Abstract:
In accordance with one embodiment of the present invention, a method for encoding data is presented comprising receiving a stream of data to be encoded comprising a plurality of frames, selecting a frame-type for each of the plurality of frame, and grouping subsets of the plurality of frames based, at least in part, on the selected frame type to facilitate optimal, random access to select groups of frames, and to facilitate motion estimation.

Description:
BACKGROUND OF THE INVENTION 
     1. Related Applications 
     This non-provisional patent application claims priority to provisional application No. 60/080,501 by Jeffrey Mc Veigh and Michael Keith for a “Method and Apparatus for Providing Real-Time MPEG-2 Image Processing”, filed on Apr. 2, 1998; as well as a continuation application Ser. No. 09/209,828 by Michael Keith for a “Simplified Predictive Video Encoder”, filed Dec. 11, 1998. Each of the foregoing applications are commonly assigned to Intel Corporation of Santa Clara, Calif. 
     2. Field of the Invention 
     The present invention relates to the field of image processing and, in particular, to a method and apparatus for simplifying real-time data encoding. 
     3. Background Information 
     Over the years, the Motion Picture Experts Group (MPEG) has developed a number of standards for digitally encoding (also commonly referred to as compressing) audio and video data (e.g., the well-known MPEG-1, MPEG-2 and MPEG-4 standards). Recently, particular attention has been drawn to the MPEG-2 standard [ISO/IEC 13818-2:1996(E), “Information technology—Generic coding of moving pictures and associated audio information: Video”, 1996], which generally describes a bit-stream syntax and decoding process for broadcast quality digitized video. The MPEG-2 standard is widely used in emerging state-of-the-art video delivery systems including digital versatile disk (DVD, sometimes referred to as digital video disk), direct broadcast satellite (DBS) (e.g., digital satellite television broadcasts) and high-definition television (HDTV). 
     The rising popularity of the MPEG-2 standard may well be attributed to its complex video compression technology that facilitates the broadcast quality video. Compression is basically a process by which the information content of an image or group of images (also referred to as a Group of Pictures, or GOP) is reduced by exploiting the spatial and temporal redundancy present in and among the image frames comprising the video signal. This exploitation is accomplished by analyzing the statistical predictability of the signal to identify and reduce the spatial and temporal redundancies, thereby reducing the amount of storage and bandwidth required for the compressed data. The MPEG-2 standard provides for efficient compression of both interlaced and progressive video content at bit rates ranging from 4 Mbps (for DVD applications) to 19 Mbps (for HDTV applications). FIG. 1 illustrates a block diagram of the complex elements of an example prior art MPEG-2 encoder for compressing video data. 
     As shown in the block diagram of FIG. 1, encoder  100  is generally comprised of an intra-frame encoder  102 , an inter-frame encoder  104  a multiplexer  106  and a buffer  108 , which controls the rate of broadcast of the compressed video data. Each of the intra-frame encoder  102  and inter-frame encoder  104  will be described in turn, below. 
     Simplistically speaking, compression by intra-frame compressor  102  may be thought of as a three-step process wherein spatial redundancy within a received video frame is identified, the frame is quantized and subsequently entropy encoded to reduce or eliminate the spatial redundancy in the encoded representation of the received frame. The identification of spatial redundancy within a frame is performed by transforming spatial amplitude data of the frame into a spatial frequency representation of the frame using the discrete cosine transform (DCT) function  110 . The DCT function is performed on 8×8 pixel “blocks” of luminance (brightness) samples and the corresponding blocks of chrominance (color differential) samples of the two-dimensional image, generating a table of 64 DCT coefficients. The block of DCT coefficients is then compressed through Quantizer (Q)  112 . Quantization is merely the process of reducing the number of bits required to represent each of the DCT coefficients. The quantizing “scale” used can be varied on macroblock (16×16 pixel) basis. The quantized DCT coefficients are then translated into a one-dimensional array for encoding  114  via variable length encoding and run length encoding. The order in which the quantized DCT coefficients are scanned into encoder  114  affects the efficiency of the encoding process. In general, two patterns for scanning the block of quantized DCT coefficients are recognized, the zigzag pattern and the alternate scan pattern, each of which are depicted in FIG. 2 as pattern  200  and  250 , respectively. Those skilled in the art will appreciate that with prior art intra-frame compression such as that employed by intra-frame encoder  102 , the zigzag scan pattern  200  is typically used as it produces long runs of zeroes, as the block of DCT coefficients are transformed run-length/value pairs for the variable length encoding process. The quantized, entropy encoded DCT coefficients along with the quantization tables are then sent to MUX  106  for broadcast and/or storage through rate control buffer  108 . 
