KR100643126B1 - Transcoder for determining intra prediction direction based on DCT coefficients and transcoding method of the same - Google Patents

Abstract

A transcoder and transcoding method for determining an intra prediction mode based on DCT coefficients is disclosed. The decoder decodes the input video stream. The information extracting unit extracts a DCT coefficient for the block of the first size decoded by the decoding unit. The edge direction calculation unit calculates the direction of the edge existing in the block of the first size based on the extracted DCT coefficient. The prediction mode determiner determines a prediction mode corresponding to the calculated edge direction. The encoder encodes and outputs a video based on the determined prediction mode. According to the present invention, it is possible to efficiently estimate the intra prediction mode to minimize the amount of computation and maintain the coding efficiency.

Transcoder, MPEG, Intra Prediction Mode, AC Coefficients, Edge

Description

Transcoder for determining intra prediction direction based on DCT coefficients and transcoding method of the same}

1 is a diagram illustrating an intra prediction mode used in MPEG-4 Part 10 (H.264),

2 is a block diagram showing the detailed configuration of a preferred embodiment of a transcoder according to the present invention;

3 is a diagram showing AC coefficients and extraction directions of a decoded block when downsampling four 8x8 blocks decoded by a variable length decoder into one 8x8 block;

4 shows the relationship between the direction of the edge and the combination of the values of AC ₀₁ , AC ₁₀ , AC ₀₂ , and AC ₂₀ , and

5 is a flowchart illustrating the implementation of a preferred embodiment of a transcoding method according to the present invention.

The present invention relates to a transcoder and a transcoding method for determining an intra prediction mode based on DCT coefficients. More particularly, the present invention relates to a DCT decoded in an input stream to reduce computational complexity and maximize image quality during transcoding of a video. The present invention relates to a transcoder and a transcoding method for determining an intra prediction mode that is an output encoding parameter based on coefficients.

Due to the development of mobile communication technology and diversification of digital media services, the necessity of transforming video data according to a transmission environment or an application is increasing. A transcoding system is a system that transforms video data into an appropriate format or changes a bit rate. However, in changing the video data in real time, it is not easy to maximize the image quality while reducing the complexity of the calculation for changing the data. That is, it is almost impossible to obtain the maximum image quality while changing the video in real time due to a large amount of calculations due to the decoding and encoding in the transcoding system that decodes and re-encodes the input video. Therefore, the research on the transcoding system aims to efficiently improve the motion prediction and macroblock type determination which require a large amount of computation in the encoding process.

In the conventionally proposed transcoding algorithm, there are a spatial domain transcoding scheme and a DCT domain transcoding scheme according to the processing region. In addition, algorithms for effectively setting output parameters to be used in encoding in each transcoding scheme have been proposed. The output parameter calculation algorithm is an algorithm that can be applied regardless of the transcoding processing area. The output parameter calculation algorithm is mainly developed for macroblock type and motion vector setting which require complex operation in encoding process.

On the other hand, the existing transcoder does not have an algorithm for efficient intra prediction mode. Therefore, intra prediction mode determination in the existing transcoder was performed based on the encoding algorithm. In the encoding compression scheme of MPEG-4 Part 10 (H.264), the intra prediction mode has nine 4Х4 prediction modes and four 8Х8 prediction modes as shown in FIG. 1. In the nine prediction modes of the Intra 4x4 mode, block prediction is performed in a vertical direction, a horizontal direction, or a diagonal direction so as to correspond to each mode name.

Intra 4 × 4 modes include a vertical mode, a horizontal mode, a DC mode, a diagonal_down_left mode, a diagonal_down_right mode, a vertical_right mode, a horizontal_down mode, a vertical_up mode, and a horizontal_up mode. In this case, the Intra 4x4 mode selects an optimal mode by calculating the cost required to obtain bit rate-distortion for all nine prediction directions. For example, if prediction mode 0 is described among 4Х4 prediction modes, prediction mode 0 performs prediction in the 90 ° direction. Pixel values in the 4Х4 block are classified into columns, and the first column performs prediction using the A value as a predictor. That is, as shown in FIG. 1, the predictor is determined according to the arrow direction, and the pixel value belonging to the arrow direction performs prediction by the predictor. In MPEG-4 Part 10 (H.264), the prediction is performed according to all the prediction modes, and then the intra prediction mode is selected in which the bit rate and distortion are optimized.

