JP2002209111A - Image encoder, image communication system and program recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform encoding with high compressibility compared with the case of performing normal encoding processing while keeping an image corresponding to low resolution when the resolution of the display device of a receiving terminal is low. SOLUTION: A pre-processing part 12 performs pre-processing to an area within image information to be inputted. In the pre-processing, the part 12 compares an n-bit binary number expressing the value of the pixel of each image information before/after the pre-processing by each bit and restricts the value of the pixel before/after the pre-processing so that the difference ranges within low-ordered m-bits (m<n). An encoding part 13 gives encoding to image information given pre-processing by this part 12. Then, image information in which the values of respective pixels are expressed by n-bits is encoded.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image encoding apparatus for encoding image information, an image communication system using the image encoding apparatus, and an image encoding apparatus and an image communication system. The present invention relates to a program recording medium storing a program.

[0002]

2. Description of the Related Art When encoding image information, it is important to increase the compression ratio and reduce the amount of data in order to reduce the amount of communication and storage capacity.

A typical coding for increasing the compression ratio is DCT (Discrete Cosine).
ne Transform (discrete cosine transform) and DPCM (Differential PC)
M) is known. Note that MPEG (M
oving Photographic Expert
In coding systems such as Group 1, MPEG2, and MPEG4, the former DCT-based coding is performed.

[0004] Encoding of image information using DCT will be described below. The image coding based on the DCT is performed by the following three-stage processing. (1) Two-dimensional DCT processing (2) Quantization processing (3) Variable length coding processing

First, in the first-stage two-dimensional DCT processing, two-dimensional DCT processing is performed on an area called a block of 8 × 8 pixels of image information to obtain 8 × 8 = 64 DCT coefficients.

Next, in the second-stage quantization process, each of the DCT coefficients is quantized to an integer value. As a result of the quantization, a DCT coefficient having a small value has a value of zero. This quantization involves a reduction in the number of bits, contributes to an improvement in the compression ratio, and is an irreversible process because the amount of information is lost.

Finally, in the variable-length coding process of the third stage, the quantized 8 × 8 = 64 DCT coefficients are scanned in a zigzag manner from the value corresponding to the low frequency, thereby making it one-dimensional. To a 64-element array. Then, Huffman coding is performed by combining the values of the respective DCT coefficients with the number of zeros (run-lengths) consecutive thereto.

In such coding based on DCT, if the input image information has a small amount of high-frequency components, as a result of the quantization processing, many DCT coefficients in the high frequency band will contain many zeros, and after the zigzag scan, In the latter half of the one-dimensional array, zeros often continue. By assigning a short coding length to this sequence of zeros in Huffman coding, the amount of information after coding is reduced. Normal image information such as a natural image often has a small amount of high frequency components, and a high compression ratio is generally expected.

Further, in order to further increase the compression efficiency, a method of performing a pre-processing using a low-pass filter or the like on image information has been conventionally performed. That is, by performing a pre-process of applying a low-pass filter to the image information before performing the encoding, the image information is further cut in a high-frequency component, so that the compression rate is improved in the subsequent encoding. However, if the effect of the low-pass filter is too strong, there is a problem that the entire image is blurred.

In order to cope with this problem, a method of making the deterioration of an image visually inconspicuous by, for example, selectively applying a low-pass filter according to image information has been proposed in, for example, Japanese Patent Application Laid-Open No. Hei 5-344346. ing.

In Japanese Patent Application Laid-Open No. Hei 5-344346,
It is better not to perform strong filtering, which represents an edge portion of an image, by extracting a high-frequency component that is an obstacle to compression from image data using a Laplacian filter and applying a median filter to the extracted high-frequency component and smoothing the extracted high-frequency component. The high-frequency component is detected, and a low-pass filter to be used for each block is selected. As a result, the deterioration of the image becomes visually inconspicuous.

Note that a low-pass filter in digital image processing is often realized by a spatial filter based on a weighted average of neighboring pixel values.