     Inter-frame compressor  104  reduces the temporal redundancies existing between frames in a group of pictures and is typically a complex process of motion estimation between frames and fields of the frames using reconstructed past and predicted future frames as a reference. Accordingly, inter-frame compressor  104  is depicted comprising motion estimator  116  which statistically computes motion vectors to anticipate scene changes between frames, anchor frame storage  118  to store reconstructed prior frame data (from the quantized DCT coefficients) and predicted frame storage  120  to store a predicted future frame based on information received from the motion estimator  116  and current frame information. In addition, inter-frame compressor  104  is depicted comprising inverse quantizer  122 , inverse DCT  124  and a summing node  126  to reconstruct the present or past frames for storage in anchor frame storage  118 . 
     Those skilled in the art will appreciate that the MPEG-2 standard provides for three types of video frames and that the type of frame determines how the motion estimation for that frame is to be accomplished. The three frame types are Intra-frame coded (I-frame), Predictably encoded frames (P-frame) and bidirectionally interpolated frames (B-frame). I-frames are encoded based only on the content within the frame itself and are typically used as reference and synchronization frames. That is, the separation between I-frames is used to denote Groups of Pictures (GOPs). P-frames are encoded based on the immediate past I- or P-frames (also referred to as anchors), and B-frames are encoded based on past or future I- and P-frames (thus the need for anchor and predicted frame storage  118  and  120 , respectively). Predicting content based on frame data is graphically illustrated with reference to FIG.  3 . 
     Turning to FIG. 3, a graphical representation of a typical GOP sequence of frames is presented  300  denoting an IBBPBBI sequence (commonly referred to as a GOP (6,3) sequence by those skilled in the art). As shown in FIG. 3, encoding of I-frame  302  does not rely on any prior or future frame. Encoding of B-frame  304  utilizes information from past frames (e.g., I-frame  302 ) as well as future I and/or P-frames (e.g., P-frame  306 ). 
     If the frame sequence contains interlaced content, field prediction is also performed in calculating the motion vector. Simplistically speaking, frames are broken into even and odd fields, and the content of each field is predicted based on the information contained in both the odd and the even fields of the past and/or future frames (depending on the frame type, P or B-frames, respectively). More specifically, the content of P- and B-frames are predicted by analyzing the even and odd fields of past and/or future anchor frames. A typical field prediction process is depicted in FIG.  4 . 
     With reference to FIG. 4, two frames  402  and  410  are depicted broken into their constituent even ( 404  and  412 ) and odd ( 406  and  414 ) fields, respectively. In this example, frame  402  is an I-frame, while frame  410  is a B-frame. In accordance with the prior art, the even field  412  of B-frame  410  is predicted from the even  404  and odd  406  field of the prior I-frame  402 . 
     Those skilled in the art will appreciate that, although the computationally intensive video encoding associated with the MPEG-2 standard provides high resolution video imagery, its implementation typically requires one or more powerful, dedicated processor(s) (e.g., a microcontroller, an application specific integrated circuit (ASIC), a digital signal processor (DSP) and the like) to encode (or, conversely decode) MPEG-2 standard video data (e.g., to/from a DVD disk). Attempts to utilize the general purpose central processing unit (CPU) of a typical home computer for MPEG-2 processing has proven computationally prohibitive, as the MPEG-2 standard processing consumed nearly all of the computational resources of the general purpose CPU, thereby rendering the computer virtually useless for any other purpose. As a consequence, providing MPEG-2 standard video technology in a personal computer has heretofore required the addition of the costly dedicated video processors described above. 
     As a result of the cost and performance limitations commonly associated with real-time video encoding described above, the roll-out of MPEG-2 video multimedia capability in the home computing market has been slowed. Consequently, a need exists for a method and apparatus for simplifying real-time data encoding that is unencumbered by the deficiencies and limitations commonly associated with the prior art. An innovative solution to the problems commonly associated with the prior art is provided herein. 
