Abstract:
A system and method for creating multi-priority streams of compressed video data. Included is a method for assigning priority to streams of compressed video data, comprising the steps of: determining a relative importance of each macroblock in a video frame based on how often each macroblock acts as a reference macroblock; and prioritizing each of the macroblocks in the video frame based on the relative importance.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to systems for processing compressed video data, and more particularly to a system and method for dividing MPEG video data into multiple streams of different priority.  
           [0003]    2. Related Art  
           [0004]    As the demand for the on-line delivery of video applications continues to grow and transmission bandwidth remains at a premium, the ability to provide highly efficient transmission schemes has become indispensable. One such scheme involves dividing MPEG coded video data into multiple streams, each having a different priority. This type of scheme, referred to herein as “multi-priority streams,” allows different portions of the video data to consume different amounts of bandwidth, depending on the importance of the particular portion.  
           [0005]    One specific scenario where multi-priority streams are useful involves applications that utilize transmission channels susceptible to errors. In these cases, it is necessary to protect the data with additional error protection bits, which may for example provide a certain amount of redundancy. Unfortunately, the inclusion of error protection bits creates additional overhead in terms of bandwidth consumption. By dividing MPEG data into a plurality of streams of differing priorities, individual streams can be assigned different levels of error protection. Accordingly, high priority streams can receive a greater amount of error protection than lower priority streams. Thus, overhead can be balanced with the corresponding priority of the streams.  
           [0006]    A second scenario where multi-priority streams are useful involves networks that offer multiple quality of service (QoS) options. In such a case, different levels of quality of service for transmitted data are made available by the underlying transport. Thus, in order to efficiently utilize such a system, it is necessary to divide the video data into streams of differing priority, and then send each stream over the connection offering the appropriate QoS.  
           [0007]    Unfortunately, existing methods of creating multi-priority streams for MPEG video are limited. For example, one approach is to divide the bitstream into different priorities based on the type of frame. In particular, since for picture quality I frames are generally more important than P frames, and P frames are generally more important than B frames, I frames receive a highest priority, P frames receive a medium priority, and B frames receive a lowest priority. However, because the actual size of a typical I or P frame can be hundreds of thousands of bits, this type of prioritization provides only marginal results since only three priority levels can be implemented. Moreover, there can be a large amount of variation in terms of importance between two I frames, or two P frames, and such variations are not taken into account in the above scheme. Thus, a need exists to create a finer prioritization among and between frames.  
           [0008]    A second approach for creating multi-priority streams for MPEG video is to create scalable video that contains a base layer and one or more enhancement layers. The base layer is assigned a higher priority than the enhancement layers. The drawback of this approach is the increased complexity required for encoding and decoding, and the need to transcode the already coded non-scalable bitstream into a scalable bitstream.  
           [0009]    Accordingly, a system is required: (1) that can provide a finer prioritization than the I, P and B frame prioritization; and (2) that can directly process a coded bitstream without incurring the complexity of a system that utilizes scalable video.  
         SUMMARY OF THE INVENTION  
         [0010]    This invention overcomes the above-mentioned problems, as well as others, by providing a system and method for prioritizing streams of compressed video data suitable for transmission over a multi-priority transmission channel. In a first aspect, the invention provides a method for assigning priority to streams of compressed video data, comprising the steps of: determining a relative importance of each macroblock in a video frame based on how often each macroblock acts as a reference macroblock; and prioritizing each of the macroblocks in the video frame based on the relative importance.  
           [0011]    In a second aspect, the invention comprises a method for assigning priority to streams of compressed video data, comprising the steps of: determining an importance value for each macroblock in a plurality of video frames based on how often each macroblock acts as a reference macroblock; grouping macroblocks into sets of macroblocks, and combining the importance values of the macroblocks within each set; and prioritizing each set of macroblocks based on the combined importance values.  
           [0012]    In a third aspect, the invention provides a system for prioritizing streams of compressed video data, comprising: a system for determining an importance value for each macroblock in a video frame based on how often each macroblock acts as a reference macroblock; and a system for prioritizing each of the macroblocks in the video frame based on the importance value determined for each macroblock.  