Such a conventional transcoder selects an optimal prediction mode by performing intra prediction modes for each type and comparing them based on a rate-distortion optimization. At this time, the process of obtaining the bit rate and the distortion for each prediction mode requires a large amount of computation, and thus it is difficult to obtain the maximum image quality while changing the video in real time.

An object of the present invention is to provide a transcoder and a transcoding method suitable for video transcoding for resolution reduction and bit rate reduction according to an application.

Another object of the present invention is to provide a computer-readable recording medium having recorded thereon a program for executing a video transcoding method for resolution reduction and bit rate reduction according to an application.

According to an aspect of the present invention, a transcoder includes: a decoder configured to decode an input video stream; An information extraction unit for extracting a DCT coefficient for the block of the first size decoded by the decoder; An edge direction calculation unit calculating a direction of an edge existing in the block of the first size based on the extracted DCT coefficients; A prediction mode determination unit determining a prediction mode corresponding to the calculated edge direction; And an encoder which encodes and outputs a video based on the determined prediction mode.

According to another aspect of the present invention, there is provided a transcoding method, comprising: decoding an input video stream; Extracting a DCT coefficient for the decoded first size block; Calculating a direction of an edge existing in the block of the first size based on the extracted DCT coefficients; Determining a prediction mode corresponding to the calculated edge direction; And encoding and outputting a video based on the determined prediction mode.

Accordingly, the intra prediction mode may be determined according to the direction of the edge selected through the extracted information when decoding the input video stream, thereby minimizing the complexity of the transcoder and maintaining the image quality.

Hereinafter, exemplary embodiments of a transcoder and a transcoding method according to the present invention will be described in detail with reference to the accompanying drawings.

2 is a block diagram showing the detailed configuration of a preferred embodiment of a transcoder according to the present invention.

2, the transcoder 200 according to the present invention includes a decoder 210, an information extractor 230, an edge direction calculator 240, a prediction mode determiner 250, and a downsampler 260. , And an encoding unit 270.

The decoder 210 decodes the input video stream. The input video stream is a video signal in units of frames compressed by compression standards such as MPEG and H.26x. The decoder 210 includes a variable length decoder (VLD) 212, an inverse quantization unit 214, an inverse DCT unit 216, a frame memory 218, a motion compensator 220, and an adder. Has 222. Since the decoder 210 is the same as the structure of an apparatus for decoding an existing MPEG video except for the configuration of providing the output of the variable length decoder 212 to the information extractor 230, a detailed description thereof will be omitted.

The encoder 270 encodes and outputs a video based on the prediction mode determined by the prediction mode determiner 250. The encoder 270 illustrated in FIG. 2 is an H.264 encoder, a DCT / quantizer 272, a variable length coder 274, an inverse DCT / dequantizer 276, a frame memory 278, A motion estimation unit 280, an intra prediction unit 282, and a subtractor 284.

The image input from the downsampler 260 is input to the DCT / quantization unit 272 and then quantized with the DCT, and then context-based adaptive variable length coding in the variable length coder 274. do. The video stream output from the variable length coder 274 is an image signal of a frame unit compressed by the compression standard of MPEG-4 Part 10 (H.264). At this time, the input image is also inputted to the inverse DCT / dequantization unit 276 to perform (DCT + Q) ^-1 . An image on which (DCT + Q) ⁻¹ is performed is optionally stored in the frame memory 278 after the block boundary is smoothed. The motion estimation unit 280 performs motion estimation with the reference image and the input image stored in the frame memory 278 to subtract the reference image from the input image according to whether the input image to be encoded is an inter frame or an intra frame. The reference image is optionally provided to the subtractor 284 based on the determination. The intra prediction unit 282 may determine an intra macroblock based on the estimation result input from the motion estimation unit 280, the prediction result input from the prediction mode determiner 250, and the input image input from the downsampler 260. An intra prediction direction for encoding is set and output to the adder 260.