    
    
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a method for encoding data is presented comprising receiving a stream of data to be encoded comprising a plurality of frames, selecting a frame-type for each of the plurality of frames, and grouping subsets of the plurality of frames based, at least in part, on the selected frame type to facilitate random access to select groups of frames, and to facilitate motion estimation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which: 
     FIG. 1 is a block diagram illustration of a typical prior art data encoder to encode data in accordance with the MPEG-2 standard; 
     FIG. 2 is a graphical representation of a block of data being encoded in accordance with a zigzag scan pattern, and a block of data being encoded in accordance with an alternate scan pattern, in accordance with one embodiment of the present invention; 
     FIG. 3 is a graphical representation of a group of pictures denoting the coding dependencies for motion estimation, in accordance with prior art encoders; 
     FIG. 4 is a graphical representation of field prediction dependencies between frames of a group of pictures, in accordance with prior art encoders; 
     FIG. 5 is flow chart illustrating an example method for intra-frame encoding in accordance with the teachings of the present invention; 
     FIG. 6 is a flow chart illustrating a method of performing virtual half-resolution (VHR) filtering in accordance with one aspect of the present invention; 
     FIG. 7 is a graphical representation of a received block of data before and after application of the VHR filter of the present invention, in accordance with the teachings of the present invention; 
     FIG. 8 is a flow chart of an example method of performing inter-frame encoding, in accordance with the teachings of the present invention; 
     FIG. 9 is a flow chart of an example method for performing unidirectional motion estimation on bi-directionally predicted frames, in accordance with another aspect of the present invention; 
     FIG. 10 is a graphical representation of motion estimation for a group of pictures in accordance with the teachings of the present invention; 
     FIG. 11 is a flow chart illustrating an example method for performing even-parity field prediction in accordance with another aspect of the present invention; 
     FIG. 12 is a graphical representation of motion estimation using even-parity field prediction in accordance with the teachings of the present invention; 
     FIG. 13 is a block diagram of an example software architecture incorporating the teachings of the present invention, in accordance with one embodiment of the present invention; 
     FIG. 14 is a block diagram of an example software architecture incorporating the teachings of the present invention, in accordance with an alternate embodiment of the present invention; and 
     FIG. 15 is a block diagram of an example storage medium having stored therein a plurality of machine executable instruction which, when executed, implement the teachings of the present invention, in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well known features are omitted or simplified in order not to obscure the present invention. Furthermore, for ease of understanding, certain method steps are delineated as separate blocks, however, those skilled in the art will appreciate that such separately delineated blocks should not be construed as necessarily conferring an order dependency in their performance. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
     Those skilled in the art will appreciate from the description to follow that the innovative encoder described herein is comprised of a number of innovative aspects, each of which provide increased performance without significant degradation to the integrity of the encoded data over prior art MPEG-2 video encoders. For ease of explanation, each of the innovative aspects of intra-frame encoding and inter-frame encoding processes of the present invention will be described in turn, and as a constituent component of the innovative encoder of the present invention. This is not to say, however, that all of the innovative aspects described herein must be present in order to practice the present invention. Indeed, a number of alternative embodiments will be presented depicting various levels of complexity incorporating one or more aspects of the present invention. Thus, those skilled in the art will appreciate from the description to follow that any of a number of embodiments of the present invention may be practiced without departing from the spirit and scope of the present invention. 
     Intra-frame Encoding 
     Turning to FIG. 5, a flow chart illustrating an example method for performing intra-frame compression and encoding in accordance with the teachings of the present invention is presented. In accordance the teachings of the present invention, method  500  begins with, in step  502 , a determination of whether virtual half-resolution (VHR) downconversion is to be performed, in accordance with a first aspect of the present invention. If VHR downconversion is not to be performed, the innovative encoder of the present invention will continue with prior art intra-frame compression, while still employing the innovative inter-frame compression aspects of the present invention to be described more fully below, step  504 . 
