Abstract:
A method of providing CABAC compliant changes such as watermarks comprises accessing encoded data such as video which comprise at least two blocks; creating or accessing a list of changes to the encoded data that include a direct change to a block; determining motion character or motion vector differential of non-immediate block, non-immediate blocks being adjacent to an immediate block that are immediately adjacent to the block; determining change to the immediate block based on original motion character of the block and the non-immediate block and the motion character of the block that would result from the application of the change; storing the change to the list if the change does not cause a difference to the immediate block; and evaluating other potential changes if other potential changes are available, wherein the other potential changes are subjected to the same process steps as the direct change.

Description:
CROSS-REFERENCE 
       [0001]    This patent application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/189,372, filed Aug. 19, 2008, and titled “CABAC STREAM COMPLIANCE”. The provisional application is expressly incorporated herein by reference in its entirety for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to embedding watermarks in Context-based Adaptive Binary Arithmetic Coding (CABAC) video streams. 
       BACKGROUND OF THE INVENTION 
       [0003]    Today, the demand for digital watermarking as an antipiracy technology is strong. To make it more difficult for pirates to circumvent watermarks it is important for many potential watermarks to be proposed and used. However, it is important for watermarks to not interfere with the intended viewing experience for the intended audience. As such, a need exists for more efficient watermarking techniques. As such, a goal of this invention is to generate a list of possible changes generally associated with watermarking that are CABAC/AVC compliant, and yet do not create visible artifacts, thereby ultimately providing an efficacious method for embedding watermarks in a CABAC video stream. 
       SUMMARY OF THE INVENTION 
       [0004]    A method of providing CABAC compliant changes such as watermarks comprises accessing encoded data such as video data which comprise at least two blocks; creating or accessing a list of changes to the encoded data that include a direct change to a block; determining motion character or motion vector differentials of non-immediate blocks, non-immediate blocks being adjacent to an immediate block that are immediately adjacent to the block; determining change to the immediate block based on original motion character of the block and the non-immediate block and the motion character of the block that would result from the application of the change; storing the change to the list if the change does not cause a difference to the immediate block; and evaluating other potential changes if other potential changes are available, wherein the other potential changes are subjected to the same process steps as the direct change. Blocks can be any size or number of a collection elements or pixels, immediate blocks can be blocks that share some finite border with the block, and non-immediate blocks can be blocks that share some finite border with the immediate blocks, but not with the block. The method can include decoding the encoded data and coding the encoded data which can include using changes that have been stored in the list. The method can include determining a context for the encoded data for at least one of the blocks and/or determining a context index for the encoded data, wherein the context index (ctxIdx) is the sum of an initial value (ctxIdxOffset) and an increment (ctxIdxInc). The method can further include calculating or computing an original increment for the immediate block and a new increment for the immediate block associated with the direct change to the block; and using the original and new increments as a criterion for determining the difference in the storing step, wherein a motion vector differential of the non-immediate block and the original and new motion vector differentials for the block can be used as a criterion for determining the difference in the storing step. If the original increment for the immediate block and a new increment for the immediate block are different for a change, the change can be removed from the list in the storing step. The method can further include the step of calculating or computing an original increment for another immediate block and a new increment for the another immediate block associated with the direct change to the block, if the original and immediate increments are not different for the immediate block; and using the original and new increments for the another immediate block as an additional criterion for determining the difference in the storing step. A motion vector differential for the another non-immediate block that is adjacent to the another immediate block and the original and the new motion vector differentials for the block can be used as an additional criterion for determining the difference in the storing step. Additionally, the change can be added to the list in the storing step if the original and new increments for the another immediate block are not different. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention will now be described by way of example with reference to accompanying drawings. 
           [0006]      FIG. 1  is a flow chart of CABAC encoding. 
           [0007]      FIG. 2  is a more detailed flowchart of CABAC encoding. 
           [0008]      FIG. 3  illustrates motion vector differentials for blocks affected by a change. 
           [0009]      FIG. 4  illustrates another set of motion vector differentials for blocks affected by a change. 
           [0010]      FIG. 5  illustrates a flow chart of a first solution for avoiding errors in encoding. 
