Digital Watermarking Lecture notes taken from the Web faculty.kfupm.edu.sa/EE/samara/WAterm arking%20Lecture%202.ppt Author Unknown Introduction • Relation to Cryptography – Cryptography is • Reversibility (no evidence) • Established – Watermarking (1990s) • Non-reversible (noise) – Information Hiding • Covert communication channel (steganography) Digital Watermarking Media • Video • Audio • Images – Our discussion will focus on this. Block Diagram Watermark of image watermarking Original Image Watermarking Algorithm Watermarked Image Applications • Copyright – The objective is to permanently and unalterably mark the image so that the credit or assignment is beyond dispute. • Digital Rights – A file may only be used by users with a license that matches the watermarked signature. • Information Hiding – Foil counterfeiters • Revision History – Tamper detection • Meta-tagging – Store keywords, descriptions, time along with images. Criteria • Main Criteria – Capacity – Payload – Computational Complexity – Transparency Require optimum relationship – Robustness Capacity • The ability to detect watermarks with a low probability of error as the number of watermarks in a single image increases. Payload • The amount of information that can be legitimately stored within a data stream – Dependent on host medium – JPEG example Computational Complexity • Difficulty in process of watermark extraction – Realtime? Transparency • Transparency refers to the perceptual quality of the data being protected. – Watermark should be invisible over all image types as well as local image characteristics. • Need to consider perceptually insignificant portion of host image for insertion for maximum transparency Robustness • Resistance to attacks on the watermark – Attack – an operation performed on the image that compromises the watermark – Active, Passive, Collusion, Forgery – Blind vs. Nonblind • Use of non-robust watermarks – eg. tamper detection Approaches and Implementation • Two Types of Encoding – Spatial watermarking (spatial domain) – Spectral watermarking (frequency-domain) • Many types due to variety of transforms • Adjustments made in frequency domain • More robust Spatial-Domain Implementation • Low-level Encoding • Use of Image Analysis Operations – eg. Edge Detection/Color Separation • Cons – Easily Attacked (Cropping) Frequency-Domain Implementation • Algorithm – Decomposition of image – Addition of Watermark • Possibly encoded/encrypted – Re-composition of Image Frequency-Domain Implementation (Discrete Cosine Transform) • Discrete Cosine Transform (DCT) – Used in today’s standard JPEG compression • Relation to DFT • Compression explained by previous groups – Image divided into non-overlapping blocks – Each block is DC transformed – Block coefficients are quantized through a special algorithm • Not ideal for human visual system Frequency-Domain Implementation (Wavelet Transform) • Wavelet Transform – Based on Short Time Fourier Transform (STFT) – Becoming more common in compression techniques • Better model of Human Visual System than DCT Examples of Wavelets Frequency-Domain Implementation (Common Wavelet Transform Algorithm - Decomposition) Filter Bank Decomposition (10 Bands) Frequency-Domain Implementation (Wavelet Transform Algorithm - Overview) Encoded Watermark Watermarked Image Frequency-Domain Implementation (Cortex Transform) • Cortex Transform – Recent – Mimics human visual system • Corresponds to known structure of human eye – Has its own disadvantages • Computational complexity – requires much more data! Other Issues • Just Noticeable Difference (JND) – Threshold based on Human Visual System • Adjustment in Frequency • Adjustments in Intensity – Important impact on transparency • Spatial adjustment of Frequency-Domain Watermark Spread Spectrum • Used to fulfill transparency criterion • The watermark in is based on spread spectrum communications – Delivers narrowband data through a noisy channel, by modulating each data symbol with a wideband (but very low amplitude) signal. – The data is a single bit – a yes or no decision on whether the given watermark is present. – The channel is the image data itself – The wideband signal is the watermark. Color Images • Scheme nearly identical to grayscale – R/G/B channels • Each color plane treated as a separate image – Luminance/Chrominance channels • Luminance = intensity • Chrominance = color Resources • • • • • • • ftp://skynet.ecn.purdue.edu/pub/dist/delp/watermark-proceedings/paper.pdf http://www.cosy.sbg.ac.at/~pmeerw/Watermarking/ http://www.cosy.sbg.ac.at/~pmeerw/Watermarking/MasterThesis/ http://www.eso.org/projects/esomidas/doc/user/98NOV/volb/node308.html http://www.jjtc.com/Steganography/ http://www.mathworks.com/matlabcentral/files/3508/digital%20watermarking.pdf Mihcak, Mehmet Kivanc. “Information Hiding Codes and Their Applications to Images and Audio”, PhD Thesis. 2002.