MPEG video decoding method and MPEG video decoder using results from analysis of motion-vector data and DCT coefficients

An MPEG video decoding method and an MPEG video decoder are provided. The method includes determining whether to perform motion compensation on motion-vector-decoded data or not depending on a value of a decoded motion vector, determining whether to perform inverse discrete cosine transformation (IDCT) on motion-compensated data or not depending on values of decoded DCT coefficients, and generating a decoded image based on the results of the two determinations.

This application claims the priority of Korean Patent Application No. 10-2002-0075398, filed on Nov. 29, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an MPEG video decoding method and an MPEG video decoder.

2. Description of the Related Art

In recent years, strenuous effort has been made to provide methods for reproducing moving pictures in a mobile device, such as a mobile phone or a personal digital assistant (PDA). Since mobile devices are required to have low power consumption, they inevitably have limitations in terms of bandwidth and storage capacity, developing the need for a moving picture decoder that can operate at higher speeds and utilize memory more efficiently.

Even though various moving picture compression standards have already been suggested, H.263 and MPEG-4 simple profiles are considered the best choices for mobile wireless communications. MPEG-4 provides tolerance to channel errors, includes various functionality applicable to limited bandwidth and defines a streaming video profile. MPEG-4 has a high data compression rate. In order to support a high data compression rate, a considerable amount of encoder and decoder calculations are required. In short, the complicated structure of MPEG-4 makes it difficult to realize software that can perform real-time operations appropriate for MPEG-4.

FIG. 1is a diagram illustrating the data hierarchy of MPEG video. The hierarchy is comprised of six levels: a sequence layer, a group of pictures (GOP) layer, a picture layer110, a slice layer120, a macroblock layer130, and a block layer140. InFIG. 1, the picture layer110and the underlying layers120,130, and140are shown.

Referring toFIG. 1, the picture layer110is a picture image comprised of slices111having a predetermined length, and the slice layer120is a band of an arbitrary number of macroblocks. The macroblock layer130is comprised of macroblocks, each containing six 8×8 pixel blocks, i.e., four blocks of a brightness signal Y and two blocks of color difference signal (Cb and Cr). The block layer140is comprised of 8×8 pixel blocks and includes discrete cosine transform (DCT) coefficient information.

The macroblock layer130includes motion vector information. The motion vector information is a value obtained by encoding the difference between motion vectors of a current macroblock and a previous macroblock.

Hereinafter, an MPEG-4 encoding method will be briefly described before explanation of the structure and operation of an MPEG-4 video decoder.

An input video object plane (VOP) is divided into macroblocks. The VOP is the basic unit of data in an MPEG-4 encoding process. During this process, each 8×8 block of a macroblock is DCTed and quantized. Thereafter, quantized DCT coefficients and a quantization width are encoded by using a variable length encoding method. This entire process is called intra-frame encoding.

A separate encoding process begins by using a motion detection method, such as block-matching. This technique consists of comparing macroblocks in temporally adjacent VOPs. After identifying the predicted macroblock having the least difference with a target macroblock, the motion variation signal, or motion vector, is obtained. The VOP of the predicted macroblock is called the reference VOP. By performing motion compensation on the reference VOP, based on the motion vector, the predicted macroblock can be obtained. Thereafter, the motion variation is DCTed and the DCT coefficients are quantized. The quantized DCT coefficients, the motion vector, and a quantization width are encoded by using a variable length encoding method. This entire process is called inter-frame encoding.

A receiving party decodes compressed and encoded VOP data by using a variable length decoding method. The difference signal is restored by performing inverse quantization and inverse DCT on quantized DCT coefficients. Thereafter, a predicted macroblock is obtained based on a motion vector and is added to the differential signal, thus reproducing image data.

FIG. 2is a block diagram of a conventional MPEG-4 video decoder. Referring toFIG. 2, a conventional MPEG-4 video decoder200includes a demultiplexer210for an MPEG-4 video bitstream input thereinto, a texture decoding unit220for performing texture decoding, a restructured VOP storing unit230, and a motion compensation unit240.

The texture decoding unit220includes a variable length decoder (VLD)221, an inverse scanner222, an inverse quantizer223, and an inverse discrete cosine transformer224. The motion compensation unit240includes a motion decoder241, a VOP memory242, and a motion compensator243.

Hereinafter, a general inter-frame reproducing method will be described. Intra-frame reproduction is the same as the inter-frame reproduction except for the process of motion compensation.