           [0011]      FIG. 6  illustrates a flow chart of a second solution and a third solution for avoiding errors in encoding. 
           [0012]      FIG. 7  illustrates a flow chart of a fourth solution for avoiding errors in encoding. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0013]    Watermark embedding is herein performed by changing data bytes in a coded video stream such as CABAC (Context-based Adaptive Binary Arithmetic Coding) coded video stream. It is important to point out that CABAC coding in the context of H.264/AVC video encoders is mentioned throughout to emphasize that the disclosed invention will be generally applicable to such CABAC coded video streams. However, the scope of the invention is generally applicable to other coded data streams and other types of encoders. 
         [0014]    With the understanding that watermarking can cause an AVC decoder failure, crash or the like, embodiments are presented that are directed to avoid negative events. 
         [0015]    Of importance is the understanding that portions of encoded video bitstreams, such as those conforming to the H.264/AVC video coding standard, cannot be blindly modified while maintaining CABAC compliance. In a system that allows modifications under the constraint of maintaining CABAC compliance, changes may be intended to mark or watermark the video data. Such changes can be designed to identify modifications that can be made such that the modifications do not affect any CABAC contexts. However, a special case can arise in which a modification can influence the contexts used for the encoding/decoding of subsequent syntax elements. In particular, a modification might not change the current context of an encoder, but might nonetheless change the context that is selected at a decoder. When this occurs, the CABAC decoder will apply the wrong context, typically causing a decoding error. This application describes one or more embodiments that identify the special cases in which this error may occur and avoid modifications that would cause such errors. 
         [0016]    The context of the invention is applicable to the operation of H.264/AVC CABAC coding algorithms, wherein an arithmetic coding system is used to improve entropy coding. CABAC achieves good compression performance through (a) selecting probability models for each syntax element according to the element&#39;s context; (b) adapting probabilities estimates based on local statistics; and (c) using arithmetic coding. 
         [0017]      FIG. 1  shows a generic block diagram for encoding a single syntax element  100  in CABAC. The coding process consists of up to three elementary steps: 1) binarization evaluation  101 ; 2) context modeling  102 ; and 3) binary arithmetic coding  103 . 
         [0018]    A more detailed view is provided in  FIG. 2 . A syntax element is provided in block  200  followed by evaluating the syntax element in decision block  201 , wherein a non-binary element advances to step  204 . The non-binary element component is uniquely mapped to a binary sequence into a so-called bin string in block  204 . The mapped sequence and syntax element which were originally binary valued in decision block  201 , will advance to decision block  205 . In decision block  205 , each element of the bin string or each binary valued element will be processed in a regular coding mode or in a bypass coding mode. 
         [0019]    In the regular coding mode, a context modeling step  206  is performed, wherein a probability model is selected such that the corresponding choice of a context depends on previously encoded syntax elements or bins. After the assignment of a context mode, the bin value  207  and its associated model  208  are passed to the regular coding engine  209 , where the final stage of arithmetic coding and a subsequent model updating are processed. 
         [0020]    In the bypass coding mode, to speed up the whole coding process, a simplified non-adaptive arithmetic coding engine or bypass coding engine  210  without the use of probability estimation and an update process is applied. 
         [0021]    A final step of H.264/AVC compression can be entropy encoding, wherein at least one of the entropy encoding methods supported by that standard is CABAC. CABAC is a binary arithmetic coding technique in which the coding of a particular AVC syntax element is performed with respect to a specific probability model which is determined by a set of variables, collectively called a context. The CABAC encoder maintains many contexts and selects one for the encoding of each syntax element or even for the encoding of each bit at different bit positions of the same syntax element. In many cases, the encoding process results in a modification of the context variables. Each context maintains a set of context variables. The encoding process can change the variables inside that context. 
