Abstract:
A method for video encoding is disclosed. The method generally includes the steps of (A) dividing a plurality of first macroblocks into at least two classes based on a plurality of first statistics, (B) measuring a respective number of bits used to encode the first macroblocks within each of the classes and (C) based on the measuring in step B, determining a quantization level in at least one of a plurality of second macroblocks that have yet to be encoded.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for video encoding generally and, more particularly, to video encoder rate control using bit usage measuring and allocating by macroblock classes. 
     BACKGROUND OF THE INVENTION 
     When lossy video compression is used, artifacts can be visible in decoded video. Use of high quantization values (i.e., coarse quantizations) reduces the bit usage but also makes quantization noise (i.e., coding noise) more visible. For a given quantization level, quantization noise is commonly more noticeable in smooth macroblocks and less noticeable in macroblocks with many pixel-to-pixel variations. 
     Referring to  FIG. 1 , a block diagram of a portion of a conventional video encoder  10  is shown. The conventional video encoder  10  (i.e., a compressor) can utilize conventional macroblock adaptive quantization (AQP) to adjust the quantization levels. A controller  12  of the encoder  10  makes measurements to determine a degree to which a macroblock will “show” quantization noise after being decoded. Typically, the measurements look at aspects of luminance sample variations and use quantization values that increase with the amount of the variations. A macroblock encoder  14  of the video encoder  10  then uses the quantization values to encode the macroblocks to generate a compressed bitstream. Another example of a conventional video encoder is a software encoder called Test Model 5. The Test Model 5 uses a block-variance based measure. 
     A linear quantization performed by a conventional encoder may be related to a quantization parameter. A linear quantization value (i.e., LINEAR_QUANT) is a number proportional to a quantization step size. In an H.264/AVC standard, a quantization parameter (i.e., QP) is an integer used to describe the quantization of a macroblock. The quantization parameter QP is related to the linear quantization by equation 1 as follows:
 
LINEAR_QUANT=2^(QP/6)  Eq. (1)
 
     Conventional adaptive quantization parameter (AQP) methods are based on an implicit model. The implicit models set the quantization parameter QP based on measured statistics to more or less equalize the visibility of quantization noise across all macroblocks. Because the models are not always accurate, the variations in minimum linear equalization to maximum linear quantization has to be limited. For example, a formula used in the Test Model 5 for AQP limits the ratio of maximum linear equalization to minimum linear quantization to 4:1 and, for normal video, the spread is typically much small (i.e., about 2:1). Even so, sequences exist where even a smaller spread in linear quantization results in better quality. Furthermore, sequences exist where a moderate spread in the linear quantization is not sufficient to provide adequate quality to very smooth macroblocks. For example, using just the measure and formulae from Test Model 5 produces a spread of 2:1, whereas a spread of 5:1 truly makes the smooth macroblocks look good. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a method for video encoding. The method generally comprises the steps of (A) dividing a plurality of first macroblocks into at least two classes based on a plurality of first statistics, (B) measuring a respective number of bits used to encode the first macroblocks within each of the classes and (C) based on the measuring in step B, determining a quantization level in at least one of a plurality of second macroblocks that have yet to be encoded. 
     The objects, features and advantages of the present invention include providing video encoder rate control using bit usage measuring and allocating by macroblock classes that may (i) allocate macroblocks among multiple classes, (ii) determine a quantization parameter per class, (iii) intelligently decide when the quantization parameter should be lowered significantly for smooth macroblocks, (iv) intelligently decide when the quantization parameters should be lowered modestly for smooth macroblocks and/or (v) provide a robust adaptive quantization parameter technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
         FIG. 1  is a block diagram of a portion of a conventional video encoder; 
         FIG. 2  is a block diagram of a portion of a circuit in accordance with a preferred embodiment of the present invention; 
         FIG. 3  is a set of equations; 
         FIG. 4  is a flow diagram of an example process performed before encoding the new picture; and 
         FIG. 5  is a flow diagram of an example process performed during and after encoding the new picture. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 2 , a block diagram of a portion of a circuit  100  is shown in accordance with a preferred embodiment of the present invention. The circuit (or system)  100  may be referred to as an encoder circuit. The encoder circuit  100  generally comprises a module (or block)  102 , a module (or block)  104  and a module (or block)  106 . A signal (e.g., IN) may be received by the module  102 . A signal (e.g., STAT) may be generated by the module  102  and presented to the module  104 . A signal (e.g., INT) may be generated by the module  102  and presented to the module  106 . The module  104  may generate a signal (e.g., MODE) presented to the module  106 . The module  106  may generate and present a signal (e.g., BITS) to the module  104 . A signal (e.g., OUT) may be generated by the module  106 . 
