JP4100067B2 - Image information conversion method and image information conversion apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image information conversion method and an image information conversion apparatus, and more particularly, to image information (bitstream) compressed by orthogonal transform such as discrete cosine transform and motion compensation, for a network such as satellite broadcasting, cable TV, and the Internet. The present invention relates to an image information conversion method and an image information conversion apparatus that are used when receiving data via a network or processing on a storage medium such as an optical disk, a magnetic disk, or a flash memory.
[0002]
[Prior art]
In recent years, when image information is handled as digital data, an image information conversion method and apparatus that realizes efficient transmission and storage of information using redundancy unique to image information has been developed between a broadcasting station and a general household. It is spreading in information distribution.
[0003]
Such an image information conversion apparatus conforms to a method of compressing image data by orthogonal transform such as discrete cosine transform and motion compensation, for example. In particular, an image coding method standardized by the Moving Picture Experts Group (MPEG) is defined in ISO / IEC 13818-2 as a general-purpose image coding method. It covers both progressively scanned images, standard resolution images and high definition images. Therefore, MPEG is expected to be used in a wide range of applications from professional use to consumer use. Using this MPEG system, for example, a standard resolution interlaced scanned image having 720 × 480 pixels has a high resolution having 1920 × 1088 pixels in a code amount of 4 to 8 Mbps (hereinafter referred to as a bit rate). Since the interlaced scanned image is assigned to a bit rate of 18 to 22 Mbps, a high compression rate can be realized while maintaining good image quality.
[0004]
In an image information conversion apparatus that compresses image data by motion compensation and discrete cosine transform as in the MPEG system, an intra-image encoded image is used in each macroblock as an encoding unit in image data, or inter-image encoding is performed. Determination of whether to use an image, determination of whether to use a forward predictive encoded image, a backward predictive encoded image, or a bidirectional predictive encoded image as a reference image frame, and a frame as motion compensated prediction It is determined whether to use prediction or field prediction.
[0005]
For example, in a moving image compression encoding system represented by MPEG2, a method for determining whether to use frame prediction or field prediction is not particularly defined as a standard.
[0006]
However, in the conventional image information conversion apparatus, generally, for the purpose of reducing the calculation amount or the circuit scale, a frame vector and a field vector are obtained using a luminance signal, and further, a field and a frame are obtained using the luminance signal. A prediction image that is determined to be advantageous for compression is selected by calculating prediction residuals between the two and comparing these prediction residuals.
[0007]
Actually, it is known that when frame prediction is used, the number of encoded vectors is halved and compression efficiency is improved. Therefore, in general, a determination criterion is set so that frame prediction is easily selected. There is also known a method of reducing the amount of motion vector computation and reducing the circuit scale involved in computation by determining which prediction method is used before obtaining a motion vector.
[0008]
[Problems to be solved by the invention]
As an example of a method for determining whether to use frame prediction or field prediction in a conventional image information conversion apparatus conforming to the MPEG system, for example, an image information conversion apparatus corresponding to a moving image compression encoding system typified by MPEG2. For example, a method of calculating a square prediction error of a motion vector when using each prediction method and comparing these is selected.
[0009]
For example, when a series of image data such as a moving image is compressed, a case will be described in which frame prediction is selected as an inter-picture prediction method for image data, particularly for an input image in 4: 2: 0 format.
[0010]
As described above, compression efficiency is usually good when frame prediction is selected as the prediction method. However, in the 4: 2: 0 format, when frame prediction is selected as a prediction method between pictures, as shown in FIG. 5, depending on the phase of the vertical component of the motion vector, the selected color difference signal may be a luminance signal. In some cases, the pixel of the field opposite to the field of. In particular, when the motion is large, it is known that when the motion vector of the luminance signal and the motion vector of the chrominance signal indicate opposite fields, the correlation of the chrominance signal decreases between the top field and the bottom field. As a result, the compression efficiency of the color difference signal is deteriorated, which may cause image quality deterioration.
[0011]
Therefore, the present invention has been proposed in view of such conventional situations, and it is determined whether or not the prediction method causes deterioration in image quality, and prediction that realizes improvement in coding efficiency and image quality at a low bit rate. An object of the present invention is to provide an image information conversion method and an image information conversion apparatus for selecting a method.
