Abstract:
A method and apparatus are disclosed for performing motion estimation and compensation to fractional pixel accuracy using polyphase prediction filters as part of a video compression/decompression technique. A motion estimator applies a set of polyphase filters to some data in the reference picture and generates motion vectors, an estimated macroblock of video data, and a residual error macroblock of video data. The data referenced in the reference picture usually have more data than a macroblock since multi-tap filtering needs to access more data. A motion compensator generates a compensated macroblock of video data in response to the reference video data, the residual error macroblock of video data, and a polyphase prediction filter decided by the motion vector. The reference video data are usually reconstructed at the compensator side.

Description:
RELATED APPLICATIONS 
   The applicants claim priority based on provisional application No. 60/325,050 filed Sep. 25, 2001, the complete subject matter of which is incorporated herein by reference in its entirety. 

   FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   [Not Applicable] 
   MICROFICHE/COPYRIGHT REFERENCE 
   [Not Applicable] 
   BACKGROUND OF THE INVENTION 
   Certain embodiments of the present invention relate to digital video compression and decompression. More specifically, certain embodiments relate to a method and apparatus for motion estimation and compensation in digital video compression and decompression. 
   Digital video compression schemes, such as MPEG-2 for example, are well known in the art. MPEG-2 uses motion compensated predictive coding to encode a sequence of pictures. This coding entails predicting a two-dimensional block of pixels by translating or interpolating a similar array of pixels from another picture (referred to as the “reference picture”) in the sequence. 
   Various compression schemes use different sizes of blocks of pixels. For example MPEG-2 uses a 16×16 or 16×8 block of pixels (referred to as a “macroblock”; the terms “block” and “macroblock” may be used interchangeably). Prediction can usually reduce the amount of data that needs to be stored or transmitted, since only the difference between the actual image macroblock and the predicted macroblock need be coded and transmitted. For example, if the predicted macroblock is similar to the actual image macroblock, then the difference between the two macroblocks is very small. Therefore the information content in the difference may be represented in a smaller number of digital bits in comparison to coding and transmitting the original image data. The more accurate the prediction is, the more effective the compression system becomes. 
   The amount of translation for the reference picture macroblock is indicated by a motion vector, which is encoded as part of the compressed data stream. The motion vector has horizontal and vertical components, indicating the spatial displacement to be applied to a reference macroblock in order to arrive at a predicted macroblock location. However, the displacement may generate a translation that does not coincide with a integer sampling grid position of the picture. The integer sampling grid positions are referred to as the “integer pixel positions” and the positions in between the integer positions are referred to as the “fractional pixel positions”. 
   The smallest fractional-pixel position in the translation process determines the accuracy of the motion vectors used for prediction. Various known prediction schemes are used in video coding. For example, MPEG-1 and MPEG-2 use ½-pixel accuracy, while MPEG-4 Video Object Plane prediction uses ½-pixel and ¼-pixel accuracy and H.26L (also known as MPEG AVC or JVT or H.264) prediction uses ¼-pixel and ⅛-pixel prediction accuracy. All of these schemes utilize interpolation in at least one step in the prediction process. For example, in MPEG-1 and MPEG-2 for example, averaging adjacent integer-position pixels produces half-pixel position values. 
   In H.26L prediction, the ¼-pixel positions are created by first performing a 6-tap interpolative filter on the integer-position pixels obtaining the nearest ½-pixel position, then the nearest integer and ½-pixel positions are averaged to obtain the desired ¼-pixel position. When calculating the ⅛-pixel positions in H.26L prediction, the nearest ¼-pixel positions are created using an 8-tap interpolative filter, then the nearest ¼-pixel positions are averaged to get the desired ⅛-pixel position. In some implementations of such codec schemes, the averaging function is combined with the 8-tap, ¼-pixel filtering function into a single 8-tap filter to produce the same result as provided previously. 
   Performing averaging to obtain pixel positions between two pixel positions results in image distortion and impaired prediction of the image macroblock, thereby reducing the effectiveness of the compression and decompression system. Furthermore, the distinct operations of filtering and averaging result in unnecessarily complex implementations compared to embodiments of the present invention. 