Next, encoding of image information using DPCM will be described. Image compression by encoding using DPCM can realize reversible encoding, unlike the case of using DCT. In this case, since there is no loss of information (lossless), the original image can be completely restored in decoding. However, DPC
Also in the encoding using M, there is a type in which information is lost (loss is present) due to a reason such as including a quantization process.

An example of the lossless coding by the DPCM will be described below. First, the value of the leftmost pixel in the row direction of the block is encoded by Huffman encoding. Subsequently, the difference between the previously encoded pixel and the pixel on the right side is obtained, and the value is encoded by Huffman encoding. Or later,
The encoding of one row in the block is completed by repeating the encoding of the difference one after another. The same process is repeated for the remaining rows.

Here, the case where the image information is divided into two-dimensional blocks has been described. However, this is not always necessary, and processing may be performed in units of one line in the entire image information. Further, after the rightward scan, a method is conceivable in which, after the rightmost pixel, the difference between the rightmost pixel and the pixel immediately below the rightmost pixel is obtained, the value is encoded, the leftward scan is performed, and this is repeated.

In an image communication system using an image coding apparatus based on the above coding, an image transmitting terminal,
In general, it is configured by a communication path and an image receiving terminal. The image transmitting terminal encodes the image information of the input image, and transmits the encoded image information to the image receiving terminal via the communication path. In the image receiving terminal,
The image information received via this communication path is decoded and CR
An image is displayed on a display device such as T. Further, such image communication may be not only one-way communication but also two-way communication.

Here, when a public line is used as a communication path and image communication is performed via a telephone terminal connected to the public line, image communication between different types of terminals may be performed. At this time, if the resolution of the display device of the image receiving terminal is insufficient compared to the resolution of the image information sent from the image transmitting terminal, the image cannot be displayed at the original image quality. In such a case, since the image cannot be displayed anyway at a high resolution, it is conceivable to improve the compression ratio by performing low-resolution encoding on the image transmitting terminal side. However, depending on the video communication standard, the encoding resolution may be determined, and the encoding must be performed at the determined resolution. In such a case, it is conceivable to improve the compression ratio by applying a low-pass filter. However, as described above, applying a low-pass filter causes image degradation.

A document which describes such encoding of image information is disclosed in Japanese Patent Application Laid-Open No. 5-3443.
No. 46, JP-A-10-25748, for example.
4, JP-A-7-274170 and JP-A-8-274170.
No. 307618, JP-A-5-114240,
JP-A-8-36642, JP-A-7-107462
JP, JP-A-10-233916, JP-A-10-233916
JP-A-285409, JP-A-5-115055, JP-A-4-219059, and the like.
Documents that describe image communication systems include:
JP-A-6-189105, JP-A-9-65111
And JP-A-5-122505.

[0019]

Since the conventional image communication system and the image coding apparatus used therein are constructed as described above, if a low-pass filter is applied to improve the compression ratio, the image quality will be degraded. There was a problem that occurs.

According to the present invention, particularly when the display device of the image receiving terminal has a low resolution, a code having a higher compression ratio as compared with a case where a normal encoding process is performed while maintaining a low resolution image quality. It is an object of the present invention to obtain an image communication system, an image encoding device used therein, and a program recording medium storing programs for them.

[0021]

An image coding apparatus according to the present invention performs pre-processing on an input image in a pre-processing unit, and converts the value of a pixel after pre-processing into the value of a pixel before pre-processing. A predetermined section including the value is restricted, and the encoding section encodes the output image of the preprocessing section.

In the image coding apparatus according to the present invention, the pre-processing unit performs a process of suppressing a difference between pixel values before and after the pre-processing to be within a maximum allowable value.

In the image coding apparatus according to the present invention, the pre-processing unit includes an n-bit two-bit value representing each pixel value before and after the pre-processing.
A process of limiting each pixel value after the pre-processing so that the difference of each bit of the radix is within the lower m bits is performed.

[0024] In the image coding apparatus according to the present invention, the pre-processing section performs high frequency component removal processing on the input image.

[0025] In the image encoding apparatus according to the present invention, the encoding section performs an encoding process by DPCM on the output image.

In the image coding apparatus according to the present invention, the coding unit performs DCT processing on an output image.

