Definitions

• the invention relates to a method of embedding a watermark in an information signal which is compressed so as to include first signal samples having a given first value and further signal samples having a different value.
• a typical example of such a compressed information signal is an MPEG2 video signal in which video images are represented by transform coefficients, a significant number of which have the first value zero.
• a known method of embedding a watermark in a compressed video signal is disclosed in F. Hartung and B. Girod: "Digital Watermarking of MPEG-2 Coded Video in the Bitstream Domain", published in ICASSP, Vol. 4, 1997, pp. 2621-2624.
• the watermark is a pseudo-noise sequence in the original signal domain.
• the watermark is discrete cosine transformed prior to embedding. Non-zero DCT coefficients of the compressed signal are modified by adding thereto the corresponding coefficients of the transformed watermark sequence.
• the prior art watermark embedding scheme has some drawbacks.
• motion-compensated coding such as MPEG2
• the modification of transform coefficients may propagate in time. Watermarks from previous frames may accumulate in the current frame and result in visual distortion.
• the prior art watermark embedder requires drift compensation.
• modification of DCT coefficients in an already compressed bit stream affects the bit rate. The prior art embedder therefore checks whether transmission of the watermarked coefficient increases the bit rate, and transmits the original coefficient if that is the case.
• the method in accordance with the invention is characterized in that the modifying step is applied to signal samples if the modified signal sample assumes the first value due to said modification. It is thereby achieved that the number of signal samples having the first value increases, which generally leads to a lower bit rate. It is not necessary to actually test the impact of a sample modification on the number of bits.
• the signal samples qualified for modification are samples having the smallest zon-zero value (i.e. MPEG video coefficients being quantized as +1 or -1). As these coefficients represent noise-like information and the changes are very small ( ⁇ quantization step), drift compensation is not necessary, and the embedded watermark is imperceptible but still detectable.
• Fig. 1 shows schematically an arrangement for carrying out the method in accordance with the invention.
• Figs. 2A-2C and 3A-3G show diagrams to illustrate the operation of the arrangement which is shown in Fig. 1.
• Fig. 1 shows a schematic diagram of an arrangement carrying out the method in accordance with the invention.
• the arrangement comprises a parsing unit 110, a VLC processing unit 120, an output stage 130, and a watermark buffer 140. Its operation will be described with reference to Figs. 2A-2C and 3A-3G.
• the arrangement receives an MPEG elementary video stream MPin which represents a sequence of video images.
• An MPEG elementary video stream MPin which represents a sequence of video images.
• One such video image is shown in Fig. 2A by way of illustrative example.
• the video images are divided into blocks of 8x8 pixels, one of which is denoted 201 in Fig. 2A.
• the pixel blocks are represented by respective blocks of 8x8 DCT (discrete cosine transform) coefficients.
• the upper left transform coefficient of such a DCT block represents the average luminance of the corresponding pixel block and is commonly referred to as the DC coefficient.
• the other coefficients represent spatial frequencies and are referred to as AC coefficients.
• the upper left AC coefficients represent coarse details of the image, the lower right coefficients represent fine details.
• the AC coefficients have been quantized.
• FIG. 3 A shows a typical example of a DCT block 300, corresponding to the pixel block 201 in Fig. 2A.
• the coefficients of the DCT block have been sequentially scanned in accordance with a zigzag pattern (301 in Fig. 3 A) and variable-length encoded.
• the variable- length encoding scheme is a combination of Huffman coding and run-length coding. More particularly, each run of zero AC coefficients and a subsequent non-zero AC coefficient constitutes a run-level pair which is encoded into a single variable-length code word.
• Fig. 3B shows the run-level pairs of the DCT block 300.
• An End-Of-Block code (EOB) denotes the absence of further non-zero coefficients in the DCT block.
• Fig. 3C shows the series of variable-length code words representing DCT block 300 as received by the arrangement,
• DCT luminance blocks and two DCT chrominance blocks constitute a macro block
• a number of macro blocks constitutes a slice
• a number of slices constitutes a picture (field or frame)
• a series of pictures constitutes a video sequence.
• Some pictures are autonomously encoded (I-pictures)
• other pictures are predictively encoded with motion compensation (P- and B-pictures).
• the DCT coefficients represent differences between pixels of the current picture and pixels of a reference picture rather than the pixels themselves.
• the MPEG2 elementary video stream MPin is applied to the parsing unit 110
• This parsing unit partially interprets the MPEG bit stream and splits the stream into variable-length code words representing luminance DCT coefficients (hereinafter: VLCs) and other MPEG codes.
• VLCs variable-length code words representing luminance DCT coefficients
• the unit also gathers information such as the coordinates of the blocks, the coding type (field or frame), the scan type (zigzag or alternate).
• the VLCs and associated information are applied to the VLC processing unit 120.
• the other MPEG codes are directly applied to the output stage 130.
• the watermark to be embedded is a pseudo-random noise sequence in the spatial domain.
• a 128x128 basic watermark pattern is "tiled" over the extent of the image. This operation is illustrated in Fig. 2B.
• the 128x128 basic pseudo-random watermark pattern is herein represented by a symbol W for better visualization.