     If, however, it is determined in step  502  that VHR downconversion is to be performed, the process continues with step  506  wherein a low-pass filter is applied to the received frame in step  506  and the frame is subsampled horizontally. In one embodiment of the present invention, for example, the frame is subsampled horizontally by a factor of two (2), which eliminates one-half of the frame of data. Turning briefly to FIGS. 6 and 7, one example embodiment of a method for performing VHR downconversion and a block of DCT coefficient data before and after VHR downconversion is presented. In accordance with one aspect of the present invention, VHR downconversion begins with step  602  wherein a block of data  700  (e.g., 8×8 block of DCT coefficients) is received and processed through a low-pass filter. In step  604 , the filtered block of data is horizontally subsampled by some scaling factor. In one embodiment, the filtered block of data is horizontally subsampled by a factor of two (2), rendering the right-half of the block null (i.e., full of zeroes). More specifically, in accordance with the teachings of the present invention, VHR downconversion is performed by application of a horizontal low-pass filter, which is applied to both the luminance and chrominance data. In one embodiment of the present invention, a [1 2 1] filter kernal is used in step  602 . For example, in one embodiment, the following filter is used: 
     
       
           h ( n )=0.25[δ( n− 1)+2δ( n )+δ( n+ 1)]  (1) 
       
     
     In one example software implementation of the present invention, suitable for execution by an Intel® Architecture processor, the following simplified version of equation (1) may be used, utilizing the pavg instruction: 
     
       
           y ( n )=PAVG( x ( n ), PAVG( x ( n− 1),  x ( n+ 1)))  (2) 
       
     
     Thus, instead of subsequently encoding the received data with a traditional 8×8 DCT and then realizing that most of the coefficients in the right half of the block, i.e., the high-frequency spatial components, are zero as a result of the foregoing filter, the block is horizontally subsampled in step  604 . In one embodiment, for example, the received blocks are subsampled by a factor of two (2) horizontally. This results in macroblocks of 8×16 and blocks of 4×8. That is, the horizontal 8-pixel DCT is replaced with a modified 4-pixel DCT. The resulting coefficients of the normal 4-pixel DCT are modified by scaling them by the square root of two (sqrt (2)) to accommodate the conversion to an 8-pixel DCT. Consequently, to an MPEG-2 compliant decoder, the VHR compressed data looks identical to full-resolution encoded MPEG-2 data. When decoded with an MPEG-2 compliant decoder, the visual effect of application of the VHR downconversion of FIG. 6 is negligible, while realizing up to a 2× improvement in data throughput. 
     Once the VHR downconversion has been completed on each block of the received frame in step  506 , discrete cosine tranform (DCT) and quantization pre-processing is performed on the VHR downconverted frame, step  508 . More specifically, in accordance with one embodiment of the present invention, the pre-processing consists of DCT type selection and macroblock quantization selection. 
     For data streams comprising interlaced video, the first step in the encoding pipeline is deciding between frame and field DCT. To improve compression efficiency, selection of the DCT type which yields smaller vertical high-frequency coefficients is preferable. In one embodiment present invention, the “vertical activity” is measured by comparing the activity of adjacent lines for both frame and field macroblocks. In one embodiment, vertical frame activity is measured by summing the absolute difference of spatial amplitudes over pairs of adjacent frame lines over a macroblock (i.e., VHR mode 8×16; non-VHR mode 16×16). In one embodiment, a psad operation may be used to sum the absolute difference of pairs and, thus, vertical frame activity is calculated by summing the result of a psad operation over pairs of adjacent frame lines over the macroblock, e.g.,              frame_activity   =       ∑     n   =   0     7                     PSAD        (       line     2      n       ,     line       2      n     +   1         )                 (   3   )                                
     Similarly, the vertical field activity for both fields is calculated by summing the absolute difference over pairs of adjacent field lines (even numbered lines contain the top field and the odd numbered lines contain the bottom field). Again, the psad operation may well be employed, e.g.,              field_activity   =         ∑     n   =   0     3                     PSAD        (       line     2      n       ,     line       2      n     +   1         )         +       ∑     m   =   0     3                     PSAD        (       line       2      m     +   1       ,     line       2      n     +   3         )                   (   4   )                                
     Low activity values indicate small vertical frequency magnitudes, while the converse is true for high activity values. In accordance with one embodiment of the present invention, the measure which provides the lowest vertical AC coefficients are selected to improve the efficiency of subsequent encoding processes. 
     In one embodiment of the present invention, the quantizer scale is selected based, at least in part, on how highly correlated the data is within each of the blocks of the macroblock. In one embodiment, if the block data is highly correlated, a lower (finer) quantization scale is used. If, however, the block data is uncorrelated (e.g., highly textured regions), a larger quantizer scale is utilized. This decision is based, in part, on the theory that the human visual system is not particularly sensitive to degenerative artifacts in highly textured regions. To estimate the activity within a macroblock, a measure of the horizontal activity is combined with a measure of the vertical activity value obtained from the DCT Type Selection (above). In one embodiment, the horizontal activity is measured using a first-order approximation of the correlation between adjacent pixels using the psad operation: 
     
       
         horizontal_activity=ΣPSAD(line[ n ]&amp; 0x00ffffffffffffff,line[ n ]&gt;&gt;8)  (5) 
       
     
     The total activity, which is the sum of the horizontal and vertical activities, then is used to select the macroblock quantizer scale to be applied. 
