# Patent

## Title
System and method for frame level bit rate control without pre-analysis

## Abstract

Methods and systems to determine a quantization parameter (QP) that may then be used in a bit rate control process during video compression and decompression. The QP for a frame may be derived by determining an initial QP, adapting this value, and applying a final control calculation. The QP for a frame may be calculated from a variety of variables, that may include the frame type (intracoded (I), predictive (P), or bipredictive (B)), the fullness of a hypothetical reference decoder (HRD) buffer, the instant and target bit rates, a total encode size, a total target size, the number of P frames between I frames in a group of pictures (GOP), the number of B frames between I frames in the GOP, and the size of previous frames.

## Background
BACKGROUND
The transmission and storage of digitized video may require considerable bandwidth. The bandwidth requirements may be such that video cannot be transmitted in real time, for example, resulting in a poor user experience.
One way in which this problem may be addressed is through the compression of a video signal. This results in fewer bits per frame having to be transmitted, and therefore allows the transmission of a greater number of frames per unit time given a channel of limited capacity. There are several disadvantages that may come with video compression, however. One disadvantage is that the quality of the video received may be poor. Compression may result in less information being transmitted. The result is that the image, as reconstructed for the user, may be inferior.
As a result, there is an historical trade-off between video quality and the extent of video compression applied to a signal. Greater compression lowers the bandwidth requirement and allows a lower bit rate, but may result in a poor experience for the user. Less compression requires more bandwidth and a higher bit rate, but the user's viewing experience may be better.
The extent of video compression may be controlled by a bit rate control algorithm. Such an algorithm affects the amount of compression performed and therefore affects the trade-off between bandwidth consumption and video quality. Typically, a quantization parameter (QP) drives the extent of video compression in a bit rate control algorithm.



BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
FIG. 1 is a flowchart illustrating the overall processing of an embodiment.
FIG. 2 is a flowchart illustrating the processing of a frame encoded according to the Advanced Video Coding standard (AVC), according to an embodiment.
FIG. 3 is a flowchart illustrating the determination of QP_deltahrd—buffer, according to an embodiment.
FIG. 4 is a flowchart illustrating the determination of QP_deltalocal—rate—ratio, according to an embodiment.
FIG. 5 is a flowchart illustrating the determination of QP_deltaglobal—rate—ratio, according to an embodiment.
FIG. 6 is a flowchart illustrating the processing of a frame encoded according to the MPEG2 standard, according to an embodiment.
FIG. 7 is a flowchart illustrating the determination of QP_deltahrd—buffer, according to an embodiment.
FIG. 8 is a flowchart illustrating the determination of QP_deltalocal—rate—ratio, according to an embodiment.
FIG. 9 is a block diagram illustrating the implementation of a software or firmware embodiment.



In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears.
DETAILED DESCRIPTION
An embodiment is now described with reference to the figures, where like reference numbers indicate identical or functionally similar elements. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the description. It will be apparent to a person skilled in the relevant art that this can also be employed in a variety of other systems and applications other than what is described herein.
Disclosed herein are methods and systems to determine a quantization parameter (QP) that may then be used in a bit rate control process during video compression. The QP for a frame may be derived by determining an initial QP, adapting this value and, in an embodiment, applying a final control process. The QP for a frame may be determined from a variety of variables; these may include the frame type (intracoded (I), predictive (P), or bipredictive (B)), the fullness of a hypothetical reference decoder (HRD) buffer, the instant and target bit rates, a total encode size, a total target size, the number of P frames between I frames in a group of pictures (GOP), the number of B frames between I frames in the GOP, and the size of previous frames.
An embodiment of the overall process is illustrated in FIG. 1. At 110, and initial quantization parameter QPinit may be calculated for a frame, based on the frame type and the previous quantization parameter. At 120, and adaptation of QPinit may be performed to create a value QPadapt, based on HRD buffer fullness and on rate ratios that will be described in greater detail below. At 130, a final quantization parameter QPfinal final may be derived by applying a control process to QPadapt.
An embodiment of the process as performed for a frame encoded according to the Advanced Video Coding standard (AVC) is illustrated in FIG. 2. At 210, QPinit may be determined as a sum of terms,

QPinit=alpha*QPprevious—I—frame+beta*QPprevious—P—frame+gamma*QPprevious—B—frame 

Here, the values QPprevious—I—frame, QPprevious—P—frame, and QPprevious—B—frame refer to quantization parameters for the previous I, P, and B frames respectively. The coefficients alpha, beta, and gamma may have the properties that each is greater than or equal to 0, and they collectively sum to 1.