[0027] In the image coding apparatus according to the present invention, the preprocessing unit changes the value of the predetermined section by setting.

[0028] An image communication system according to the present invention includes a preprocessing unit for limiting a pixel value after preprocessing of an input image to a predetermined section including a pixel value before preprocessing, and an output from the preprocessing unit. An image transmitting terminal provided with an encoding unit for encoding an image and an image receiving terminal for decoding and displaying an output image from the image transmitting terminal are connected by a communication path.

[0029] In the image communication system according to the present invention, the preprocessing unit performs processing for suppressing a difference between values of respective pixels before and after the preprocessing to be within a maximum allowable value.

[0030] In the image communication system according to the present invention, the pre-processing unit performs the post-processing after the pre-processing so that the difference of each bit of the n-bit binary number representing the value of each pixel before and after the pre-processing falls within the lower m bits. The processing for limiting each pixel value is performed.

In the image communication system according to the present invention, the information of the display resolution is transmitted from the image receiving terminal to the image transmitting terminal, and the image transmitting terminal performs the preprocessing only when the display resolution is low. It is something to do.

In the image communication system according to the present invention, the image receiving terminal transmits display resolution information to the image transmitting terminal, and the image transmitting terminal changes a predetermined section based on the display resolution information. It is like that.

In the program recording medium according to the present invention, the procedure for removing high-frequency components of an input image by pre-processing, the difference between each bit of an n-bit binary number representing the value of each pixel before and after the pre-processing is the lower m bits It stores a program for executing a procedure for limiting each pixel value after pre-processing so as to fall within the range, and a procedure for encoding.

[0034] The program recording medium according to the present invention comprises m
In which a program for changing the value of is stored.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below. Embodiment 1 FIG. FIG. 1 is a block diagram illustrating a configuration of an image communication system according to Embodiment 1 of the present invention. In the figure, reference numeral 1 denotes an image transmitting terminal for encoding and transmitting image information of an input image having n bits representing the value of each pixel, and 2 for receiving image information encoded by the image transmitting terminal 1. This is an image receiving terminal that decodes and decodes and displays an image based on the decoded image. Reference numeral 3 denotes a communication network as a communication path for transmitting image information transmitted and received between the image transmitting terminal 1 and the image receiving terminal 2.

In the image transmission terminal 1, reference numeral 11 denotes an image input unit for inputting an input image having n bits representing the value of each pixel, and is realized by, for example, a camera. Reference numeral 12 denotes a pre-processing unit that performs pre-processing on an area in the image information of the input image, and reference numeral 13 denotes an encoding unit that performs lossless DPCM compression processing on the output of the pre-processing unit 12. Reference numeral 14 denotes a transmission-side communication unit that transmits the image information encoded by the encoding unit 13 to the image receiving terminal 2 via the communication network 3 as a bit stream.

In the image receiving terminal 2, reference numeral 15 denotes a receiving side communication unit for extracting a bit stream sent from the transmitting side communication unit 14 of the image transmitting terminal 1 and performing communication control. 16 is the receiving side communication unit 1
Reference numeral 17 denotes a decoding unit for decoding the extracted bit stream. Reference numeral 17 denotes a display unit such as a CRT for displaying an image based on the image information decoded by the decoding unit 16.

Further, in the pre-processing unit 12, 21
Is an original image storage unit for storing image information of an input image input to the image input unit 11 as an original image;
Reference numeral 22 denotes a horizontal spatial filter for performing a low-pass filter process for removing high-frequency components on the image information read from the original image storage unit 21. Reference numeral 23 denotes a comparison limiting unit that compares the output of the horizontal spatial filter 22 with the image information read from the original image storage unit 21 to correct the value of each pixel.

Reference numeral 31 denotes an image used in the image communication system, which comprises the pre-processing unit 12 including the original image storage unit 21, the horizontal spatial filter 22, and the comparison and restriction unit 23, and the encoding unit 13. It is an encoding device.