Through syntax-parsing of the bitstream, the header is separated and image data is extracted. Then, the variable length decoder221creates DCT coefficients through Huffman decoding of the image data, and the inverse scanner222creates data having the same order as the image data through inverse scanning.

The inverse quantizer223inversely quantizes the inversely-scanned data, and the inverse discrete cosine transformer224creates a differential image by performing DCT. When creating the differential image, a VOP for the differential image is generated by repeatedly performing macroblock-wise decoding. Then the restructured differential image VOP is stored in the VOP memory242. When the differential image VOP is completed through texture decoding, motion decoding is performed utilizing a motion vector.

The motion decoder241creates a predicted image by decoding a motion vector. The motion compensator243adds this predicted image to the differential image stored in the VOP memory242and creates a decoded image.

FIG. 3is a flowchart of a decoding method300performed in the conventional MPEG-4 video decoder200shown inFIG. 2. Referring toFIG. 3, information on the current macroblock is obtained in step S301by decoding the header of the current macroblock. A motion vector is obtained in step S302, and the decoded motion vector is stored in motion vector memory in step S303. Thereafter, DCT coefficients are decoded in step S304.

A differential image macroblock is created by inversely scanning the decoded DCT coefficients in step S305, inversely quantizing the inversely-scanned DCT coefficients in step S306, and inversely discrete-cosine-transforming the inversely quantized DCT coefficients in step S307. The differential image macroblock is recorded in a frame buffer in step S308.

Thereafter, in step S309, it is determined whether or not all macroblocks belonging to one frame have been decoded.

If it is determined that all the macroblocks belonging to one frame have not yet been decoded, then steps S301through S308are repeated until one frame is generated.

If it is determined that all the macroblocks belonging to one frame have been decoded, i.e., if one frame is completed, then motion compensation is performed in step S310. Motion compensation represents a process of creating a predicted image macroblock. Through reference to motion vector memory, the motion vector is obtained and applied to previous image data to produce the predicted image macroblock.

Thereafter, in step S311, the motion-compensated predicted image frame is added to a differential image stored in a frame buffer. When the frame of the motion-compensated predicted image is added to the frame of the differential image recorded in the frame buffer, a decoded image frame is created.

FIG. 4is a diagram illustrating unit frames used to generate a decoded image, according to a conventional MPEG-4 video decoding method. Referring toFIG. 4, in the prior art, a decoded image VOP430is generated by adding a predicted image VOP410and a differential image VOP420.

Specifically, macroblocks1through16of the predicted image constitute the predicted image VOP410, and macroblocks1′ through16′ of the differential image constitute the differential image VOP420. Thereafter, macroblocks1″ through16″ of the decoded image VOP430are generated by adding the macroblocks1through16of the predicted image VOP410to the macroblocks1′ through16′ of the differential image VOP420. In other words, the macroblock1′ of the decoded image VOP430is generated by adding the macroblock1of the predicted image VOP410and the macroblock1′ of the differential image VOP420. The macroblock2″ of the decoded image VOP430is generated by adding the macroblock2of the predicted image VOP410and the macroblock2′ of the differential image VOP420. In the same manner, the rest of the macroblocks3″ through16″ of the decoded image VOP430are generated by adding the macroblocks3through16of the predicted image VOP410and their corresponding macroblocks3′ through16′ of the differential image VOP420.

According to the preceding conventional decoding method, a decoded image is stored in the current frame memory and preserved until a next frame is decoded. The current frame memory is always filled with the most recent decoded images. Accordingly, even when an image currently being decoded is the same as the previous image, the current image unnecessarily undergoes the same procedures used to generate the previous image. This aspect of the conventional method crates inefficiency in the decoding process.

Furthermore, according to the conventional decoding method, inverse DCT is performed first, a complete differential image frame is stored in a frame buffer, and then motion compensation is performed. Therefore, even in a block with a motion vector of 0, it is necessary to create a predicted image based on a previous image and store macroblocks of the predicted image. Since having a motion vector of 0 indicates portions of the previous image and the predicted image are the same, it is a waste of memory space to store the predicted image separately from the previous image.

Moreover, according to the present invention, texture decoding and motion compensation are sequentially performed. Thus, a motion vector generated during a variable length decoding process is stored in memory until referenced for motion compensation. In which case motion vector memory must be sufficient to support the number of macroblocks in a VOP.

In the case of decoding high compression rate data, inverse DCT and motion compensation may not always be necessary. However, the performance advantages gained by omitting these routines are not applicable to the conventional MPEG-4 video decoder. In creating a decoded image, by adding a differential image and a predicted image, the conventional MPEG-4 video decoder is further constrained by memory demands.

