JPH0686258A - Orthogonal transform encoder and decoder - Google Patents

Abstract

PURPOSE:To improve the encoding efficiency of the entire picture, and to attain an encoding in which the quantization noise is not noticeable by changing the dimension of an orthogonal transformation and selecting an optimal transformation system for each block. CONSTITUTION:A picture signal inputted from a picture input 1 is DCT-operated in a horizontal direction by a horizontal DCT in the block state of 8X8 picture elements, and transmitted to a vertical DCT 3 and a quantizer 8. The data area DCT-operated in a vertical direction by the DCT 3, and the two-dimensional DCT signal is transmitted to a quantizer 4. Then, a weighting corresponding to a visual characteristic is operated to each DCT coefficient by the quantizers 4 and 8. The outputs of the quantizers 4 and 8 are encoded by variable length encoders 5 and 9, the output of the optimal transformation system, that is, the encoding amounts of each block are smaller discriminated by a minimum value discriminator 10 is selected by a selector 6, the data are outputted from an data output 7, and a mode is outputted from a mode output 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high efficiency coding for efficiently coding and decoding a signal with a smaller code amount in a recording, transmitting and displaying apparatus for processing a digital signal, and more particularly to a DCT. The present invention relates to an encoding device and a decoding device that perform orthogonal transformation such as.

[0002]

2. Description of the Related Art <Orthogonal Transform Coding> A high efficiency coding system using orthogonal transform such as DCT (Discrete Cosine Transform)
It is widely used in standard systems for high-efficiency coding because it can efficiently use the correlation of images to reduce data.
Here, orthogonal transformation is generally performed in two dimensions, vertical and horizontal. This is because in one dimension only horizontal or vertical one-way correlation can be used, whereas in two dimensions, both vertical and horizontal can be handled, and data can be reduced more efficiently.

Larger transform blocks are advantageous in that the correlation can be effectively used, but there is not much difference when the size is 8 × 8 pixels or more. On the other hand, since the quantization error spreads over the entire block, it is preferable that the block is visually small, and the processing amount is small. Therefore, it is most common to perform conversion by a block of 8 × 8 pixels of vertical 8th order and horizontal 8th order. Further, in moving image coding, it is general to perform predictive coding between frames in the time direction and spatially perform orthogonal transformation on prediction residuals.

<Encoding Device> FIG. 6 shows the configuration of a conventional encoding device. The image signal given from the image input 1 is
For each two-dimensional block of 8 × 8 pixels, the horizontal DCT 2 performs the 8th DCT in the horizontal direction, and the vertical DCT 3 subsequently performs the 8th DCT in the vertical direction. The converted signal is quantized by the quantizer 4 with a quantization step width such that an error is visually inconspicuous, and is given to the variable length encoder 5. Most of the coefficients of the quantized signal are 0.
In the variable length encoder 5, the signals in the two-dimensional block state are array-converted into the one-dimensional state in the order shown in FIG. 9A (so-called zigzag scan), and the number of consecutive 0 coefficients is 0. The value of the non-zero coefficient is encoded by a variable length code (VLC) such as Huffman code. Variable length encoder 5
Is output as compressed data from the data output 7 to the decoding device.

<Decoding Device> FIG. 7 shows the structure of a conventional decoding device. The processing operation of the decoding device is the reverse of that of the encoding device. The compressed data given from the data input 21 is
The variable length decoder 71 converts the variable length code back to the fixed length code,
The inverse transformation of the array is performed and a signal in a two-dimensional block state of 8 × 8 pixels is provided to the inverse quantizer 72. In the inverse quantizer 72, the code is converted into a quantized representative value, and the vertical inverse DCT
Given to 24. The quantized representative value is the vertical inverse DCT24.
Inverse DCT is performed in the vertical direction, and then horizontal inverse DCT2
In step 6, inverse DCT is performed in the horizontal direction to form a reproduced image signal, which is output from the image output 27.