     Once the pre-processing of step  508  is completed, the VHR downconverted frame is discrete cosine transformed into the frequency domain, step  510 . As provided above, the DCT is but one means of transforming the spatial amplitude data of a frame to a spatial frequency representation. Within the context of the present invention, any of a number of known techniques for performing DCT may well be employed. However, in the instance where the VHR filter has been employed, the transformation to the frequency domain need only be performed on the lower frequency 4×8 pixels of the block (i.e., the left half of the 8×8 block). In one embodiment, the well known fast DCT-SQ algorithm is utilized for eight and four pixel DCT&#39;s. 
     With continued reference to FIG. 5, the downconverted, DCT coefficients resulting from the DCT process of step  508  are quantized in step  512 , before entropy encoding in block  514 . In accordance with one embodiment of the present invention, only the left-side, i.e., the low-frequency components, of the DCT transformed block are quantized, thereby increasing throughput by a factor of two. 
     As described above, the entropy encoding process  514  translates the two-dimensional block of quantized DCT coefficients into a one dimensional representation. Since the quantized DCT coefficients in the right half of the 8×8 block are always zero, as a result of the VHR downconversion, the alternate scan pattern  250  (described above) and run length encoding provides the most efficient entropy encoding process. That is because application of the alternate scan pattern  250  guarantees that almost the entire left half of the block is traversed before traversing the right half. In one embodiment, the run-length encoding process compresses the quantized data further into a form of (run_of_zeroes, next non-zero value). For example, a sequence of “070003000002” would be encoded as (1,7),(3,3),(5,2) and so on. As provided above, the goal is to maximize the run of zeroes for maximum compression efficiency. 
     Those skilled in the art will appreciate, based on the foregoing, that the VHR method of FIG. 6 facilitates up to a 2× improvement in intra-frame compression by replacing the right-half of the received blocks with zeroes, thereby eliminating the need for DCT and quantization of nearly 50% of the received data, while improving encoding efficiency. Thus, those skilled in the art will appreciate that the VHR aspect of the present invention provides for high-quality video while increasing data throughput through the innovative encoder of the present invention. 
     Inter-frame Compression/Encoding 
     Having described the innovative intra-frame compression process above with reference to FIGS. 5 through 7, the innovative inter-frame compression process will now be described with reference to FIGS. 8 through 12. Those skilled in the art will appreciate, that the innovative frame prediction and field prediction motion estimation aspects of the present invention, to be described more fully below, facilitate the additional processing speed improvements associated with the present invention. More specifically, disclosed herein is an innovative temporally constrained, unidirectional interpolation of bidirectional interpolated frames, and the even-parity field prediction, each of which will be described in greater detail below. We begin with reference to FIG. 8 which presents an example method for removing temporal redundancies between frames (i.e., inter-frame compression) is presented, in accordance with one embodiment of the present invention. 
     As shown, inter-frame compression process  800  begins upon the receipt of one or more frames of video. In the instance where more than one frame of video is received, they are classified in step  801  as either I-, B-, or P-frames, as described above. In accordance with one embodiment of the present invention, the assignment of frame type follows a predetermined sequential pattern to achieve the desired GOP sequence, to be described more fully below. In an alternate embodiment, the received frames are buffered and analyzed to determine whether a scene change occurs within any of the buffered frames. If so, the scene change will be placed between two inter-frame encoded frames, e.g., two B-frames, to maximize coding efficiencies and motion estimation of the B-frames (to be described more fully below). 
     In accordance with one aspect of the present invention, the innovative encoding process of the present invention utilizes a constrained GOP sequence of GOP (3,3), i.e., 3 frames separating I-frames, with a maximum of 3 frames separating anchor frames. By limiting the inter-frame encoding to the GOP structure identified, the innovative encoder of the present invention provides fast access to particularly fine quantities of video (e.g., facilitating editing, post-production, etc.). Moreover, the constrained GOP structure of the present invention facilitates motion estimation by limiting the number of frames which must undergo motion estimation. 
     In step  802 , a decision is made of whether VHR downconversion is to be performed. If not, the process continues with step  806  offering the innovative frame-prediction and field prediction aspects of the inter-frame compression process. If VHR downconversion is to be performed, the VHR filter (see, e.g., FIG. 6) is applied in step  804 , and the process continues with motion estimation in step  806 . Those skilled in the art will appreciate, based on the foregoing, that the VHR method of FIG. 6 facilitates up to a 2× improvement in inter-frame compression by replacing the right-half of the received blocks with zeroes, thereby eliminating the need for DCT and quantization of nearly 50% of the received data, while improving encoding efficiency. Thus, those skilled in the art will appreciate that the VHR aspect of the present invention provides for high-quality video, while reducing encoding complexity thereby enabling real-time video encoding of the present invention. 