   It is well known in the art how to design motion estimation and compensation systems for video compression, using the various fractional pixel interpolation techniques described above. 
   Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with embodiments of the present invention as set forth in the remainder of the present application with reference to the drawings. 
   A need exists for an approach to perform efficient video compression and decompression to fractional pixel accuracy with a simply implemented architecture. 
   BRIEF SUMMARY OF THE INVENTION 
   An embodiment of the present invention provides motion estimation and compensation to fractional pixel accuracy using polyphase filters as part of a video compression and decompression technique. An embodiment of the present invention uses a polyphase filter framework to directly produce a translation of an image macroblock. The polyphase framework has the same number of phases as the number of fractional pixel positions required by the video compression/decompression technique. 
   A method is provided for performing motion estimation and compensation to fractional pixel accuracy using polyphase filters as part of a video compression/decompression technique. A set of polyphase prediction filters are selected based on a desired fractional pixel resolution of the motion estimation. A current macroblock of video data is examined and a reference macroblock of video data is selected from a reference video image in response to the current macroblock of video data such that the reference macroblock of video data is most similar to the current macroblock of video data in certain criterion such as Minimum Absolute Error. A set of estimated macroblocks of video data is generated, having the fractional pixel resolution, in response to the reference macroblock of video data and the set of polyphase prediction filters. A macroblock of video data is selected from the set of estimated macroblocks of video data in response to the current macroblock of video data such that the estimated macroblock of video data is most similar to the current macroblock of video data, or similar selection algorithm at the discretion of the video encoder. A motion vector and a residual error macroblock of video data are generated in response to the reference macroblock of video data, the current macroblock of video data, and the estimated macroblock of video data. The reference macroblock of video data, the motion vector, and the residual error macroblock of video data may be used to re-create the current macroblock of video data. 
   Apparatus is provided for performing motion estimation and compensation to fractional pixel accuracy using polyphase prediction filters as part of a video compression and decompression technique. The apparatus includes a motion estimator applying a set of polyphase filters to some data in the reference picture and generating motion vectors, an estimated macroblock of video data, and a residual error macroblock of video data. The data referenced in the reference picture usually have more data than a macroblock since multi-tap filtering needs to access more data. The apparatus also includes a motion compensator generating a compensated macroblock of video data in response to the reference video data, the residual error macroblock of video data, and a polyphase prediction filter decided by the motion vector. The reference video data are usually reconstructed at the compensator side in video decoders. 
   Certain embodiments of the present invention afford an approach to facilitate efficient video compression and decompression of video data by reducing the residual error data to be stored or transmitted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flowchart of a method for performing motion estimation and compensation to fractional pixel accuracy using polyphase prediction filters as part of a video compression and decompression technique in accordance with an embodiment of the present invention. 
       FIG. 1   a  is a schematic block diagram of video encoder using motion estimation and compensation as part of its compression process in accordance with an embodiment of the present invention 
       FIG. 2  is a schematic block diagram of a system including an apparatus for performing motion estimation and compensation to fractional pixel accuracy using polyphase prediction filters as part of a video compression/decompression technique in accordance with an embodiment of the present invention. 
       FIG. 3  is a schematic block diagram of the motion estimator element of the apparatus of  FIG. 2  in accordance with an embodiment of the present invention. 
       FIG. 3   a  is a schematic block diagram of the data interface components of  FIG. 2  in accordance with an embodiment of the present invention. 
       FIG. 4  is a schematic block diagram of the motion compensator element of the apparatus of  FIG. 2  in accordance with an embodiment of the present invention. 
       FIG. 5  is an exemplary schematic of a video image. The current image to be coded is usually made up of macroblocks of video data in accordance with an embodiment of the present invention. A reference image may have a reference macroblock starting at any position, including fractional ones. 
       FIG. 6  illustrates the movement of a pixel position between a reference macroblock and a current macroblock of video data. 
       FIG. 7  illustrates the relationship between integer pixels and fractional pixels and also shows an exemplary schematic of a polyphase filter structure in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a flowchart of a method for performing motion estimation and compensation to fractional pixel accuracy using polyphase prediction filters as part of a video compression/decompression technique in accordance with an embodiment of the present invention. 