         [0022]    A first step of H.264/AVC decompression is entropy decoding. When the stream is CABAC-encoded, the decoding is CABAC decoding. The CABAC decoder maintains many contexts and uses different contexts to decode different syntax elements or even bits at different bit positions of the same syntax element. Each context maintains a set of context variables. In many cases, the decoding process results in a modification of the context variables of the corresponding context. Different contexts are identified by a context index. It is important that the data be coded in such a way so that the decoder and encoder remain synchronized. The decoder needs to use an appropriate context for any particular element (i.e., which context was used to encode that element). The variables of that context, or the state of that context, need to be as predicted in the encoder. 
         [0023]    It is possible and useful to modify certain CABAC-encoded syntax elements in a bitstream without modifying any of the context states. However, it is possible that a special case can occur in which the modifications result in the wrong context being used for decoding of a subsequent syntax element. 
         [0024]    Now, the binarization of a motion vector differential (MVD) can consist of a prefix, and if the MVD is larger than 9, a suffix, wherein the binarized MVD can be denoted or defined as MVDBIN. The CABAC context used to decode the prefix of the MVDBIN of an inter-predicted block is determined by the context index “ctxIdx” as specified in the H.264/AVC standard. This index is calculated from the sum of two variables, ctxIdxOffset and ctxIdxInc, an initial index value and an increment, respectively, as shown in the following equation: 
         [0000]        ctxIdx=ctxIdx Offset+ ctxIdxInc    
         [0025]    The ctxIdxOffset is specified in the standard. Specifically, ctxIdxOffset is 40 for the horizontal component of the MVDBIN and 47 for the vertical component of the MVDBIN. The ctxIdxInc is different for different bit positions of the MVDBIN prefix. For bits other than the first bit, the ctxIdxInc is determined by bit position. Specifically, ctxIdxInc is 3, 4, 5, 6, 6, 6 at bit positions 1, 2, 3, 4, 5, 6 (or greater), respectively. For the first bit (bit position 0) of the MVD prefix, the CABAC encoder and decoder determine the value of the increment, ctxIdxInc, by examining the absolute values of the MVDs (the entire MVD) of the neighboring blocks, specifically, the neighbor to the left (A) and the neighbor above (B), as shown in  FIG. 3  where the current block is labeled “Cur.” The bit position determines the ctxIdxInc for bits other than the first of a prefix of an MVDBIN. Blocks A and B in  FIG. 3  can be referred to as immediate blocks, because they share a border with the current block. 
         [0026]    To encode/decode the first bit of a prefix of the MVDBIN of an intra-predicted block, there are three possible contexts that can be used, as explained below. The first context is indexed by ctxIdxOffset, the second by ctxIdxOffset+1, and the third by ctxIdxOffset+2. Thus, the variable ctxIdxInc can take the values 0, 1, or 2 and this variable is used to select from the three available contexts. The first of these contexts is used for cases in which the current MVD is expected to be small, the second is used when the value of the current MVD is expected to be moderate, and the third is used when the current MVD is expected to be large. The expectation is based on the neighboring MVD values as follows: 
         [0027]    If (absMvd_A+absMvd_B) is less than 3, ctxIdxInc is set equal to 0. 
         [0028]    If (absMvd_A+absMvd_B) is in the range of 3 to 32, inclusive, ctxIdxInc is set equal to 1. 
         [0029]    If (absMvd_A+absMvd_B) is greater than 32, ctxIdxInc is set equal to 2. 
         [0030]    In short, the (absMvd_A+absMvd_B) can fall into three regions: [0, 3), [3, 32] and (32, +∞). (Notations are order theory notations: right-open interval, closed interval and open interval, respectively.) 
         [0031]    One or more of the embodiments in the attached application can possibly result in modifications to MVDs of some blocks while taking care not to cause any context states to change. However, if the MVD of block A or that of block B change or if both change due to the modifications in such an embodiment, the new value of (absMvd_A+absMvd_B) could fall into a different region than the original one. In that case, the ctxIdxInc will be changed and the wrong context will be used for the decoding of the prefix of the MVDBIN of the current block. Thus, the CABAC decoder will behave differently in decoding the MVD of block Cur and decoding errors can result. A first solution to these decoding errors pertaining to potential modifications can be made that does not induce any changes in context states and is illustrated in  FIG. 5 . Each of these potential modifications can be evaluated by a number of criteria including its ability to represent data, its impact on the fidelity of the decoded imagery, the robustness of the resulting pixel changes, etc. Of the list of potential modifications identified, only those that meet the particular application dependent criteria are generally selected for use in the watermarking process and the rest are generally discarded. 