The performance of the transcoder 200 according to the present invention is determined according to the efficiency of the information extractor 230, the edge direction calculator 240, and the prediction mode determiner 250. Therefore, the algorithm for extracting meaningful information from the decoder 210 by the information extractor 230 should efficiently extract information without increasing the complexity of the decoding process in the entire transcoder 210. In addition, the edge direction calculator 240 and the prediction mode determiner 250 should calculate the coding parameter to be close to the optimum using the extracted information. If the encoding unit 210 in the transcoder 200 calculates the entire encoding parameter, the image quality is optimal, but the overall complexity increases, which causes problems in real-time applications. In addition, if the encoding parameter is incorrectly selected, the overall image quality is lowered, which lowers the quality of the actual video service.

The information extracting unit 230 extracts the DCT coefficient AC coefficient for the 8x8 block decoded by the decoding unit 210. At this time, the information extraction unit 230 extracts the AC coefficient in the zigzag scan order. FIG. 3 shows AC coefficients and extraction directions of the decoded blocks when downsampling four 8x8 blocks decoded by the variable length decoder 212 into one 8x8 block. Referring to FIG. 3, the information extracting unit 230 sequentially extracts AC ₀₁ , AC ₁₀ , AC ₀₂ , and AC ₂₀ from the 8 × 8 block shown in the upper left side of the decoded block. The 8Х8 block illustrated in FIG. 3 affects only the 4Х4 block of the encoder 270 of the transcoder 200. Although the case in which the resolution of the output image is reduced compared to the input image is shown in FIG. 3, the intra prediction mode determination algorithm of the present invention may be applied even when the resolution is not reduced.

The edge direction calculation unit 240 calculates the direction of the edge existing in the 8 × 8 block decoded based on the AC coefficient extracted by the information extraction unit 230. At this time, the edge direction calculation unit 240 determines whether there is an edge component in a certain direction existing in the block by the following equation. The conversion equation for calculating the AC coefficients in order to determine the characteristics of the DCT coefficients extracted from the block as shown in FIG. 3 is defined as in Equation 1.

In Equation 1, the expressions of AC ₀₁ and AC ₁₀ may be modified and expressed as Equation 2 and Equation 3, respectively.

Since AC ₀₁ of Equation 2 is equivalent to expressing the difference of the pixel information in the horizontal direction as the sum of the DCTs, it is provided as information for determining the presence or absence of the vertical edge in the block. Since AC ₁₀ of Equation 3 is the same as expressing the difference of the pixel information in the vertical direction as the sum of the DCTs, AC ₁₀ is provided as information for determining the presence or absence of the horizontal edge in the block. Therefore, if AC ₀₁ is "0", it means that there is no vertical edge, and if AC ₁₀ is "0", it means that there is no horizontal edge. In conclusion, the combination of values of AC ₀₁ , AC ₁₀ , AC ₀₂ , and AC ₂₀ can determine the presence or absence of edge components in the directions of 0 °, 90 °, 45 °, 135 °, 67.5 °, and 112.5 °.