     FIG. 1   a  is a schematic block diagram of a video encoder which uses motion compensation as part of its video compression process. This  FIG. 1A  is representative of video encoders compliant with such standards as MPEG-1, MPEG-2, MPEG-4, H.263, and H.26L (proposed). 
     FIG. 2  is a schematic block diagram of a system including an apparatus for performing motion estimation and compensation to fractional pixel accuracy using polyphase prediction filters as part of a video compression/decompression technique in accordance with an embodiment of the present invention.  FIG. 2  shows a video encoder  100  and a video decoder  200 .  FIG. 2  shows a motion estimator  101  and a motion compensator  201  as elements of the video encoder  100 , and it shows a motion compensator  201  as an element of video decoder  200 . A communication channel  150  is shown interfacing video encoder  100 , presumably at a first location, to video decoder  200 , which is presumably at a second location. Communication channel  150  provides a means for transmitting compressed video data from video encoder  100  to video decoder  200 . 
   Alternatively, video encoder  100  and decoder  200  may be co-located and the communication channel  150  used to transmit compressed video data to a storage medium such as an image server. The video data may then be compressed, transmitted to the storage medium, retrieved from the storage medium, and decompressed. 
     FIG. 3  is a schematic block diagram of the motion estimator  101  of the apparatus of  FIG. 2  in accordance with an embodiment of the present invention. Motion estimator  101  includes a reference image buffer  110 , a current image buffer  120 , a polyphase filter  170 , a video block comparator  130 , a video block estimator  140 , and a polyphase filter coefficient bank  160 . 
   In  FIG. 3 , the output of the reference image buffer  110  interfaces to the polyphase filter  170  and also to the video block comparator  130 . The outputs of the polyphase filter  170 , the reference image buffer  110  and of the current image buffer  120  interface to inputs of video block comparator  130  in order to input macroblocks of video data to video block comparator  130 . The video block comparator can select between the reference image buffer  110  and the output of polyphase filter, as one of its inputs; the signal from reference image buffer  110  can be utilized in place of the signal from polyphase filter  170  when phase shift or pixel interpolation is not required. An output of video block comparator  130  connects to an input of video block estimator  140 . An output of video block estimator  140  connects to an input of video block comparator  130 , and another output of video block estimator  140  connects to an input of the polyphase filter coefficient bank  160 . An output of polyphase filter coefficient bank  160  connects to an input of polyphase filter  170 . 
     FIG. 3   a  illustrates encoder data interface  180  and decoder data interface  190 . Encoder data interface  180  interfaces to signals  131 ,  133  and  135  from motion estimator  101 , and to communications channel  150 . Decoder data interface  190  interfaces to communications channel  150  and to signals  131 ,  133 , and  135  to the motion compensator in the decoder. 
     FIG. 4  is a schematic block diagram of the motion compensator  201  of the apparatus of  FIG. 2  in accordance with an embodiment of the present invention. Motion compensator  201  includes a polyphase filter coefficient bank  260  and a video compensation module  270 . 
   An output of polyphase filter coefficient bank  260  connects to an input of video compensation module  270 . A motion vector signal  131  connects to an input  131  of polyphase filter coefficient bank  260 . A residual error macroblock signal  133  connects to a first input  133  of video compensation module  270 . A reference macroblock signal  135  connects to a second input  135  of video compensation module  270 . In an embodiment of the present invention, connections  131 ,  133 , and  135  may be separate, dedicated interfaces to motion estimator  101 , for example in a video encoder  100 . In an alternative embodiment of the present invention connections  131 ,  133 , and  135  may be interfaces to decoder data interface  190 , for example in a video decoder  200 . 
   The various elements illustrated in  FIGS. 2 ,  3 ,  3   a , and  4  may be dedicated hardware elements such as circuit boards with digital signal processors or may be software running on a general purpose computer or processor such as a commercial, off-the-shelf PC. Also, the various elements may be embedded in a single video processing chip. The various elements may be combined or separated according to various embodiments of the present invention. 
