KR20020015885A - Loop/post filtering method for video coder - Google Patents

Abstract

PURPOSE: A loop/post filtering method of a video coder is provided to improve picture quality with a predetermined level while reducing the quantity of calculations to facilitate real-time processing. CONSTITUTION: An additional function is adaptively set for each pixel of a compressed image using various information items capable of being used when a motion picture is compressed and transmitted. The additional function defined with respect to each pixel is differentiated in order to attain a restored image from which blocking and ringing effects were eliminated. A normalized parameter is obtained in order to attain pixels restored from the differentiated additional function.

Description

Loop / post filtering method for video coders {LOOP / POST FILTERING METHOD FOR VIDEO CODER}

The present invention relates to a loop and post filtering technology for removing blockages occurring in a video video coder developed under ITU-T. The present invention relates to a loop / post filtering method of a video coder adapted to improve image quality.

Since 1997, next-generation video compression has proven superior in performance to other existing compression standards. The main difference from other compression standard methods is that video compression method uses 4 × 4 block-based change and encoding, performs motion estimation and compensation of transform block size, and one variable length code (VLC). Length Code) is used.

As described above, the conventional video compression scheme is superior in performance to other conventional compression standardization schemes, but the conventional video compression scheme has a large amount of computation at the encoding stage and adopts a transformation scheme based on a 4 × 4 block size. There is a drawback that the blocking phenomenon is more prominent than the standardization method.

Filtering methods to remove such blocking phenomenon include a loop filtering method processed inside the encoder and the decoder, a post filtering method processed at the next stage of the decoder, and a filtering method including a combination of the two methods. Can be mentioned.

In general, if the blocking phenomenon is removed by a loop filtering method, the difference between the input video and the motion video is affected, resulting in a higher coding rate in a specific case. In addition, since it is installed inside the video coder, the complexity of the coding unit is increased.

In this case, since the complexity of the loop filter is determined according to the number of filter taps and the feasibility of implementing the algorithm, the ideal loop filter has a small number of taps, and a simple algorithm is preferable. On the other hand, the advantages and disadvantages of post filters are the inverse of the loop filter advantages and disadvantages.

However, in the test model for video compression standardization, the encoder is quite complicated. The deblocking loop filter attached to the basic configuration of the test model is composed of 7 tap filters, Due to the large amount of computations involved, the complexity of the entire system is biased towards the encoder stage, which causes a lot of difficulties in real time processing.

Accordingly, an object of the present invention is to provide a video coder that guarantees a certain level of image quality even though the computation amount is reduced to facilitate real-time processing in implementing a loop / post filter to block blocking and ringing effects of a compressed video. It provides a loop / post filtering method.

1 is a signal flow diagram of a loop / post filtering method of a video coder according to the present invention.

Figure 2 is an illustration of a post filter according to the present invention.

3 to 8 are graphs showing experimental results of a conventional filtering method and a filtering method according to the present invention.

*** Description of the symbols for the main parts of the drawings ***

1: Current Macro Block 2: Upper Macro Block

A first aspect of the present invention is to adaptively set an additional function for each pixel of a compressed image by using various information in a video compression decoding stage.

A second aspect of the present invention is to estimate the degree of correlation with adjacent pixels so as to enable real time processing.

A third aspect of the present invention is to remove block and ring phenomena according to the position of a pixel and the encoding form of a macroblock in an encoding and decoding stage.

A loop / post filtering method of a video coder for achieving the object of the present invention comprises the first step of adaptively setting an additional function for each pixel of the compressed image using a variety of information available when the video is compressed and transmitted; A second step of differentially processing the addition function defined for each pixel to obtain a restored image from which blocking and ringing are removed; Comprising a third process of obtaining a normalization parameter value in order to obtain a pixel to be restored from the differentially processed addition function, which will be described in detail with reference to FIGS. 1 to 4 attached to the operation of the present invention. .

In a typical video compression method, the encoding unit transmits time-space compressed information and additional information necessary for decoding by using a technique of removing spatial and temporal excess information. The decoder is constructed in the reverse order of the encoder.

First, a process of blocking a ring and removing a ring phenomenon by predicting an original image from a quantization variable (QP) and a macroblock type (mtype) available at a decoding end by using correlation between pixels will be described.

When the original image f is transmitted in a compressed form, the image reconstructed by the decoding end may be expressed as follows.

In Equation (1), g, f, and n denote a MM × 1 sized compressed image, an original image, and a column vector of quantization error. Equation 1 may be expressed as follows to express the relationship of each pixel for processing in a pixel unit.

When the reliability of the original pixel and the one-dimensional smoothing function for each pixel are used together with the normalization technique, the following additional functions can be defined. First consider the horizontal direction.

Of the above formula (3) Is a function representing the degree of reliability and flattening in the left and right directions for the pixel f (i, j), and these functions are defined as follows.

The first terms of the right term of each function of Equation 4 are pixels Denotes an irregularity with adjacent pixels with respect to the second term, and the second terms indicate reliability with respect to the original pixel. Also, Is a kind of normalization parameter representing the ratio of the irregular term and the confidence term defined in each function. In this manner, an additional function is defined for each pixel of the video, and the function definition in the vertical direction is defined by changing i instead of the variable j in Equation (4).

