KR20130112448A - Method and apparatus for controling bit-rate - Google Patents

Abstract

The present invention relates to a method and apparatus for controlling a bit rate, the method for controlling a bit rate according to the present invention comprises the steps of: allocating a bit amount for an intra interval, allocating a bit amount for a group of pictures (GOP), and a bit amount per frame. And allocating the bit rate per frame by using an initial quantization parameter, layering a bit amount allocation weight using a relative ratio of each result bit amount after the first frame encoding for each frame type / layer level. Can be applied by level.

Description

Bit rate control method and apparatus {METHOD AND APPARATUS FOR CONTROLING BIT-RATE}

The present invention relates to a video compression technique, and more particularly, to a bit rate allocation and control in a video encoding / decoding process.

Recently, as broadcasts having high definition (HD) resolution have been expanded and serviced not only in Korea but also in the world, many users are getting used to high resolution and high quality images. Accordingly, many video service related organizations are speeding up the development of next generation video equipment.

In addition, as interest in Ultra High Definition (UHD), which has four times the resolution of HDTV, is increasing along with HDTV, a technology for compressing and processing higher resolution and higher quality images is required.

In order to compress and process an image, an inter prediction technique for predicting a pixel value included in a current picture from a previous and / or subsequent picture in time, and a pixel value included in a current picture using pixel information in the current picture Intra-prediction techniques for predicting the C-type, entropy encoding techniques for assigning short codes to symbols with high appearance frequency and long codes to symbols with low appearance frequency may be used.

In the video compression technology, it is necessary to control the bit rate in the encoding process in consideration of the network environment and the performance of the video device. In particular, in consideration of the increasing demand for ultra-high definition video, it is necessary to effectively perform real-time bit rate control in order to provide an effective video service in the process of encoding a large video. In addition, unlike the conventional coding technology, it is necessary to consider the hierarchical coding structure reflected in the current coding technology in bit rate control.

In view of the hierarchical coding structure, the present invention provides a real-time bit rate control technique for ultra-high definition video encoding.

An object of the present invention is to provide a method and apparatus for real-time bit rate control for ultra-high definition video encoding.

An object of the present invention is to provide a method and apparatus for controlling a bit rate in video compression considering a hierarchical coding structure.

An embodiment of the present invention is a method for controlling a bit rate, comprising: allocating a bit amount for an intra interval, allocating a bit amount for a group of pictures (GOP), and allocating a bit amount per frame, In the step of allocating the bit amount per frame, the bit amount allocation weight using the relative ratio of each result bit amount after the first frame encoding for each frame type / layer level may be applied for each layer level by using an initial quantization parameter.

According to the present invention, in real time transmission of ultra-high definition video, the bit rate can be adaptively controlled, and further, high image compression efficiency can be obtained.

1 is a block diagram showing a configuration of an image encoding apparatus according to an embodiment of the present invention.
2 is a block diagram illustrating a configuration of an image decoding apparatus according to an embodiment of the present invention.
3 schematically illustrates an example of a hierarchical coding structure to which the present invention is applied.
4 is a flowchart schematically illustrating a method of controlling a bit rate according to the present invention.
5 is a flowchart schematically illustrating an example of a method of performing frame rate control according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In describing the embodiments of the present specification, when it is determined that a detailed description of a related well-known configuration or function may obscure the subject matter of the present disclosure, the detailed description may be omitted.

When an element is referred to herein as being "connected" or "connected" to another element, it may mean directly connected or connected to the other element, Element may be present. In addition, the content of " including " a specific configuration in this specification does not exclude a configuration other than the configuration, and means that additional configurations can be included in the scope of the present invention or the scope of the present invention.

Terms such as first and second may be used to describe various configurations, but the configurations are not limited by the terms. The terms are used to distinguish one configuration from another. For example, without departing from the scope of the present invention, the first configuration may be referred to as the second configuration, and similarly, the second configuration may also be referred to as the first configuration.

