Chotikakamthorn, Nopporn

Preferred name

Chotikakamthorn, Nopporn

Alternative Name

Chotikakamthorn, N.

Chotikakamthornb, Nopporn

Main Affiliation

School of Information Technology

Email

nopporn.ch@kmitl.ac.th

Now showing 1 - 6 of 6

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Comparative study of two projective-invariant digital watermarking methods using cross-ratios and line intersections
(2005-12-01)
Pantuwong, Natapon
;
Chotikakamthorn, Nopporn
Recently, based on a set of four coplanar points, a projective-invariant digital watermarking has been developed. A combination of line intersection and the cross-ratio relationship of collinear points is used to calculate watermarking embedding locations. The four coplanar points used to derive embedding locations can be obtained from either the image corners or the feature points extracted from the image content. It is shown in this paper that based on the same principle, an alternative algorithm for calculating the watermark embedding locations improves computational efficiency. The proposed method heavily uses the cross-ratio relationship of collinear points, while minimizing the use of line intersection in obtaining embedding locations. This is in contrast with the original method, where embedding locations are mainly obtained from intersections of line pairs. Both analytical and experimental results are provided to demonstrate the improvement. © 2005 IEEE.
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Attacks on feature-based affine-invariant watermarking methods
(2005-12-01)
Chotikakamthorn, Nopporn
;
Pantuwong, Natapon
A class of attacks targeting at a feature-based affine-invariant digital watermarking technique is addressed in this paper. First, it is pointed out that the use of feature points to revert an affine transform operation is equivalent to the use of a known image size or boundary for image normalization. Therefore, any method based on feature points suffers from point shifting in much the same manner as how the image normalization scheme suffers from cropping or reframing. Some simple methods to perform a (feature) point shifting attack are next described. Although use of a sufficient large number of candidate feature points can solve the problem, the paper further points out that this solution, like any secure template-based watermarking method, results in high detection complexity. A two-stage search scheme is proposed to avoid the attack, while retaining the overall detection complexity to a reasonable level. © 2005 IEEE.
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Projective-invariant digital image watermarking technique using four co-planar feature points
(2005-12-01)
Chotikakamthorn, Nopporn
;
Pantuwong, Natapon
;
Yawai, Wiyada
The problem of digital image watermarking invariant to planar projective transformation is considered in this paper. Methods that are invariant to certain spatial transformations, such as 2D geometric, and a more general affine transformation, are now widely available. However, those methods fail if a possibly watermarked image has undergone a projective transformation. A method based on a well-known projective invariant property of triangle area ratios, constructed from a set of four co-planar feature points, is described in this paper. The proposed method makes use of such invariant property to compute watermark embedding locations. Formation of the required four feature points for such computation is described. Experimental results are shown to demonstrate failure of an existing affine-invariant watermarking method even at very small projective distortion, as well as the success of the proposed method in withstanding distortion due to the same transformation over wide projection angles. © 2005 IEEE.
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Digital watermarking technique for projective-distorted images using collinear points
(2005-01-01)
Pantuwong, Natapon
;
Yawai, Wiyada
;
Chotikakamthorn, Nopporn
Detection of a digital watermark in images distorted by geometric transforms is made difficult by the so-called synchronization problem. Many watermarking methods have been developed to be robust against an affine transform. Based on such assumption, those methods fail under a more general projective transform. In this paper, a method that is invariant to planar projective transform is presented. The method is based on a projective invariance property of the cross-ratio of four collinear points. Based on four coplanar points as obtained by extracting feature points from the host image, two sets of three collinear points are obtained. From each set of three collinear points, each watermark embedding location can be obtained through its cross-ratio relationship with those aforementioned three collinear points. A method to increase the number of embedding points in addition to those primarily derived from these sets of collinear points is also proposed. An algorithm to form a set of four coplanar points from feature points is also described.
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Alpha channel digital image watermarking method
(2008-12-01)
Pantuwong, Natapon
;
Chotikakamthorn, Nopporn
In this paper, a method for embedding a digital watermark onto a host image's alpha channel is described. Blending factors which form an alpha channel define the proportion of foreground and background colors which in turn are combined to result in an actual color for each pixel. Here, the blending factors corresponding to areas along edges in the host image is of interest. The host image is first divided into blocks of subimages. A dominant edge found in any subimage is used to divide the pixels on that block into three groups:the group of foregroundcolor only pixels, the group of background-color only pixels, and the group of foreground-background blended color pixels. The blending factors for the pixels in the last group are modified to embed each bit of a watermarking pattern. Because the modified pixels belong to the area around an image edge, change to the original image due to watermark embedding is less perceptible. © 2008 IEEE.
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Line watermark embedding method for affine transformed images
(2007-12-01)
Pantuwong, Natapon
;
Chotikakamthorn, Nopporn
This paper describes a new content-based affine-invariant digital watermarking technique. With the proposed embedding and detecting scheme, the watermark patterns are located along the lines constructed from pairs of image's salient points. By using a pair of feature points, the method can perform a search for a watermark faster than other schemes that require at least three correspondence points. The problem of limited embedding capacity, which results in weak robustness, is addressed. A solution to increase the method robustness using multiple line averaging is described. In addition, by utilizing a certain constraint on cropping pattern, the method can be used to provide an estimation of a cropping boundary. ©2007 IEEE.