         [0032]    This evaluation can be extended by examining each potential modification to determine if it would induce a change in ctxIdxInc of any other block, thus possibly causing a decoding error. Potential modifications that do induce a change in ctxIdxInc of another block are, in at least one implementation, discarded. 
         [0033]    For a potential modification in block A that will affect the MVD of block A, the MVDs of its two neighboring blocks, R (Right) and D (Down), might be affected as shown in  FIG. 4 . Here blocks R and D are immediate blocks and M and N are non-immediate blocks, because they do not share a border with the current block A, but share a border with the immediate blocks. 
         [0034]    To check if the modification will cause a decoding error, this solution checks whether this modification of the MVD in block A will cause a change in the ctxIdxInc of the prefix of the MVD in block R or the prefix in block D. This leads to an algorithm described in  FIG. 5 . Here, in step  505 , alternative MVDs in block A are determined. For a potential modification that affects the MVD of block A, the ctxIdxInc in block R and block D are examined or determined. In each of these two cases, the old ctxIdxInc that would have been used if the change in block A had not been made is calculated. Also, the new ctxIdxInc that would have been used if the change in block A is made in steps  515  for the R block and in step  525  for the D block is calculated, wherein these example adjacent block to the current A block are shown in  FIG. 5 . In either case, for the R block in decision block  520  and for the D block in decision block  535 , in at least one embodiment, if the old ctxIdxInc and the new ctxIdxInc differ, then the potential modification is discarded in step  545 , because it has the potential to cause the decoding error described above. If the old and new ctxIdxInc are the same, then the alternative MVD in block A is saved in step  540 . 
         [0035]    Note that the ctxIdxInc for block R is dependent, not only on the MVD of block A, but also on the MVD of block M, which is immediately adjacent to block R. The ctxIdxInc for block R is determined by the range in which the (absMvd_A+absMvd_M) falls. Similarly, the ctxIdxInc of block D depends on both the MVD of block A and the MVD of block N, an adjacent block to D, as it is determined by the range in which the (absMvd_A+absMvd_N) falls. 
         [0036]    The algorithm of  FIG. 5  can be considered to be “Block A centric.” Here a potential change in block A is considered and the impact downstream is evaluated. 
         [0037]    There is a case in which the algorithm described above and shown in  FIG. 5  may not detect or identify all decoding errors. Referring to  FIG. 3 , the evaluation process of  FIG. 5  will not necessarily completely address the case when the MVDs of both block A and block B are modified. Another solution is proposed to consider this case. 
         [0038]    A second solution and a third solution are illustrated in  FIG. 6  which shows extended versions of that of  FIG. 5 . The second and third solutions differ in that the second solution uses the step  640   a  option and the third solution uses the step  640   b  option. 
         [0039]    Regarding the second solution, essentially, for each alternative MVD in block A, the effects on the ctxIdxInc of blocks D and R in  FIG. 4  are considered or processed. What is different in this solution is that in the cases in which the MVDs of blocks M and N themselves, the alternative values are considered. Here, it is desirable to retain block A&#39;s alternative MVDs that will not change the ctxIdxInc of blocks D or R, regardless of which alternatives can be selected for blocks M or N. 
         [0040]      FIG. 6  shows one algorithm for doing this. For each alternative MVD for block A, all possible MVDs for block M are considered or processed first. Each combination can be evaluated to determine if any combination will change the ctxIdxInc of block R. If any combination changes the ctxIdxInc of block R, that alternative MVD for block A can be discarded. If none of the combinations (for a given alternative MVD for block A) change the ctxIdxInc of block R, then all possible MVDs for block N can be considered. Each combination can be evaluated to determine if any combination will change the ctxIdxInc of block D. If any combination changes the ctxIdxInc of block D, that alternative MVD for block A is discarded. If none of the combinations change the ctxIdxInc of block D, this alternative MVD for block A is saved. 
