Patent Publication Number: US-2022239993-A1

Title: Client forensic watermarking device, system, and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a client forensic watermarking device, system, and method and, more particularly, to a client forensic watermarking device, system, and method enabling a client to display a forensic watermark using a watermark mask. 
     2. Description of the Prior Art 
     A method of embedding a forensic watermark in a server requires a large number of server resources and has a low real-time responsiveness. Therefore, for a server to embed a forensic watermark, it is necessary to overcome a difficulty of embedding user information in real time. To overcome the difficulty, a method is employed in which a 0 content file in which watermark 0 is embedded in video content and a 1 content file in which watermark 1 is embedded in the video content are generated through a preprocessing procedure and then a forensic watermark is embedded by combining the 0 content file and the 1 content file, generated in the preprocessing procedure, in real time based on user information. However, this method has a fatal weakness that the forensic watermark can be easily paralyzed by a plurality users combining respectively sections of content in their possession. For example, two users can download content sections respectively and mix them to make a completely different content file, thereby paralyzing information embedded by a service provider. 
     Further, when hardware digital rights management (DRM) is applied to video content, it is impossible to embed a watermark by fabricating an image. Therefore, there is a need for a method for displaying a forensic watermark that can also be embedded in video content to which hardware DRM is applied. 
     SUMMARY OF THE INVENTION 
     To solve the foregoing problems, an aspect of the present disclosure is to provide a client forensic watermarking device, system, and method enabling a client to display a forensic watermark using a watermark mask. 
     In view of the foregoing aspect, a forensic watermarking device capable of communicating with a content server selecting a watermark mask area in which a watermark mask is displayed from video content and storing watermark area information about the watermark mask area in a storage unit according to an embodiment of the present disclosure may include: a downloading unit requesting the video content to be played from the content server and receiving the video content and the watermark area information from the content server; a watermark mask generation unit outputting the watermark mask using the watermark area information inputted from the downloading unit; and an overlay unit superimposing the watermark mask inputted from the watermark mask generation unit on the watermark mask area of the video content inputted from the downloading unit. 
     The downloading unit may further receive a client session ID from the content server, and the watermark mask generation unit may generate and output the watermark mask using the client session ID inputted from the downloading unit. 
     The watermark mask generation unit may generate a mask frame including the watermark mask, and the overlay unit may superimpose the mask frame inputted from the watermark mask generation unit on an image frame including the watermark mask area, of the video content inputted from the downloading unit. 
     The mask frame may be divided into a background and a foreground, and different alpha values representing a transparency level may be imparted to the background and the foreground for invisibility. 
     The watermark mask generation unit may extract a luminance value from the watermark area information, and may select a darkness alpha value for the background when the luminance value is a darkness value based on a predetermined threshold value. 
     The forensic watermarking device may further include a synchronization management unit synchronizing the mask frame with the image frame so that the mask frame is superimposed on the image frame including the watermark mask area, of the video content. 
     When overlay duration indicating that there is no change in an image frame following the image frame including the watermark mask area is received, the synchronization management unit may synchronize the mask frame with the image frame so that the mask frame is superimposed on the image frame for the overlay duration. 
     The forensic watermarking device may further include an attack-defense unit monitoring, with respect to the watermark mask, whether the client session ID received from the content server is valid, whether the synchronization management unit operates based on the overlay duration, or whether an operation of the overlay unit is removed and only the video content is displayed. 
     A forensic watermarking system according to another embodiment of the present disclosure may include: a content server selecting a watermark mask area in which a watermark mask is displayed from video content and storing watermark area information about the watermark mask area; and the foregoing forensic watermarking device, wherein, when client metadata is received along with a request for the video content to be played from the downloading unit, the content server may generate the client session ID for specifying a client of the video content and may store the client session ID along with the client metadata in the storage unit, thereby achieving the foregoing aspect. 
     The content server may analyze blocks of a predetermined frame of GOP content of the video content and may store location information about blocks in which the watermark mask is displayed as the watermark area information. 
     A forensic watermarking method performed by a forensic watermarking device capable of communicating with a content server selecting a watermark mask area in which a watermark mask is displayed from video content and storing watermark area information about the watermark mask area in a storage unit according to another embodiment of the present disclosure may include: a step of requesting the video content to be played from the content server and receiving the video content and the watermark area information from the content server; a step of outputting the watermark mask using the watermark area information received in the receiving step; and a step of superimposing the watermark mask outputted in the outputting step on the watermark mask area of the video content received in the receiving step, thereby achieving the foregoing aspect. 