[0006]

The conventional two-dimensional orthogonal transform coding is efficient at a portion where the intra-image correlation is relatively high, but is not necessarily suitable for a portion where the correlation is low such as an edge portion of the image. The two-dimensional orthogonal transform is not necessarily an appropriate encoding because the tendency becomes particularly strong in the inter-frame prediction residual signal and the intra-image correlation of the residual signal is considerably low.
There is a portion where the efficiency is higher in the one-dimensional conversion only in the horizontal direction or the vertical direction than the two-dimensional conversion.

Further, when the two-dimensional conversion is performed, the quantization error spreads within the two-dimensional block, and the noise component around the edge becomes conspicuous. That is, if the coding error is equivalent, one-dimensional conversion with a smaller conversion block is visually desirable. On the other hand, a coding method such as DPCM is visually desirable because noise components are inconspicuous, but the intra-image correlation cannot be used effectively, and the basic efficiency is not sufficient.

The present invention has been made by paying attention to the above points. By changing the dimension of orthogonal transformation for each block and selecting the optimum transformation method, the efficiency of the orthogonal transformation is high and the noise component is not noticeable. An object is to provide an encoding device and a decoding device.

[0008]

In order to achieve the above object, the present invention is an encoding device for orthogonally transforming an image signal in a plurality of dimensions, and an orthogonal transformation in which the combinations of dimensions are different, and A plurality of types of transform coding means for performing corresponding variable length coding, and a means for transform coding the image signal by the plurality of types of transform coding means, and selecting a transform coding output with the smallest generated code amount. An orthogonal transform encoding device is provided.

Further, there is provided a decoding device for orthogonally inversely transforming an encoded image signal in a plurality of dimensions, the orthogonality inverse transformation means being capable of changing the combination of dimensions for orthogonality inverse transformation, and the aforementioned orthogonal inverse transformation means. The present invention provides an orthogonal transform decoding device characterized by comprising a variable length decoding means capable of changing the decoding method corresponding to the above.

Further, there is provided an encoding device for orthogonally transforming an image signal in a plurality of dimensions, and a plurality of types of transform encoding means for performing an orthogonal transform having different combinations of dimensions and a variable length coding corresponding thereto. , A mode determination means for detecting the degree of change in each dimensional direction of the image, and for the dimensions determined to be relatively large by the mode determination means,
An orthogonal transform coding device is provided which is characterized in that orthogonal transform is not performed.

[0011]

According to the orthogonal transform coding and decoding apparatus configured as described above, the single transform method is changed by changing the dimension of the orthogonal transform (for each block) and selecting the optimum transform method. Optimal encoding can be performed locally for the encoding method used, and the encoding efficiency of the entire image can be improved.
In addition, since one-dimensional conversion is used at edges and the like, quantization noise is less noticeable than in a device that performs uniform two-dimensional conversion, and subjective image quality is better than efficiency improvement.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Coding Device> FIG. 1 is a block diagram showing a first embodiment of an orthogonal transform coding device of the present invention.
The same parts as those in the conventional example of FIG. 6 are designated by the same reference numerals. The encoding device of FIG. 6 includes a quantizer 8, a variable length encoder 9,
The difference is that a minimum value determiner 10, a selector 6, and a mode output 11 are added.

In FIG. 1, the image signal supplied from the image input 1 is a horizontal DCT in a block state of 8 × 8 pixels.
DCT is performed in the horizontal direction by 2 and applied to the vertical DCT 3 and the quantizer 8. Vertical DCT3 is horizontal DCT
The obtained signal is subjected to DCT in the vertical direction, and the two-dimensional DCT signal is given to the quantizer 4. Therefore, the quantizer 8 quantizes the DCT signal only in the horizontal direction, and the quantizer 4 quantizes horizontal and vertical two-dimensional DC.
The T signal is quantized.

Here, in the case of conversion in the horizontal direction only, DC
The T block has one-dimensional 8 pixels, but for convenience of switching to the two-dimensional processing, it is convenient to bundle eight DCT blocks in the vertical direction and handle them in a block state of 8 × 8 pixels.
This state is shown in FIG. 8B. In the figure, DC indicates the DC component of the DCT coefficient, AC indicates the AC component, and the thick line frame indicates the DCT block. In quantizers 4 and 8,
If uniform quantization is performed on each coefficient, the processing is the same, but weighting corresponding to the visual characteristics is performed on each DCT coefficient (for example, coarser at high frequencies).
If the case is different. Quantization weighting is performed by the quantizer 4
Has a two-dimensional characteristic, the quantizer 8 repeats a one-dimensional characteristic.

