KR20040031948A - Method for moving picture decoding - Google Patents

Abstract

PURPOSE: A method for decoding motion pictures is provided to prevent blocks with errors from being used as estimation blocks in decoding video frames, thereby preventing the accumulation of errors and the deterioration of the quality. CONSTITUTION: A motion picture bit stream applied to a decoder is decoded(S501). In decoding the motion picture bit stream, information judging the existence of errors in macro blocks within a video frame is stored(S502). It is judged whether error blocks exist in the video frame(S503). If so, the error-generated macro blocks are not used as reference blocks for intra mode estimation(S504). If not, the macro blocks without the errors are used as the reference blocks for intra mode decoding(S505).

Description

How to decode video {METHOD FOR MOVING PICTURE DECODING}

The present invention relates to a video decoding method. More specifically, when an error occurs in macroblocks of an image frame decoded by a decoder, when decoding is performed according to intra mode prediction, the blocks in which the error occurs are reference blocks for prediction. The present invention relates to a video decoding method that can prevent error accumulation and improve image quality by not using it.

The current and future communication environments are rapidly changing beyond the distinction between wired and wireless areas and regional countries. In particular, future communication environments, such as IMT-2000, require users as well as video and audio. The trend is to build an environment that provides various information in real time or comprehensively.

In addition, the development of the personal mobile communication system is not only a voice communication in the cellular phone or PCS, but also to transmit text information as well as to access the Internet wirelessly using a personal mobile communication terminal, or to watch videos on TV It is being developed to transmit the data.

In particular, a digital television system that processes a video into digital data, transmits the data in real time, receives the video, and displays the video, and the video transmitted in real time is becoming an essential element in a personal portable terminal using the IMT2000.

In the related art, the portable terminal transmits and receives only human voice. However, due to the development of multimedia and the development of digital information processing technology, various information such as voice and video can be transmitted.

Above all, this technology has been commercialized. First of all, the analog video signal is subjected to special digital processing such as quantization and variable length encoding, and then included in the digital information and transmitted. The development of video compression technology, which allows abundant information transmission and reception, contributed greatly.

Recently, with the development of digital signal processing technology, methods of compressing and transmitting a large amount of video information through a limited bandwidth transmission channel have been developed. However, if an error occurs on the transmission channel, the quality of the reconstructed image is greatly reduced. Occurs.

At this time, error concealment techniques have been studied to recover near to the original image by supplementing the lost information from the neighboring information that is normally restored without using an error correction code to maximize the limited bandwidth.

In particular, in the case of an MPEG (Moving Picture Experts Group), if an error occurs, all information until the next sync signal is found is lost. In addition, since the motion compensation encoding technique is used, the influence of the damaged portion is continued over the subsequent frames.

The present invention is an IMT-2000 video terminal that applies an optimized motion decision value and an intra mode compression algorithm to a hardware accelerator structure that can enable fast encoding and quality improvement of video data. Motion decision value of MPEG compression method, which is generally used for wireless image communication, is the most important because it has the largest amount of computation and has a direct influence on the image quality and the amount of bits.

Therefore, many studies on motion judgments have been made continuously, and recently, the motion judgments have the same image quality and bit rate as the full search using the diamond search pattern, and are most commonly used. A three step search method is used.

1 is a view showing a video encoder system according to the prior art.

As shown in FIG. 1, when a video signal photographing a real object is received, a digital conversion is performed, and then an encoding operation is performed to compress the video signal. The digitally converted video signal is subjected to discrete cosine transform in DCT (Discrete Cosine Transform 1), and the image signal converted in frequency form is quantized in quantization unit 3 and then entropy in VLC (Variable Length Coding 10). It is sent to the outside through the conversion process.

In this case, when the image to be hatched is an inter mode, a motion vector capable of predicting the previous image is generated from a motion predictor 9. The decoded image frame is reconstructed by the inverse quantization unit 5 and the inverse DCT 7 and stored in the image memory 8. The image stored in the image memory is a reference for predicting motion of an image to be encoded later. Use it as a video.

When the next input image is defined as a P picture as an inter mode, after motion prediction and compensation are performed on a block of each image frame, the macro block of the intra mode converts the current image value to an intra image frame encoding. Encode in the same way.

The video bitstream encoded by the encoder is transmitted to the decoder and decoded through the same procedure as that encoded by the encoder.

