Abstract:
A method and apparatus for inserting a low frequency watermark into a compressed data stream carrying compressed content is disclosed. A portion of the compressed data stream is decoded to generate decoded content. The decoded content is analyzed to generate watermark insertion information. The compressed content is embedded with a low frequency watermark using the watermark insertion information.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of U.S. provisional patent application Ser. Nos. 60/479,775, filed Jun. 19, 2003, and 60/480,307, filed Jun. 20, 2003, which are herein incorporated by reference. 

   This invention was made with U.S. government support under contract number NIST 70NANB1H3036. The U.S. government has certain rights in this invention. 

   BACKGROUND OF THE INVENTION 
   Embodiments of the present invention generally relate to digital watermarking. Digital watermarks can be applied to image content, moving (video) or stationary (still pictures). These watermarks can serve a variety of purposes, including the tracking of unauthorized copies back to the party who licensed the use of the content and who was responsible for preventing its further distribution. 
   There are several watermarking techniques for images/video, covering a wide range of properties. These techniques are generally applicable in the pixel domain, i.e., they can insert watermarks in the raw (uncompressed) images. 
   In practice, video is usually compressed before being distributed on a physical medium (e.g., DVD) or over a network (e.g., soft copy downloadable over the Internet). If the watermark payload is different for every copy and the watermark is applied in the pixel domain, then each individual copy needs to be compressed, since it is different from every other copy. This concept is illustrated in  FIG. 1 . Compression can be a very expensive operation both from a computing resource standpoint (it requires a lot of computational power) as well as from a human resource standpoint (it is very common to have a human inspect the results of the compression algorithm and adjust parameters to improve the visual result). The cost of the per-copy operations renders the watermark insertion in the pixel domain untenable for these applications. 
   For watermarking of material already in the compressed domain (e.g., DVDs, Internet downloads), it is important that the watermark embedding process does not result in extensive changes in the bitstream, because this could undermine compression choices made at the time of the initial encoding both to optimize the perceived quality of the encoding, and to maintain rate control, bit-rate, and other profile constraints of the intended application. 
   Therefore, there is a need in the art for watermarking content in the compressed domain. There is also a need in the art for inserting a watermark in the compressed domain with minimal change to the bitstream. 
   SUMMARY OF THE INVENTION 
   The present invention generally discloses a method and apparatus for inserting a low frequency watermark in the compressed domain. In one embodiment, a portion of the compressed data stream is decoded to generate decoded content. The decoded content is analyzed to generate watermark insertion information. The compressed content is embedded with a low frequency watermark using the watermark insertion information 
   In another embodiment a low frequency watermark is inserted into the compressed domain by positioning the watermark in a central frame based on a maskability calculation. A trajectory of a center of gravity of the watermark is determined. A reduced amplitude version of the watermark at frames neighboring the central frame is inserted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is an illustration of watermark insertion in the pixel domain; 
       FIG. 2  is an illustration of watermark insertion in the compressed domain in accordance with one embodiment of the present invention; 
       FIG. 3  illustrates a system in accordance with one embodiment of the present invention; 
       FIG. 4  illustrates a DCT domain embedder in accordance with one embodiment of the present invention; 
       FIG. 5  illustrates one embodiment of DCT adder/recoder in accordance with one embodiment of the present invention; 
       FIG. 6  illustrates a method  600  in accordance with one embodiment of the present invention; 
       FIG. 7  illustrates a method  700  in accordance with one embodiment of the present invention; 
       FIG. 8  illustrates insertion of a watermark in the compressed domain in accordance with one embodiment of the present invention; 
       FIG. 9  illustrates a method in accordance with one embodiment of the present invention; and 
       FIG. 10  illustrates a block diagram of an image processing device or system in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention discloses techniques for performing watermarking-related operations on compressed content. The following discussion focuses on video, but the techniques are equally applicable to still pictures or other audio/visual content. 
   Watermarking is the modification of content (e.g., pixel values or transform coefficients) in order to represent some auxiliary data. This auxiliary data can be characterized as a payload and usually comprises a sequence of binary bits. Applying the modifications to the original content yields a marked copy. Applying the modifications to a flat field (all pixels and all transform coefficients of all frames have constant value—essentially a blank picture) yields a watermark sequence. A unique payload results in a unique watermark sequence. 
   Typically, if one were to watermark compressed content, a decode operation and subsequent re-encode operation (i.e., the compression per copy operation of  FIG. 1 ) would need to be performed. This operation is not acceptable since quality of the watermarked content would be reduced. 
   Ideally, one would like to embed the watermarks after compression, so that compression is performed only once. This is illustrated in  FIG. 2 . 
     FIG. 3  illustrates a system in accordance with one embodiment of the present invention. Compressed content is decoded by decoder  305 . The decoded content is evaluated by analyzer  310 . Analyzer  310  creates a watermark carrier list, which comprises spatiotemporal locations for watermarks. The watermark carrier list is used by embedder  315  to insert watermarks into the compressed content. The embedding may be accomplished in real time. In one embodiment, embedder  315  is a Discrete Cosine Transform (DCT) domain embedder. It should be noted that the present invention discloses the insertion of watermarks in the transform domain and not the pixel domain. Thus, the present invention is able to insert watermarking in the compressed domain. 