         [0041]    Specifically,  FIG. 6  begins with a calculation of the original ctxIdxInc of blocks R and D based on the original MVD pairs of blocks A and M and block A and N in step  605 . In step  610 , each alternative MVD in block A is examined, followed by examining each MVD in block M in step  615 . In step  620 , a calculation is performed with the modified ctxdxInc of R based on the alternative MVD of M and the alternative MVD of M and the alternative of MVD of A. This is followed by decision step  625 , wherein if the Block R modified ctxIdxInc equals the original ctxIdxInc, then additional block M alternative MVDs are examined in step  630  and if the Block R modified ctxIdxInc does not equal the original ctxIdxInc, then the alternative MVD in block A is discarded in step  640   a . Following the discarding step  640   a  is another decision step  645  in which additional alternative block A MVDs are considered. If there are no further alternative block A MVDs, then the second solution is complete, which is represented by step  680 . On the other hand, if there are additional alternative block A MVDs, the solution advances to step  650 , in which the next alternative MVD in block A is accessed, followed by looping to step  610  and to the steps that followed step  610  as mentioned above. 
         [0042]    With further reference to  FIG. 6 , when the Block R modified ctxIdxInc equals the original ctxIdxInc in step  625  and there are additional block M alternative MVDs in decision step  630 , then the process advances to evaluating the next available alternative MVD in block M in step  635 , followed by looping back to step  615  and to the steps that followed step  615  as mentioned above. 
         [0043]    Additionally with reference to  FIG. 6 , when the Block R modified ctxIdxInc equals the original ctxIdxInc in step  625  and there are no additional block M alternative MVDs in decision step  630 , then the process advances to accessing each alternative MVD in block N in step  655  and then to calculating the modified ctxdxInc of D based on the alternative MVD of N and the alternative MVD of A in step  660 . Following step  660 , if block D modified ctxIdxInc equals the original ctxIdxInc, then in decision block  665  the process advances to decision block  670 , wherein additional block N alternative MVDs are accessed. If there are additional block N alternative MVDs, one advances to block  675 , wherein the next MVD in block N is accessed followed by looping back to step  655  and to the steps that followed step  655  as mentioned above. If block D modified ctxIdxInc does not equal the original ctxIdxInc, then in decision block  665  the process advances to step  640   a  and advances according to the steps mentioned above with respect to step  640   a.    
         [0044]    If there are no additional block N alternative MVDs in decision step  670 , the solution advances to step  685  in which the alternative MVD or MVDs in block A are saved, followed by advancing to decision step  645 , in which additional block A alternative MVDs are accessed. If there are no more alternative block A MVDs, then the second solution is complete, which is represented by step  680 . On the other hand, if there are additional alternative block A MVDs, the solution advances to step  650 , in which the next alternative MVD in block A is accessed, followed by looping to step  610  and to the steps that followed step  610  as mentioned above. 
         [0045]    This solution does discard any alternative MVD for block A that has a combination with any of the alternative MVDs for blocks M or N such that the combination will cause the ctxIdxInc of blocks D or R to change. 
         [0046]    In some cases, removing the alternative MVD for block A may not be preferable or may need to be avoided. As such, a third solution is illustrated in  FIG. 6  in which the step  640   b  option is used instead of the step  640   a  option. The third solution removes the other alternative MVD such as the one associated with block M or N, rather than the alternative for block A. This modification of the second solution makes it such that potentially detrimental combinations are recorded in a discard list. The discard list can later be processed to decide which alternatives should be removed. 
         [0047]    There are many approaches that can be taken to process the discard list. Here are a few:
       1. One approach is to consider any combination on the discard list for which one of the alternative MVDs is an original MVD. This combination is removed from the list by removing the other alternative MVD of the combination.   2. Another approach is to minimize the number of alternative MVDs removed by first building a histogram and for every alternative MVD on the discard list, count the number of times it appears. This is followed by removing the alternative MVD that appears most often, thereby removing from the discard list any combinations containing that alternative MVD. Additionally, alternative MVDs can be continually removed until the discard list is empty, from most frequently appearing to least frequently appearing,   3. In a third approach a histogram can be built for every alternative MVD on the discard list and the number of times it appears can be counted. Then, a fidelity model and/or a robustness model can assign a cost to each alternative MVD. This cost can be used, along with the histogram of frequency of occurrence, as a basis for removal of alternative MVDs until the discard list is empty.       