     The motion estimation step  806  calculates motion vectors which are stored/broadcast along with the compressed video data to facilitate broadcast quality decoding. As described above, motion estimation may well be performed on a frame- or field-basis. In accordance with one aspect of the present invention, the motion estimation of step  806  is comprised of an innovative frame-based motion estimation technique and/or an innovative even-parity field prediction motion estimation technique. With reference to the first of these two aspects of the present invention, an innovative unidirectional interpolated B-frame prediction technique is described more fully with reference to FIGS. 9 and 10. 
     Turning briefly to FIG. 9 an innovative method for performing temporally constrained, unidirectional B-frame motion estimation  900  is presented. In accordance with the illustrated example embodiment, the method begins upon receipt of a B-frame which is to be inter-frame encoded, step  902 . In step  904 , a single anchor frame is selected from which the content of the B-frame is to be predicted. In accordance with one embodiment of the present invention, the temporally closest anchor frame, whether preceding or superseding the B-frame is selected. In step  906 , in contradiction to the well established method for predicting B-frame content, the content of the B-frame is unidirectionally interpolated from the content of the above identified temporally closest anchor frame, in accordance with one aspect of the present invention. More specifically, in accordance with one embodiment of the present invention, the content of the B-frame is unidirectionally interpolated using the content of the temporally closest anchor frame and a motion vector calculated based on the temporally closest anchor frame. In one embodiment, the motion vector is the sum of absolute differences (SAD) of the activity within the anchor frame, e.g., within each scan line of the anchor frame. 
     Graphically, the temporally constrained, unidirectional interpolation of a B-frame is presented with reference to FIG.  10 . As shown in FIG. 10, rather than bidirectionally interpolating the content of B-frame  1004  from past and future anchor frames, the content of B-frame  1004  is unidirectionally interpolated by the closest anchor frame, i.e., I-frame  1002 , in accordance with one aspect of the present invention. Similarly, B-frame  1006  is unidirectionally interpolated from the temporally closest anchor frame, P-frame  1008 , in accordance with this aspect of the present invention. As shown, inter-frame encoding of P-frame  1008  is premised on the nearest past anchor frame, in this example, I-frame  1002 . 
     Although contrary to the well established practice for predicting B-frame content, the innovative temporally constrained, unidirectional B-frame technique of FIG. 9 has been empirically shown to provide substantially the same decoded picture quality as video encoded using computationally intensive standard B-frame encoding process. Those skilled in the art will appreciate, based on the foregoing, that this aspect of the present invention, namely, the temporally constrained unidirectional interpolation of B-frames greatly reduces the computation complexity of inter-frame compression, thereby facilitating greater encoding throughput with minimal degredation to the quality of the encoded data. 
     In addition to the innovative frame-based motion estimation technique described above with reference to FIGS. 9 and 10, innovative motion estimation process  806  also includes an enhanced field prediction process, namely, an innovative even-parity field prediction motion estimation technique. In accordance with this aspect of the present invention, attention is drawn to FIGS. 11 and 12, wherein a method and graphical representation for performing even-parity field motion estimation is presented. 
     Turning to FIG. 11, an example method for performing even-parity field prediction is presented, in accordance with one aspect of the present invention. As shown in FIG. 11, the method begins upon the receipt of a subject frame of interlaced (or progressive) video which is to be inter-frame encoded, step  1102 . In step  1104 , each of a plurality of fields of a past or future (i.e., the temporally closest anchor frame, as described above) anchor frame are analyzed to predict the content of corresponding fields in the subject frame, step  1106 . In one embodiment, the even-field of the anchor frame is used to predict the even-field of a subject frame, while an odd-field of an anchor frame is used to predict the odd-field of the subject frame. In one embodiment, the odd-field of an anchor frame is used to predict the even-field of a subject frame, while the even-field of the anchor frame is used to predict the odd-field of the subject frame. In one embodiment, the content of the even- or odd-field of the anchor frame is scaled by a motion vector to predict the content of corresponding even- or odd-fields of the subject frame. In one embodiment, the motion vector is computed by measuring the sum of absolute differences of the activity within the respective field of the anchor frame. 
     Graphically, the even-parity field prediction process is presented with reference to FIG.  12 . As shown in FIG. 12, two frames are presented an I-frame  1302  and a subsequent B-frame  1308 . In accordance with the even-parity field prediction process of the present invention, the even field  1310  of B-frame  1308  is predicted from the corresponding even field  1304  of the temporally closest reference frame, i.e., I-frame  1302  in this example. Similarly, the odd field  1312  of B-frame  1308  is inter-frame encoded based on the content of the odd field  1306  of reference frame  1302 . In an alternate embodiment, odd-parity field prediction may well be used, wherein the even field of the subject frame is inter-frame encoded based on the content of the odd field of the reference frame, and vice versa. 