           [0013]    In a fourth aspect, the invention provides a system for prioritizing streams of compressed video data, comprising: a system for determining an importance value for each macroblock in a plurality of video frames based on how often each macroblock acts as a reference macroblock; a system for grouping macroblocks into sets of macroblocks and combining the importance values for macroblocks in each set; and a system for prioritizing sets of macroblocks based on the combined importance values.  
           [0014]    In a fifth aspect, the invention provides a decoder system for decoding multi-priority compressed video data, comprising: a system that correlates an error protection scheme to each of a plurality of data streams; and a system that interprets each data stream based on the error protection scheme; wherein the error protection scheme is determined by a prioritization system that prioritizes each data stream based on how often macroblocks act as reference macroblocks.  
           [0015]    In a sixth aspect, the invention provides a program product stored on a recordable media, that when executed, prioritizes streams of compressed video data, the program product comprising: means for determining an importance value for macroblock data in video frames based on how often each of a plurality of macroblocks act as reference macroblocks; and means for prioritizing macroblock data based on the determined importance values.  
           [0016]    In a further aspect, each of the prioritization systems and methods described above may further include a system for assigning different error protection schemes to streams of different priority. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The preferred exemplary embodiment of the present invention will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements, and:  
         [0018]    [0018]FIG. 1 depicts a block diagram of a multi-priority coding system in accordance with a preferred embodiment of the present invention.  
         [0019]    [0019]FIG. 2 depicts a P frame analysis in accordance with the present invention.  
         [0020]    [0020]FIG. 3 depicts an I frame analysis in accordance with the present invention.  
         [0021]    [0021]FIG. 4 depicts an indirect analysis in accordance with the present invention.  
         [0022]    [0022]FIG. 5 depicts a partial reference block analysis in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    Referring now to FIG. 1, a multi-priority coding system  10  is depicted. System  10  includes an encoder  12  for converting a bitstream  18  of MPEG data into a set of multi-priority streams  16 , and a decoder  14  for interpreting the set of streams  16 . Encoder  12  could be incorporated directly into an MPEG encoder, or alternatively, operate on already encoded MPEG data. Similarly, decoder  14  could be incorporated into an MPEG decoder or reside separately from an MPEG decoder. Each of the multi-priority streams  16  generated by encoder  12  are encoded with a different error protection scheme. Thus, for example, where the data is being transmitted over a channel susceptible to errors (e.g., the Internet), more important data can be afforded a greater amount of bandwidth to allow for a greater amount of error protection. On a receiving end, decoder  14  decodes the set of multi-priority streams  16  and outputs decoded video data  20 . While this preferred embodiment is described as processing MPEG data, it should be understood that the invention could apply to any data compression scheme using predictive coding.  
         [0024]    Encoder  12  comprises an importance analysis system  22  that examines macroblock data and determines the relative importance of each macroblock or set of macroblocks. Stream prioritization system  24  then assigns a relative priority to each macroblock or group of macroblocks based on the above analysis. Error protection system  25  then encodes the macroblock data with the appropriate error protection scheme for transmission as one of the plurality of multi-priority streams  16 .  
         [0025]    Importance analysis system  22  includes a P frame analysis system  26 ; an I frame analysis system  28 ; a partial macroblock analysis system  27 ; an indirect analysis system  29 ; and a residual analysis system  19 . P frame analysis system  26  and I frame analysis system  28  examine macroblocks within P and I frames, respectively, to determine the relative importance of the macroblock data. Specifically, when a P or I frame is analyzed, system  26  or  28  systematically examines each macroblock, and a relative importance value is calculated as each macroblock (i.e., the “current macroblock”) is examined. Importance is based on how often the current macroblock acts as a reference macroblock or partial reference block. (Note that for the purposes of this invention, the term “reference macroblock” may comprise a complete or partial reference block.)  
         [0026]    Because P and I frames are used for forward and backward prediction, P frame analysis system  26  and I frame analysis system  28  analyze the motion vectors of previous and subsequent B frames, and a subsequent P frame (if applicable), to determine how often a current macroblock within either a P or I frame acts as a reference macroblock. An importance value is determined based on the number of target macroblocks that reference the current macroblock in either the P or I frame (i.e., the number of predictions). Examples of this process are described below with reference to FIGS. 2 and 3. (Note that certain P frames are followed by I frames, and therefore will not have a subsequent P frame to analyze.)  