In the illustrated embodiment, the QP adaptation 120 is shown as sequence 220-250. At 220, a value QP_deltahrd—buffer may be determined. This represents a value that may be added to QPinit, and that depends on the fullness of the HRD buffer. For greater fullness of this buffer, QPinit may be increased by applying a higher value of QP_deltahrd—buffer. Given less fullness, QP_deltahrd—buffer may be given a lesser value, or even a negative value.
At 230, a value for QP_deltalocal—rate—ratio may be determined. This value depends on a local rate ratio, which is the ratio of the instant bit rate (calculated in a manner to be described below, according to an embodiment) to a target bit rate. For higher local rate ratios, the value for QP_deltalocal—rate—ratio may be greater; for lower local rate ratios, the value for QP_deltalocal—rate—ratio may be lower.
Note however that, if fullness of the HRD buffer is high and the local rate ratio is high, their combined effect on QPinit may be minimal. Likewise, if fullness of the HRD buffer is low and the local rate ratio is low, their combined effect on QPinit may be minimal.
At 240, a value for QP_deltaglobal—rate—ratio may be determined. This value depends on a global rate ratio, which is the ratio of a total encode size to a total target size. For higher global rate ratios, the value for QP_deltaglobal—rate—ratio may be greater; for lower global rate ratios, the value for QP_deltaglobal—rate—ratio may be lower.
At 250, QPadapt may be calculated as the sum

QPadapt=QPinit+QP_deltahrd—buffer+QP_deltalocal—rate—ratio+QP_deltaglobal—rate—ratio 

In the illustrated embodiment, the determination of QPcontrol is shown as the sequence 260-270. At 260, QPfinal may be calculated as the sum of QPadapt and a value QPcontrol (frame type, QPadapt). In an embodiment, this latter value may be a relatively small value, between −1 and +1, that may be added to QPadapt if the frame is a reference frame. This serves to make the weight of a reference frame greater with respect to non-reference frames. Generally, higher values for QPcontrol (frame type, QPadapt) may be used for higher bit rates. The appropriate value for QPcontrol (frame type, QPadapt) may be decided empirically in an embodiment.
At 270, a clipping process may be performed in an embodiment. Here, if QPfinal is less than QPmax then QPfinal may be set to QPmin. If QPfinal is greater than QPmax then QPfinal may be set to QPmax. QPfinal may be unchanged otherwise. In an embodiment, the values for QPmax and QPmin may be supplied by the user.
FIG. 3 illustrates the determination of QP_deltahrd—buffer (220 of FIG. 2) in greater detail, according to an embodiment. At 310, a value hrddeviation may be calculated according to the equation

hrddeviation=hrdbuffer—fullness−(hrdbuffer—size/2)

where hrddeviation refers to the amount of memory currently used in the HRD buffer, and hrdbuffer—size refers to the size of this buffer. At 320, a value for QP_deltahrd—buffer is determined on the basis of hrddeviation. In an embodiment, QP_deltahrd—buffer may be looked up in a lookup table. As discussed above, given less fullness, QP_deltahrd—buffer is given a lesser value, or even a negative value, but may be given a larger value if fullness of the HRD buffer is greater. QP_deltahrd—buffer may therefore depend on the extent of fullness of this buffer. The lookup table may be generated empirically in an embodiment, and may depend on the particular system and configuration in which this process is used.

FIG. 4 illustrates the determination of QP_deltalocal—rate—ratio (230 of FIG. 2) in greater detail, according to an embodiment. At 410, the current bit rate, instbitrate, may be calculated. In the illustrated embodiment, this may be calculated according to the equation

instbitrate=[prev—I_framesize+(prev—P_framesize*Num—P_in_GOP)+(prev—B_framesize*Num—B_in_GOP)]/GOP_size.

Here, prev_I_framesize refers to the size of the previous I frame, prev_B_framesize refers to the size of the previous B frame, and prev_P_framesize refers to the size of the previous P frame. GOP refers to a group of pictures between two I frames. Num_P_in_GOP refers to the number of P frames in the GOP, while Num_B_in_GOP refers to the number of B frames in the GOP. GOP_size refers to the number of frames in the GOP.