     The encoder circuit  100  may be compliant with an H.264/AVC standard. The H.264/AVC specification (ISO/IEC 14496-10 AVC) is published by the International Telecommunication Union-Telecommunication Standardization Sector, Geneva, Switzerland. In other embodiments, the encoder circuit  100  may be compliant with other standards that have a variable quantization capability. Suitable standards include, but are not limited to H.261 and H.263. 
     The signal IN may be an input signal to the encoder circuit  100 . The signal IN generally carries pictures (fields or frames) of unencoded digitized video. The pictures may be received by the module  102  in a normal display order sequence. 
     The signal STAT may be an internal signal to the encoder circuit  100 . The signal STAT may carry statistical information regarding the spatial activity of the unencoded macroblocks making up the unencoded pictures received via the signal IN. The statistical information may include an average of activities for a picture and a number of macroblocks in each class. The activity may be computed as a weighted sum of an absolute value of Haddamard transform coefficients. The activities may be used as a previous activity value (e.g., PREV_ACT) in encoding a future picture. The number of macroblocks in each class may be used to compute a count value of macroblocks in each class (e.g., ACT_CNT). The module  104  may assign quantization levels based on the statistics. 
     The signal INT may be an internal signal to the encoder circuit  100 . The signal INT generally comprises the unencoded pictures and class information. The unencoded pictures in the signal INT may be identical to the unencoded pictures received in the signal IN. The class information may identify a class for each macroblock in each unencoded picture. 
     The signal BITS may be an internal signal to the encoder circuit  100 . The signal BITS generally carries historical information about the number of bits used to encode previously-encoded macroblocks in previously-encoded pictures. The number of bits may be arranged per class. 
     The signal MODE may be an internal signal to the encoder circuit  100 . The signal MODE may convey information about what type of picture (e.g., Intra, Predicted or Bidirectional) and what quantization levels should be used for encoding yet-to-be-encoded macroblocks in each class. 
     The signal OUT may be an output signal from the module  106  and an internal signal relative to the encoder circuit  100 . The signal OUT may undergo additional processing in the encoder circuit  100  after leaving the module  106 . The signal OUT generally comprises a compressed bitstream of the pictures received in the signal IN. The compressed pictures in the signal OUT may vary in sequence from the normal display order sequence. 
     The module  102  may be referred to as a macroblock classifier module. The macroblock classifier module  102  may be operational to examine unencoded macroblocks received in the signal IN. Based on the examination, the macroblock classifier module  102  may allocate the unencoded macroblocks among two or more different classes (e.g., very smooth, smooth, textured, rough). The macroblock classifier module  102  may also be operational to generate statistical information about the unencoded pictures. The statistical information may be presented in the signal STAT. 
     The module  104  may be referred to as a controller module. The controller module  104  may be operational to generally control the encoding functionality of the encoder circuit  100 . The controller module  104  may use (i) the statistical information of an incoming unencoded picture in the signal STAT and (ii) historical statistical information for one or more previously encoded pictures received in the signal BITS to determine how the unencoded macroblocks in the signal INT should be processed. Once the controller module  104  has determined how to encode each macroblock class, the controller module  104  may pass the information (e.g., picture type and quantization levels) along to the module  106  in the signal MODE. 
     The module  106  may be referred to as a macroblock encoder module. The macroblock encoder module  106  may be operational to encode the unencoded macroblocks received in the signal INT. The encodings may be controlled by the picture type information and the class-specific quantization level information received from the controller module  104  in the signal MODE. The encoded macroblock may be presented in the signal OUT as part of a compressed bitstream. 
     The macroblock encoder module  106  generally measures and/or estimates a number of bits used by the previously-encoded macroblocks in each of the different AQP classes (or categories). The number of bits may be used to determine how to set future quantization levels between the different classes of unencoded macroblocks. Specifically, the encoder circuit  100  may perform several measurements and allocations to determine the future quantization levels. 1) The macroblock classifier module  102  may make several measurements to classify each unencoded macroblock as belonging to one of a number of classes. The classification, generally separates the unencoded macroblocks into different AQP classes (e.g., different macroblocks with similar sensitivity to quantization noise and/or using about the same quantization for a certain visual quality may be put in the same class). 2) Measurements and/or estimations may be made by the macroblock encoder module  106  for the number of bits, historically used in each class. 3) Based on the measurements from step 2, an allocation of bits may be performed by controller module  104  among the classes for yet-to-be-encoded pictures. (4) Based on the allocations from step 3, a QP value may be set by the controller module  104  for each class for the yet-to-be-encoded pictures. 
