Abstract:
A method and apparatus for run length encoding video data for transform based coders. The video data is separated into blocks of pixels. The pixel values are transformed to another set of values which can be represented with less data. The transformed values are quantized by generating a quantized magnitude and sign for multiple transformed values at a time, while removing branch misprediction errors during the quanitizing process. The quantized values are run length encoded by removing branch misprediction errors during the encoding process.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the compression of video signals. More particularly, the invention relates to a method and apparatus for quantizing and run length encoding transform coefficients in a video coder. 
     A number of transform-based video coders utilize a quantization algorithm and a run length encoding algorithm to compress a video signal. In general, quantization refers to reducing the magnitude of transform coefficients, and increasing the number of zero valued transform coefficients. Run length encoding is a technique wherein a series of repetitive data symbols are compressed into a shorter code which indicates the length of a code and the data being repeated. 
     The purpose of a video coder is to reduce the data required to represent the video signal while maintaining an acceptable viewing quality. Thus, there exists a need in the art for efficient compression techniques to further this purpose. One method of increasing the efficiency of a compression technique is to reduce the number of processing cycles required to implement the technique. 
     Quantization and run length encoding are two of the more expensive steps in a video coder in terms of the number of processing cycles required to implement these techniques. Increased coding efficiency during these steps of a compression algorithm would free processing cycles the video coder could use to improve video compression. For example, a video coder could use the extra processing cycles to increase the number of encoded frames which would increase the viewing quality of the decompressed video signal, or perform better motion estimation in block motion compensated transform based video coders. 
     One reason conventional run length encoding and quantization techniques are expensive is that they are susceptible to branch misprediction errors. For example, run length encoding techniques for block motion compensated transform based video coders reduce a serial bit stream to a sequence of a triple of values. Each triple of values comprises a run value representing the number of consecutive values of zero, a coefficient value representing a non-zero value, and a sign for the non-zero coefficient value. Such a run length algorithm could be implemented using the following pseudo-code: 
     Initialize run length to zero 
     Traverse coefficients in run order 
     result=abs(coeff/quantizer) 
     if result equal to zero then 
     increment run length 
     else 
     write run length 
     write result 
     write sign of coeff 
     initialize run length to zero 
     end if 
     end. 
     It can be appreciated that the above pseudo-code utilizes a conditional IF branching statement to determine whether the algorithm should count the values of zero (increment run length) or write out the non-zero coefficient (result) and current zero value count (run length). To minimize processing cycles, modern central processing units (CPU), such as Intel&#39;s Pentium™ and Pentium Pro™ microprocessors, attempt to predict which branch of the branching statement the algorithm is going to select based upon previous branch selections. If the CPU mispredicts which branch is selected, the CPU must utilize extra CPU cycles to correct the misprediction. This is referred to as a branch misprediction penalty. As the branch misprediction penalty increases, coding efficiency decreases. Modern run length encoding techniques as the one described above lead to significant branch misprediction penalties for Pentium™ processors, and even higher penalties for Pentium Pro™ processors. It is estimated that the branch misprediction penalty running conventional run length encoding algorithms on Pentium™ processors is 3 cycles per coefficient, and for Pentium Pro™ processors is 10 cycles per coefficient. 
     In addition, conventional quantization techniques are expensive since they quantize only one value at a time. This requires a large number of calculations which further decreases coding efficiency. 
     In view of the foregoing, it can be appreciated that a substantial need exists for quantizing and run length encoding algorithms for improving the coding efficiency for transform based video coders. 
     SUMMARY OF THE INVENTION 
     This and other needs are met by a method and apparatus for quantizing and run length encoding video data for transform based coders. The video data is separated into blocks of pixels. The pixel values are transformed to another set of values which can be represented with less data. The transformed values are quantized by generating a quantized magnitude and sign for multiple transformed values at a time, while removing branch misprediction errors during the quantizing process. The quantized values are run length encoded while removing branch misprediction errors during the run length encoding process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a partial compression circuit for use with a transform-based video coder in accordance with an embodiment of the invention. 
     FIG. 2 is a diagram of zig-zag pattern in accordance with an embodiment of the invention. 
     FIG. 3 illustrates a block flow diagram of a conventional quantization and run length encoding scheme. 
     FIG. 4 illustrates a block flow diagram of a run length encoding scheme used in accordance with an embodiment of the invention. 