In particular, the macro block type information and the like, which are inter mode encoded, are inserted into the macro block header information together with the macro blocks of the encoded video frame and transmitted.

The decoder decodes the macroblock type information included in and inserted into the video bitstream as described above, and performs decoding according to each macrotype information.

Although shown in the figure, 2a and 2b, which are not described, represent adders.

2A to 2B are diagrams for explaining a process of intra mode encoding in an MPEG standard according to the prior art.

As shown in FIG. 2A, the encoder encodes a video signal for each image frame. The blocks in the image frame are subjected to discrete cosine transforming in DCT, and then each block A, B, C, D obtained here. , Quantize by dividing the DCT coefficient values for X, Y) into direct current (DC) and alternating current (AC) values. Among the quantized DCT coefficients, the DC coefficient represents a pixel coefficient at the top of the image block. In the intra mode prediction mode, a block X currently encoding the immediately previous block DC coefficient A and the DC coefficient C of the upper block. The difference value is encoded to refer to a DC coefficient value for prediction.

When the video image is encoded in the intra mode, since the previous reference image does not exist, the image must be predicted from the image frame in the self. The encoding is performed so that the image can be predicted from the adjacent block of the block to be encoded.

When the encoded data is decoded by the decoder, the encoded data is reproduced in the same manner as that of the encoder, so that the image can be easily reproduced. As such, the method of predicting the blocks A, B, C, D, X, and Y in the intra frame in the intra mode compares the direct current (DC) value and the alternating current (AC) value of the adjacent block of the current block to be encoded. Set a predictive value.

Since the direct current value represents the coefficient value of the first starting pixel of the 8 * 8 block of the image frame, as shown in the drawing, the direct current of the block existing in the adjacent region of the block X encoded in the same image frame ( DC) coefficient values are used to predict the DC coefficient of the block X to be encoded. Since there is no GOB unit in the H.263 + standard, prediction is made using the DC coefficients of three blocks adjacent to the block to be encoded.

As shown in FIG. 2B, when a video signal is encoded, a macroblock constituting an image frame and four 8 * 8 blocks constituting the macroblock indicate a first pixel coefficient value among block coefficient values. The AC value indicating the value and the remaining pixel values are encoded independently of each other.

Accordingly, the encoding is performed by obtaining a prediction value so that pixels representing the AC coefficient of each block can also be used as a prediction value of the block to be encoded.

That is, the AC coefficient values of the first vertical pixels in the previous block A of the block X currently encoded and the AC values of the first horizontal pixels in the upper block C of the block X are encoded. The encoding is performed with the predicted value of the AC value.

Since MPEG does not have a GOB unit that defines blocks in an image frame, prediction is performed using three adjacent blocks for each 8 * 8 block constituting a macro block.

2A and 2B, the intra-encoded video is compressed into a series of bitstreams and transmitted to the decoder, and the decoder obtains coefficient values of a block currently decoded with a prediction direction for the blocks in the image frame. Because of the reconstruction, the coding efficiency can be improved.

However, although the video signal is encoded as described above and the prediction value is obtained from previous image blocks existing in the image frame in the intra mode, the image is reproduced. However, data loss inevitably occurs in a transmission system using a wireless network. If the blocks are continuously predicted and decoded with reference to the lost blocks, the reproduced image quality is very bad.

In addition, when intra-mode decoding is performed with reference to a block in which an error occurs, a problem arises in that an error accumulates and propagates in the blocks to be decoded.

According to an embodiment of the present invention, when an error occurs in blocks of an image frame to be decoded due to a channel error in an image bitstream transmitted to a wireless network such as IMT-2000, among the blocks in the image frame referred to in decoding in an intra mode It is an object of the present invention to provide a video decoding method capable of preventing error accumulation and deterioration of image quality by excluding an errored block from a prediction block.

1 illustrates a video encoder system according to the prior art.

2A to 2B are diagrams for explaining a process of intra mode encoding in an MPEG standard according to the prior art.

3 is a block diagram illustrating a structure of a video decoder for error correspondence according to the present invention;

4 is a diagram illustrating a process of obtaining a prediction value for a block to be decoded in an intra mode image frame according to the present invention.

5 is a flowchart for explaining an intra mode decoding method according to the present invention.