[0012]
[Means for Solving the Problems]
In order to achieve the above-described object, an image information conversion method according to the present invention is an image information conversion method in which image data is compressed by applying orthogonal transform and motion compensation to obtain compressed image information. When performing motion compensation prediction in a frame in which the top field and the bottom field are combined as unit motion compensation prediction, a pixel in a field opposite to the field in which the motion vector in the chrominance signal has a pixel pointed to by the motion vector in the luminance signal exists , The motion compensated prediction of the coding unit is changed to field prediction.
[0013]
Here, when the magnitude of the motion vector of the luminance signal exceeds a predetermined threshold, the determination of the encoding mode can be changed to field prediction.
[0014]
In particular, the image compression information preferably conforms to the MPEG (Moving Picture Experts Group) standard.
[0015]
Also, the image information conversion apparatus according to the present invention is the top as motion compensation prediction in the encoding unit of image data in the image information conversion apparatus that obtains image compression information by compressing image data by applying orthogonal transform and motion compensation. When motion compensation prediction is performed in a frame in which the field and the bottom field are combined, it is determined whether or not the motion vector in the color difference signal indicates a pixel in a field opposite to the field in which the pixel indicated by the motion vector in the luminance signal exists And a prediction mode control means for controlling whether the motion compensated prediction of the coding unit is to be frame prediction or field prediction according to the motion vector, wherein the motion vector in the chrominance signal is a luminance When the pixel of the field opposite to the field where the pixel indicated by the motion vector in the signal exists If it is the cross-sectional, the prediction mode control means is characterized in that changing the motion compensation prediction of the coding unit in the field prediction.
[0016]
Here, when the magnitude of the motion vector exceeds a predetermined threshold, the determination of the encoding mode can be changed to field prediction.
[0017]
In particular, the image compression information preferably conforms to the MPEG (Moving Picture Experts Group) standard.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
An image information conversion apparatus shown as a specific example of the present invention is an image information conversion apparatus that compresses image data by motion compensation and discrete cosine transform, and in each macroblock as a coding unit in the image data, the motion of the image data Enhancing coding efficiency and image quality at a low bit rate by using not only luminance information but also information such as motion vector values and phases in determining whether to use frame prediction or field prediction as compensation prediction It is a device that realized.
[0019]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The configuration of an image information conversion apparatus shown as a specific example of the present invention will be described with reference to FIG.
[0020]
An image information conversion apparatus 1 shown in FIG. 1 is an image information conversion apparatus that conforms to the MPEG (Moving Picture Experts Group) method, particularly the MPEG2 method, and mainly receives image data. A configuration for pre-processing, a configuration for determining an encoding mode by motion prediction and motion compensation, a configuration for performing discrete cosine transform (DCT) and quantization, and a configuration for outputting image data compressed by these transform processing It has.
[0021]
Specifically, the image information conversion apparatus 1 includes an image data input terminal 11 and a screen rearrangement circuit 12 as a configuration for inputting and preprocessing image data.
[0022]
This image data is a video signal composed of luminance (Y) and color difference signals (Cb, Cr), and although not shown, for example, it is digitized before being input from the input terminal 11, and thereafter The format has been converted to the spatial resolution used in the encoding.
[0023]
In the screen rearrangement circuit 12, this image data is converted into three pictures, i.e., I picture (intra-frame predictive encoded image), P picture (inter-frame forward predictive encoded image), and B picture (inter-frame bi-directional predictive encoded image). Sorted by each picture type. Each picture of the input image data is subjected to motion compensation and DCT encoding in this encoding order in a motion compensation circuit 24 and a DCT circuit 14 described later.
[0024]
The image information conversion apparatus 1 further includes, as a configuration for determining an encoding mode by motion prediction and motion compensation, and a configuration for performing discrete cosine transform (DCT) and quantization, specifically, a differencer 13 and a DCT circuit 14. A quantization circuit 15, a motion compensation circuit 24, and a motion prediction circuit 25. The motion compensation circuit 24 and the motion prediction circuit 25 may be integrated.