         [0027]    It should be appreciated that importance analysis system  22  can analyze individual macroblocks for their relative priority or sets of macroblocks (e.g., an entire frame or even a set of frames such as a group of pictures). In the case where sets of macroblocks are being analyzed for their importance, importance analysis system  22  would first group sets of macroblocks together based on a predetermined scheme. The importance value of the set is then determined by combining (e.g., summing, weighting, etc.) the importance values of each macroblock in the set. Priority is thus decided, for instance, based on cumulative importance of the macroblocks in each set.  
         [0028]    Importance analysis system  22  further comprises a partial macroblock analysis system  27  that analyzes macroblock importance when reference macroblocks do not exactly coincide with the current macroblock being analyzed (i.e., when a current macroblock acts a partial reference block). Specifically, in cases where only part of a current macroblock is used as the reference macroblock, partial macroblock analysis system  27  computes the overlap (in terms of pixels) between the current macroblock and the reference macroblock. Thus, for example, if there were an overlap of 128 of 256 pixels, the importance value would be scaled by 50%. An example of this is described below with respect to FIG. 5.  
         [0029]    An exemplary algorithm for calculating an importance value of a macroblock in a P frame would be as follows:  
                                                                                                                     read a current macroblock;           set importance_value (of the current macroblock) = 0;           identify target macroblocks in B frames and the subsequent P           frame that reference            the current macroblock;                for each identified target macroblock:                examine the corresponding reference macroblock;           if the reference macroblock exactly coincides           with the current macroblock,                then importance_value = importance_value + 1;                else                compute pixel overlap (maximum is 16x16=256);           importance_value = importance_value +           (pixel overlap/256);                end.                      
 
         [0030]    As an alternate embodiment, importance analysis system  22  may also include an indirect analysis system  29  that examines subsequent indirect predictions in determining importance. Indirect analysis system  29 , while more computationally expensive, provides a more accurate valuation scheme. For example, in MPEG coding, macroblocks in I frames are used to “directly” predict P frame macroblocks, which in turn are used to “indirectly” predict subsequent P frame macroblocks and B frame macroblocks, and so forth. So in computing the importance of an I or a P frame macroblock, importance analysis system  22  may be used to not only examine direct predictions, but also examine subsequent indirect predictions. Thus, if an I macroblock acts as a reference for motion prediction for a macroblock in a P frame (direct prediction), and subsequently the P frame macroblock acts as a reference for other macroblocks in subsequent P and B pictures (indirect prediction), the importance values of the indirect predictions can be added, or otherwise factored into the importance values of the direct prediction. Accordingly, the relative importance among and between macroblocks in both I and P frames can be computed and prioritization can be based on such results.  
         [0031]    As a further alternate embodiment, the importance value can be calculated (or further calculated) based on discrete cosine transform (DCT) residual values. A residual is the difference between a target macroblock and a reference macroblock. Thus, the smaller the residual, the closer the target macroblock matches the reference macroblock, and the greater the importance. Accordingly, residual analysis system  19  can examine the residual of each identified target macroblock and compute a function of each residual (e.g., the absolute or weighted sum of the coefficients). The importance value of the current macroblock can then be calculated based on, for example, a cumulative value of the residual computations from each target macroblock. It should be appreciated that this embodiment can be combined or used separately from the other embodiments described herein.  
         [0032]    Once the importance values have been obtained, stream prioritization system  24  will assign priority to individual macroblocks or groups of macroblocks. In general, the greater the number of predictions for which the current macroblock is referenced by a target macroblock, the greater its priority, since the macroblock has a relatively higher importance for the visual quality of the decoded video stream.  
         [0033]    In a preferred embodiment, since B frames are not used as references in predictive coding, the macroblocks in B frames are assigned the lowest priority. The macroblocks in the P frames are then assigned a relative higher priority, with the individual P frame macroblock data being prioritized in the manner described above. Finally, the macroblocks in the I frames are assigned the highest priority, again with the macroblock data being further prioritized based on the methods discussed above.  
         [0034]    Decoder  14  includes a stream decoding system  30  that comprises a correlation system  21  and an interpretation system  23 . Correlation system  21  correlates each of the multi-priority streams  16  with a particular error protection scheme, and interpretation system  23  interprets the data and packages it accordingly.  