At 420, the local rate ratio (localrate—ratio) may be determined as instbitrate divided by a target bit rate (targetbitrate). At 430, a value for QP_deltalocal—rate—ratio may be determined on the basis of localrate—ratio. In an embodiment, this determination may be performed using a lookup table. This table may be structured such that for higher local rate ratios, the value for QP_deltalocal—rate—ratio may be greater; for lower local rate ratios, the value for QP_deltalocal—rate—ratio may be lower, or even negative. The lookup table may be derived empirically in an embodiment, and may depend on the system in which this process is used, and the configuration of this system.
FIG. 5 illustrates the determination of QP_deltaglobal—rate—ratio according to an embodiment. At 510, globalrate—ratio may be calculated as a ratio of a totalencode—size to a totaltarget—size. At 520, the value of this ratio may be used in a lookup table to determine a corresponding value for QP_deltaglobal—rate—ratio. This table may be derived empirically in an embodiment, and may depend on the system in which this process is used, and the configuration of this system.
As discussed above with respect to FIG. 2, the value for QP_deltaglobal—rate—ratio may then be used along with the values for QP_deltahrd—buffer and QP_deltalocal—rate—ratio to determine QPadapt.
FIGS. 2-5 relate to the case of controlling bit rate for frames coded according to the AVC standard, according to an embodiment. FIG. 6 illustrates an embodiment of for controlling the bit rate for frames that are encoded according to the MPEG2 standard. At 610, QPinit may be determined as a sum of terms,

QPinit=alpha*QPprevious—I—frame+beta*QPprevious—P—frame+gamma*QPprevious—B—frame 

Here, the values QPprevious—I—frame, QPprevious—P—frame, and QPprevious—B—frame refer to quantization parameters for the previous I, P, and B frames respectively. The coefficients alpha, beta, and gamma may have the properties that each is greater than or equal to 0, and they collectively sum to 1.

The sequence 620-640 corresponds to QP adaptation 120 of FIG. 1. At 620, a value QP_deltahrd—buffer (QPinit) may be determined, in a manner to be described in greater detail below. At 630, a value QP_deltalocal—rate—ratio (QPinit) may be determined. The determination of this value is also discussed below. At 640, QPadapt may be calculated as the sum

QPadapt=QPinit+QP_deltahrd—buffer(QPinit)+QP_deltalocal—rate—ratio(QPinit).

In an embodiment, the value by which QPinit is modified may also depend on the previous value of QP. In such an embodiment, if the previous QP is higher, then the value of QPadapt is further increased.

At 650, QPfinal may be calculated as the sum

QPfinal=QPadapt+QPcontrol(frame type,QPadapt).

The last term, QPcontrol (frame type, QPadapt), may have the same properties as discussed in the case of AVC encoded frames. Specifically, this value may be a relatively small value, e.g., between −1 and +1, that may be added to QPadapt if the frame is a reference frame. This serves to make the weight of a reference frame greater with respect to non-reference frames. As in the case of the embodiment of FIG. 2, higher values for QPcontrol (frame type, QPadapt) may be used for higher bit rates. The appropriate value for QPcontrol (frame type, QPadapt) may be decided empirically in an embodiment.
FIG. 7 illustrates the determination of QP_deltahrd—buffer (QPinit) (620 of FIG. 6) in greater detail, according to an embodiment. At 710, a value hrddeviation may be determined by the calculation

hrddeviation=hrdbuffer—fullness−(hrdbuffer—size/2).

At 720, the value hrddeviation may be used to access a lookup table. Using this value, a corresponding value for QP_deltahrd—buffer (QPinit) may be determined. The lookup table may be derived empirically in an embodiment, and may depend on the system in which this process is used, and the configuration of this system.

FIG. 8 illustrates the determination of QP_deltalocal—rate—ratio (QPinit) (630 of FIG. 6) according to an embodiment. At 810, an instant bit rate instbitrate may be determined according to the calculation

instbitrate=[prev—I_framesize+(prev—P_framesize*Num—P_in_GOP)+(prev—B_framesize*Num—B_in_GOP)]/GOP_size.

As in the case of 410 of FIG. 4, prev_I_framesize refers to the size of the previous I frame, prev_B_framesize refers to the size of the previous B frame, and prev_P_framesize refers to the size of the previous P frame. GOP refers to a group of pictures between two I frames. Num_P_in_GOP refers to the number of P frames in the GOP, while Num_B_in_GOP refers to the number of B frames in the GOP. GOP_size refers to the number of frames in the GOP.