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

1: buffer section 3: VLD

5: error detection / recovery unit 7: inverse quantization unit

9: reverse DCT10: display unit

11: summer 12: MC (Motion Compensation)

15: Video frame memory

In order to achieve the above object, a video decoding method according to the present invention,

Decoding a video bitstream applied to the decoder;

Storing information determining whether there is an error of macroblocks in an image frame while decoding the video bitstream; And

When intra mode prediction is performed on macro blocks of an image frame using error information of the macro blocks, intra prediction is sequentially performed using a block in which there is no error as a reference block of a block currently performing intra mode decoding. It is characterized by including; proceeding.

Here, the error-prone block among the macroblocks is not used as a reference block for prediction in the decoding process, and an error does not occur when decoding the macroblocks in the image frame by intra mode prediction. Intra prediction may be performed using only macro blocks.

According to the present invention, by using the characteristic of determining an errored block among the blocks of an image frame decoded by the decoder, the decoding calculation amount is reduced by not referring to the errored blocks when decoding an intra mode image frame. Therefore, there is an advantage that can prevent the image quality deterioration due to an error.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 is a block diagram illustrating a structure of a video decoder for error correspondence according to the present invention.

As shown in FIG. 3, when the encoded image data enters the decoder unit in the form of a bit stream, decoding is performed as opposed to encoding performed by an encoder in order to restore the original image.

Therefore, the decoder which decodes the compressed bitstream performs a decoding operation in order to reproduce the bitstream stored in the buffer unit 1 and the bitstream stored in the buffer unit 1 as an image temporarily. A variable length decoder (VLD 3), an error detection / reconstruction unit (5) for error detection, motion vector restoration and image reproduction in a bitstream decoded from the variable length decoder (3); The inverse quantization unit 7 and the inverse quantization unit 7 perform inverse quantization and inverse discrete cosine transformation in order to reverse the compression scheme performed by the encoder in the error detection / recovery unit 5. Inverse Discrete Cosine Transform: 9), a motion compensation (MC 12) for compensating for the motion vector reconstructed by the error detection / recovery unit (5), and displaying a video. The video frame memory 15 stores previous images from the display unit 10.

The process of reproducing the compressed video by the decoder having the above structure is as follows.

When an analog image signal is received by a decoder in a compressed bit stream state, the variable frame coded image frame signal is received through a discrete cosine transform and a quantization at an encoder. First, it is stored in the buffer unit 1 to temporarily store it. The compressed video bitstream stored in the buffer unit 1 converts the value, length, etc., of the compressed bitstream by the variable length decoder 3 into two-dimensional encoding, and then detects the error / The restoration unit 5 detects a motion vector value and whether an error occurs.

Then, the compressed image applied from the error detection / recovery unit 5 is reconstructed in the opposite direction as when the encoder is compressed by the encoder. The inverse quantization unit 7 performs inverse scan and inverse quantization, and then performs inverse discrete cosine transformer (IDCT) 9 (103) for IDCT conversion.

The video signal, which has been subjected to the Inverse Discrete Cosine Transform, is input to the adder 11. After determining whether the INTER mode image exists, the display unit 10 directly reproduces the image because there is no motion vector in the case of the intra mode image.

In case of determining the inter mode, since the motion vector exists, the motion vector is extracted by the error detection / recovery unit 5 from the compressed bitstream and then compensated for by the motion compensator 12. send.

The motion compensator 12 obtains and decodes a difference of motion vectors extracted from the error detection / recovery unit 5 from the image frame memory 15 storing a previous image frame reproduced by the display unit 10. Compensate the video.

Therefore, the motion compensator 12 transmits the difference obtained by subtracting the current motion vector value from the previous video frame to the adder 11, and when the compressed bitstream is an inter mode in which the previous video frame exists, The image is reproduced by adding the inverse DCT values.

The reconstructed image frame is divided into macroblock units, and the data of each macroblock includes a motion vector while being decoded, and these motion vectors indicate a predetermined direction. Video compression technology reduces the compression rate by taking a macro block of a previous video frame and restoring the current video frame by using the motion vector.

In particular, in video standard codecs such as h.263 + and MPEG4, the blocks in which an error occurs in the decoding process can be known in advance. When decoding by intra mode prediction using such characteristics, It is not used as a reference block for prediction.

4 is a diagram illustrating a process of obtaining a prediction value for a block decoded in an intra mode image frame according to the present invention.

As shown in FIG. 4, when intra mode decoding is performed in the decoder, when the MPEG 4 standard is used, it is possible to know whether an error has occurred in blocks of the decoded image frame based on information about blocks encoded in the encoder. have. As described above, when decoding an image bitstream, intra mode prediction is performed by using characteristics of the MPEG4 standard that can determine an error block.