     The following definitions and equations may be used in calculating the quantization levels for each class. Most of the equations may be found in  FIG. 3 . A complexity value (e.g., COMPLEXITY) may be defined by equation 2 as follows:
 
COMPLEXITY=LINEAR_QUANT×bits used  Eq. (2)
 
     An average macroblock complexity (e.g., MBCOMPLEXITY[type] [class]) may be a function of picture type and class (category or set). Picture types generally include I (e.g., Intra coded), P (e.g., Predicted) and B (e.g., Bidirectional predicted). Class is generally a grouping in which the unencoded macroblock is placed for AQP purposes. In some embodiments, four classes may be implemented for each of the three picture types resulting in twelve macroblock classes. Other numbers of picture types and other numbers of macroblock classes (categories or sets) may be implemented to meet the criteria of a particular application. 
     A linear quantization multiplier (e.g., QMUL[type]) may be a function of the picture type. The targeted linear quantization level in a picture of type “type” is generally proportional to QMUL[type]. Typically, QMUL[I]&lt;QMUL[P]&lt;QMUL[B], because using a lowest-quantization in I pictures and a highest quantization in B pictures generally yields a best quality. In various embodiments, the values of QMUL[x] may be determined by observing the quality in test video sequences. In some embodiments, the values of QMUL[x] may vary over time. 
     A number of activity sets (e.g., classes/categories) value (e.g., NUM_ACT_SETS) generally identifies an actual number of classes per picture type used for AQP purposes. In some embodiments, the value of NUM_ACT_SETS may be four. Larger or smaller numbers of classes may be implemented to meet the criteria of a particular application. 
     The count of macroblocks in each spatial activity class value ACT_CNT[class] generally defines a number of macroblocks in each class. The value of ACT_CNT[class] may be set equal to the number of macroblocks in a previously encoded picture that was categorized as being in a class “class”. 
     Macroblocks are generally classified by the macroblock classifier module  102  based on a measure of spatial activity. In some embodiments, four classes may be defined for the unencoded macroblocks. The activity measure for a common (e.g., 16×16) macroblock may be computed as follows. 1) Use a 16×16 Intra DC prediction to predict luminance samples of the macroblock. An example of the Intra DC prediction may be found in the H.264/AVC specification. 2) Compute a Haddamard transform of a prediction difference for determining if the Intra DC prediction should be used. An example of the Haddamard transform may be found in the JVT encoder software version 7.3 available from http://iphome.hhi.de/suehring/tml/download/old_jm/. 3) Compute an activity value (e.g., ACTIVITY) for the macroblock as a weighted sum of absolute transform coefficients. 
     Once the activity value of the macroblock has been calculated, the macroblock may be classified based on (i) previous activity of one or more frames that have already been encoded and (ii) a set of thresholds. For example, let the previous activity value PREV_ACT be the average activity of a previous frame that has already been encoded and is closest in time (of all encoded frames) to a new frame being encoded. The class of a new macroblock in the new frame may be determined by: 
     If (ACTIVITY&lt;PREV_ACT×M 0 ), then class=0 
     Else if (ACTIVITY&lt;PREV_ACT×M 1 ), then class=1 
     Else if (ACTIVITY&lt;PREV_ACT×M 2 ), then class=2 
     Else class=3 
     Where each of the thresholds M 0 , M 1  and M 2  may be constants of increasing value (e.g., M 0 &lt;M 1 &lt;M 2 ). As such, each class may have a unique range of activity values. 
     Referring to  FIG. 4 , a flow diagram of an example process  120  performed before encoding the new picture is shown. The process (or method)  120  generally comprises a step (or block)  122 , a step (or block)  124 , a step (or block)  126 , a step (or block)  128 , a step (or block)  130 , a step (or block)  132 , a step (or block)  134  and a step (or block)  136 . The process  120  may be performed between the controller module  104  and the macroblock encoder module  106 . 
     The process  120  may begin in the step  122  with the macroblock encoder module  106  computing a coding rate value (e.g., CODING_RATE). The value CODING_RATE may be determined by modifying a target bit rate based on having under-utilized bits or over-utilized bits in previously encoded pictures. For example, if (i) the average bit rate (in bits/second) has a predetermined value (e.g., BIT_RATE), (ii) a picture rate (in pictures/second) has a value (e.g., PICTURE_RATE), and (iii) B bits were used to encode N pictures, the value CODING_RATE may be computed per equation 3 in  FIG. 3 . If the encoder circuit  100  has overproduced bits, CODING_RATE&lt;BIT_RATE. If the encoder circuit  100  has under-produced bits, CODING_RATE&gt;BIT_RATE. The step  122  may feed back an indication of bit over production or bit under production to achieve a correct long-term rate. 