Next, the operation will be described. In the following description, the image is a monochrome grayscale image, the value of each pixel is represented by n = 8 bits, and the value range is from 0 to 255. The display resolution on the display unit 17 of the image receiving terminal 2 is 64 tones from 0 to 63. In this case, the display has a lower resolution than the original pixel value of the image information received by the image receiving terminal 2 and the display unit 1 of the image receiving terminal 2
At 7, the pixel value is truncated by 2 bits and converted to 64 gradations for display.

Here, in the first embodiment, eight pixels continuous in the row direction are used as units of encoding. Hereinafter, the unit of eight pixels in the row direction will be referred to as a coding unit. FIG. 2 is a diagram illustrating an example of a coding unit in image information of an input original image. In the coding unit shown in the figure, eight pixels are arranged left and right. The value of each pixel is represented by 8 bits, and its value range is between 0 and 255. Hereinafter, the manner in which the coding unit shown in FIG. 2 is processed by image coding apparatus 31 in the first embodiment will be described.

FIG. 3 is a diagram showing an example of a coding unit output from the horizontal spatial filter 22 in the preprocessing unit 12. The horizontal spatial filter 22 receives the coding unit shown in FIG. 2 as an input, and outputs the filtered coding unit shown in FIG. In the horizontal spatial filter 22, a weighted average of pixels and neighboring pixels is obtained. In the first embodiment, the following equation is used.

Y (i) = (X (i-1) + 2X (i) +
X (i + 1)) / 4 (1 <i <6) Y (0) = Y (1) Y (7) = Y (6) However, the pixel value of the original image (before filtering) is X
(I), the pixel value after the filter processing is represented by Y (i) (0 <i <
7).

The horizontal spatial filter 22 sends the coding unit after the filtering to the comparison limiting unit 23. Comparison limiter 2
3 reads out a corresponding coding unit of the original image stored in the original image storage unit 21 and converts it into a horizontal spatial filter 22.
The value of each pixel is corrected in comparison with the transmitted coding unit.

Next, the value of each pixel is corrected in the comparison limiting unit 23 by performing the processing represented by the following equation. However, the output of the comparison limiting unit 23 is Z (i) (0 <i
<7) The operation of shifting a to the right by b is shr (a,
b) and multiplication by *.

Shr (Y (i), 2) = shr (X
When (i), 2) Z (i) = Y (i) When shr (Y (i), 2)> shr (X (i), 2) Z (i) = shr (X (i), 2) * 4 + 3 When shr (Y (i), 2) <shr (X (i), 2) Z (i) = shr (X (i), 2) * 4

In this equation, the pixel value Y (i) after the filtering process by the horizontal spatial filter 22 includes the pixel value X (i) before the filtering process in the section [4n, 4n + 3] (0 <n
<63), the filtered pixel value Y
(I) is forcibly restricted within the section. For example, when the pixel value X before filtering of the image information of the input original image is 5 and the pixel value Y after filtering is 9, the section [4] containing 5 is the pixel value Y after filtering. , 7] (n = 1), the comparison limiting unit 23 sets the pixel value in the section [4, 7] (n =
Forcibly change to the value 7 in 1).

FIG. 4 shows a case where the coding unit shown in FIG. 2 from the original image storage unit 21 and the coding unit shown in FIG. It is a figure showing an example of an encoding unit. As shown in the figure, the value of each pixel is a value obtained by changing the pixel value after the filtering process by the horizontal spatial filter 22 by the above equation.

As described above, in the preprocessing unit 12, the comparison limiting unit 23 converts the pixel value Y (i) after the filtering process by the horizontal spatial filter 22 into the pixel value X (i) before the filtering process. Is forcibly restricted so as not to exceed a predetermined section including. In particular,
The pixel value Y (i) of the image after the filtering process by the horizontal spatial filter 22 is represented by a binary number (n bits) representing the pixel value X (i) before the process and the pixel value Y (i) after the process. The base number is compared with each bit (n bits), and processing is performed so that the difference is limited to a range within the lower m bits (m <n).