The outputs of the quantizers 4 and 8 are encoded by the variable length encoders 5 and 9, and both are given to the selector 6 and the minimum value determiner 10. In the variable length coding, the array conversion (scan) differs between the variable length encoders 5 and 9, and the variable length encoder 5 has the same zigzag scan as in the conventional example as shown in FIG. Variable-length encoder 9 for three-dimensional DCT
Scan the vertical direction of FIG. 9 (B). As described above, in the case of the one-dimensional DCT, the array conversion is performed by straddling the blocks of the orthogonal transformation, but this method is the "variable length coding method and its apparatus" by the same inventor and the same applicant as the present invention. It is the same as that shown in (Japanese Patent Application No. 1-213939). In this way, one-dimensional conversion is 8
By bundling and processing, it can be handled like a two-dimensional DCT,
The efficiency of 0 run length coding can be improved. in this case,
The same VLC table can be used for the variable length encoders 5 and 9, but if they are optimized separately, the efficiency can be improved slightly.

In the minimum value judging device 10, the variable length encoder 5 is used.
And 9 are compared in code amount (bit number) of each block, and a control signal is output as mode information so that the selector 6 selects the smaller output. Here, the unit for comparing the code amounts and switching the processing does not have to be the same as the DCT block (8 × 8 pixels), and a plurality of DCT blocks may be bundled. Especially when color difference signal sub-sampling is performed on a color image, a plurality of luminance signal blocks and one color difference signal block form a pair according to the ratio of the sub-samples. It is convenient to switch the processing.

The mode information is output from the mode output to the decoding device and is also given to the selector 6. The selector 6 selects the one having the smaller data amount from the outputs of the variable length encoders 5 and 9, and outputs it from the data output 7. As a result, when the correlation in the vertical direction is small, the one-dimensional (horizontal) DCT is selected and the two-dimensional DCT is not performed, so that the orthogonal transformation is performed with high efficiency and in which the noise component is inconspicuous.

In such processing, if the quantization weighting is set so that the distortion due to the quantization becomes equal in each transformation and the one with smaller data amount is selected in that state, the distortion amount is constant, The total amount of data will definitely decrease. At this time, if the quantization of the quantizer 8 is made slightly coarser, the one-dimensional conversion is easier to be selected, and the encoding error amount is slightly increased, but the encoding is visually desirable.

In the present embodiment, only two-dimensional and horizontal ones are shown, but three types of two-dimensional and vertical only, vertical only and horizontal only, two-dimensional and vertical only and horizontal, and orthogonal transformation are performed. It is possible to perform adaptive processing including the case where it does not exist.

<Decoding Device> FIG. 2 shows the configuration of the decoding device of the embodiment corresponding to FIG. The same parts as those in the conventional example of FIG. 7 are designated by the same reference numerals. The decoding device of FIG. 7 has a mode input 28 and a selector 25, and the variable length decoder 2
2. The operation of the inverse quantizer 23 is different. The compressed data supplied from the data input 21 is guided to the variable length decoder 22. On the other hand, the mode information input from the mode input 28 includes the variable length decoder 22, the dequantizer 23, and the selector 25.
Given to. The operations of the variable length decoder 22 and the inverse quantizer 23 are basically the same as in the conventional example, but the inverse conversion table of the array is changed in the variable length decoder 22 according to the mode information, and the inverse quantizer 23 in the inverse quantizer 23. The weighting table is changed.

The output of the inverse quantizer 23 is the vertical inverse DCT2
4 and the selector 25, and the vertical inverse DCT 24 inversely transforms the DCT in the vertical direction. The selector 25 is controlled by the mode information, and in the two-dimensional DCT block, the output of the vertical inverse DCT 24 is selected, and in the case of only horizontal, the output of the inverse quantizer 23 is selected, and the horizontal inverse DCT is selected.
The signal is guided to T and the DCT inverse transformation is performed in the horizontal direction. As a result, in the two-dimensional DCT block, both vertical and horizontal inverse DCT are performed, and in the case of only horizontal, only horizontal inverse DCT is performed, and the obtained reproduced image is output from the image 27.