     In the step  124 , a reserved fraction value (e.g., RES_FRAC) may be set to a predetermined value stored in the controller module  104 . The value RES_FRAC may be a fraction of a total number of bits allocatable among the classes without regard to complexity. The value RES_FRAC generally ranges from zero (e.g., no AQP) to one (e.g., bit allocation is proportional to a number of macroblocks in each class). In various embodiments, the value of RES_FRAC may be set to approximately 0.5 (e.g., a mixed approach to AQP). 
     For each class and type, a scaled complexity per second value (e.g., SCALED_TYPE_COMPLEXITY_SEC) may be computed by the controller module  104  in the step  126 . The value SCALED_TYPE_COMPLEXITY_SEC may be the average macroblock complexity times a frequency of the picture type per second times a number of macroblocks in the class divided by the linear quantization multiplier. Equation 4 in  FIG. 3  generally shows how to calculate the value SCALED_TYPE_COMPLEXITY_SEC as a function of both picture type and a macroblock class. 
     For each class, a scaled complexity per second value (e.g., SCALED_CLASS_COMPLEXITY_SEC) may be computed by the controller module  104  in the step  128 . The value SCALED_CLASS_COMPLEXITY_SEC may be a summation over all picture type values of the scaled complexity per second values. Equation 5 in  FIG. 3  generally shows how to calculate the scaled complexity per second value. 
     A total scaled complexity per second value (e.g., SCALED_COMPLEXITY_SEC) may be computed by the controller module  104  in the step  130 . The value SCALED_COMPLEXITY_SEC may be a summation of all of the class scaled complexity per second value over all classes. Equation 6 in  FIG. 3  generally shows how to calculate the scaled complexity per second value. 
     In the step  132 , the controller module  104  may allocate the reserved fraction of bits to each class. The allocation (e.g., BITS_PRESEC_CLASS) to any particular class may be in proportion to the number of macroblocks in the particular class. The remaining bits may be allocated to the classes in proportion to the activity in each class. A calculation for the allocation may be provided by equation 7 in  FIG. 3 . 
     The controller module  104  generally sets a linear quantization value (e.g., LINEAR_QUANT) for each class for the upcoming picture in the step  134 . The linear quantization as a function of class may be the scaled class complexity times the picture type class multiplier divided by the class bit rate. Equation 8 in  FIG. 3  generally shows how to calculate the values LINEAR_QUANT. 
     The value QP for each class may be computed as a function of the class by the controller module  104  in the step  136 . The value QP is generally set as several (e.g., 6) times a base 2 logarithm of the linear quantization for the class. Equation 9 in  FIG. 3  generally shows how to calculate the value QP. 
     To understand why the process  100  works, consider the following three cases. For a case 0 (e.g., RES_FRAC=0), the bits per class may be allocated in proportion to class complexity. Since class complexity may be proportional to linear quantization and an upcoming linear quantization may be proportional to complexity, the upcoming linear quantization will generally be the same for all classes. Therefore, the upcoming values QP may be the same for all classes. For case 0, the encoder system  100  generally does not perform any adaptive quantization. 
     For a case 1 (e.g., RES_FRAC=1), the bits per class may be allocated in proportion to the number of macroblocks in the class. If the lower classes (e.g., lower activity classes) may be “easier” to encode (e.g., use fewer bits for a given quantization level), the lower classes generally use a finer quantization (e.g., a smaller linear quantization) and therefore smaller values for QP. 
     Case 2 (e.g., 0&lt;RES_FRAC&lt;1) is generally somewhere in-between case 0 and case 1 (e.g., RES_FRAC≈0.5). Therefore, if the lower classes are easier to encode, the lower classes may use lower values of QP. However, the lower values of QP in case 2 are generally not as low as the values of QP in case 1. For both case 1 and case 2, when the lower classes are much easier to encode than the higher classes, the lower classes may receive a much lower value of QP than the higher classes. 
     Case 2 generally reallocates (e.g., takes away) at most half of the bits from any class. As a result, the value QP in any class losing bits may be increased by at most 6 (e.g., double linear quantization). Thus, the process  120  generally has (i) an unlimited lowering potential for lowering quantization in the “easy” classes and (ii) a firm limit on how the value QP may be increased in the “hard” classes. A conventional encoder (e.g., Test Model 5) does not have the behavior of the class 2. 