In addition, the components shown in the embodiments of the present invention are independently shown to represent different characteristic functions, and do not mean that each component is made of separate hardware or one software component unit. In other words, each component is listed as a component for convenience of description, and at least two of the components may form one component, or one component may be divided into a plurality of components to perform a function. The integrated and separated embodiments of each component are also included in the scope of the present invention without departing from the spirit of the present invention.

In addition, some of the components are not essential components to perform essential functions in the present invention, but may be optional components only to improve performance. The present invention can be implemented including only the components necessary to implement the essentials of the present invention except for the components used for improving the performance, the structure including only the essential components except for the optional components used for improving the performance Also included within the scope of the present invention.

1 is a block diagram showing a configuration of an image encoding apparatus according to an embodiment of the present invention.

1, the image encoding apparatus 100 includes a motion prediction unit 111, a motion compensation unit 112, an intra prediction unit 120, a switch 115, a subtractor 125, a transform unit 130, A quantization unit 140, an entropy encoding unit 150, an inverse quantization unit 160, an inverse transformation unit 170, an adder 175, a filter unit 180, and a reference image buffer 190.

The image encoding apparatus 100 may encode an input image in an intra mode or an inter mode and output a bit stream. In the intra mode, the switch 115 is switched to intra, and in the inter mode, the switch 115 is switched to inter. The image encoding apparatus 100 may generate a prediction block for an input block of the input image, and may then code the difference between the input block and the prediction block.

In the intra mode, the intraprediction unit 120 may generate a prediction block by performing spatial prediction using the pixel value of the already coded block around the current block.

In the inter mode, the motion predicting unit 111 may find a motion vector by searching an area of the reference image stored in the reference image buffer 190, which is best matched with the input block, in the motion estimation process. The motion compensation unit 112 may generate a prediction block by performing motion compensation using a motion vector and a reference image stored in the reference image buffer 190.

The subtractor 125 may generate a residual block by a difference between the input block and the generated prediction block. The transforming unit 130 may transform the residual block to output a transform coefficient. The quantization unit 140 may quantize the input transform coefficient according to the quantization parameter to output a quantized coefficient.

The entropy encoder 150 entropy codes a symbol according to a probability distribution based on the values calculated by the quantizer 140 or an encoding parameter value calculated in the encoding process to generate a bit stream. You can print The entropy encoding method is a method of receiving a symbol having various values and expressing it as a decodable column of numbers while removing statistical redundancy.

Here, the symbol means a syntax element, a coding parameter, a value of a residual signal, and the like that are encoded / decoded. The residual signal can be referred to as a residual block in block units.

When entropy coding is applied, a small number of bits are allocated to a symbol having a high probability of occurrence, and a large number of bits are allocated to a symbol having a low probability of occurrence, so that the size of a bit string for the symbols to be coded Can be reduced. Therefore, the compression performance of the image encoding can be enhanced through the entropy encoding.

For entropy encoding, an encoding method such as exponential golomb, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC) may be used. For example, the entropy encoder 150 may store a table for performing entropy coding, such as a variable length coding (VLC) table, and the entropy encoder 150 stores the stored variable length coding. Entropy coding may be performed using a Variable Length Coding (VLC) table. Further, the entropy encoding unit 150 derives a binarization method of a target symbol and a probability model of a target symbol / bin, and then performs entropy encoding using the derived binarization method or probability model You may.

The quantized coefficients can be inversely quantized in the inverse quantization unit 160 and inversely transformed in the inverse transformation unit 170. The inverse quantized and inverse transformed coefficients can be added to the prediction block through the adder 175 and a reconstruction block can be generated.

The restoration block passes through the filter unit 180 and the filter unit 180 applies at least one of a deblocking filter, a sample adaptive offset (SAO), and an adaptive loop filter (ALF) can do. The reconstructed block that has passed through the filter unit 180 may be stored in the reference image buffer 190.

2 is a block diagram illustrating a configuration of an image decoding apparatus according to an embodiment of the present invention.

2, the image decoding apparatus 200 includes an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an intra prediction unit 240, a motion compensation unit 250, 260 and a reference image buffer 270.