<Second Coding Device> FIG. 3 is a block diagram showing a second embodiment of the orthogonal transform coding device of the present invention. The difference from the first embodiment is that the conversion mode is determined by means other than the main coding system. The configuration is different in that it does not have two systems of quantizers and variable length coding, but has a vertical change detector 31, an activity detector 32, and a mode determiner 33 as a mode determiner.

In FIG. 3, the image signal given from the image input 1 is a horizontal DC signal as a block signal of 8 × 8 pixels.
DCT is performed in the horizontal direction at T2, and is guided to the vertical DCT3 and the selector 6. In the vertical DCT 3, DCT is performed in the vertical direction, and the two-dimensional DCT signal is guided to the selector 6.

On the other hand, in the mode discriminating section, the input signal input from the image input 1 is guided to the vertical change detector 31 and the activity detector 32, and the vertical change detector 31 obtains the degree v of vertical change, Activity detector 32
Then, the block variance value d of the image is obtained. The degree of change v in the vertical direction and the block variance value d are given by the following equations (where dc is an average value).

[0025]

[Equation 1]

The mode discriminator 33 compares the two pieces of information and outputs a control signal such that only the horizontal DCT is obtained when the degree of change d in the vertical direction is large with respect to the variance value v. This is because if a signal that varies greatly in the vertical direction is subjected to DCT in the vertical direction, the number of high frequency components increases, and the efficiency becomes worse than that in the case where no conversion is performed. That is, orthogonal transformation is not performed in the dimension for which the mode determination unit has determined that the change is relatively large, thereby avoiding deterioration of conversion efficiency. The quantizer 4 and the variable length encoder 5 are the same as those in FIG.
The basic operation is the same as that of the conventional example as in the decoding device embodiment, and the weighting table is changed in the quantizer 4 and the array conversion is changed in the variable length encoder 5 according to the mode information. In the case of this embodiment, the main encoding is only one system, and the processing amount is smaller than that in the embodiment of FIG.

<Third Coding Device> FIG. 4 is a block diagram showing a third embodiment of the orthogonal transform coding device of the present invention. The same parts as those in FIG. 3 are designated by the same reference numerals. 3 is different from FIG. 3 in interframe predictive coding, in that orthogonal prediction is performed on a prediction residual signal. In terms of configuration, it has a predictive subtractor 41, a frame memory 42, and a local decoding unit (inverse quantizer 23, horizontal inverse DCT 26, vertical inverse DCT 24, selector 25).

The predictive signal supplied from the frame memory 42 is subtracted from the predictive subtractor 41 from the image signal supplied from the image input 1 to form a residual signal, which is supplied to the horizontal DCT 2 and the mode determining section 34. Mode determination unit 34, horizontal DC
The operations of T2, the vertical DCT 3, the selector 6, the quantizer 4, and the variable length encoder 5 are the same as those of the encoder of the second embodiment of FIG. Inverse quantizer 23 which is a local decoding unit, horizontal inverse D
The operations of the CT 26, the vertical inverse DCT 24, and the selector 25 are the same as those of the first embodiment decoding apparatus shown in FIG. The locally decoded residual signal is added with the prediction signal by the adder 43 to be a reproduced image signal, which is given to the frame memory 42. The reproduced image delayed by one frame from the frame memory 42 is used as a prediction signal to predict the subtractor 41 and the adder 4.
Given to 3.

The intra-image correlation of the prediction residual signal is lower than that of the normal image signal, and the proportion of the one-dimensional DCT used is higher than that of the normal image signal. That is, when the motion of the image is small, the prediction residual is small and the intra-image correlation is small, so that the two-dimensional DCT is not performed and the one-dimensional DCT is not performed.
By this, the orthogonal transformation is performed efficiently and without making noise noticeable. In reality, motion compensation is often applied to the inter-frame prediction signal, but there is essentially no difference from the motion compensation that is omitted in the present embodiment.

