Lecture 17: How to Spend those Bits we Saved…AnnouncementsGoals for TodayTypes of Coding (Orsak et al. Sec. 6.1+)Big Picture of CodingWhy Code???General View of CodingTypes of Coding: Lecture PlanDigital WatermarkingResources for ExamplesVisible WatermarkingInvisible WatermarkingInformation HidingThose Imperceptible Bits…Basic IdeaDigital vs. Traditional WatermarksIssues in WatermarkingDemosThings to Think About…Lecture 17: How to Spend those Bits we Saved…The Digital World of MultimediaProf. Mari OstendorfEE299 Lecture 1714 February 2007Announcements No HW due this week Work on your lab! Holiday next Monday 2/18 Exam Friday 2/22 Review in class 2/20 Last year’s exam is on the course web pageEE299 Lecture 1714 February 2007Goals for Today Big picture of signal coding Spending vs. saving bits Different reasons for different types of coding Building on ideas from lossy compression Watermarking, information hidingEE299 Lecture 1714 February 2007Types of Coding (Orsak et al. Sec. 6.1+) Formatting Describes what type of file it is & how to translate bits into asignal Compression (Source Coding) Reduces the number of bits needed to represent the signal (lossless & lossy) Watermarking Adds unobservable information to the signal so someone can trace where it came from Error detection and correction (Channel Coding) Adds extra bits to make it possible to detect and sometimes correct errors Encryption Modifies the signal or bit sequence to conceal the true valueEE299 Lecture 1714 February 2007Big Picture of Codingoriginal signalSignal to be stored or transmittedEncodingDecoding: reverse the processcompresswatermarkformaterror codeencryptEE299 Lecture 1714 February 2007Why Code??? Formatting Practical implementation Compression  Faster communications, less storage Watermarking Establish ownership, information hiding Error Coding Reduce impact of errors Encryption Privacy, securityEE299 Lecture 1714 February 2007General View of Coding Change the representation of the signal Transforms, scrambling Quantization (lossy compression) Expanding (error coding) Need mapping functions to get “original”: Undoing the expansion How bits map to numbers  Which bits correspond to different aspects of the signalEE299 Lecture 1714 February 2007Types of Coding: Lecture Plan Formatting Describes what type of file it is & how to translate bits into asignal Compression (Source Coding) Reduces the number of bits needed to represent the signal (lossless & lossy) Watermarking Adds unobservable information to the signal so someone can trace where it came from Error detection and correction (Channel Coding) Adds extra bits to make it possible to detect and sometimes correct errors Encryption Modifies the signal or bit sequence to conceal the true valueLast weekNext timeTodayNot coveredAfter examEE299 Lecture 1714 February 2007Digital Watermarking What is digital watermarking? Imperceptible or barely noticable transformation of a digital signal that embeds a message in it Mainly used for audio, image and video Multiple purposes: Indicate creator/owner and/or purchaser Information hiding Tamper detection Multiple types Visible: Lighten or darken colors Invisible: Use the bits below the noise floor (leverage human perception)EE299 Lecture 1714 February 2007Resources for Examples[1] http://www.dlib.org/dlib/december97/ibm/1 2lotspiech.html[2] www.sagesecure.com[3] en.wikipedia.org[4] www.catchlock.com[5] www.caci.comEE299 Lecture 1714 February 2007Visible WatermarkingDifferent uses:• To remind viewer who owns image• To let viewer know where to find image• For safeguarding previews(from [1], Vatican library image)(from [3])(from [1])EE299 Lecture 1714 February 2007Invisible Watermarking(from [4])Different uses:• Digital rights• Trace bootlegged movie previews• Identify illegal copies • Find source of illegal copies• Access control• Information hiding• Descriptive information (date, source) • Title for broadcast auditing• Secret messageEE299 Lecture 1714 February 2007Information Hiding(from [5])EE299 Lecture 1714 February 2007Those Imperceptible Bits… Recall from audio coding lecture: Below a certain threshold, humans can’t hear the sound (analogous ideas for images) From the bit perspective: Idea in compression: don’t send those bits Idea in watermarking: put a message in those bits1 0 1 1 0 1 1 1 0 1 0 0 1 1 0 1Least significant bits Æbelow the noise floorBits associated with high amplitudeEE299 Lecture 1714 February 2007Basic Idea(from [2])EE299 Lecture 1714 February 2007Digital vs. Traditional WatermarksTraditional Paper Elaborate & costly Visible (though may be hard to see) Lost in copy process Aimed mainly at safeguarding user (by ensuring authenticity)Digital Fast, cheap & easy Often not visible, need computer processing to read Usually kept when signal is copied Aimed mainly at safeguarding ownerEE299 Lecture 1714 February 2007Issues in Watermarking Factors to trade off (interrelated) Number of bits What information is encoded Is encryption needed for privacy Visibility (intentional or unintentional) Robustness to signal modification One big watermark is more fragile than repeats of small watermarks Different media types are subject to different degrees of perceptual masking (watermarking text is very hard!)EE299 Lecture 1714 February 2007Demos Embed text in an image Embed text in a sound file Embed a soundfile in an image Embed an image in an image Approach: use least significant bits of carrier signal to hold hidden signalEE299 Lecture 1714 February 2007Things to Think About… Is identification of the purchaser an invasion of privacy? How vulnerable are watermarks to attacks? Can watermarks be used as evidence in a legal