 
         [0051]    In contrast to the first three solutions, a fourth solution can be considered “Block C centric” or “Block Cur centric” in that a potential change in block Cur is evaluated and block A or block B are checked to determine if either block A or B has changes that can impact the ctxIdxInc of block Cur. This forth solution is illustrated in  FIG. 7 . In at least one implementation, the process of  FIG. 7  is performed for every slice that has alternative MVDs in the image sequence. 
         [0052]    The first step  705  is to create a list of potential modifications that apply to the blocks in the current slice. It should be pointed out that embodiments of the inventions are intended to include the feature of accessing and/or modifying an existing list. In this regard, a list can be compiled, accessed or generated having all alternative MVDs for a slice. 
         [0053]    Next, in step  710 , for a given inter-predicted block, all possible MVDs are considered. All inter-predicted blocks will have at least one MVD; however, for some, there are potential changes that have been identified. Each potential change will result in a different MVD for the current block, block C, wherein the MVD is referred to as MVDc. For the current block, Block C, there are two neighbors of interest, block A to the left and block B above as shown in  FIG. 3 . Each of these blocks will have a MVD; however, for some neighbors, there are potential changes that have been identified. Each potential change will result in a different MVD for that neighboring block, block A or block B. For MVDc in block C one considers or gathers all possible combinations or alternative values of MVDa and MVDb in step  715  and the alternative MVDs of blocks A and B respectively, including the original MVDs of those blocks. At step  720 , all of possible combinations of alternatives are accumulated and staged to send through to step  725 . Here in step  725 , for each combination, ctxIdxInc is calculated for block C. In decision step  730 , if any combination causes a change in ctxIdxInc from the value generated using the original MVDs of blocks A and B, one of the alternative MVDa or alternative MVDb is discarded from the list in step  735  and the next possible combination begins to be considered in step  720 . If the new ctxIdxInc is not different than the original for block C, then the change is removed from the list in step  720 . Similar approaches as which are proposed to process the discard list in Solution 3 can be used. 
         [0054]    An alternative implementation examines a previously created list and extracts the subset of those that apply to blocks in the current slice. The method then considers every inter-predicted block in the current slice, one at a time. 
         [0055]    Several of the implementations and features described in this application can be used in the context of the H.264/MPEG-4 AVC (AVC) standard. However, these implementations and features can be used in the context of another standard (existing or future), or in a context that does not involve a standard. Thus, provided herein is one or more implementations having particular features and aspects. However, features and aspects of described implementations can also be adapted for other implementations. 
         [0056]    The implementations described herein can be implemented in, for example, a method or process, an apparatus, a software program, a datastream, or a signal. Even if only discussed in the context of a single form of implementation such as only being mentioned as a method, the implementation or features discussed can also be implemented in other forms such as an apparatus or program. An apparatus can be implemented in, for example, appropriate hardware, software, and firmware. The methods can be implemented in, for example, an apparatus such as, for example, a computer or other processing device. Additionally, the methods can be implemented by instructions being performed by a processing device or other apparatus, and such instructions may be stored on a computer readable medium such as a CD, or other computer readable storage device, or an integrated circuit. Further, a computer readable medium can store the data values produced by an implementation. 
         [0057]    As should be evident to one of skill in the art, implementations may also produce a signal formatted to carry information that can, for example, be stored or transmitted. The information may include, for example, instructions for performing a method, or data produced by one of the described implementations. For example, a signal can be formatted to carry a watermarked stream, an unwatermarked stream, or watermarking information. 
         [0058]    Additionally, many implementations can be implemented in one or more of an encoder, a decoder, a post-processor processing output from a decoder, or a pre-processor providing input to an encoder. Further, other implementations are contemplated by this disclosure. For example, additional implementations can be created by combining, deleting, modifying, or supplementing various features of the disclosed implementations.