     FIG. 5 illustrates a block flow diagram of a run length encoding scheme used in accordance with another embodiment of the invention. 
     FIG. 6 illustrates a block flow diagram of a quantizing scheme used in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now in detail to the drawings wherein like parts are designated by like reference numerals throughout, there is illustrated in FIG. 1 a block diagram of a partial compression circuit for use with a transform-based video coder in accordance with an embodiment of the invention. 
     As shown in FIG. 1, discrete cosine transform (DCT) circuit  22  has a digital image data input  20  and a DCT coefficient output  24 . Quantizer  28  has a quantizer input  26  which is coupled to DCT coefficient output  24  for receiving DCT coefficients from DCT circuit  22 , and a quantization table input  27  for receiving quantization information from quantization table  30 . Quantizer  28  also has a quantizer output  34 . A run length encoding circuit  38  has a run length encoding input  36  coupled to quantizer output  34  for receiving quantized DCT coefficients, and a run length encoding output  40 . 
     In operation, DCT circuit  22  receives a block of video data through digital image data input  20 . The block is an 8×8 block of pixels (or pixel differences) from a video frame or picture, although any size block falls within the scope of the invention. DCT circuit  22  transforms the values for the 8×8 block of pixels into another set of values which can be transmitted with less data. More particularly, DCT circuit  22  transforms the values for the block to 16-bit DCT coefficients. The DCT coefficients represent the original pixel values (or pixel differences) in the frequency domain. The signal power for the resultant block is concentrated in specific frequency components. 
     Quantizer  28  receives the DCT coefficients for the block through quantizer input  26 . Quantizer  28  quantizes the received DCT coefficients according to quantization table  30  supplied by the application. Run length encoding circuit  38  receives the quantized DCT coefficients through run length encoding input  36 . Circuit  38  linearizes the quantized DCT coefficients according to a zig-zag pattern shown in FIG.  2 . 
     FIG. 2 is a diagram of zig-zag pattern used in accordance with an embodiment of the invention. As shown in FIG. 2, the quantized DCT coefficients are stored in a DCT coefficient matrix. A zig-zag pattern traverses the DCT coefficient matrix from the lowest frequency DCT coefficient (i.e., the DCT DC coefficient) which is at the upper left corner of the matrix, to the highest frequency DCT coefficient which is at the lower right corner. 
     In addition to linearizing the quantized DCT coefficients according to the zig-zag pattern shown in FIG. 2, run length encoding circuit  38  run length encodes the quantized DCT coefficients. Circuit  38  run length encodes the quantized DCT coefficients by generating a sequence of triples, where each triple consists of a run value representing the number of consecutive quantized DCT coefficient having a value of zero, a coefficient value representing a quantized DCT coefficient having a non-zero value, and a sign for the non-zero value. 
     Run length encoding circuit  38  takes advantage of the fact that, for typical video images, after DCT transformation and quantization, there are few non-zero quantized DCT coefficients, and they are usually distributed in the upper left corner of the (8×8) block of FIG. 2 (i.e., they tend to be low frequency). The result is run-length encoding that produces a small number of triples of run values, coefficient values, and signs, with a long run of quantized DCT coefficients (or quantized DCT coefficient differences) having a value of zero at the end of the block. This final string of zeros can be identified by any conventional technique, such as an end of block (EOB) marker. 
     FIG. 3 illustrates a block flow diagram of a conventional quantization and run length encoding algorithm. A DCT coefficient i is selected at step  50 . At step  52 , coefficient i is divided by a number (QUANTIZER) to produce RESULT. At step  54 , RESULT is tested. If RESULT=0, then a run length counter (RLC) is incremented by 1 at step  56 . At step  58 , the algorithm checks if all coefficients for a block have been tested. If so, coding continues according to the particular compression algorithm. For example, in a block motion compensated transform based compression algorithm, the run length encoded values are further encoded using entropy encoding. If not, the next coefficient i is selected and run length encoded. 
     If at step  54  RESULT is not equal to zero, RLC is outputted at step  60 , RESULT is outputted at step  62 , the sign of RESULT (SIGN) is outputted at step  64 , and the output pointer (OUTP) for the run length encoding array storing the outputted values is incremented by three to prepare for outputting the next set of triples. The sign of each coefficient is used during the entropy encoding phase of a video coder, which is not described herein. 