   In an embodiment of the present invention, a motion estimation method provides motion compensation prediction to minimize the data bits that are required to be transmitted. An embodiment of the present invention also provides for less expensive and less complex structures and mechanisms for encoding and decoding video. One of the motion estimation methods searches for the (locally) most-similar macroblock (denoted as the reference macroblock) in the integer positions first and then refines to fractional positions by using the reference macroblock and data around it. Referring to  FIG. 5 , the reference macroblock  112  is first found in the reference video image  111  and its surrounding data are used in the refining prediction (estimation) process through filtering using a set of polyphase prediction filters. 
   In an embodiment of the present invention, polyphase prediction filters perform motion compensated prediction to fractional pixel accuracy. A polyphase filter structure is used to directly produce an optimized translation of the predicted video macroblock. The polyphase filter structure has the same number of phases as the number of fractional pixel positions required by the video compression algorithm. For example, in a system with ¼-pixel resolution, the polyphase prediction filters have four unique phases for the horizontal axis, and four unique phases for the vertical axis. In the case of ⅛-pixel resolution, for example, the polyphase filters have eight unique phases for each axis. It is also recognized that the 8-phase filter can also be used in ¼-pel position interpolation. Other embodiments are contemplated in which the set of coefficients for one axis is the same as the set of coefficients for the other axis. 
   For example, in  FIG. 6 , a current macroblock  122  of video data has moved with respect to a reference macroblock  112  of video data. More specifically, any given pixel  123  in the current macroblock  122  has moved ¼ of a pixel in the horizontal direction and ⅜ of a pixel in the vertical direction. In other words, the current macroblock  122  of video data has moved to a new fractional pixel position  113  with respect to the reference macroblock  112 . Polyphase prediction filters may be used to translate the reference macroblock  112  accounting for the fractional pixel movement and associated phase shift. In the example, the polyphase filter structure supports eight phases or fractional pixel positions between integer pixel positions, thus providing a fractional pixel accuracy of ⅛ pixel. 
   In H.26L prediction, it is necessary to perform a series of tests depending on the relative position of the desired sub-pixel location with respect to the integer-pixel positions. An embodiment of the present invention uses a well-defined and regularized method that applies equally to all fractional-pixel positions without having to consider the relative position of the desired interpolated pixel with respect to the integer-pixel data. The regular structure enables the design of simple hardware for the application of motion translation of reference macroblock data. 
   Embodiments of the present invention are simple to implement and do not require performing tests on the relative positions of fractional-pixels. Further embodiments of the present invention do not require performing different levels of filtering. The simplicity reduces production and operation costs. For example, the polyphase structure may be expressed by a single FIR filter with loadable coefficients. Particular coefficients are selected by a simple decision based on the fractional-pixel position. The implementation may be effected by very simple hardware and software. 
   Referring to  FIG. 1 , in step  10 , a particular set of polyphase prediction filters is selected based on a desired fractional pixel resolution. If a ⅛ pixel fractional pixel resolution is desired, then a set of eight polyphase prediction filters are selected, one filter for each phase between integer pixel locations. The same eight filters may be used for both horizontal and vertical directions or separate, dedicated sets of filters may be selected for each axis. 
   As an illustration and corresponding to the motion estimation method described above, steps  20  and  30  in  FIG. 1  show one of the embodiments of the invention. In step  20 , a current macroblock  122  of video data is compared to the macroblocks whose top-left corner starts at integer positions in a reference image  111 . The comparison is accomplished by video block comparator  130  (see  FIG. 3 ). Video block comparator  130  selects a reference macroblock  112  that is closest (or locally closest) to the current macroblock  122  (see  FIG. 6 ). 