As described above, in the above description, the comparison restricting unit 23 performs processing so that the difference between the preprocessed pixel value and the preprocessed pixel value for each bit falls within the lower m bits. However, the processing in the comparison limiting unit 23 is | Y (i) -X
(I) |, that is, processing may be performed such that the absolute value of the difference between Y (i) and X (i) is kept within the maximum allowable value. In this case, it is impossible to accurately achieve lossless display, but it is considered that there is a certain effect.

The encoding unit 13 encodes the encoding unit output from the comparison limiting unit 23 of the preprocessing unit 12 using a lossless DPCM. That is, the value of the leftmost pixel of the coding unit is encoded, and then the difference between the value of the leftmost pixel and the value of the pixel on the right is taken, and the difference value is sequentially subjected to variable-length encoding. go. The encoding of the leftmost pixel is performed by simply expressing the pixel value with a fixed length of 8 bits.

FIG. 5 is a diagram showing an encoding table which the encoding unit 13 refers to when encoding the difference between pixel values. Since the difference can take a value in the range of -255 to 255, there are 511 items in the encoding table, and FIG. In this coding table, the coding length L of the code for the difference d is as follows.

L = 1 (d = 0) L = 2d (d> 0) L = 2 * (− d) +1 (d <0)

FIG. 6 is a diagram showing an intermediate calculation result when the encoding unit 13 performs encoding. In this figure, the output from the comparison limiting unit 23 is displayed as it is for the leftmost pixel, and subsequently the difference between the pixels is calculated.

FIG. 7 is a diagram showing an example of a bit stream output from the encoding unit 13. In the figure, the corresponding values are shown below the bit stream. The encoding unit 13 encodes the leftmost pixel with a fixed length of 8 bits (the first 000000 of the bit stream shown in FIG. 7).
1). For other pixel differences, variable-length coding is performed with reference to the coding table shown in FIG. 5 to obtain the obtained bit stream (0, 0, 1110, 111111).
111111111110, 10, 0, 0).

The coding length of this bit stream is as follows. First, the leftmost pixel is 8 bits. The coding length of the difference between the subsequent pixels is calculated by L in the above equation, and is 36 as follows. 8 + 1 + 1 + (2 * 2) + (2 * 9) + (2 * 1) +1
+ 1 = 36

Here, with respect to the above processing in the first embodiment, the coding unit of the image information of the input original image is directly encoded without passing through the horizontal spatial filter 22 and the comparison limiting unit 23. Assuming that the data is input to the coding unit 13 and coded, the coding length is large in this example.

FIG. 8 is a diagram showing an intermediate calculation result of the encoding unit 13 when the encoding unit of the image information of the original image shown in FIG.

The coding length of the bit stream in this case is 8+ (2 * 2 + 1) + (2 * 2) + (2 * 2 + 1) +
(2 * 15) + (2 * 3 + 1) + (2 * 3) + (2 * 3
+1) = 72, which is larger than the previous case. This is considered to be the result of applying the horizontal spatial filter 22, and it is generally considered that there is an effect in cases other than this example.

This bit stream is transmitted to the transmitting side communication unit 14.
The data is sent to the image receiving terminal 2 via the communication network 3. In the image receiving terminal 2, it is received by the receiving side communication unit 15 and decoded by the decoding unit 16, and an image based on it is displayed on the display unit 17. Since the encoding method in the first embodiment is lossless, the decoding unit 16 of the image receiving terminal 2 reproduces the encoding unit of the output of the comparison limiting unit 23 shown in FIG. 4 as it is. This encoding unit is the display unit 1
At 7, two bits are truncated.

FIG. 9 is a diagram showing the coding unit after the truncation processing in the display unit 17. This is obtained by simply truncating the output of the comparison limiter 23 shown in FIG. 4 by 2 bits. It should be noted that the same result is obtained even if the coding unit shown in FIG. 2 is truncated by 2 bits. This means that the image quality of the original image can be obtained as it is by encoding the image information of the original image and inputting it to the display unit 17 by the processing of the comparison limiting unit 23 in the present invention.

On the other hand, if the coding unit output from the horizontal spatial filter 22 is truncated by 2 bits, the same image quality cannot be guaranteed.