     For an 8×8 block of pixels, the run length encoding scheme described with reference to FIG. 3 executes 64 branches. This leads to branch misprediction errors which increases the number of CPU cycles required to process the algorithm. Thus, a run length encoding scheme which removes the need for branching decisions would remove the possibility of branch misprediction errors, and would result in a concomitant savings in CPU cycles. An embodiment of the invention illustrating this type of run length encoding scheme is discussed in detail with reference to FIG.  4 . 
     Run Length Encoding 
     FIG. 4 illustrates a block flow diagram of a run length encoding scheme used in accordance with an embodiment of the invention. As shown in FIG. 4, each quantized coefficient is processed one at a time using steps  70 ,  72 ,  74 ,  76 , and  78 . For each coefficient i, a sequence of triples are written out to a run length encoding array, wherein each triple consists of a value for RLC, RESULT and SIGN, at steps  72 ,  74 , and  76 , respectively. 
     At step  78 , OUTP for the run length encoding array is assigned a value where OUTP=OUTP+Table 1  [RESULT]. Table 1  is an array of values, wherein the zero address of Table 1  contains a value of zero, and the remaining addresses contain a value of three. Thus, if RESULT is zero, OUTP is incremented by the value stored in the zero address of Table 1 , which is zero. If RESULT is any other number, OUTP is incremented by the value stored in the address corresponding to the other number of Table 1 , which will always contain the value of three. Consequently, if the quantized coefficient is a zero, OUTP is not incremented, and RLC, RESULT and SIGN are written out to the same addresses in the run length encoding array as before. If the quantized coefficient is a non-zero value, however, OUTP is incremented by three, which moves the OUTP pointer to where the next set of triples is to be written to in the run length coding array. 
     To ensure that a run of zeros is properly accounted for by RLC without the use of conditional branching statements, RLC uses a scheme similar to that used for incrementing OUTP. At step  80 , RLC is assigned a value where RLC=RLC+Table 2  [RESULT]. Table 2  is an array of values, wherein the zero address of Table 2  contains a value of one, and the remaining addresses contain a value of zero. Thus, if RESULT is zero, RLC is incremented by the value stored in the zero address of Table 2 , which is one. If RESULT is any other number, RLC is incremented by the value stored in the address corresponding to the other number of Table 2 , which will always contain the value of zero. Consequently, if the quantized coefficient is a zero, RLC is incremented. If the quantized coefficient is a non-zero value, however, RLC is not incremented, and RLC is reinitialized to zero at step  82 . 
     Step  82  also utilizes a table referred to as Table 3 . As shown in FIG. 4, RLC is incremented using a bit wise AND function and Table 3 . Table 3  is an array of values, wherein the zero address of Table 3  contains a value of all ones, and the remaining addresses contain a value of zero. Thus, if RESULT is zero, RLC is bit wise AND&#39;d using the value stored in the zero address of Table 3 , which is all ones, thus not changing the value of RLC. If RESULT is any other number, RLC is bit wise AND&#39;d using the value stored in the address corresponding to the other number of Table 3 , which will always contain the value of zero, thereby reinitializing RLC to zero. Coding for all coefficients continues at step  84 . 
     FIG. 5 illustrates a block flow diagram of a run length encoding scheme used in accordance with another embodiment of the invention. This embodiment also run length encodes transform coefficients without using a conditional branching statement, or tables shown in the embodiment discussed with reference to FIG.  4 . As with FIG. 4, Steps  70 ,  72 ,  74 ,  76  and  84  of FIG. 5 remain the same. OUTP and RLC, however, are updated without using tables, as shown in steps  86 ,  88 ,  90 ,  92  and  94 . 
     At step  86 , RESULT=RESULT−1. If RESULT is equal to zero, the borrow bit for the processor is set to one. If RESULT is a non-zero value, the borrow bit is set to zero. 
     At step  88 , RLC is incremented by the value of the borrow bit. Thus, if RESULT is zero, the borrow bit for the processor is set to one, which means RLC is incremented by one. If RESULT is non-zero, the borrow bit for the processor is set to zero, which means RLC is incremented by zero. 