The video decoding apparatus 200 receives the bit stream output from the encoder and decodes the video stream into the intra mode or the inter mode, and outputs the reconstructed video, that is, the reconstructed video. In the intra mode, the switch is switched to the intra mode, and in the inter mode, the switch can be switched to the inter mode. The image decoding apparatus 200 may obtain a residual block reconstructed from the input bit stream, generate a prediction block, and then add the reconstructed residual block and the prediction block to generate a reconstructed block, that is, a reconstruction block. have.

The entropy decoder 210 may entropy decode the input bit stream according to a probability distribution to generate symbols including symbols in the form of quantized coefficients.

When the entropy decoding method is applied, a small number of bits are assigned to a symbol having a high probability of occurrence, and a large number of bits are assigned to a symbol having a low probability of occurrence, so that the size of a bit string for each symbol is Can be reduced.

The quantized coefficient is inversely quantized by the inverse quantizer 220 and inversely transformed by the inverse transformer 230, and as a result of the inverse quantization / inverse transformation of the quantized coefficient, a reconstructed residual block may be generated.

In the intra mode, the intraprediction unit 240 can generate a prediction block by performing spatial prediction using the pixel value of the already coded block around the current block. In the inter mode, the motion compensation unit 250 can generate a prediction block by performing motion compensation using a motion vector and a reference image stored in the reference image buffer 270. [

The restored residual block and the prediction block are added through the adder 255, and the added block is passed through the filter unit 260. The filter unit 260 may apply at least one of the deblocking filter, SAO, and ALF to the restoration block or the restored picture. The filter unit 260 outputs a reconstructed image, that is, a reconstructed image. The reconstructed picture may be stored in the reference picture buffer 270 to be used for inter prediction.

Meanwhile, the bit rate control in the encoding process may be performed by the quantizer, the quantizer, and the entropy encoder, or may be performed by a separate module.

For bit rate control, a technique for determining an initial quantization parameter value, allocating bit amounts for each coding unit, setting a bit rate control basic model, and determining related parameters is required.

Hereinafter, bit rate control according to the present invention will be described in detail with reference to the drawings.

3 schematically illustrates an example of a hierarchical coding structure to which the present invention is applied.

Referring to FIG. 3, frames from an encoding target I frame at level 0 to the next I frame constitute one group of pictures (GOP).

3 illustrates a structure in which one frame B1 at level 1, two frames at level 2, and four frames at level 3 exist, this is only an example of a hierarchical coding structure to which the present invention can be applied. Each level may consist of more frames or fewer frames.

In addition, a frame of each level may be referred to encoding / decoding of a lower level frame or a same level frame than itself. In other words, a frame of each level may be encoded / decoded by referring to a frame of a higher level or the same level than itself.

For example, B2-2 may refer directly to the B1 frame and / or the I / B0 frame, as shown in FIG. 3, or may not refer to the B2-1 frame, although not shown. In addition, B2-2 may be referred to encoding / decoding of B3-3 and / or B3-4, which is a lower level frame.

4 is a flowchart schematically illustrating a method of controlling a bit rate according to the present invention. In the present specification, for convenience of description, it will be described that the bit rate control is performed by the bit rate controller. As described above, the bit rate controller for performing the bit rate control may be included in the quantization unit, the quantization unit / entropy encoding unit, or may be configured as a separate module.

Referring to FIG. 4, the bit rate controller determines an initial quantization parameter value (S410). The bit rate control unit is a bit rate that can be allocated per pixel using a given target bit rate, frames per second (fps), and an image resolution value to remove the bit rate (BPP). Calculate.

An initial quantization parameter QPI (Quantization Parameter of I-frame) for encoding an I frame image is determined according to a range of values of BPP. The QPI may vary depending on the resolution of the video to be encoded.

For example, the QPI shown in Equation 1 is a QPI for an image having a resolution of 2560x1600, and the QPI shown in Equation 2 is a QPI for an image having a resolution of 1920x1080.