     According to the present disclosure, with the foregoing configuration, a client may display a forensic watermark, thereby dealing with a collusion attack. 
     Further, according to the present disclosure, it is possible to display a forensic watermark even when hardware digital rights management (DRM) is applied to video content. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the present disclosure will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating a forensic watermarking device according to an embodiment of the present disclosure; 
         FIG. 2  illustrates an example of blocks selected by a block analysis unit of  FIG. 1 ; 
         FIG. 3  illustrates an example of a watermark mask frame generated by a watermark mask generation unit of  FIG. 1 ; 
         FIG. 4  illustrates an example of overlay duration managed by a synchronization management unit of  FIG. 1 ; 
         FIG. 5  illustrates an example of an image superimposed by a content overlay unit of  FIG. 1 ; and 
         FIG. 6  is a flowchart illustrating a forensic watermarking method according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Hereinafter, exemplary embodiments of a client forensic watermarking device, system, and method according to the present disclosure are described with reference to the accompanying drawings. For reference, in describing the present disclosure below, terms referring to components of the present disclosure are named in consideration of the functions of the respective components and thus should not be construed as limiting the technical components of the present disclosure. 
       FIG. 1  is a block diagram illustrating a client forensic watermarking system according to an embodiment of the present disclosure,  FIG. 2  illustrates an example of blocks selected by a block analysis unit of  FIG. 1 ,  FIG. 3  illustrates an example of a watermark mask frame generated by a watermark mask generation unit of  FIG. 1 ,  FIG. 4  illustrates an example of overlay duration managed by a synchronization management unit of  FIG. 1 , and  FIG. 5  illustrates an example of an image superimposed by a content overlay unit. 
     As illustrated in  FIG. 1 , the forensic watermarking system includes a content server  110  and a forensic watermarking device  120 . 
     The content server  110  may include a frame extraction unit  112 , a block analysis unit  114 , a content storage unit  116 , and a database storage unit  118 . 
     The frame extraction unit  112  extracts an intra frame from video content. The video content may include a plurality of groups of pictures (GOPs). Here, a GOP includes an intra frame (I frame) that is an intra-frame coded image, a predictive frame (P frame) that is an inter-frame forward predictive coded image, and a bidirectional predictive frame (B frame) that is a bidirectional predictive coded image. For example, one GOP may include I, B, B, P, B, B, P, B, B, P, B, B, P, B, and B. Here, the intra frame is merely used as an example of a frame extracted by the frame extraction unit  114 , and it is not intended to exclude a predictive frame (P frame) and a bidirectional predictive frame (B frame). 
     The block analysis unit  114  may receive the intra frame extracted and outputted by the frame extraction unit  112  as an input, may analyze blocks capable of expressing intact watermark information, and may select a frame in which a watermark mask is displayed in the forensic watermarking device  120 . In this case, it is possible to select the frame by analyzing whether there are blocks capable of expressing all payloads in one frame. 
     The block analysis unit  114  selects a block of a predetermined size or less from the intra frame of an encoded image, that is, the smallest block of the encoded image. The block analysis unit  114  may select, for example, a luminance signal of a 4×4 prediction block from the intra frame. Accordingly, a block encoded to be larger than the predetermined size in the intra frame is excluded from a candidate group of a block in which watermark information is embedded. The reason for excluding the block encoded to be larger than the predetermined size from the candidate group is that an image of this block is nearly a smooth image and thus a watermark is highly likely to be noticed when embedded in this block. Therefore, according to the HEVC standard, when a block is encoded as any one of 32×32, 32×16, 16×32, and 16×16 blocks, the block may be considered as being nearly a smooth image. Further, in the AVC standard, when a block is encoded as any one of 16×16, 8×16, and 16×8 blocks, the block may be considered as being nearly a smooth image. 
     The block analysis unit  114  selects a macroblock comprised of only blocks of the predetermined size or less. For example, according to the AVC standard, the block analysis unit  114  may select a 16×16 macroblock comprised of only 4×4 subblocks selected from the intra frame of the encoded image. 
     The block analysis unit  114  identifies whether the number of nonzero quantized coefficients (NNZs) among quantized coefficients of the selected macroblock is greater than a predetermined number Nt. When the number of NNZs is greater than the predetermined number Nt, the block analysis unit  114  selects the blocks as blocks in which the watermark information can be embedded. When the number of NNZs is small, original information of the macroblock is substantially lost in an image decoding process, which may increase synchronization errors. Thus, it is important to set the predetermined number Nt in consideration of this aspect. 
