Effect of Block Sizes on the Attributes of Watermarking Digital Images

Abstract: This work examines the effect of block sizes on attributes (robustness, capacity, time of watermarking, visibility and distortion ) of watermarked digital images using Discrete Cosine Transform (DCT) function. The DCT function breaks up the image into various frequency bands and allows watermark data to be easily embedded. The advantage of this transformation is the ability to pack input image data into a few coefficients. The block size 8 x 8 is commonly used in watermarking. The work investigates the effect of using block sizes below and above 8 x 8 on the attributes of watermark. The attributes of robustness and capacity increase as the block size increases (62-70db, 31.5-35.9 bit /pixel). The time for watermarking reduces as the block size increases. The watermark is still visible for block sizes below 8 x 8 but invisible for those above it. Distortion decreases sharply from a high value at 2 x 2 block size to minimum at 8 x 8 and gradually increases with block size. The overall observation indicates that watermarked image gradually reduces in quality due to fading above 8 x 8 block size. For easy detection of image against piracy the block size 16 x 16 gives the best output result because it closely resembles the original image in terms of visual quality displayed despite the fact that it contains a hidden watermark.