     Although contrary to the well established practice of field prediction used to encode video data, the innovative even-parity field prediction technique has been empirically shown to encode data which, when decoded in accordance with the MPEG-2 standard, provides substantially similar results to the comprehensive field prediction technique of the prior art. Accordingly, those skilled in the art will appreciate that the innovative frame and field prediction techniques presented above, greatly reduce the complexity of motion estimation, facilitating greater encoder throughput while retaining the required and expected video integrity of the MPEG-2 encoded data. 
     In one embodiment, except for the innovative frame and field prediction constraints described above, motion estimation in accordance with prior art MPEG-2 encoders is performed, albeit at a greatly increased rate due to the innovative constraints. In alternate embodiments, process enhancements to the motion estimation process can be made by multi-resolution decomposition (also referred to as hierarchical decomposition) of the received video into two or more levels, and performing coarse motion estimation on certain levels, while performing fine motion estimation on other levels. 
     Once motion estimation step  806  is complete, coding decisions of whether intra- or inter-frame encoding is required are performed, step  810 . In accordance with one embodiment of the present invention, the vertical and horizontal activity measures described above are utilized in step  806  to determine whether intra- or inter-frame encoding is more suitable. In one embodiment of the present invention, intra-frame encoding is performed per the innovative method of FIG. 5, step  812 . If inter-frame encoding is to be performed (i.e., B- or P-frames), the block difference is calculated, step  814 . The block difference is the residual between the original and motion compensated blocks, for both the luminance and chrominance data in the block. In one embodiment, this residual is calculated only over even-numbered lines to reduce computational complexity. 
     Once the block residual is calculated in step  814 , a determination of whether the block is empty can be made, step  816 . If so, further determinations of whether the end of the macro-block or frame has been reached in steps  820  and  822 , before the encoding process is complete. If, however, the block is not empty, the block is inter-frame encoded (DCT, quantization, entropy encoding, etc.) per FIG. 5 is performed in step  818 . 
     Having described the innovative intra-frame and inter-frame compression and encoding techniques of the present invention, above, some alternate embodiments for the present invention will be presented with reference to FIGS. 13 through 15. 
     Turning to FIG. 13, a block diagram of an example software architecture  1400  implemented on an electronic appliance incorporating the teachings of the present invention is presented, in accordance with one embodiment of the present invention. In accordance with the illustrated example embodiment of FIG. 13, software architecture  1400  is shown comprising a plurality of applications  1402  including a video encoder application  1404 , operating system  1406  with associated device drivers and dynamic link libraries (DLL)  1406 , cooperatively coupled as depicted. In accordance with one embodiment of the present invention, the innovative elements of intra-frame compressor/encoder  500  and inter-frame compressor/encoder  800  are embodied within distinct DLL&#39;s  1408 , which can be called by any of a number of applications  1402 , including the video encoder application  1404 . 
     In accordance with this example embodiment, DLL&#39;s  1408  include a VHR filter DLL  1410 , a frame motion estimation DLL  1412  and, and a field motion estimation DLL  1414  each incorporating the teachings of the present invention described above with reference to FIGS. 5-12. In an alternate embodiment, video encoder application  1404  includes the innovative aspects of intra-frame encoder  500  and inter-frame encoder  800 , described above, as sub-routines within the application itself. 
     Whether resident within a stand-alone application (e.g., video encoder  1404 ) or as a number of discrete DLL&#39;s  1408  which are called when required, the innovative aspects of the present invention are embodied as a plurality of executable instructions which, when executed by an appropriate processor/controller, implement the methods of FIGS.  5  and/or  8  and their referenced progeny enabling the innovative MPEG-2 encoder technique presented above. 
     In accordance with the teachings of the present invention, VHR filter DLL  1410  downconverts the received block of data by a factor of two by replacing the data in the right half of the received block with all zeroes (see, e.g., FIG.  7 ). The frame motion estimation DLL  1412  employs the innovative temporally constrained unidirectionally interpolated B-frame technique described above with reference to FIG.  9 . The field motion estimation DLL  1414  employs the innovative even-parity field prediction technique described above with reference to FIG.  11 . In alternate embodiments of the present invention, one or more of the innovative aspects of the present invention are provided within the DLL library  1408  or within video encoder application  1404  facilitating the use of encoders with different levels of computational complexity with minimal differentiation in the integrity of the encoded data. 