     In general, the value QP in the easy classes may be lowered by a large amount when the easy classes may be “very easy” to encode. Lowering the value QP a significant amount for the very easy macroblocks is generally beneficial if (i) a cost to the hard macroblocks is not high and (ii) the lower classes normally do not draw a reasonable number of bits with modest values of QP. Failure to draw a reasonable number of bits may result in many macroblocks in the lower classes with no coded transform coefficients. Such macroblocks generally have very noticeable quantization noise. The quantization noise may appear in the form of blockiness and/or a too smooth appearance. 
     Referring to  FIG. 5 , a flow diagram of an example process  140  performed during and after encoding the new picture is shown. The process (or method)  140  generally comprises a step (or block)  142 , a step (or block)  144 , a step (or block)  146  and a step (or block)  148 . 
     While encoding a new picture, each macroblock is generally classified by the macroblock classifier module  102  in the step  142 . The classification of each new macroblock may be into one of the NUM_ACT_SETS classes (e.g., NUM_ACT_SETS=4 in various embodiments). Classification may be based on measured statistics for the new macroblocks. The value QP used for each new macroblock may be subsequently set to a class value QP calculated earlier while encoding one or more previous pictures. 
     The following statistics may be computed by the encoder module  106  while encoding a new picture. An average of activity statistic may be used as the previous activity value PREV_ACT for picture(s) to be encoded in the future. A bits-used-by-class statistic may be used to compute MBCOMPLEXITY[type] [class], as described above. A number of macroblocks in each class statistic may be used for computing MBCOMPLEXITY[type] [class]. The number of macroblocks in each class statistic may also be used in other steps before encoding the next picture. In a first embodiment, the actual number of bits by class may be counted and used for the statistic (e.g., ACT_CNT[class]). In a second embodiment, an approximation of the number of bits by class may be generated as the statistic. 
     After a new picture is encoded, the macroblock complexity by class and picture type may be recalculated for use in encoding subsequent new, yet-to-be-encoded pictures. In one embodiment, the calculation of the value MBCOMPLEXITY may be determined by equation 10 in  FIG. 3 . In another embodiment, an estimation of a number of bits per class may be determined by equation 11 in  FIG. 3 . 
     Assuming that relationships between the values QP, the bits used per class and the picture type are only slowly changing, control may be exercised over a degree to which using lower QP values for more “sensitive” macroblocks cause an excessive amount of bits to be consumed by the sensitive macroblocks. The control may be based on how many bits are generally used in each class. The number of bits used in each class may be measured and/or estimated. For example, if the classification step separates smooth macroblocks (e.g., macroblocks that get a lower value of QP) from spatially more complex macroblocks (e.g., macroblocks that get a higher value of QP), the smooth macroblocks may not utilize a high percentage of the overall number of available bits. Since the smooth macroblock classes do not consume many available bits, a big drop in the value QP for the smooth macroblocks may not be expensive. In particular, the smooth macroblock classes may not take a large absolute percentage of the total bit rate. Therefore, the encoder system  100  generally allows the smooth macroblock classes to have low QP values. 
     In cases where a low QP for smooth macroblocks may consume a large percentage of the total bit rate, the encoder circuit  100  may limit the degree to which the smooth macroblocks get the lower QP. Because the encoder circuit  100  generally measures bits by class, the encoder circuit  100  may intelligently decide when the value QP should be lowered (i) by a lot or (ii) by a little for the smooth macroblocks. 
     In situations where the QP values for the not-smooth macroblocks do not significantly increase due to a large decrease in QP values for the smooth macroblocks, a big QP difference between smooth macroblock classes and not-smooth macroblock classes may be implemented by the encoder system  100 . In situations where the QP values for the not-smooth macroblocks increase significantly due to a large decrease in the QP values for the smooth macroblocks, a big QP difference between the smooth macroblock classes and the not-smooth macroblock classes are generally not implemented by the encoder circuit  100 . The result may be an AQP method that is more robust than conventional methods. 
     The function performed by the flow diagrams of  FIGS. 4 and 5  may be implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant art(s). 
     The present invention may also be implemented by the preparation of ASICs, FPGAs, or by interconnecting an appropriate network of conventional component circuits, as is described herein, modifications of which will be readily apparent to those skilled in the art(s). 
     The present invention thus may also include a compute product which may be a storage medium including instructions which can be used to program a computer to perform a process in accordance with the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disk, optical disk, CD-ROM, magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, Flash memory, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.