     FIG. 4  illustrates a DCT domain embedder in accordance with one embodiment of the present invention. Based on information received from analyzer  310 , a watermark sequence is created by watermark signal creator  410 . The watermark sequence is encoded by a “slave” encoder  415 . Coding decision extractor  405  reads the encoding decisions made by the encoder that produced the compressed version of the original video and passes the encoding decisions to slave encoder  415 . Slave encoder  415  encodes the watermark sequence using the same decisions. DCT adder/recoder combines the encoded watermark sequence and the compressed content. Thus, the original video sequence and the watermark sequence are encoded in a compatible way and can be combined efficiently in the DCT (or other linear transform) domain without full decoding. 
     FIG. 5  illustrates one embodiment of DCT adder/recoder  420 . Bitstream X is, for example, a compressed data stream carrying compressed content. Bitstream X is variable length decoded (VLD) by VLD  520  to recover the quantized DCT coefficients for each macroblock in DCT recovery block  516 . Bitstream W is also variable length decoded (VLD) by VLD  536  to recover the quantized DCT coefficients for each macroblock in DCT recovery block  540 . The temporal reference, motion vectors and mode decisions must also be variable length decoded in VLD  524 , recovered in recovery block  528  and used to partially encode the watermark W. The recovered absolute DCT coefficients of X (main bitstream) and W (watermark bitstream) are added by adder  512  in the DCT domain. 
   In one embodiment, the addition of the DCT coefficients is accomplished by piping  64  floating-point numbers per block into the adder. If desired, the addition of the DCT coefficients may be accelerated. 
   In one embodiment, the acceleration of the addition of DCT coefficients is accomplished by sending the DCT coefficients in run-length format. This action drastically reduces the piping bandwidth for those blocks that only have a few non-zero coefficients. In another embodiment, the acceleration of the addition of DCT coefficients is accomplished by only performing the addition for blocks in the watermark bitstream that have non-zero DCT coefficients. 
   In one embodiment, the recovered absolute DCT coefficients of X (main bitstream) and W (watermark bitstream) are inverse quantized by inverse quantizers  504 ,  508  prior to addition by adder  512  in the DCT domain. In this embodiment, quantization follows addition in quantizer  548 . Inverse quantizers  504 ,  508  and quantizer  548  are optional elements, however, when utilized, these optional elements allow proper handling of the non-linear quantization option allowed by MPEG-2, and also allows requantization. Requantization may be required in applications that require strict control of the watermarked bit count (e.g., watermarked bit count for each coded frame must be no more than 0.5% higher than original bit count). 
   Block  532  determines the length of each segment. It should be understood that segment length may be determined by a number of factors, including, but not limited to, group of picture (GOP) size, the number of frames, and time. 
   The quantized DCT coefficients of partially decoded X and partially encoded W are added together, and new variable-length codes (VLCs)  556 ,  564  are produced as a function of the new quantized DCT coefficients (Xeq+Weq)  552  and the Motion Vectors (MVx)  560  and Mode Decisions (MDx)  572  from bitstream X. The new quantized DCT coefficients  552 , Motion Vectors  560  and Mode Decisions  572  are combined by multiplexer  568 . 
   For applications in which playback of the new bitstream is from a digital storage medium, such as an optical or hard disk, Video Buffering Verifier (VBV) violations are allowed and will not affect the quality of playback. For streaming applications, however, the VBV trajectory of the new bitstream must be made compliant. 
   Because of the extremely low spatio-temporal variation of the watermark, it is highly certain that the only modification would be changes in the DC component of macroblocks (MBs) containing the watermark. Since bitstream W is created with an offset, e.g., a midlevel of  128 , block  544  is needed to remove this offset. The modification of the DC component of MBs containing the watermark is accomplished in block  544 . The DC differentials for these MBs would be slightly larger, and the VLCs will be slightly longer. This means that bit counts for frames containing watermark data will be slightly larger. For MPEG-2 bitstreams coded using the “VBV Delay Method”, there is a danger that the VBV will, over time, underflow. For bitstreams coded using the “0xFFFF” method, the VBV trajectory usually “rides high” in the buffer, and there is a much lower chance that the VBV will underflow, even over the long term. However, to further guard against occasional buffer underflow in the “0xFFFF” case, the sequence-level bit_rate parameter can be made slightly higher, or it can be forced to the maximum allowed by the MPEG-2 profile and level. 
   If MPEG-2 bitstream X is coded using the “VBV Delay Method”: 1. replace the 16-bit vbv_delay value in the picture header with “0xFFFF”, and 2. increase the 18-bit bit_rate_value in the sequence header and the 12-bit bit_rate_extension in the sequence extension to the maximum allowed by the MPEG-2 profile and level. 
   If MPEG-2 bitstream X is coded using the “0xFFFF Method”: 1. increase the 18-bit bit_rate_value in the sequence header and the 12-bit bit_rate_extension in the sequence extension to the maximum allowed by the MPEG-2 profile and level. 