At 820, the local rate ratio may be determined according to a calculation

localrate—ratio=instbitrate/targetbitrate 

as in 420 of FIG. 4.

At 830, a look up table may be used to determine a value for QP_deltalocal—rate—ratio base on the calculated value localrate—ratio. The lookup table may be derived empirically in an embodiment, and may depend on the system in which this process is used, and the particular configuration of this system.
One or more features disclosed herein may be implemented in hardware, software, firmware, and combinations thereof, including discrete and integrated circuit logic, application specific integrated circuit (ASIC) logic, and microcontrollers, and may be implemented as part of a domain-specific integrated circuit package, or a combination of integrated circuit packages. The term software, as used herein, refers to a computer program product including a non-transitory computer readable medium having computer program logic stored therein to cause a computer system to perform one or more features and/or combinations of features disclosed herein.
A software or firmware embodiment of the processing described herein is illustrated in FIG. 9. In this figure, system 900 may include a processor 920 and a body of memory 910 that may include one or more computer readable media that may store computer program logic 940. Memory 910 may be implemented as a hard disk and drive, a removable media such as a compact disk, a read-only memory (ROM) or random access memory (RAM) device, for example, or some combination thereof. Processor 920 and memory 910 may be in communication using any of several technologies known to one of ordinary skill in the art, such as a bus. Computer program logic 940 contained in memory 910 may be read and executed by processor 920. One or more I/O ports and/or I/O devices, shown collectively as I/O 930, may also be connected to processor 920 and memory 910.
Computer program logic 940 may include QPinit determination logic 950. This logic may be responsible for performing the processing illustrated in references 110, 210, and 610 of FIGS. 1, 2, and 6 respectively. Logic 950 may therefore include, for example, logic for the computation of QPinit based at least in part on one or more QP values from previous frames.
Computer program logic 940 may also comprise QP adaptation logic 960. Logic 560 may be responsible for calculating QPadapt as discussed above with respect to reference 220-250 in FIG. 2, and reference 620-640 of FIG. 6, for example. Logic 960 may therefore include logic for adapting the value QPinit on the basis of HRD buffer fullness and on local and/or global rate ratios.
Computer program logic 940 may also comprise QP control logic 970. Logic 970 may be responsible for performing the processing discussed above with respect to references 260-270 of FIG. 2 and reference 650 of FIG. 6, for example. Logic 970 may therefore include logic for calculating the final value of the QP by adding a control value QPcontrol based at least in part on the frame type, and in the case of AVC encoded frames, performing a clipping process.
Methods and systems are disclosed herein with the aid of functional building blocks illustrating the functions, features, and relationships thereof. At least some of the boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed.
While various embodiments are disclosed herein, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made therein without departing from the spirit and scope of the methods and systems disclosed herein. Thus, the breadth and scope of the claims should not be limited by any of the exemplary embodiments disclosed herein.

## Claims

1. A method, comprising:
in a video compression process, performing bit rate control by determining an initial quantization parameter QPinit for a frame based on the type of frame and a quantization parameter QPprevious of frame a previous frame of the same type;
adapting QPinit to form an adapted quantization parameter QPadapt based on fullness of a hypothetical reference decoder (HRD) buffer, and on a local rate ratio of an instant bit rate to a target bit rate;
controlling QPadapt to form a final quantization parameter OP final based on the type of frame; and
compressing the frame using QPfinal,
wherein the frame is encoded according to the MPEG2 standard, and said determining of QPinit comprises:





  
    calculating
    ⁢
    
      
    
    ⁢
    
      QP
      init
    
  
  =
  
    
      alpha
      *
      
        QP
        
          previous_I
          ⁢
          _frame
        
      
    
    +
    
      beta
      *
      
        QP
        
          previous_P
          ⁢
          _frame
        
      
    
    +
    
      gamma
      *
      
        QP
        
          previous_B
          ⁢
          _frame
        
      
    
  



where alpha+beta+gamma=1, where each ≧0,
QPprevious—I—frame is the quantization parameter of most recent I frame,
QPprevious—P—frame is the quantization parameter of the most recent P frame, and
QPprevious—B—frame is the quantization parameter of the most recent B frame.