SUMMARY OF THE INVENTION

The present invention provides an MPEG video decoding method and an MPEG video decoder, which are capable of conserving memory capacity and increasing the speed of decoding.

According to an illustrative aspect of the present invention, there is provided an exemplary MPEG video decoding method. The method includes determining whether to perform motion compensation on motion-vector-decoded data or not depending on a value of a decoded motion vector, determining whether to perform inverse discrete cosine transformation (IDCT) on motion-compensated data or not depending on values of decoded DCT coefficients, and generating a decoded image based on the results of the two determination steps.

Preferably, but not necessarily, determining whether to perform motion compensation or not includes determining whether or not the decoded motion vector is 0, and determining not to perform motion compensation if the decoded motion vector is 0 and determining to perform motion compensation if the decoded motion vector is not 0.

Preferably, but not necessarily, determining whether to perform inverse DCT or not includes determining whether or not the value of each decoded DCT coefficient is 0, and determining not to perform inverse DCT if the value of each decoded DCT coefficient is 0 and determining to perform inverse DCT if the value of any decoded DCT coefficient is not 0.

According to another illustrative aspect of the present invention, there is provided an exemplary MPEG video decoding method. The method includes generating a predicted image macroblock, generating a differential image macroblock, generating a decoded image macroblock by adding the predicted image macroblock and the differential image macroblock, writing the decoded image macroblock in a frame buffer, and filling the frame buffer with decoded image macroblocks by circularly performing the previous steps.

According to another illustrative aspect of the present invention, there is provided an exemplary MPEG video decoding method. The method includes generating a predicted image macroblock depending on a value of a decoded motion vector, writing the predicted image macroblock in a macroblock buffer, generating a differential image macroblock depending on values of decoded DCT coefficients, generating a decoded image macroblock by adding the differential image macroblock to the predicted image macroblock written in the macroblock buffer, and writing the decoded image macroblock in a frame buffer.

Preferably, but not necessarily, generating the predicted image macroblock includes determining whether or not the decoded motion vector is 0, determining a previous image macroblock as the predicted image macroblock if the decoded motion vector is 0, and generating the predicted image macroblock by performing motion compensation on the previous image macroblock if the decoded motion vector is not 0.

Preferably, but not necessarily, generating the differential image macroblock includes determining whether or not the value of each decoded DCT coefficient is 0, determining not to generate the differential image macroblock if the value of each decoded DCT coefficient is 0, and generating the differential image macroblock by performing inverse DCT if the value of any decoded DCT coefficient is not 0. Here, if the differential image macroblock is not generated, adding the differential image to the predicted image is skipped.

According to another illustrative aspect of the present invention, there is provided an exemplary MPEG video decoder. The MPEG video decoder includes a motion vector determiner determining whether to perform motion compensation or not depending on a value of a decoded motion vector, and a DCT coefficient determiner determining whether to perform inverse discrete cosine transform (IDCT) or not depending on values of decoded DCT coefficients. Here, an MPEG video stream is decoded based on determinations of the motion vector determiner and the DCT coefficient determiner.

Preferably, but not necessarily, the motion vector determiner determines not to perform motion compensation if the decoded motion vector is 0, and determines to perform motion compensation if the decoded motion vector is not 0.

Preferably, but not necessarily, the DCT coefficient determiner determines not to perform inverse DCT if the value of each decoded DCT coefficient is 0, and determines to perform inverse DCT if the value of any decoded DCT coefficient is not 0.

According to another illustrative aspect of the present invention, there is provided an exemplary MPEG video decoder. The MPEG video decoder includes a predicted image calculation unit generating a predicted image macroblock, a differential image calculation unit generating a differential image macroblock, a macroblock buffer where the predicted image macroblock and the differential image macroblock are added, and a frame buffer where a decoded image macroblock is written, after the decoded image macroblock is generated by adding the predicted image macroblock and the differential image macroblock in the macroblock buffer.

According to another illustrative aspect of the present invention, there is provided an exemplary MPEG video decoder. The MPEG video decoder includes a predicted image calculation unit generating a predicted image macroblock depending on a value of a decoded motion vector, a differential image calculation unit generating a differential image macroblock depending on values of decoded DCT coefficients, a macroblock buffer where the predicted image macroblock and the differential image macroblock are added, and a frame buffer where a decoded image macroblock is written, after the decoded image macroblock is generated by adding the predicted image macroblock and the differential image macroblock in the macroblock buffer.