     FIG. 6  illustrates a method  600  in accordance with one embodiment of the present invention. Method  600  starts in step  605  and proceeds to step  610 . 
   Decoded content is analyzed in step  610 . Step  610  creates a watermark carrier list that provides spatiotemporal locations for low frequency watermarks. 
   Step  615  embeds compressed content with a low frequency watermark. Based on information, e.g. watermark carrier list, received from analyzer  310 , embedder  315  creates a watermark sequence. Embedder  315  reads the encoding decisions made by the encoder that produced the compressed version of the original video and encodes the watermark sequence using the same decisions. Embedder  315  combines the encoded watermark sequence and the compressed content. 
     FIG. 7  illustrates a method  700  in accordance with one embodiment of the present invention. Method  700  starts in step  705  and proceeds to step  710 . 
   In step  710 , a first bitstream is decoded by decoder  520 . In step  715 , discrete cosine transform coefficients for a plurality of macroblocks of the first bitstream are recovered by recovery module  516 . In step  720  a second bitstream is decoded by decoder  536 . In step  725 , discrete cosine transform coefficients for a plurality of macroblocks of the second bitstream are recovered by recovery module  540 . In step  730 , temporal reference, motion vectors, and mode decisions of the first bitstream are decoded by decoder  524 . In step  735 , temporal reference, motion vectors, and mode decisions of the first bitstream are recovered by recovery module  528 . In step  740 , the discrete cosine transform coefficients of the first bitstream and the discrete cosine transform coefficients of the second bitstream are added by adder  512 . In step  745 , the added discrete cosine transform coefficients of the first and second bitstreams are quantized by quantizer  548 . In step  750 , the quantized discrete cosine transform coefficients, the motion vectors and the mode decisions are combined into a bitstream by multiplexer  568 . 
   When embedding a watermark that is intended to persist over many frames, one can insert the watermark in an I-frame only, and then observe the “bleeding” of the mark to P- and B-frames in the temporal neighborhood of this I-frame. Furthermore, to the extent that motion estimation is tracking moving objects, this bleeding of the mark will tend to track the objects as well, with some dispersion as the temporal distance from the I-frame increases. This concept is illustrated in  FIG. 8 . 
     FIG. 9  illustrates a method  900  of inserting a watermark in the compressed domain according to one embodiment of the present invention. Method  900  starts in step  905  and proceeds to step  910 . 
   In step  910 , a watermark is positioned in a central frame according to a maskability calculation. In one embodiment, the central frame is selected based on the peak of a spatiotemporal Guassian. 
   In step  915 , a trajectory of a center of gravity of the watermark is determined. In one embodiment, the trajectory of the center of gravity of the watermark is determined over previous and subsequent I-frames. 
   In step  920 , a reduced amplitude version of the watermark at frames neighboring the central frame is inserted. In one embodiment, the reduced amplitude version of the watermark is inserted at neighboring I-frames. In another embodiment, instead of inserting a reduced amplitude version of the watermark at neighboring I-frames, a different method may be used in order to reduce the visibility of any effect of abrupt changes from a dispersed image of the mark (e.g., in P- or B-frames) to a pure Gaussian in a non-central I-frame. In this embodiment, the “dispersed” version of the watermark, can be copied from a neighboring P- or B-frame onto the desired I-frame, with appropriate amplitude reduction to mimic the temporal fall-off of the pure Gaussian. The intended effect here is for the Gaussian watermark to reduce in amplitude and disperse as one proceeds further from the central I-frame in both temporal directions. In either case, the selected path and geometry of each watermark pattern is saved for use in subsequent detection operations. 
     FIG. 10  illustrates a block diagram of an image processing device or system  1000  of the present invention. Specifically, the system can be employed to insert low frequency watermarks in the compressed domain. In one embodiment, the image processing device or system  1000  is implemented using a general purpose computer or any other hardware equivalents. 
   Thus, image processing device or system  1000  comprises a processor (CPU)  1010 , a memory  1020 , e.g., random access memory (RAM) and/or read only memory (ROM), compressed domain watermark insertion module  1040 , and various input/output devices  1030 , (e.g., storage devices, including but not limited to, a tape drive, a floppy drive, a hard disk drive or a compact disk drive, a receiver, a transmitter, a speaker, a display, an image capturing sensor, e.g., those used in a digital still camera or digital video camera, a clock, an output port, a user input device (such as a keyboard, a keypad, a mouse, and the like, or a microphone for capturing speech commands)). 
   It should be understood that the compressed domain watermark insertion module  1040  can be implemented as one or more physical devices that are coupled to the CPU  1010  through a communication channel. Alternatively, the compressed domain watermark insertion module  1040  can be represented by one or more software applications (or even a combination of software and hardware, e.g., using application specific integrated circuits (ASIC)), where the software is loaded from a storage medium, (e.g., a magnetic or optical drive or diskette) and operated by the CPU in the memory  1020  of the computer. As such, the compressed domain watermark insertion module  1040  (including associated data structures) of the present invention can be stored on a computer readable medium, e.g., RAM memory, magnetic or optical drive or diskette and the like. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.