<Decoding Device> FIG. 5 is a block diagram showing an embodiment of a decoding device corresponding to the coding device of FIG. 4 of the present invention. The same parts as those in the embodiment of FIG. 2 are designated by the same reference numerals. The decoding device of FIG. 2 is different in that a frame memory 42 and an adder 43 are provided. The operations of the variable-length code decoder 22, the inverse quantizer 23, the vertical inverse DCT 24, the horizontal inverse DCT 26, and the selector 25 are the same as those of the first embodiment decoding apparatus in FIG. The operations of the frame memory 42 and the adder 43 are shown in FIG.
This is the same as the third embodiment of the encoding device.

[0031]

According to the encoding of the present invention, by changing the dimension of the orthogonal transformation (for each block) and selecting the optimal transformation method, the local transformation can be performed with respect to the coding method using a single transformation method. Optimal encoding can be performed for each of them, and the encoding efficiency of the entire image can be improved. In particular, when inter-frame prediction residuals are applied, the intra-image correlation is small in the prediction residuals, so that orthogonal transformation is extremely efficient.

In addition, since one-dimensional conversion is used at edges and the like, quantization noise becomes less noticeable, and subjective image quality becomes better than error reduction. As described above, the orthogonal transform encoding device and the decoding device of the present invention have extremely excellent practical effects.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of an orthogonal transform coding device of the present invention.

FIG. 2 is a block diagram showing a first embodiment of an orthogonal transform decoding device of the present invention.

FIG. 3 is a block diagram showing a second embodiment of the orthogonal transform encoding device of the present invention.

FIG. 4 is a block diagram showing a third embodiment of the orthogonal transform encoding device of the present invention.

FIG. 5 is a block diagram showing a third embodiment of the orthogonal transform decoding device of the present invention.

FIG. 6 is a block diagram showing a conventional example of an orthogonal transform encoding device.

FIG. 7 is a block diagram showing a conventional example of an orthogonal transform decoding device.

FIG. 8 is a diagram showing a configuration of two-dimensional and one-dimensional DCT blocks.

FIG. 9 is a diagram showing the order of two-dimensional and one-dimensional array conversion (scan order).

[Explanation of symbols]

1 ... Image input, 2 ... Horizontal DCT, 3 ... Vertical DCT, 4,
8 ... Quantizer, 5, 9 ... Variable encoder, 6, 25 ... Selector, 7 ... Data output, 10 ... Minimum value determiner, 11 ... Mode output, 21 ... Data input, 22, 71 ... Variable length decoding , 23, 72 ... Inverse quantizer, 24 ... Vertical inverse DCT, 2
6 ... Horizontal inverse DCT, 27 ... Image output, 28 ... Mode input, 31 ... Vertical change detector, 32 ... Activity detector, 33 ... Mode decision unit, 34 ... Mode decision unit, 41 ...
Predictive subtractor, 42 ... Frame memory, 43 ... Adder.

Claims (3)

[Claims] 1. An encoding device for orthogonally transforming an image signal in a plurality of dimensions, comprising a plurality of types of transform encoding means for performing an orthogonal transform having different combinations of dimensions and a variable length coding corresponding thereto. An orthogonal transform coding device, comprising: a unit for transform-encoding an image signal by the plurality of types of transform-encoding units and selecting a transform-encoded output with the smallest generated code amount. 2. A decoding device for orthogonally inversely transforming an encoded image signal in a plurality of dimensions, the orthogonal inverse transforming means being capable of changing the combination of the dimensions of the orthogonal inverse transforming, and the orthogonal inverse transforming means. An orthogonal transform decoding device, comprising: a variable length decoding means capable of changing a decoding method corresponding to the above. 3. An encoding device for orthogonally transforming an image signal in a plurality of dimensions, comprising a plurality of types of transform encoding means for performing an orthogonal transform having different combinations of dimensions and a variable length encoding corresponding thereto. , A mode determination unit that detects the degree of change in each dimension of the image, and that the orthogonal transformation is not performed for the dimension determined to be relatively large by the mode determination unit. A characteristic orthogonal transform coding device.