The prediction mode determiner 250 determines a prediction mode corresponding to the calculated edge direction. 4 shows the relationship between the direction of the edge and the combination of the values of AC ₀₁ , AC ₁₀ , AC ₀₂ , and AC ₂₀ . When all four 8Х8 blocks shown in FIG. 3 have edges in the same direction, the prediction mode determiner 250 declares a mode corresponding to the edge direction and compares the 4Х4 prediction mode with the 8Х8 prediction mode. For example, if AC ₁₀ > 0, AC ₀₁ > 0, and AC ₀₂ = 0 in the block in the upper left corner of the block shown in FIG. 3, the prediction mode determiner 250 determines the prediction mode of the output 4Х4 block. It is determined as "7" from the 4Х4 prediction mode shown in FIG. In addition, when all of the blocks shown in FIG. 3 are determined to be "0" in the 4Х4 prediction mode, the prediction mode determiner 250 predicts the 4Х4 prediction mode "7" of four blocks and four blocks one 8Х8. The prediction mode is determined to have better performance compared to the result predicted by 8Х8 prediction mode "0". The same applies when all blocks shown in FIG. 3 are determined to be "0", "1", "2", and "3" in the 4Х4 prediction mode.

When the prediction mode is determined in a direction coinciding with the direction of the edge by the method as described above, the probability of existence of pixel values having a large difference value is reduced by intra prediction, so that the intra that is closest to the optimum without having to actually perform all intra predictions is determined. You can set the prediction mode. In the intra prediction mode, the smaller the difference value according to the prediction, the better the output image can be calculated.

5 is a flowchart illustrating the implementation of a preferred embodiment of a transcoding method according to the present invention.

Referring to FIG. 5, the variable length decoder 212 decodes and outputs an input video stream (S500). The information extracting unit 230 extracts a DCT coefficient for calculating the edge direction from the decoded block (S510). At this time, the information extraction unit 230 extracts only AC ₀₁ , AC ₀₁ , AC ₁₀ , AC ₀₂ , and AC ₂₀ , in a zigzag scan order, from the DCT coefficients. The edge direction calculation unit 240 calculates the direction of the edge in the block by using the characteristics of the extracted AC coefficient (S520). An edge means an edge that exists in a block, and the direction of the edge is selected only in one of the directions of 0 °, 90 °, 45 °, 135 °, 67.5 °, and 112.5 °. The prediction mode determiner 250 determines the intra prediction mode according to the calculated direction of the edge (S530). The efficiency of the intra prediction mode is higher when the value used for prediction is similar to the pixel value of the current block. Therefore, when the direction by the prediction mode matches the edge direction, the value used for the prediction and the current block predicted are The efficiency is increased because the difference in the pixel values of is reduced. Due to such a property, unlike the actual encoding algorithm, the prediction mode is determined by simple calculation without obtaining the bit rate-distortion of the entire prediction mode. Therefore, the complexity of the calculation performed by the prediction mode determiner 250 in the transcoder 200 is determined. There is an advantage to reduce. The encoder 270 encodes and outputs the downsampled image based on the determined intra prediction mode (S540).

In the above, the output is MPEG-4 Part 10 (H.264) and the transcoder down-sampling the horizontal and the vertical half is used for intra prediction mode, which is an encoding parameter of the intra macroblock of MPEG-4 Part 10 (H.264). Although the case of the determination has been described, the intra prediction mode determination algorithm proposed in the present invention can be applied even when the resolution is not reduced.

The transcoder and the transcoding method for determining the intra prediction mode according to the characteristics of the DCT coefficients according to the present invention can be written in a computer program and implemented in a chip. Codes and code segments constituting the program can be easily inferred by a computer programmer in the art. The program is also stored in a computer readable media, which is read and executed by a computer to implement a transcoding method. The information storage medium includes a magnetic recording medium, an optical recording medium, and a carrier wave medium. The computer readable storage medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion.

Although the preferred embodiments of the present invention have been shown and described above, the present invention is not limited to the above-described specific preferred embodiments, and the present invention belongs to the present invention without departing from the gist of the present invention as claimed in the claims. Various modifications can be made by those skilled in the art, and such changes are within the scope of the claims.