   In step  30 , polyphase prediction filters are applied to the selected reference macroblock  112  and its neighboring data according to the fractional pixel positions to generate estimated macroblocks to compare with the current macroblock and get the final closest estimated reference macroblock, which may start at a fractional pixel.  FIG. 7  shows an example of a fractional pixel array bounded by four integer pixels  450  on the corners and comprising a set of fractional pixels  460 . The array comprises a total of 81 pixel positions. The polyphase filter structure  400  is used to generate the set of estimated macroblocks of video data. In one embodiment of the present invention, the polyphase filter structure  400  comprises a bank of polyphase filter coefficients  410  from which to select, and a 6-tap FIR filter structure  420  to which the filter coefficients may be applied. In  FIG. 3 , the structure is represented by the polyphase filter coefficient bank  160  and polyphase filter  170 . 
   Each phase of the polyphase filter structure, in accordance with an embodiment of the present invention, is designed to perform the correct phase shift for the fractional-pixel location, improving the resulting picture quality. The improvement is especially noticeable where fine horizontally or vertically oriented detail in the picture travel across the active displayed region. When only bilinear interpolation is used, the picture may exhibit phase shifts that are characterized by a pulsating effect on such details. The use of the correct interpolative phases in accordance with an embodiment present invention minimizes the pulsating effect and other artifacts generated as a consequence of improperly shifting the interpolative fractional-pixel phase. 
   For each row of pixels in the pixel array of  FIG. 7 , there is a corresponding set of polyphase filter coefficients for vertical interpolation filtering. Similarly, for each column of pixels in the array, there is a corresponding set of polyphase filter coefficients for horizontal interpolation filtering which may be the same or different from the set of filter coefficients for the rows. To generate a particular estimated macroblock of video data from the original reference macroblock, the 6-tap FIR filter is loaded up with integer pixel values from the reference macroblock. 
   For example, to generate the estimated macroblock corresponding to a shift in position of 3 fractional pixel positions in the horizontal direction (i.e. the ⅜ shift shown in  FIG. 6 ), the polyphase filter coefficients corresponding to the third phase of the eight phases is applied to the FIR filter  420 . The FIR filter is loaded with six integer pixel values from the reference macroblock  112  at filter input  430 , three integer pixels  114  to the left of the fractional pixel of interest  113  and three integer pixels  115  to the right. Each integer pixel value in the filter  420  is multiplied by its corresponding filter coefficient. The products are then summed to generate the new fractional pixel value at filter output  440  corresponding to the horizontal component of fractional pixel position  113 . The filtering process is performed for all integer pixels in the reference macroblock to generate the estimated macroblock corresponding to horizontal movement of three-phases (i.e. three fractional pixel locations in the horizontal direction). Filtering is then performed similarly in the vertical direction on the horizontally translated data to obtain the final estimated macroblock of video data. As shown in the example of  FIG. 6 , the vertical shift corresponds to ¼= 2/8 or two fractional pixel positions. The filtering process of step  30  is performed for each of the possible fractional pixel positions  460 . The result is multiple estimated macroblocks of video data, one macroblock for each fractional pixel position. Video block estimator  140  is structured to select the filter coefficients from polyphase filter coefficient bank  160  via connection  146 , which coefficients are provided to polyphase filter  170  via connection  147 . The polyphase filter  170  creates an estimated macroblock from reference image data using the coefficients so provided. The video block estimator also directs the video block comparator  170  via signal  145  to compare the estimated macroblocks from the polyphase filter. 
   In an embodiment of the present invention, a set of eight phases of 6-tap filter coefficients include the following:
         Phase 0 filter: 0 0 256 0 0 0   Phase 1 filter: 5 −21 249 30 −8 1   Phase 2 filter: 8 −34 228 68 −17 3   Phase 3 filter: 9 −38 195 111 −27 6   Phase 4 filter: 8 −35 155 155 −35 8   Phase 5 filter: 6 −27 111 195 −38 9   Phase 6 filter: 3 −17 68 228 −34 8   Phase 7 filter: 1 −8 30 249 −21 5       

   In step  40 , the video block comparator  130  compares the estimated macroblocks to the current macroblock  122  to determine the estimated macroblock that is most similar to the current macroblock  122 . If the current macroblock  122  did indeed move by an exact number of fractional pixel positions, then the chosen estimated macroblock is typically very similar to the current macroblock  122  and it may have exactly the same value. However, it is often the case that the movement of the current macroblock is in between the fractional pixel locations. Even though the fractional pixel prediction may get close to estimating the current macroblock, a residual error will typically still exist between the two. 