• the spatial pixel values of the basic watermark are transformed to the same representation as the video content in the MPEG stream.
• the 128x128 basic watermark pattern is divided into 8x8 blocks, one of which is denoted 202 in Fig. 2B.
• the blocks are discrete cosine transformed and quantized. Note that the transform and quantizing operation needs to be done only once.
• the DCT coefficients thus calculated are stored in the 128x128 watermark buffer 140 of the arrangement.
• the watermark buffer 140 is connected to the VLC processing unit 120, in which the actual embedding of the watermark takes place.
• the VLC processing unit decodes (121) selected variable-length code words representing the video image into run-level pairs, and converts (122) the series of run-level pairs into a two-dimensional array of 8x8 DCT coefficients.
• the watermark is embedded, in a modification stage 123, by adding to each video DCT block the spatially corresponding watermark DCT block.
• the DCT block representing watermark block 202 in Fig. 2B is thus added to the DCT block representing image block 201 in Fig. 2 A.
• only DCT coefficients that are turned into zero coefficients by this operation are selected for the purpose of watermarking.
• the AC coefficient having the value 2 in Fig. 3 A will be modified only if the corresponding watermark coefficient has the value -2.
• the number of zero coefficients in the DCT block is increased by this operation, so that the watermarked video DCT block can be more efficiently encoded than the original DCT block.
• This is particularly the case for MPEG compressed signals, because the new zero coefficient will be included in the run of another run-level pair (run merge).
• the re-encoding is performed by a variable-length encoder 124 (Fig. 1).
• the watermarked block is applied to the output stage 130, which regenerates the MPEG stream by copying the MPEG codes provided by the parsing unit 110 and inserting regenerated VLCs provided by the VLC processing unit 120. Furthermore, the output stage 130 may insert stuffing bits to make the output bit rate equal to the original video bit rate.
• Fig. 3D shows a typical example of a watermark DCT block 302 corresponding to the spatial watermark block 202 in Fig. 2B.
• Fig. 3E shows a watermarked video DCT block 303 obtained by addition of watermark DCT block 302 to video DCT block 300.
• Fig. 3F shows the run-level pairs of the watermarked DCT block. Note that the former run-level pairs (1,-1) and (0,2) have been replaced by one run-level pair (2,2).
• Fig. 3G shows the corresponding output bit stream. The run merge operation appears to save one bit in this example.
• Fig. 2C shows the watermarked image represented by the output signal MPout of the arrangement.
• the pixel block denoted 203 in this Figure corresponds to the watermarked video DCT block 303 in Fig. 3E.
• the amount of watermark embedding varies from tile to tile and from block to block.
• the watermark coefficient values +1 and -1 in the embodiment described above may also be assigned to mean the direction (positive and negative, respectively) in which the corresponding image coefficient is to be modified.
• a given range of negative DCT coefficients for example, -2 and -1 are turned into zeroes by the watermark coefficient value +1, whereas a range of positive DCT coefficients (for example, +2 and +1) are turned into zeroes by watermark coefficient value -1.
• an MPEG2 elementary video stream may include field-coded DCT blocks and frame-coded DCT blocks.
• the watermark buffer 140 may be arranged to contain two watermark patterns, one for field- coded blocks and one for frame-coded blocks. The pattern being used for embedding the watermark is then selected by the field/frame selection identification signal accommodated in the input video stream.
• a level is not an actual value of an AC coefficient, but a quantized version thereof.
• a further embodiment of the embedding method includes the step of controlling the number and/or positions of coefficients being modified in dependence upon the quantizer step size.
• inverse quantization is achieved by multiplying the received level x(n) with the quantizer step size.
• the quantizer step size is controlled by a weighting matrix W(n) which modifies the step size within a block and a scale factor QS which modifies the step size from (macro-)block to (macro-)block.
• W(n) which modifies the step size within a block
• QS which modifies the step size from (macro-)block to (macro-)block.
• X(n) x(n) x W(n) ⁇ QS where n denotes the index in order of the zigzag scan.
• the maximum number N of coefficients that are allowed to be modified in a block is a function of the quantizer scale factor QS such that N decreases as QS increases.
• QS quantizer scale factor
• N —
• the quantizer scale factor QS is accommodated in MPEG bit streams as a combination of a parameter quantizer _scale_code and a parameter qjscale ype.
• the parameter quantizer _scale_code is a 5-bit code.
• the parameter q_scale_type indicates whether said code represents a linear range of QS-values between 2 and 62, or an exponential range of values between 1 and 112. In both cases, the code is indicative for the step size. Accordingly, the term QS in the above-mentioned function may also be replaced by the parameter quantizer _scale_code.
• the watermark (a spatial noise pattern) is embedded by selectively discarding the smallest quantized DCT coefficients. The discarded coefficients are subsequently merged in the runs of the remaining coefficients. The decision whether a coefficient is discarded or not is made on the basis of a pre-calculated watermark buffer and the number of already discarded coefficients per 8x8 DCT block.
• the advantages of this method are (i) a very simple bit rate control system and (ii) no need for drift compensation.
• the algorithm can be implemented in a very efficient manner with respect to memory requirements and computational complexity.

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