         [0035]    Referring now to FIG. 2, an example of how an importance value is calculated for a P frame macroblock is shown. A stream of MPEG video data  32  is depicted comprised of a sequence of frames (P,B,B,P,B,B,P). In accordance with the methods discussed above, the macroblock data within P frame  33  is being analyzed for its importance. Specifically, a current macroblock  31  is examined (as shown by the arrows) to determine how often the current macroblock  31  acts as a reference macroblock for target macroblocks in previous and subsequent B frames  70 , and target macroblocks in the subsequent P frame  34 . As can be seen, current macroblock  31  acts as a reference macroblock for nine target macroblocks (shown as squares with a diagonal line). The target macroblocks could be any one of the 16×16 blocks (not shown) in the neighboring frames  70  and  34 . Assuming exact coincidence between the current macroblock  31  and the corresponding reference macroblock, the macroblock would be assigned an importance value of nine. Accordingly, macroblock  31  would be assigned a relative priority based on this value as compared to the other macroblocks in P frame  33 . Note that in this case a subsequent P frame  34  follows P frame  33 . In other cases (not shown), P frame  33  may be followed by an I frame, in which cases the subsequent I frame would not be analyzed for target macroblocks.  
         [0036]    Referring now to FIG. 3, a similar example of how an importance value is calculated for an I frame macroblock is shown. In this case, a stream of frames  38  (P,B,B,I,B,B,P) is shown, and the macroblock data of I frame  36  is being analyzed to determine the relative priority of each macroblock in I frame  36 . Here again, target macroblocks are identified by examining the motion vectors in the subsequent P frame and neighboring B frames. In this case, there are eight target macroblocks that are predicted by the current macroblock. Assuming exact coincidence between current macroblock  35  and the corresponding reference macroblock, an importance value of eight would be assigned to the current macroblock  35 .  
         [0037]    Referring now to FIG. 4, an example is shown of how an importance value is calculated for an alternate embodiment utilizing indirect analysis system  29 . Specifically, a stream of frames  40  (P,B,B,P,B,B,P) is shown, with the macroblock data in P frame  42  being analyzed to determine relative priority. It can be seen that a current macroblock  41  acts as a reference macroblock for a total of five target macroblocks in both B frame  44  and P frame  46 . In addition, the target macroblock  43  in P frame  46  further acts as an “indirect” reference macroblock for a total of six indirect target macroblocks in B frame  48  and P frame  50 . Assuming no other target macroblocks in P frame  46  act as reference macroblocks, the importance value for current macroblock  41  of P frame  42  would be eleven. Although not shown, a more complex chain of indirect calculations could be utilized. For example, the target macroblocks in P frame  50  could be further examined to determine how often they act as reference macroblocks, etc.  
         [0038]    Referring now to FIG. 5, an example is shown of how a partial reference block is taken into account by partial macroblock analysis system  27 . Specifically, frame  52  (e.g., I or P) includes a current macroblock  54  that is being analyzed for importance, a reference macroblock  56  that does not exactly coincide with current macroblock  54 , and an overlap portion  60  that represents the portion where macroblock  54  and reference macroblock  56  coincide. In this example, only 25% of the pixels in the current macroblock  54  are used as a reference macroblock for target macroblocks in other frames. Thus, the importance value for this particular macroblock would be scaled (e.g., by 25%) to account for the overlap.  
         [0039]    It is understood that the systems, functions, methods, and modules described herein can be implemented in hardware, software, or a combination of hardware and software. They may be implemented by any type of computer system or other apparatus adapted for carrying out the methods described herein. A typical combination of hardware and software could be a general-purpose computer system with a computer program that, when loaded and executed, controls the computer system such that it carries out the methods described herein. Alternatively, a specific use computer, containing specialized hardware for carrying out one or more of the functional tasks of the invention could be utilized. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods and functions described herein, and which—when loaded in a computer system—is able to carry out these methods and functions. Computer program, software program, program, program product, or software, in the present context mean any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: (a) conversion to another language, code or notation; and/or (b) reproduction in a different material form.  
         [0040]    The foregoing description of the preferred embodiments of the invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teachings. Such modifications and variations that are apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.