     At step  90 , RLC is updated using a temporary holding value TEMP 1 . TEMP 1  equals TEMP 1  minus TEMP 1  minus the borrow bit. Thus, if the borrow bit is zero, which means RESULT is non-zero, TEMP 1  equals zero. If the borrow bit is one, which means RESULT is zero, TEMP 1  equals negative one. 
     At step  92 , OUTP equals OUTP plus three times TEMP 1  plus three. Thus, if RESULT is zero, TEMP 1  is negative one, which means OUTP is not incremented. If RESULT is non-zero, TEMP 1  is zero, which means OUTP is incremented by three. 
     At step  94 , RLC is reinitialized to zero if RESULT is non-zero. RLC is equal to RLC bit wise AND&#39;d with TEMP 1 . Thus, if RESULT is zero, TEMP 1  is negative one, which means RLC remains unchanged. If RESULT is non-zero, TEMP 1  is zero, which means RLC is assigned the value of zero. 
     Quantizing 
     Conventional quantizing algorithms suffer from two problems. First, these algorithms quantize only one coefficient at a time. Second, these algorithms quantize a coefficient and a sign value for the coefficient in two separate steps, thus requiring the use of a conditional branching statement. 
     By way of contrast, an embodiment of the invention uses a quantizing algorithm which quantizes a plurality of coefficients at a time. This embodiment quantizes a set of coefficients at a time, with each set of coefficients containing between 2 to 8 coefficients, with an advantageous set containing 4 coefficients. This results in the processor running the quantization algorithm having to use fewer processing cycles. This is accomplished using an instruction provided by the MMX™ instruction set. 
     In addition, this embodiment quantizes the sign for each coefficient within a set of coefficients at the same time each coefficient is quantized. This removes the need for using conditional branching statements, which removes the possibility of branch misprediction penalties. 
     FIG. 6 illustrates a block flow diagram of a quantizing scheme used in accordance with an embodiment of the invention. As shown in step  96 , SIGN is equal to the DCT coefficient shifted right by the number of bits in the coefficient minus one. SIGN is then exclusive-OR&#39;d with the coefficient, and the resulting value is assigned to TEMP 1  at step  98 . Thus, if SIGN is negative, TEMP 1  is assigned the one&#39;s complement of the coefficient. IF SIGN is positive, TEMP 1  is simply assigned the value of the coefficient. At step  100 , the value ABSCOEFF is derived by subtracting SIGN from TEMP 1 . Again, if SIGN is positive, ABSCOEFF is simply assigned the coefficient. If SIGN is negative, however, ABSCOEFF is assigned the one&#39;s complement of the coefficient plus one (i.e., ABSCOEFF equals two&#39;s complement of the coefficient). At step  102 , the quantized coefficient is generated by dividing ABSCOEFF by a quantizing value (QUANTIZER). 
     Accordingly, the quantizing algorithm described with reference to FIG. 6 generates a quantized coefficient and a sign value for each quantized coefficient without using any conditional branching statements, thereby removing the possibility of any branch misprediction errors. Moreover, this embodiment of the invention quantizes multiple coefficients at a time, thus reducing the overall number of calculations required for a block, thereby freeing processing cycles for other coding tasks. 
     CONCLUSION 
     A video coder using the run length encoding algorithm described with reference to FIGS. 4 and 5, and the quantizing algorithm described with reference to FIG. 6, would result in significant savings over the prior art in terms of the number of CPU cycles required to implement each device. It is estimated that a video coder using a Pentium™ processor can save approximately 3 processing cycles per coefficient, and a video coder using a Pentium Pro™ processor can save approximately 10-30 processing cycles per coefficient. These extra cycles could be used to process a higher number of frames, thereby increasing the viewing quality of a compressed video signal. 
     Although this embodiment of the invention is illustrated as a series of steps using pseudo-code, it can be appreciated that person of ordinary skill in the art could implement this embodiment in either software or hardware. This embodiment of the invention assumes a software based video coder implemented on a personal computer, which at a minimum has a CPU, memory, input device, and output device. It is further assumed for illustrative purposes that the CPU is a Pentium™, Pentium Pro™ or MMX™ microprocessor, using standard operating software for use with these microprocessors. 
     Although embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, although an embodiment of the invention was illustrated using a transform coder using DCT, it can be appreciated that any transform coding technique falls within the scope of the invention. For another example, although various embodiments of the invention were illustrated using steps based in software, it can be appreciated that these embodiments can be implemented in hardware as well.