[0025]
The subtractor 13 calculates a prediction error by taking the difference between the I picture or P picture supplied from the image rearrangement circuit 12 and the image obtained by the prediction, and supplies this value to the DCT circuit 14. The DCT circuit 14 converts the image data into DCT coding coefficients. The quantization circuit 15 quantizes the DCT coding coefficient here based on the quantization scale determined by the rate control circuit 16. The quantized data is variable-length encoded in the variable-length encoding circuit 17 together with the motion vector and encoding mode information, and then stored in the buffer 18 and output as an MPEG video bit stream in accordance with the target bit rate. Output from terminal 19.
[0026]
Also, if the image data is an I picture and a P picture, it is necessary to use this DCT coding coefficient as a reference screen for motion compensation prediction in later processing. The image is locally decoded by the inverse DCT circuit 21, restored as the same image as that obtained by a decoder (not shown), passes through the adder 22, and is stored in the image memory 23.
[0027]
Next, the flow of the encoding prediction process in the image information conversion apparatus 1 shown in FIG. 1 will be described with reference to FIG. FIG. 2 shows a series of prediction mode determination processes in the image information conversion apparatus 1 as processing blocks. The motion vector calculation block 41 and the frame prediction / field prediction determination block 42 shown in FIG. This corresponds to the processing in the prediction circuit 24. The encoding block 43 corresponds to the processing in the configuration excluding the input terminal 11, the output terminal 19, the screen rearrangement circuit 12, and the motion prediction circuit 25 of FIG.
[0028]
The image data including the input color difference signal (input image color difference signal 30) and the luminance signal (input image luminance signal 31) is sent to the encoding block 43. At this time, the input image luminance signal 31 is also sent to the motion vector calculation block 41.
[0029]
The field prediction motion vector 32 and the frame prediction motion vector 33 calculated from the input image luminance signal 31 by the motion vector calculation block 41 are sent to the encoding block 43. The field prediction residual 34 and the frame prediction residual 35 calculated by the motion vector calculation block 41 are sent to the frame prediction / field prediction determination block 42. At this time, the field prediction motion vector 32 and the frame prediction motion vector 33 are also sent to the frame prediction / field prediction determination block 42. In the frame prediction / field prediction determination block 42, whether to use frame prediction or field prediction is selected from the field prediction motion vector 32, the frame prediction motion vector 33, the field prediction residual 34, and the frame prediction residual 35.
[0030]
The motion compensation circuit 24 is, for example, a DSP (digital signal processor). As one method, the motion compensation circuit 24 calculates a motion vector square prediction error in the case of field prediction and frame prediction, and compares them to determine the minimum. The prediction method is determined. Here, the size comparison may be performed by adding an offset to one of the prediction residuals so that frame prediction with good compression efficiency is easily selected.
[0031]
The determination result of the frame / field prediction in the frame prediction / field prediction determination block 42 is sent to the encoding block 43 as the frame / field prediction selection signal 36, and the encoding block 43 uses the motion vector of the selected prediction method. To encode. Then, the bit stream 38 is output from the variable length coding circuit 17 shown in FIG.
[0032]
Next, an example of the encoded prediction mode determination process in the motion compensation circuit 24 and the motion prediction circuit 25 will be described with reference to FIG. In the image information conversion apparatus 1 shown as a specific example of the present invention, when determining the encoding prediction mode, the phase or magnitude of the value of the motion vector is used for the encoding prediction mode determined from the luminance information. An optimal prediction mode with ideal encoding efficiency and little image degradation is selected.
[0033]
First, in step S1, the motion compensation circuit 24 determines whether to use frame prediction or field prediction as a coding unit of image data from luminance information based on a conventional general-purpose method.
[0034]
When field prediction is selected in step S1, the encoding prediction method described above is repeated for the next macroblock without changing the prediction method for this macroblock.