     When the number of macroblocks capable of embedding the watermark information is a predetermined number, for example, 64, or greater of the number of payloads of the watermark information, the block analysis unit  114  selects the macroblocks as frames in which the watermark information is displayed. An example of blocks according to an analysis result by the block analysis unit  114  is shown in  FIG. 2 . In  FIG. 2 , when frames correspond to intra frames respectively, an intra frame Frame-1 of a first GOP and an intra frame Frame-3 of the first GOP are selected as frames in which a watermark mask is displayed in the forensic watermarking device  120 . 
     Temporal flicker may occur in an image on which the watermark mask is superimposed in the forensic watermarking device  120 . To prevent or reduce the temporal flicker, the block analysis unit  114  may calculate a pseudo motion vector, thereby determining final macroblocks in which the watermark mask is displayed. 
     When the final macroblocks in which the watermark mask is displayed are determined, the block analysis unit  114  stores frame information (frame number) and location information (block-location) about the blocks in the frame in the database storage unit  118 . The block analysis unit may store luminance information about the final macroblocks in which the watermark mask is displayed. 
     The content storage unit  116  may store content that is exactly the same as the video content, and depending on an apparatus, may store content in which a sequence header contains information that the watermark mask is later displayed on the frame. However, an image frame itself is not changed, and thus all of the foregoing types of content are collectively used as video content. 
     The database storage unit  118  stores information about the frame selected by the block analysis unit  114 , the location information about the blocks in the frame, and the luminance information as watermark area information. The watermark area information is transmitted to the forensic watermarking device  120  to display forensic watermark information later. When the video content is requested from the forensic watermarking device  120  and a client session ID for specifying the forensic watermarking device  120  or a user is generated at a content playback time, the client session ID is also stored along with client metadata in the database storage unit  118 . 
     The forensic watermarking device  120  may include a downloading unit  122 , a watermark mask generation unit  124 , a synchronization management unit  126 , an overlay unit  128 , a display  130 , and an attack-defense unit  132 . 
     The downloading unit  122  requests the content that user wants to see from the content server  110 , and manages and plays downloaded content. The downloading unit  122  receives not only video content but also a client session ID, watermark area information, and overlay duration from the content server  110 . 
     The watermark mask generation unit  124  receives the client session ID and the watermark area information from the downloading unit  122 . The watermark mask generation unit  124  generates a watermark mask using the client session ID and provides the watermark mask to blocks of a mask frame using the watermark area information. Although  FIG. 1  shows that the watermark mask is generated in the forensic watermarking device  120 , the watermark mask may also be received from the content server  110 . 
     The watermark mask is superimposed on the video content in a semi-transparent form. Therefore, the watermark mask cannot be visually identified. An example of a watermark mask frame generated by the watermark mask generation unit  124  is illustrated in  FIG. 3 . 
     As illustrated in  FIG. 3 , for example, when the client session ID is ABC, the watermark mask may be displayed as ABC in the mask frame. In this case, for invisibility, the watermark may be displayed with alpha indicating a transparency level, in which 0.0 denotes □fully transparent□ and 1.0 denotes □fully opaque□. In this embodiment, as alpha indications, for example, {circle around (1)} Background: Black Alpha 0.0 and Foreground: White Alpha 0.1, {circle around (2)} Background: Black Alpha 0.1 and Foreground: White Alpha 0.0, {circle around (3)} Background: White Alpha 0.0 and Foreground: Black Alpha 0.1, {circle around (4)} Background: White Alpha 0.1 and Foreground: Black Alpha 0.0 are used. These alpha values may be selected using the luminance information included in the watermark area information. For example, when the luminance of the content image is low, black alpha is selected for the background and a black alpha value is selected according to a luminance value, and when the luminance of the content image is high, white alpha is selected for the background and a white alpha value is selected according to a luminance value. Here, the alpha values in this embodiment are for illustration and may be changed according to the luminance information. 
     The synchronization management unit  126  synchronizes a content frame and the mask frame so that the watermark mask is superimposed on the video content at an appropriate time. One watermark mask may be overlapped on one or more frames. An example of overlay duration managed by the synchronization management unit  126  is illustrated in  FIG. 4 . As illustrated in  FIG. 4 , watermark mask  1  (overlay-mask- 1 ) may be overlapped on three frames, and watermark 2 (overlay-mask- 2 ) may be overlapped on only one frame. 