Preferably, but not necessarily, the predicted image calculation unit includes a motion vector determiner determining whether or not the decoded motion vector is 0, and a motion compensator performing motion compensation depending on a result of the determination.

Preferably, but not necessarily, the differential image calculation unit includes a DCT coefficient determiner determining whether or not the value of each decoded DCT coefficient is 0, and an inverse discrete cosine transformer performing inverse DCT depending on a the result of the determination.

DETAILED DESCRIPTION OF THE INVENTION

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

First of all, an MPEG-4 video decoding method, according to an exemplary embodiment of the present invention, will be described in the following paragraphs. More specifically, the description will detail how unit data is processed to generate a decoded image.

By processing data at the macroblock level, rather than the VOP level, the present invention conserves memory capacity necessary for MPEG-4 decoding. In the present invention, a predicted image macroblock is generated and then recorded in a macroblock buffer. Thereafter, a differential image macroblock is generated and added to the predicted image macroblock in the macroblock buffer. Thereafter, the result of the adding is written in a frame buffer. For example, as shown inFIG. 5, a macroblock510of a predicted image is written in a macroblock buffer530, and then a macroblock520of a differential image is added to the macroblock510written in the macroblock buffer530, thus generating a macroblock of a decoded image. Thereafter, the decoded image macroblock is written in a predetermined part, e.g.,541,542or543, of a frame buffer540.

In the present invention, when generating a predicted image macroblock and a differential image macroblock, it is determined first whether or not a motion vector is 0 and whether or not the number of DCT coefficients is 0. Specifically, motion compensation is performed only if a motion vector is not 0. When the motion vector is 0, motion compensation is not performed. A differential image is generated only when a value of a DCT coefficient is not 0. When the value of each DCT coefficient is 0, a previous differential image is directly used rather than generating a new differential image.

FIG. 6is a block diagram of an MPEG-4 video decoder600according to an exemplary embodiment of the present invention. Referring toFIG. 6, the MPEG-4 video decoder600includes a macroblock-wise processing unit610comprised of a predicted image calculation unit620and a differential image calculation unit630, a macroblock buffer640, and a frame buffer650.

The predicted image calculation unit620decodes a motion vector, determines whether or not the decoded motion vector satisfies a predetermined condition, and then performs motion compensation depending on the result of that determination. The differential image calculation unit630decodes DCT coefficients, determines whether or not the decoded DCT coefficients satisfy a predetermined condition, and then generates a differential image based on the result of this determination.

The macroblock buffer640generates decoded image macroblocks by adding the predicted image macroblocks created by the predicted image calculation unit620and the differential image macroblocks created by the differential image calculation unit630.

The frame buffer650receives decoded image macroblocks from the macroblock buffer640individually, and then stores them in a single frame.

FIG. 7is a block diagram of the predicted image calculation unit620shown inFIG. 6. Referring toFIG. 7, the predicted image calculation unit620includes a motion vector decoder621, a motion vector determiner622, and a motion compensator623.

The motion vector decoder621decodes a motion vector by using a variable length decoding method. The motion vector determiner622determines whether or not the motion vector, decoded by the motion vector decoder621, is 0. Whether to perform motion compensation or not depends on the result of this determination. If the decoded motion vector is 0, motion compensation is not performed, and macroblocks of a previous frame are used in subsequent decoding processes. If the decoded motion vector is not 0, motion compensation is performed.

The motion compensator623performs motion compensation on the macroblocks of the previous frame referring to the decoded motion vector. When the decoded motion vector is 0, the macroblocks of the previous frame are written directly in the macroblock buffer640. Otherwise, macroblocks obtained by performing motion compensation on the macroblocks of the previous frame are written in the macroblock buffer640.

Therefore, the predicted image macroblocks are written in the macroblock buffer640by the predicted image calculation unit620.

FIG. 8is a block diagram of the differential image calculation unit630shown inFIG. 6. Referring toFIG. 8, the differential image calculation unit630includes a DCT coefficient decoder631, a DCT coefficient determiner632, an inverse quantizer633, and an inverse discrete cosine transformer634.

The DCT coefficient decoder631decodes DCT coefficients by using a variable length decoding method. The DCT coefficient determiner632determines whether or not a value of each DCT coefficient, decoded by the DCT coefficient decoder631, is 0. Whether to perform inverse quantization and inverse DCT or not depends on the result of this determination. If the value of each decoded DCT coefficient is 0, inverse DCT is not performed, and accordingly, a differential image is not generated. If the value of any decoded DCT coefficient is not 0, inverse DCT is performed.