[0035]
On the other hand, if frame prediction is selected in step S1, whether or not the motion vector of the chrominance signal points to a pixel on the opposite field to the pixel indicated by the motion vector of the luminance signal in the frame prediction reference image in step S2 Is determined. Here, for example, the determination is made based on the following conditions. That is, when the remainder of dividing the vertical component of the frame prediction motion vector by 4 is 2, the selected color difference signal is a pixel on the opposite field (reverse phase).
if ((vertical component of the magnitude of the frame prediction motion vector)% 4 == 2) At this time, the color difference signal is in reverse phase.
[0036]
else Not in reverse phase.
[0037]
If it is determined in step S2 that the selected color difference signal is a pixel on the opposite field (reverse phase), the compression efficiency is reduced in the frame prediction, so the field prediction is changed in step S3. In addition, if it is not detected that the motion vector of the color difference signal indicates a pixel in reverse phase, the compression efficiency is not lowered even in the frame prediction, and the frame compression is applied because good compression efficiency is obtained.
[0038]
If the determination of all macroblock types in this frame is completed in step S4, the process proceeds to step S5, and the encoding prediction mode of the next frame is determined.
[0039]
According to the above-described processing, by determining the encoding prediction mode using the phase and magnitude of the motion vector value in addition to the luminance signal, the motion vector of the color difference signal indicates a pixel having an opposite phase and the compression efficiency is improved. Since it is possible to avoid the case where the deterioration is caused, it is possible to realize high image quality and reduce the amount of calculation. Therefore, the circuit scale can be reduced.
[0040]
By the way, when the amount of motion is small, the value of the motion vector is considered to be substantially the same in the top field and the bottom field of the color difference signal. Therefore, when the amount of motion is small, the amount of motion vector motion is introduced to determine the coding mode. Next, this method will be described.
[0041]
Steps S11, S12, S14, S15, and S16 in FIG. 4 correspond to steps S1, S2, S3, S4, and S5 shown in FIG. 3, respectively. In this method, if it is determined in step S12 that the motion vector of the color difference signal indicates a pixel in reverse phase, in step S13, it is determined whether or not the compression efficiency is reduced by using the magnitude (motion amount) of the motion vector. Judging.
[0042]
For example, if the vertical component and the horizontal component of the magnitude of the motion vector of the luminance signal are each smaller than a predetermined threshold, the amount of motion is assumed to be small, and even if the motion vector of the color difference signal points to a pixel in reverse phase, the compression efficiency Therefore, the frame prediction is applied as it is without changing to the field prediction.
[0043]
On the other hand, if the vertical and horizontal components of the motion vector size exceed a predetermined threshold, the selected color difference signal becomes a pixel on the opposite field (reverse phase) and the deterioration of image quality becomes noticeable. Change mode to field prediction.
[0044]
Therefore, according to the image information conversion apparatus 1 described above, using the motion vector value, the phase, and the magnitude of the luminance information in addition to the luminance information, the frame prediction is used as the prediction mode of the encoding unit or the field prediction is used. Therefore, it is possible to select an optimal encoding prediction mode in which image degradation is minimized at a low bit rate.
[0045]
It should be noted that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention.
[0046]
【The invention's effect】
As described above in detail, the image information conversion method according to the present invention performs the color difference signal when performing the motion compensation prediction in the frame in which the top field and the bottom field are combined as the motion compensation prediction of the coding unit of the image data. If the motion vector in the field indicates a pixel in a field opposite to the field in which the pixel pointed to by the motion vector in the luminance signal exists, the reduction in compression efficiency is avoided by changing the motion compensation prediction in the coding unit to field prediction. High image quality can be achieved. In addition, the amount of calculation can be reduced and the circuit scale can be reduced.
[0047]
In particular, in the determination step, a predetermined threshold is set for the magnitude of the motion vector of the luminance signal, and when this threshold is exceeded, the encoding mode determination is changed to field prediction to avoid a reduction in compression efficiency. High image quality can be achieved. In addition, the amount of calculation can be reduced and the circuit scale can be reduced.
