Ordinal measure of DCT coefficients for image correspondence and its application to copy detection

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Abstract
Detecting unauthorized copies of digital media (images, audio and video) is a basic requirement for Intellectual Property Right (IPR) protection. This paper proposes a novel method to detect copies of digital images. This copy detection scheme can be used as either an alternative approach or a complementary approach to watermarking. A test image is first reduced to 8×8 sub-image by intensity averaging, and then, the AC coefficients of its discrete cosine transform (DCT) are used to compute distance from those generated from the query image, of which a user wants to find copies. A challenge arises when copies are processed to avoid copy detection or enhance image quality. We show the ordinal measure of DCT coefficients, which is based on relative ordering of AC magnitude values and using distance metrics between two rank permutations, are robust to various modifications of the original image. The optimal threshold selection scheme using the maximum a posteriori (MAP) criterion is also addressed. Through simulations on the database of 40,000 images we show the effectiveness of the proposed system.

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