According to the transcoder and the transcoding method according to the present invention, in order to store a video stream in a medium suitable for a storage medium, a format is converted, a digital TV service is converted into a wireless Internet broadcast or a digital multimedia broadcasting (DMB) service, or a bandwidth of a network. When transcoding with MPEG-4 Part 10 (H.264) to adjust the bit rate to fit the, the edge direction is selected based on the extracted information when decoding the input video stream, and according to the selected edge direction. By determining the intra prediction mode, the complexity of the transcoder can be minimized and the image quality can be maintained. In addition, by adjusting the bit rate or converting the format at the network gateway, it is possible to provide a service suitable for the network band, and the MPEG-4 Part 10 (H.264) moving image can be converted to MPEG-1, 2, 4 encoded video contents. It can be transformed into content efficiently.

Claims (12)

A decoder which decodes the input video stream; An information extraction unit for extracting a DCT coefficient for the block of the first size decoded by the decoder and extracting an AC coefficient from the DCT coefficient; An edge direction calculation unit that determines a direction of an edge existing in the decoded block of the first size based on the extracted AC coefficient; A prediction mode determination unit determining a prediction mode corresponding to the calculated edge direction; And An encoder which encodes and outputs a video based on the determined prediction mode, The edge direction calculation unit determines whether an edge exists in the horizontal or vertical direction based on the extracted values of the DCT coefficients, and the block of the first size decoded by the combination of edge directions determined for the respective DCT coefficients. Determines the edge direction of The prediction mode determiner includes a plurality of first intras having a predetermined prediction direction for a block of a second size, based on a comparison result of a predetermined reference value with a value of AC coefficients selected in a zigzag scan direction among the extracted DCT coefficients. And a prediction mode corresponding to the decoded first size block among prediction modes. delete delete delete The transcoder according to claim 1, wherein the value of the AC coefficient is calculated by the following equation.

. The plurality of second intra prediction modes according to claim 1, wherein all of the prediction modes of the second size corresponding to each of the plurality of decoded first sizes of blocks are assigned a predetermined prediction direction with respect to the blocks of the first size. And when the prediction mode is determined to be one of prediction modes belonging to the first mode group including the first intra prediction mode having the same prediction direction, the prediction mode determining unit does not belong to the plurality of decoded first size blocks. The second prediction block belonging to the prediction mode belonging to the first mode group corresponds to a result of predicting the prediction mode selected from the first intra prediction mode and a block of a second size including the plurality of decoded first size blocks. And comparing a result predicted by the second intra prediction mode to determine a mode having excellent performance as a final prediction mode. Decoding the input video stream; Extracting a DCT coefficient for the decoded first sized block; Determining whether edges exist in the horizontal or vertical direction based on the extracted values of the DCT coefficients, and determining the edge direction of the decoded first size block by a combination of the edge directions determined for each of the DCT coefficients. An edge direction calculating step comprising determining; Comparing the values of the AC coefficients selected in the zigzag scan direction with a predetermined reference value among the extracted DCT coefficients, and a plurality of first intras having a predetermined prediction direction for a second block size based on the comparison result; Determining a prediction mode corresponding to the calculated edge direction, including determining a prediction mode corresponding to the decoded first size block among prediction modes; And And encoding and outputting a video based on the determined prediction mode. delete delete 8. The transcoding method of claim 7, wherein the value of the AC coefficient is calculated by the following equation.

. The method of claim 7, wherein the prediction mode determination step, All of the prediction modes of the second size corresponding to each of the plurality of decoded first size blocks are the same prediction direction as the plurality of second intra prediction modes in which a predetermined prediction direction is specified for the block of the first size. Checking whether the information belongs to a first mode group configured in one intra prediction mode; If all of the prediction modes of the second size corresponding to each of the plurality of decoded first size blocks belong to the first mode group, the plurality of decoded first size blocks do not belong to the first mode group. A prediction result determined from a prediction mode belonging to the first mode group, wherein the second prediction block belonging to the first mode group is a result of the prediction using the prediction mode selected from the first intra prediction mode. And comparing the results predicted by the second intra prediction mode to determine a mode having excellent performance as the final prediction mode. A computer recording medium storing a program for executing the transcoding method according to claim 7 on a computer.

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