The inverse quantizer633inversely quantizes a quantized DCT coefficient, and the inverse discrete cosine transformer634performs IDCT on the inversely-quantized DCT coefficient.

Accordingly, macroblocks of a differential image are generated by the differential image calculation unit630. In a case where the differential image is not generated because the value of each DCT coefficient is 0, a process of adding the macroblocks of the differential image and the macroblocks of the predicted image is omitted. Only when the differential image is generated, is the adding process performed and the results of the adding process are written in the macroblock buffer640.

The macroblock buffer640is filled with decoded image macroblocks, which are sent individually to the frame buffer650.

FIG. 9is a flowchart of a decoding method S900performed in an MPEG-4 video decoder according to an exemplary embodiment of the present invention. Referring toFIG. 9, a header of a macroblock is decoded in step S901.

A motion vector is decoded in step S902using a variable length decoding method.

It is determined whether or not the decoded motion vector is 0 in step S903.

The method proceeds to step S906if the decoded motion vector is 0. This transmission indicates that a macroblock of a previous image frame has not had any motion variations, and thus the macroblock of the previous image frame is the same as the macroblock of the current image frame. In this scenario, motion compensation is not performed and the macroblock of the previous image frame is written in the macroblock buffer as the macroblock of the current image frame. Therefore, if the current image is the same as the previous image, the macroblock buffer utilizes previous image information stored in frame memory.

If the motion vector is not 0, motion compensation is performed on the macroblock of the previous image frame in step S904referring to the decoded motion vector.

The result of the motion compensation is written in the macroblock buffer in step S905.

Thereafter, DCT coefficients are decoded using a variable length decoding method in step S906.

It is determined whether or not each decoded DCT coefficient is 0 in step S907. If each decoded DCT coefficient is 0, which means there is no difference between the current image and the previous image, the method directly proceeds to step S911without performing texture decoding, i.e., without performing inverse DCT.

However, if a decoded DCT coefficient is not 0, a differential image is generated by performing inverse quantization in step S908and performing inverse DCT in step S909.

Thereafter, in step S910, the differential image macroblock is added to the predicted image macroblock already written in the macroblock buffer, and the result of the adding is written in a frame buffer in step S911.

Thereafter, in step S912, it is determined whether or not all macroblocks belonging to one frame have been decoded. If all the macroblocks of a single frame have already been decoded, the whole decoding process is complete. But, if there are any remaining macroblocks to be decoded, the method returns to step S901.

Since the present invention allows both motion compensation and texture decoding to be performed immediately on each macroblock, there is no need for motion vector memory to support the number of macroblocks in a VOP.

Hereinafter, with reference toFIGS. 10A through 10C, various performance simulations of an MPEG video decoding method, according to an exemplary embodiment of the present invention, will be described.

In the simulations, shown inFIGS. 10A through 10C, an optimized variation of the Microsoft reference source was used on a Windows 2000-based Intel Pentium III 866 MHz PC platform. In addition, Akiyo (QCIF), Foreman (CIF), and Mobile (CIF) were used as test images. Data was compressed to have an I frame-to-a P frame ratio of 1:30, and bilateral prediction was not performed. Motion compensation has been carried out. An H.263 quantization method was used, and then nine images were obtained by applying different quantization parameters (QP), i.e., 5, 12, and 20 to each of the test images Akiyo, Foreman, and Mobile.

FIG. 10Ashows rates of blocks, which have not been subject to texture decoding in each of the test images, andFIG. 10Bshows rates of blocks, which have not been subject to motion compensation in each of the test images.

FIG. 10Cis a table showing how the nine images were used for comparing the performance of a conventional algorithm with that of an optimized algorithm.FIG. 10Cshows that the optimized algorithm, i.e., the decoding method of a preferred embodiment of the present invention, is superior in terms of decoding speed to the conventional algorithm, i.e., the Microsoft reference. In the case of decoding the test images Akiyo and Foreman, the optimized algorithm has decoding speeds at least two times faster than the conventional algorithm irrespective of the quantization parameter (QP).

According to an exemplary embodiment of the present invention, it is possible to conserve memory capacity by generating a decoded image macroblock by macroblock in MPEG video decoding. In addition, in an exemplary embodiment of the present invention, motion compensation is not performed when a motion vector is 0, and inverse DCT is not performed when the value of each DCT coefficient is 0. Thus, the speed of decoding can be considerably increased.