     As depicted herein, applications  1402  are intended to represent any of a number of specialty applications known in the art which are executable by an electronic appliance. Similarly, except for the teachings of the present invention, operating system  1406  is also intended to represent any of a number of alternative general operating systems and device drivers known in the art. Those skilled in the art will appreciate that the execution of operating system  1406  is initiated from within a basic input/output system (BIOS) (not shown). Operating system  1406  is a general software service which provides an interface between applications  1402 , a video encoder application  1404  and, the DLL&#39;s  1408  incorporating the teachings of the present invention, described above. According to one embodiment of the present invention, operating system  912  is the Windows™95 operating system, available from Microsoft Corporation of Redmond, Wash. However, it is to be appreciated that the present invention may be used with any other conventional operating system, such as other versions of Microsoft Windows™ (for example, Windows™3.0, Windows™3.1, Windows™NT, or Windows™CE), Microsoft DOS, OS/2, available from International Business Machines Corporation of Armonk, N.Y., the Apple Macintosh Operating System, available from Apple Computer Incorporated of Cupertino, Calif., the NeXTSTEP™ operating system available from Apple Computer Incorporated, the UNIX operating system, available from Santa Cruz Operations of Santa Cruz, Calif., the Be operating system from Be, Inc. of Menlo Park, Calif., and the LINUX operating system. 
     Turning to FIG. 14, a block diagram of an example data encoder incorporating the teachings of the present invention is presented. In accordance with the teachings of the present invention, encoder  1500  is depicted comprising VHR filter  1502 , intra-frame encoder  1504  and inter-frame encoder  1506 , in addition to multiplexer  106  and rate control buffer  108 , each cooperatively coupled as depicted. Except for the teachings of the present invention, encoder  1500  is typical of prior art encoders. In particular, VHR filter  1502  is a low-pass filter that effectively replaces the right-half of a received block of data with all zeroes (see, e.g., FIG.  7 ). Accordingly, the computation resources of the DCT and Quantization phases of frame encoder  1504  are greatly reduced, with minimal impact to decoded video image. In accordance with another aspect of the present invention stemming from the VHR filter  1504 , entropy encoder  1514  employs run-length encoding utilizing the alternate scan pattern, as described above. 
     In addition to the innovative encoding techniques described above, the inter-frame encoder  1506  utilizes a computationally efficient motion estimator  1508 , which employs the temporally constrained unidirectional B-frame encoding and the even-parity field encoding techniques described above. Moreover the innovative inter-frame encoder  1506  of the present invention does not rely on reconstructed past frames as a reference, but rather utilizes the original frame, thereby eliminating the need for the reconstructing circuitry (e.g., DCT −1 , Q −1  and Summing stage) and additional storage typical of prior art encoders. In one embodiment, innovative encoder  1500  is implemented on a video board accessory board of a typical home computer system, or as a constituent member of a special purpose video processing station. 
     In accordance with another embodiment of the present invention, the innovative encoding techniques of the present invention are embodied in software. Accordingly, FIG. 15 illustrates an example storage medium  1602  having stored thereon machine executable instructions  1604  which, when processed by a controller transforms an appropriately configured machine executing machine executable instructions  1604  into a data encoder incorporating one or more of the innovative aspects of the present invention described above. In accordance with the illustrated example embodiment of FIG. 15, storage medium  1602  is intended to represent any of a number of alternative storage media including, but not limited to, floppy disks, magnetic tape, compact disk, digital versatile disk, optical disks, and the like. Further, those skilled in the art will appreciate that the machine executable instructions need not be located within the an executing machine itself, but may be accessed from coupled network devices. 
     Those skilled in the art will appreciate that innovative encoder  1500  may well be embodied in any of a number of different forms. In addition to the embodiments described above, those skilled in the art will appreciate that the teachings of the present invention may well be integrated with a single integrated circuit (not shown). That is, those skilled in the art will appreciate that advances in IC fabrication technology now enable complex systems to be integrated onto a single IC. Thus, in accordance with one embodiment of the present invention, the teachings of the present invention may be practiced within an application specific integrated circuits (ASIC), programmable logic devices (PLD), microcontroller, processor and the like. 
     Thus, alternative embodiments for a method and apparatus for simplifying real-time video encoding has been described. While the method and apparatus of the present invention has been described in terms of the above illustrated embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described. Thus, those skilled in the art will appreciate that the present invention can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the descriptions thereof are to be regarded as illustrative instead of restrictive on the present invention.