When an image frame decoded by the decoder is reproduced by intra mode prediction, when an error occurs in a block to be referred to by a block to be decoded using the block information in which error occurs, it is not used as a reference block for intra prediction.

In general, when an error occurs in any one of the blocks of the image frame, the error concealment method is replaced with the most similar image block and error concealment is then decoded.

As described above, when decoding an image frame according to an error concealment method, a certain portion is compensated, but as a rule, blocks containing errors continue to accumulate and cause image quality deterioration. There is a problem that the amount of calculation increases.

In the present invention, the deterioration of image quality and the accumulation of errors are fundamentally prevented by not referring the macroblock in which an error occurs to the macroblock currently decoded from the beginning.

As shown in the figure, an 8 * 8 block in a macroblock in which an error does not occur among blocks referenced by an intra mode decoding among 8 * 8 blocks of a macroblock in which no error occurs is used for intra prediction. In addition, 8 * 8 blocks in the macro block in which the adjacent error occurs are not referenced during intra mode decoding.

That is, when there is a macroblock in which an error occurs, it is possible to prevent error accumulation and to improve image quality by not using the prediction block when performing intra mode prediction decoding.

In addition, since the error block is not compensated or referenced according to an error concealment process, the decoding calculation amount is reduced, and thus the decoding efficiency can be improved.

5 is a flowchart for explaining an intra mode decoding method according to the present invention.

As shown in FIG. 5, the decoding method of the intra mode image frame displaying the first image in operation S501 of decoding the image bitstream applied to the decoder.

That is, since the previous video frame has not yet been decoded at the decoder because it is the first video frame of the video bitstream to be decoded, intra mode decoding is usually performed.

However, if there is a previous decoded image frame for the decoded image frame, the difference vector value of the image block of the previous image frame corresponding to the block of the image frame currently decoded in the inter mode and the intermediate value of the previous image block are determined. To decode.

In the case of the inter mode as described above, since the information of the previously decoded video frame is used and the encoder encodes only the difference vector of the referenced video block, the coding efficiency can be increased.

The present invention relates to intra mode predictive decoding for reproducing an image by referring to an image block in itself by performing intra mode prediction when a previously decoded image frame does not exist or a macro block in the image frame is an intra mode block.

When the video bitstream applied to the decoder is decoded to make the first video frame, the MPEG 4 standard may determine an error block in the video frame while decoding.

Using such a characteristic, the macroblock or 8 * 8 block information in the image frame in which an error occurs is stored (S502) and then decoded using the same in intra mode prediction.

If an error block exists due to the decoding of the intra mode image frame (S503), a block in which an error occurs among three previous image blocks referred to by the block currently decoded by intra mode prediction is used as a reference block for prediction. Do not do it.

As such, when there are macroblocks and 8 * 8 blocks in which an error occurs, error accumulation is prevented by fundamentally not using them in the decoding process by intra mode prediction.

If no error block exists in the decoded video frame (S503), a prediction value is obtained by referring to three adjacent blocks according to a general scheme, and intra-mode decoding is performed using the same.

As described in detail above, the present invention uses an intra mode prediction method when an error occurs in a block of an image frame decoded by a decoder due to a channel error in an image bitstream transmitted through an IMT-2000 wireless network. When decoding is performed, the image quality deterioration is prevented by not using it as a reference block for prediction.

In addition, since a block in which an error occurs is not used as a reference block for prediction, the amount of decoding calculation is reduced and an error accumulation is prevented.

The present invention is not limited to the above-described embodiments, and various changes can be made by those skilled in the art without departing from the gist of the present invention as claimed in the following claims.

Claims (3)

Decoding a video bitstream applied to the decoder; Storing information determining whether there is an error of macroblocks in an image frame while decoding the video bitstream; And When intra mode prediction is performed on macro blocks of an image frame using error information of the macro blocks, intra prediction is sequentially performed using a block in which there is no error as a reference block of a block currently performing intra mode decoding. The video decoding method comprising a; step. The method of claim 1, The block in which the error occurs among the macroblocks is not used as a reference block for prediction in the decoding process. The method of claim 1, And decoding the macroblocks in the image frame by intra mode prediction using the intra mode prediction, thereby performing intra prediction on only macroblocks in which no error occurs.