   In step  50 , the video block comparator  130  computes the residual error between the chosen estimated macroblock and the current macroblock  122  for each pixel position. The result is a residual error macroblock that is output from video block comparator  130  at output  133 . If the difference between the chosen estimated macroblock and the current macroblock  122  is small, then the residual error macroblock may be represented with a small number of digital bits. 
   In step  60 , video block estimator  140  computes a motion vector that is output from video block estimator  140  at output  131 . The motion vector represents the fractional pixel movement component between the final reference macroblock as described above and the current macroblock  122 . The motion vector may also be represented as a small number of digital bits. The reference macroblock  112  is also output from the video block comparator  130  at output  135 . 
   In step  70 , the motion vector, and the residual error macroblock, usually after they are processed and coded in certain form, are transmitted over a communication channel  150  to either a video storage device or to a video decoder  200  at a remote location. The (usually processed and coded) motion vector and the residual error macroblock represent the compressed video data for the current macroblock  122  of video data. As a result, video data may be transmitted much more efficiently and may be re-created later. 
   Once the data is compressed and transmitted and/or stored as a motion vector and a residual error macroblock, a current macroblock may be re-created by applying the motion vector and residual error macroblock to the reference macroblock of video data as shown in step  80 . Referring to  FIG. 4 , the motion vector at input  131  selects the correct polyphase filter coefficients from the polyphase filter coefficient bank  260  in motion compensator  200  corresponding to the fractional pixel motion of the current macroblock with respect to the reference macroblock. The video compensation module  270  applies the selected polyphase filter coefficients to the reference macroblock  112  and its neighboring data that is input to the video compensation module  270  at input  135  to generate the estimated macroblock. The video compensation module  270  filters the reference macroblock  112 , as previously described, to generate the estimated macroblock. The video compensation module  270  then applies the residual error macroblock, input to video compensation module  270  at input  133 , to the estimated macroblock to generate the compensated macroblock which is a reconstruction of the current macroblock  122 . The process is known as video reconstruction. 
   As a result, the current macroblock  122  is reconstructed. The reconstructed data may be selectively stored to be used as reference pictures for future pictures. 
   An embodiment of the present invention comprises applying a polyphase interpolative structure for the H.26L codec in the form of an 8-phase, 6-tap polyphase filter. The method may be applied to other codecs that require more accurate fractional pixel prediction, bearing in mind that the number of phases in the structure corresponds to the number of desired fractional pixel positions. There is no restriction on the number of taps in each phase. 
   In summary, certain embodiments of the present invention afford an approach to achieve efficient video compression/decompression of macroblocks of video data to fractional pixel accuracy by reducing the residual error data to be stored or transmitted. In an embodiment of the present invention used in H.26L prediction, a general form polyphase filter structure computes the fractional-pixel prediction for macroblocks of pixel data. The polyphase structure consists of 8 phases, each phase comprising six coefficients. The number of phases corresponds to the number of fractional pixel locations needed to perform up to ⅛-pixel prediction. Each ⅛-pixel location is assigned a set of coefficients corresponding to one phase of the polyphase filter structure. The computation of a fractional pixel position is done using a six-tap filter structure. The six-tap filter is implemented using one embodiment of a FIR filter structure. In the embodiment, it is not necessary to perform any other operation for the desired fractional pixel position other than selecting the set of coefficients assigned to the position as indicated by the corresponding phase of the polyphase filter. For two-dimensional prediction, the polyphase interpolation is performed first in one direction (horizontal or vertical) and then the resulting data is polyphase interpolated in the other direction. The regularized structure of an embodiment of the present invention enables the interpolation for any fractional pixel location regardless of the number of fractional pixel positions. Further, the same set of coefficients may be used to perform larger fractional-pixel positions. For example, the same set of coefficients used for ⅛-pixel interpolation may be used for ¼-pixel interpolation provided the correct phase is selected. For example, the selected phases may be 0, 2/8, 4/8, 6/8. 
   While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.