a careful inspection. Invisible-Robust watermark is embedded is such a way changes made to the pixel value are perceptually unnoticed. Invisible –Fragile watermark is embedded in such a way that any manipulation of the content would alter or destroy the watermark. Sometimes another watermarking called dual watermarking is used. Dual watermark is a combination of a visible and an invisible watermark [1]. In this type of watermark an invisible watermark is used as a back up for the visible watermark as clear from the following diagram.

Fig. 4. Schematic representation of dual watermarking

From application point of view, digital watermarking could also be [2]: source based and destination based. In source based a unique watermark identifying the owner is introduced to all the copies of particular content being distributed. Destination based is where each distributed copy gets a unique watermark identifying the particular buyer. Different types of watermarks are shown in the figure. 5.

Current digital image watermarking techniques can be grouped into two major classes: spatial-domain and frequency-domain watermarking techniques [3]. Compared to spatial domain techniques [4], frequency-domain watermarking techniques proved to be more effective with respect to achieving the imperceptibility and robustness requirements of digital watermarking algorithms [5].

Besides, we utilized the accuracy rate *AR* to evaluate the robustness of a copyright

CP AR

Where *NP* is the number of pixels of the watermark image and *CP* is the number of correct

Watermarking techniques can be divided into four categories according to the type of document to be watermarked as follows [1]: Text Watermarking, Image Watermarking,

In the case of images from implementation point of view, watermarks can be applied in spatial domain and in frequency domain. In Spatial domain, pixels of one and two randomly selected subsets of an image are modified based on perceptual analysis of the original image. In Frequency domain, values of certain frequencies are altered from their

According to human perception, digital watermarks can be divided into three categories as follows [2]: Visible, Invisible-robust and Invisible-Fragile. Visible watermark is where the secondary translucent overlaid into the primary content and appears visible on a careful inspection. Invisible-Robust watermark is embedded is such a way changes made to the pixel value are perceptually unnoticed. Invisible –Fragile watermark is embedded in such a way that any manipulation of the content would alter or destroy the watermark. Sometimes another watermarking called dual watermarking is used. Dual watermark is a combination of a visible and an invisible watermark [1]. In this type of watermark an invisible watermark

From application point of view, digital watermarking could also be [2]: source based and destination based. In source based a unique watermark identifying the owner is introduced to all the copies of particular content being distributed. Destination based is where each distributed copy gets a unique watermark identifying the particular buyer. Different types

Current digital image watermarking techniques can be grouped into two major classes: spatial-domain and frequency-domain watermarking techniques [3]. Compared to spatial domain techniques [4], frequency-domain watermarking techniques proved to be more effective with respect to achieving the imperceptibility and robustness requirements of

is used as a back up for the visible watermark as clear from the following diagram.

NP (6)

protection scheme for a specific attack. The formula for *AR* is shown below:

pixels in the extracted watermark image.

original.

**1.3 Classification of watermarking techniques** 

Audio Watermarking and Video Watermarking.

Fig. 4. Schematic representation of dual watermarking

of watermarks are shown in the figure. 5.

digital watermarking algorithms [5].

Fig. 5. Types of watermarking techniques

Commonly used frequency-domain transforms include the Discrete Wavelet Transform (DWT), the Discrete Cosine Transform (DCT) and Discrete Fourier Transform (DFT). The host signal is transformed into a different domain and the watermark is embedded in selective coefficients. Here we have described DFT and DWT domain techniques.