2. The method of claim 1, wherein said adapting of QPinit comprises:
calculating QPadapt=QPinit+QP_deltahrd—buffer(QPinit)+QP_deltalocal—rate—ratio(QPinit)
where
QP_deltahrd—buffer(QPinit) is a value dependent on the fullness of the HRD buffer, and
QP_deltalocal—rate—ratio(QPinit) is a value dependent on the local rate ratio.

3. The method of claim 1, wherein said controlling of QPadapt adapt comprises:
calculating QPfinal=QPadapt+QPcontrol(frame type, QPadapt)
where QPcontrol is dependent on the frame type and on QPadapt.

4. A system, comprising:
a processor; and
a memory in communication with said processor, said memory for storing a plurality of processing instructions for directing said processor to:
perform bit rate control by determining an initial quantization parameter QPinit for a frame based on the type of frame and a quantization parameter QPprevious of a previous frame of the same type; previous of
adapt QPinit to form an adapted quantization parameter QPadapt based on fullness of a hypothetical reference decoder (HRD) buffer, and on a local rate ratio of an instant bit rate to a target bit rate;
control QPadapt to form a final quantization parameter QPfinal based on the type of frame; and
compress the frame using QPfinal,

wherein the frame is encoded according to the MPEG2 standard, and said determining of QPinit comprises:





  
    calculating
    ⁢
    
      
    
    ⁢
    
      QP
      init
    
  
  =
  
    
      alpha
      *
      
        QP
        
          previous_I
          ⁢
          _frame
        
      
    
    +
    
      beta
      *
      
        QP
        
          previous_P
          ⁢
          _frame
        
      
    
    +
    
      gamma
      *
      
        QP
        
          previous_B
          ⁢
          _frame
        
      
    
  



where alpha+beta+gamma=1, where each ≧0,
QPprevious—I—frame is the quantization parameter of most recent I frame,
QPprevious—P—frame is the quantization parameter of the most recent P frame, and
QPprevious—B—frame is the quantization parameter of the most recent B frame.

5. The system of claim 4, wherein said adapting of QPinit comprises:
calculating QPadapt=QPinit+QP_deltahrd—buffer(QPinit)+QP_deltalocal—rate—ratio(QPinit)
where
QP_deltahrd—buffer(QPinit) is a value dependent on the fullness of the HRD buffer, and
QP_deltalocal—rate—ratio(QPinit) is a value dependent on the local rate ratio.

6. The system of claim 4, wherein said controlling of QPadapt comprises:
calculating QPfinal=QPadapt+QPcontrol(frame type, QPadapt)
where QPcontrol is dependent on the frame type and on QPadapt.

7. A computer program product comprising a non-transitory computer useable medium having control logic stored therein, the computer control logic comprising:
logic configured to cause a processor to perform bit rate control by determining an initial quantization parameter QPinit for a frame based on the type of frame and a quantization parameter QPprevious of a previous frame of the same type;
logic configured to cause a processor to adapt QPinit to form an adapted quantization parameter QPadapt based on fullness of a hypothetical reference decoder (HRD) buffer, and on a local rate ratio of an instant bit rate to a target bit rate;
logic configured to cause a processor to control QPadapt to form a final quantization parameter QPfinal based on the type of frame; and
logic configured to cause a processor to compress the frame using QPfinal, wherein the frame is encoded according to the MPEG2 standard, and the determining of QPinit comprises:





  
    calculating
    ⁢
    
      
    
    ⁢
    
      QP
      init
    
  
  =
  
    
      alpha
      *
      
        QP
        
          previous_I
          ⁢
          _frame
        
      
    
    +
    
      beta
      *
      
        QP
        
          previous_P
          ⁢
          _frame
        
      
    
    +
    
      gamma
      *
      
        QP
        
          previous_B
          ⁢
          _frame
        
      
    
  



where alpha+beta+gamma=1, where each ≧0,
QPprevious—I—frame is the quantization parameter of most recent I frame,
QPprevious—P—frame is the quantization parameter of the most recent P frame, and
QPprevious—B—frame is the quantization parameter of the most recent B frame.

8. The computer program product of claim 7, wherein the adapting of QPinit comprises:
calculating QPadapt=QPinit+QP_deltahrd—buffer(QPinit)+QP_deltalocal—rate—ratio(QPinit)
where
QP_deltahrd—buffer(QPinit) is a value dependent on the fullness of the HRD buffer, and
QP_deltalocal—rate—ratio(QPinit) is a value dependent on the local rate ratio.