<Formula 1>

<Formula 2>

When the QPI is determined, quantization parameter values for encoding pictures corresponding to the remaining frame types may be determined based on the determined QPI. For example, the initial quantization parameters for each level of frame may be determined by giving a predetermined offset value to the QPI value. Equation 3 shows an example of determining an initial quantization parameter value for each level of the frame.

<Formula 3>

Initial quantization parameter QPB0 = QPI + 1 of frame B0

Initial quantization parameter QPB1 = QPI + 2 of B1 frame

Initial Quantization Parameter QPB2 = QPI + 3 in B2 Frame

Initial Quantization Parameter QPB3 = QPI + 4 in B3 Frame

The encoder may perform encoding on the first I frame of an image (picture) based on the quantization parameter value determined by the bit rate controller (S420). The encoder / decoder may perform encoding on subsequent frames based on the first I frame. Although the encoder has been described herein, the decoder may perform decoding on the first I frame of an image (picture) based on the determined quantization parameter value.

Meanwhile, the bit rate controller may allocate a bit amount for each coding unit. The bit rate controller allocates a bit amount for each corresponding unit in each encoding step according to a given target bit rate.

Encoding may be divided into an encoding step for an intra period, an encoding step for a GOP, and an encoding step for a frame. Each encoding step is performed in the order of an encoding step for an intra interval → an encoding step for a GOP → an encoding step for a frame.

In the encoding step for the intra period, the coding unit becomes an intra period, in the encoding step for the GOP, the coding unit becomes a GOP, and in the sub-code step for the frame, the coding unit becomes a frame.

The bit rate controller first performs bit allocation on the intra period (S430). The intra interval means an interval in which random access may be performed in the random access setting. The bit rate controller may perform bit allocation for the current intra period in consideration of the target bit rate, the frame rate (fps), the number of frames per intra period, the remaining bit amount of the previous intra period, and the like. Equation 4 shows an example of a bit amount allocation method for an intra period.

<Equation 4>

The bit rate controller performs bit allocation on the GOP according to the bit amount allocated for the intra period (S440). The bit rate controller may perform bit allocation for the current GOP in consideration of the amount of bits allocated to the current intra period, the number of GOPs per intra period, the remaining bit amount of the previous GOP, and the like. Equation 5 shows an example of a bit amount allocation method for the GOP.

&Lt; EMI ID =

The bit rate controller performs frame unit bit rate control of the hierarchical encoding structure (S450). The bit rate control unit may assign appropriate weights according to the frame type and the hierarchical level when allocating bit amounts for each frame in a GOP.

The bit rate control in units of frames will be described later.

After performing the bit rate control on a frame basis, the bit rate controller determines whether the bit rate control for the current GOP is completed (S460). If the bit rate control for the current GOP is not completed, the bit rate control for other frames in the GOP is performed.

If the bit rate control for the current GOP is completed, the bit rate controller determines whether the bit rate control for the current intra period is completed (S470). If the bit rate control for the current intra period is not completed, bit rate allocation for the GOP in the current intra period is performed.

If the bit rate control for the current intra period is completed, the bit rate controller determines whether bit rate control for all images is completed (S480). If the bit rate control for all the images is not completed, bit rate allocation for another intra section in the image is performed.

When the bit rate control for all the images is completed, the bit rate controller terminates the bit rate control process.

5 is a flowchart schematically illustrating an example of a method of performing frame rate control according to the present invention. As illustrated in FIG. 5, the frame rate control described in FIG. 5 is an example of an operation performed by the bit rate controller in the frame rate control step S450 of FIG. 4. Accordingly, each step of FIG. 5 may be located between the bit allocation step S440 for the GOP of FIG. 4 and the step S460 of determining whether the bit rate control for the current GOP has ended.

Referring to FIG. 5, the bit rate controller performs encoding and bit allocation on pictures of respective hierarchical levels in the GOP (S510). For example, the bit rate controller may encode first images of each layer level in the first GOP of the video, calculate bit allocation weights, and calculate a bit rate control model parameter. In addition, the bit rate controller may perform encoding by allocating target bits for the remaining images of each hierarchical level in the GOP and calculating a quantization parameter.