     In  FIG. 4 , watermark mask  1  (overlay-mask- 1 ) is overlapped on three frames, which may mean, for example, that watermark mask  1  is displayed in two consecutive frames following an I frame in a GOP of a video. In this case, it may mean that there is no temporal change in scenes of the two consecutive frames following the I frame, and this information may be obtained from GOP header information of the content. 
     The overlay unit  128  superimposes the mask frame on the played content frame. An example of an image superimposed by the content overlay unit  128  is illustrated in  FIG. 5 . If necessary, the overlay unit  128  may superimpose the mask frame by changing the density of the mask frame in consideration of the color and density of the content frame. 
     The display  130  displays the video superimposed by the overlay unit  128 . In this case, the watermark mask cannot be recognized with the naked eye in the video displayed on the display unit  130 . 
     The attack-defense unit  132  monitors whether there is an external attack of removing the watermark mask. The attack-defense unit  132  monitors whether the watermark area information and the client session ID provided to the overlay unit  128  are valid and monitors whether the synchronization management unit  126  operates based on the watermark area information and overlap duration information. The attack-defense unit  132  also monitors whether the operation of the overlay unit  128  is removed and only the video content is displayed on the display  130 . When an external attack is detected during monitoring, playback is stopped so that the video content is not displayed on the display  130 . 
       FIG. 6  is a flowchart illustrating a forensic watermarking method according to another embodiment of the present disclosure. 
     The frame extraction unit  112  extracts a predetermined frame, for example, an intra frame, from video content (S 602 ). The block analysis unit  114  receives the intra frame extracted and outputted by the frame extraction unit  112  as an input, analyzes blocks capable of expressing intact watermark information, and selects a frame in which a watermark mask is displayed in the forensic watermarking device  120  (S 604 ). When macroblocks in which the watermark mask is displayed are determined, the block analysis unit  114  stores frame information (frame number) and location information (block-location) about the blocks in the frame in the database storage unit (S 606 ). The block analysis unit may further store luminance information about the macroblocks in which the watermark mask is displayed. 
     The content storage unit  116  may store original content, that is, content that is exactly the same as the video content (S 608 ). The content storage unit  116 , depending on the content server  110 , may store content in which a sequence header contains information that the watermark mask is later displayed on the frame. However, an image frame itself is not changed, and thus all of the foregoing types of content are collectively used as original content. 
     When the video content is requested from the forensic watermarking device  120  and a client session ID capable of specifying the forensic watermarking device  120  or a user is generated at a content playback time, the client session ID is also stored along with client metadata in the database storage unit  118  (S 610 ). 
     The downloading unit  122  receives the video content and watermark area information from the content server  110  (S 612 ). The downloading unit  122  may further receive the client session ID and overlay duration in addition to the watermark area information. 
     The watermark mask generation unit  124  receives the client session ID and the watermark area information from the downloading unit  122 . The watermark mask generation unit  124  generates a watermark mask using the client session ID and provides the watermark mask to blocks of a mask frame using the watermark area information, thereby outputting the mask frame (S 614 ). 
     The synchronization management unit  126  synchronizes a content frame and the mask frame so that the watermark mask is superimposed on the video content at an appropriate time (S 616 ). One watermark mask may overlap on one or more frames. 
     The overlay unit  128  superimposes the mask frame on the played content frame (S 618 ). An example of an image superimposed by the content overlay unit  128  is illustrated in  FIG. 5 . If necessary, the overlay unit  128  may superimpose the mask frame by changing the density of the mask frame in consideration of the color and density of the content frame. 
     The display  130  displays the video superimposed by the overlay unit  128  (S 620 ). In this case, the watermark mask cannot be recognized with the naked eye in the video displayed on the display unit  130 . 
     Although an example in which blocks are analyzed and selected has been described in the embodiments of the present disclosure, fixed blocks may be selected and used in some cases. 
     The embodiments of the present disclosure described above are provided merely to explain the technical idea of the present disclosure, and the scope of the present disclosure should be construed as being defined by the appended claims. Further, it will be apparent to those having ordinary skill in the art to which this disclosure belongs that various modifications and variations can be made in the disclosure without departing from the essential characteristics of the disclosure, and thus any technical ideas within the appended claims and equivalents thereof should be construed as being included in the scope of the disclosure.