[0048]
In addition, the image information conversion apparatus according to the present invention performs motion compensation prediction on a frame in which a top field and a bottom field are combined as motion compensation prediction of a coding unit of image data. A determination unit for determining whether or not a pixel in a field opposite to a field in which a pixel indicated by a motion vector exists is present, and whether motion compensation prediction of a coding unit is set to frame prediction or field prediction according to the motion vector. Prediction mode control means for controlling whether or not, and in the discrimination means, it is determined that the motion vector in the color difference signal indicates a pixel in a field opposite to the field in which the pixel indicated by the motion vector in the luminance signal exists. The prediction mode control means changes the motion compensation prediction of the coding unit to field prediction. More can be realized avoiding image quality deterioration of compression efficiency. In addition, the amount of calculation can be reduced and the circuit scale can be reduced.
[0049]
In particular, a predetermined threshold is set for the magnitude of the motion vector of the luminance signal, and when this threshold is exceeded, the encoding mode determination is changed to field prediction, thereby avoiding a reduction in compression efficiency and improving the image quality. realizable. In addition, the amount of calculation can be reduced and the circuit scale can be reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of an image information conversion apparatus shown as a specific example of the present invention.
FIG. 2 is a diagram illustrating processing blocks in the image information conversion apparatus shown in FIG. 1;
FIG. 3 is a flowchart for explaining a field / frame prediction determination process in the image information conversion apparatus shown in FIG. 1;
FIG. 4 is a flowchart for explaining another example of the field / frame prediction determination process in the image information conversion apparatus shown in FIG. 1;
FIG. 5 is a schematic diagram for explaining a case where a motion vector of a color difference signal indicates a pixel on a field opposite to a luminance signal in frame prediction.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Image information converter, 11 Input terminal, 12 Screen rearrangement circuit, 13 Differentiator, 14 DCT circuit, 15 Quantization circuit, 16 Rate control circuit, 17 Variable length encoding circuit, 18 Buffer, 19 Output terminal, 20 Reverse Quantization circuit, 21 inverse DCT circuit, 22 adder, 23 image memory, 24 motion compensation circuit, 25 motion prediction circuit, 30 input image chrominance signal, 31 input image luminance signal, 32 field prediction motion vector, 33 frame prediction motion vector , 34 frame prediction residual, 35 field prediction residual, 36 frame prediction / field prediction selection signal, 37 reference image data, 38 bit stream, 41 motion vector operation block, 42 frame prediction / field prediction determination block, 43 encoding block

Claims (6)

In an image information conversion method for obtaining image compression information by compressing image data by applying orthogonal transformation and motion compensation,
When motion compensation prediction is performed in a frame in which a top field and a bottom field are combined as motion compensation prediction in a coding unit of image data, a field in which a motion vector in a color difference signal includes a pixel indicated by a motion vector in a luminance signal An image information conversion method characterized by changing motion-compensated prediction of a coding unit to field prediction when referring to a pixel in the opposite field. 2. The image information conversion method according to claim 1, wherein when the magnitude of the motion vector of the luminance signal exceeds a predetermined threshold, the motion compensated prediction of the coding unit is changed to field prediction. 2. The image information conversion method according to claim 1, wherein the image compression information conforms to an MPEG (Moving Picture Experts Group) standard. In an image information conversion device that obtains image compression information by compressing image data by applying orthogonal transformation and motion compensation,
When motion compensation prediction is performed in a frame in which a top field and a bottom field are combined as motion compensation prediction in a coding unit of image data, a field in which a motion vector in a color difference signal includes a pixel indicated by a motion vector in a luminance signal A discriminating means for discriminating whether or not it points to a pixel in the opposite field;
Prediction mode control means for controlling whether motion compensation prediction of a coding unit is to be frame prediction or field prediction according to a motion vector;
When the determination unit determines that the motion vector in the color difference signal indicates a pixel in a field opposite to the field in which the pixel indicated by the motion vector in the luminance signal exists, the prediction mode control unit An image information conversion apparatus characterized by changing compensation prediction to field prediction. 5. The image information conversion apparatus according to claim 4, wherein when the magnitude of the motion vector of the luminance signal exceeds a predetermined threshold, the motion compensated prediction of the coding unit is changed to field prediction. 5. The image information conversion apparatus according to claim 4, wherein the image compression information conforms to an MPEG (Moving Picture Experts Group) standard.

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