In detail, the bit rate controller encodes the first frame for each type / layer level belonging to the first GOP of the video by using an initial quantization parameter. As a result, the ratio of the bit amount of the first frame for each type / layer level and the bit amount of the B0 frame is determined based on the bit amount of the I frame and the B0 frame. The bit rate ratio for each type / layer level determined as described above is a weight value added for each type / layer level in bit allocation to be used later.

Equation 6 shows an example of a method of allocating a bit amount per frame by considering a hierarchical structure in a GOP and adding weights.

&Lt; EMI ID =

 In addition, the bit rate controller applies a bit rate control model, determines the relevant parameters and updates them (S520).

Equation 7 shows a Cauchy-density-based model as an example that can be used as a bit rate control model.

Equation (7)

In the case of an I frame, the bit rate controller quantizes transform coefficients, for example, Discrete Cosine Transform (DCT) or Discrete Sine Transform (DST) coefficients, using an initial quantization parameter value QPI, and the quantized value Generate count information. The transform coefficients are quantized to QPI-2 and QPI + 2, and the number information of the quantized coefficients whose non-quantized values are equal to zero is obtained as before. A complexity related parameter value of Equation 7 may be calculated based on the quantization step values corresponding to the three quantization parameter values and the number information of non-zero quantization coefficients at that time.

Equation 8 shows an example of a method of calculating the value of α among the complexity-related parameters. Where a 'is a predetermined coefficient representing a proportional relationship.

<Formula 8>

In the case of the B0 frame, the α value may be determined in the same manner as in the case of the I frame. However, in the case of the B0 frame, α may be calculated by using quantization coefficient information corresponding to each other using various quantization parameter values such as QPB0-2, QPB0, and QPB0 + 5, and then the optimal α value may be determined.

In the case of B1 to B3 frames, if the value of the quantization parameter of each frame is in the range of QPB0-2 to QPB0 + 5, the α value of the B0 frame can be equally used.

In addition, the value a may be determined from Equation 7 based on α obtained in Equation 8, an initial quantization parameter used in each type / layer level frame, and a bit rate of encoding result.

The bit rate control unit also needs to update a and alpha values as necessary, for example, to update and apply a and alpha values every frame. Equation 9 shows an example of a method of updating a value every frame.

Equation (9)

If the quantization parameter obtained in the new B0 frame is out of the range of the quantization parameter value defined for the previous α value calculation, the bit rate controller sets a new range and calculates the α value again. In this case, the bit rate controller can recalculate the α value in the manner described above using Equation 8.

The bit rate controller may recalculate the new a value in the manner described above using the newly calculated value of α, the quantization step of the currently used quantization parameter, and the number of bits generated after the actual encoding.

The bit rate controller can obtain a quantization parameter by applying the bit rate control model equation using the newly obtained a value and α value from a subsequent frame.

As described above, the bit rate controller may calculate a quantization parameter value corresponding to the allocation target bit amount every frame based on the bit rate control model.

The quantization unit and / or entropy encoding may perform quantization and / or entropy encoding using a quantization parameter calculated according to the allocation target bit amount.

Meanwhile, the bit rate controller may limit the amount of change in the quantization parameter in order to minimize the change in image quality from the previous image. Equation 10 shows an example of a method of limiting the amount of change in the quantization parameter.

<Formula 10>

In the above-described exemplary system, the methods are described on the basis of a flowchart as a series of steps or blocks, but the present invention is not limited to the order of the steps, and some steps may occur in different orders or simultaneously . In addition, the above-described embodiments include examples of various aspects. For example, combinations of the embodiments are also to be understood as one embodiment of the present invention.

Claims (1)

Allocating bit amounts for intra intervals;
Allocating a bit amount for a group of pictures (GOP); And
Allocating bits per frame;
In the step of allocating the bit amount per frame,
And a bit rate allocation weight using a relative ratio of the resultant bit amounts after the first frame encoding for each frame type / layer level by using an initial quantization parameter.

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