In order to obtain the reconstructed image from which the blocking and ringing phenomenon is eliminated, the derivative of the above-described addition function defined for each pixel is obtained as follows.

In the above formula (5),

From the above Equations 5 and 6, the pixel f (i, j) to be restored in the horizontal direction can be obtained as follows.

Since the normalization parameter of Equation 7 has a value between 0 and 1, Equation 7 can be defined as follows.

Referring to Equation 8, the deblocking image of the compressed image is determined by two pixel values and a normalization parameter based on the (i, j) th image. Since the left and right two pixel values are values that can be used by the encoder and the blocker, two normalization parameter values may be set to obtain a reconstructed image. For this purpose, applying the set theoretic theory to the reconstruction method of the pixel unit, the normalization coefficients can be expressed as follows.

Of the above formula (9) Is an adaptive parameter determined by the position of the pixel because the degree of nonuniformity between the block boundary and the pixels in the block is different, and QP is a quantization parameter of the macro block to which f (i, j) belongs. In general, the adaptation parameters are determined statistically.

Using Equation 8 and Equation 9, a reconstructed image obtained by removing the blocking phenomenon may be obtained, but since the value of the normalization parameter requires a floating-point operation, it may be a problem in terms of computation. . Therefore, the equations (8) and (9) are modified as follows for integer arithmetic.

And,

The algorithm of the present invention is used to generate a normalization parameter in the form of integer using Equation (11) and then store it in the form of a lookup table. Therefore, the above-described equation (10), which is a blocking filter expression, can be obtained by knowing the position and quantization parameter values of two pixels around f (i, j).

For reference, the capacity of the memory for constructing the lookup table needs 8 bits × 31 (number of quantization parameter types) × 2 (type of adaptive parameter used) × 256 (type of difference between pixels) = 127kbits.

Meanwhile, a method of using the above filter as a loop and a post filter will be described.

The loop filter as in the above description is effectively used to reduce the computational burden of the video codec. First, when the type of each encoded macroblock is determined and the macroblock type including the pixel to be processed is found to be an encoded macroblock (intra or inter macroblock), the equations (10) and (11) are block boundary Applies only to pixel values in the area. In this case, the adaptation parameters of Equation 11 are as follows.

The post filter as described above is installed after the video codec decoder, and as shown in FIG. 2, the current macroblock 1 type, which is a macro block of the current processing image, and the upper macro block 2 located above are copied copy blocks. In this case, the information of the previous reconstructed image is used, otherwise it is obtained by using the above expressions (10) and (11). In other words,

In addition, the adaptation parameter of the normalization parameter of Equation 11 is determined as follows.

For reference, FIGS. 3 to 8 are graphs showing experimental results of the conventional filtering method (eg, TML4) and the filtering method according to the present invention, and it can be seen that the real-time processing is easy and the image quality is improved.

As described in detail above, the present invention provides a digital video apparatus using a video compression technique, which is configured to adaptively set an additional function for each pixel of a compressed image based on various available information, and to perform a real-time processing. By estimating the degree of relationship with the pixels and removing the blocking and ring phenomena according to the position of the pixels and the encoding form of the macroblocks, there is an effect that can contribute to the improvement of the image quality of the compressed image where low bit rate or high speed processing is desired.

Claims (9)

A first step of adaptively setting an additional function for each pixel of the compressed image by using various pieces of information available when compressing and transmitting the moving image; A second step of differentiating the addition function defined for each pixel to obtain a restored image from which blocking and ringing are removed; And a third step of obtaining a normalization parameter value to obtain a pixel to be reconstructed from the differentiated addition function. The loop / post filtering method of a video coder according to claim 1, wherein the additional function is defined as follows. : A function indicating the degree of reliability and leveling in the left and right directions for the pixel f (i, j) The method of claim 2, wherein the function ( ) Is a loop / post filtering method of a video coder, characterized by the following equation. 2. The loop / post filtering method of a video coder according to claim 1, wherein the derivative of the additional function defined for each pixel is defined as follows. The loop / post filtering method of a video coder according to claim 1, wherein the normalization parameter is defined as follows. 6. The loop / post filtering method of a video coder according to claim 5, wherein the normalization coefficient of the normalization parameter is defined as follows. 2. The loop / post filtering method of a video coder according to claim 1, wherein the following equation is used for integer operations on normalization parameters. A first step of adaptively setting an additional function for each pixel of the compressed image by using various pieces of information available when compressing and transmitting the moving image; A second step of differentiating the addition function defined for each pixel to obtain a restored image from which blocking and ringing are removed; When a normalization parameter value is obtained to obtain a pixel to be restored from the differentiated addition function, if the type of the macroblock is an encoded macroblock, only the pixel value of the block boundary is applied so as to be suitable for use in a loop filter. And a third step of obtaining the image using previous reconstructed image information so as to be suitable for use in a post filter. 10. The loop / post filtering method of a video coder according to claim 8, wherein the normalization parameter is generated in an integer form using the following equation and stored in a lookup table.