Article

A Visual Cryptographic Technicque for Chromatic Images Using Multi-Pixel Encoding Method.

Journal of Research and Practice in Information Technology (Impact Factor: 0.22). 01/2005; 37(2).
Source: DBLP

ABSTRACT

Visual cryptography is a secret sharing method that uses human eyes to decrypt the secret. Most visual cryptographic methods utilize the technique of pixel expansion, which causes the size of the shares to be much larger than that of the secret image. This situation is more serious for grey-level and chromatic images. In this paper, we propose a multi-pixel encoding method for grey-level and chromatic images without pixel expansion. We simultaneously encrypt r successive white or black pixels each time. The probability of these r pixels being coloured black depends on the ratio of blacks in the basis matrices. Afterward, we incorporate the techniques of colour decomposition and halftoning into the proposed scheme to handle grey-level and chromatic images. The experimental results show that the shares are not only the same size as the secret image, but also attain the requirement of security. The stacked images have good visual effect as well. Besides, our method can be easily extended to general access structure.

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    • "Given a fixed average contrast, the recovered secret image with a smaller variance has better visual quality, especially the evenness of the recovered secret image. In fact, similar observation also has been mentioned by Hou and Tu [13], in this paper, we thoroughly verify, analytically and experimentally, the effectiveness of the variance to be a criterion for evaluating the visual quality of the recovered secret image. And based on the observation, we propose two ME-SIVCS's (Constructions 3 and 4) under different security levels (ME-SIVCS-1 and ME-SIVCS-2), where the proposed schemes satisfy the following properties. "
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    ABSTRACT: In order to reduce the pixel expansion of visual cryptography scheme (VCS), many size invariant visual cryptography schemes (SIVCS’s) were proposed. However, most of the known SIVCS’s have bad visual quality and thin line problems, hence the known SIVCS’s are only suitable to encrypt coarse secret images. In this paper, we notice that the variance of the darkness levels of the pixels also reflects the visual quality of the recovered secret image, as well as the average contrast. We verify, analytically and experimentally, the effectiveness of the variance to be a criterion for evaluating the visual quality of the recovered secret image. Furthermore, we propose two multi-pixel encryption size invariant visual cryptography schemes (ME-SIVCS’s) which improve the visual quality of the recovered secret image by reducing the variance of the darkness levels. In addition, the proposed ME-SIVCS’s can be used to encrypt fine secret images since they avoid some known thin line problems. Experimental results and comparisons are also given to show the effectiveness of the proposed ME-SIVCS’s. Finally, we give suggestions on obtaining good visual quality for the recovered secret image.
    Full-text · Article · Feb 2012 · Journal of Visual Communication and Image Representation
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    • "Gray 1 k=n=2 2 levels × HT [23] √ 1 √ 2 levels × "
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    ABSTRACT: A k-out-of-n Extended Visual Cryptography Scheme (EVCS) is a secret sharing scheme which hides a secret image into n shares, which are also some images. The secret image can be recovered if at least k of the shares are superimposed, while nothing can be obtained if less than k shares are known. Previous EVCS schemes are either for black-and-white images or having pixel expansion. In this paper, we propose the first k-out-of-n EVCS for color images with no pixel expansion. The scheme also improves the contrast of the n shares and the reconstructed secret image (i.e. the superimposed image of any k or more shares) by allowing users to specify the level of each primary color (i.e. Red, Green and Blue) in the image shares as well as the reconstructed secret image.
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    • "So the encoding efficiency is promoted largely. The idea of encoding block by block is also called multi-pixel encoding method by Hou and Tu in [10] [11] "
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    ABSTRACT: In visual secret sharing (VSS) schemes, a secret image can be visually revealed from overlapping shadow images without additional computations. However, the contrast of reconstructed image is much lost. Employing reversing operation to reverse black and white pixels as well as increasing encoding runs is an effective way to improve the contrast. A novel VSS scheme with reversing is presented in this paper. It achieves really ideal contrast within only [m/h] encoding runs (where m and h are the number of the total columns and the number of the wholewhite columns in the basis matrix to encode white pixels, respectively) and no pixel expansion occurs. It encodes the secret image block by block. A block consists of m pixels, which means that m pixels together join into each encoding step. It is suitable for all access structures and can be applied to encrypt black-white, gray-scale and chromatic images. The experimental results, analyses and comparisons show that the proposed scheme is optimal among those schemes with reversing in encoding runs, pixel expansion, complexity, system capacity and encoding efficiency.
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