FIG. 10 shows the horizontal spatial filter 2 shown in FIG.
2 shows a result in a case where a 2-bit truncation is tentatively performed on a coding unit of output No. 2. According to this,
The result is different from FIG. This indicates that the image quality has deteriorated due to the horizontal spatial filter 22.

In the above description, the lossless type DPCM is used as the encoding method of the encoding unit 13, but a lossy type DPCM may be adopted. In this case, it is considered that a loss occurs in the encoding, so that a loss in the display is inevitable. However, the operation of the low-pass filter and the comparison limiting unit 23 is considered to have a certain effect in improving the compression ratio. Can be

The encoding method in which the encoding unit 13 performs the compression process is not limited to the method using the DPCM, but may employ the encoding using the DCT. Although a loss occurs in this case as well, it is considered that the operation of the low-pass filter and the comparison limiting unit 23 has a certain effect in improving the compression ratio.

As described above, according to the first embodiment, it is possible to reduce the coding length of the bit stream by removing the high frequency band by the low-pass filter processing by the horizontal spatial filter 22. Since the difference between the pixel values before and after the pre-processing is limited to be within the lower m bits, the reproducibility of the displayed image with respect to the input image is improved, and the original image is directly displayed on the low-resolution image receiving terminal 2. Thus, an effect that the same image quality as that obtained when the image is obtained can be obtained.

Embodiment 2 Next, an image communication system according to Embodiment 2 of the present invention will be described. The second embodiment provides an image communication system that makes the display resolution of the image receiving terminal 2 variable. Since the system configuration is the same as that of the first embodiment shown in FIG. 1, the description is omitted here.

In an image communication system in which various types of image receiving terminals 2 are connected to the image transmitting terminal 1 via the communication network 3, the display resolution of the display unit 17 of the image receiving terminal 2 has various values. For example, the first embodiment
In the above, the display resolution of the display unit 17 of the image receiving terminal 2 is assumed to be 64 gradations, but may be 32 gradations depending on the image receiving terminal 2 connected to the communication network 3.

In the second embodiment, when starting image communication between the image transmitting terminal 1 and the image receiving terminal 2,
The receiving side communication unit 15 of the image receiving terminal 2 to its display unit 17
Is transmitted to the transmission-side communication unit 14 of the image transmission terminal 1 via the communication network 3.

In the image transmitting terminal 1, when the transmitting communication unit 14 receives the display resolution from the image receiving terminal 2, the display resolution is transferred to the comparison limiting unit 23 of the preprocessing unit 12. Comparison limiter 23
Changes its operation based on the information on the display resolution of the image receiving terminal 2 from the transmitting side communication unit 14. here,
The transmitting communication unit 14 determines that the display resolution of the image receiving terminal 2 is 64.
When the information indicating the gradation is received, the comparison restricting unit 23 operates in the same manner as in the first embodiment. on the other hand,
When the information that the display resolution of the image receiving terminal 2 is 32 gradations is received, the comparison limiting unit 23 outputs the output Z
The calculation method of (i) is changed as shown in the following equation.

Shr (Y (i), 3) = shr (X
When (i), 3) Z (i) = Y (i) When shr (Y (i), 3)> shr (X (i), 3) Z (i) = shr (X (i), 3) * 8 + 7 When shr (Y (i), 3) <shr (X (i), 3) Z (i) = shr (X (i), 3) * 8

FIG. 11 shows the comparison limiting unit 2 when the display resolution on the display unit 17 of the image receiving terminal 2 is 32 gradations.
FIG. 6 is a diagram illustrating an example of an output output from a third example. in this case,
Unlike the case of 64 gradations shown in FIG. 4, the output has the same contents as the output of the horizontal spatial filter 22 shown in FIG. This means that the display becomes coarse due to the 32 gradations, so that even if the same value as in FIG. 3 is transmitted, the difference is no longer shown on the display of the display unit 17 on the image receiving terminal 2 side. I do.

FIG. 12 is a diagram showing an intermediate calculation result when the encoding unit 13 of the image transmission terminal 1 performs encoding.

The coding length of the bit stream in this case is calculated in the same manner, and 8 + 1 + 1 + (2 * 3) + (2 * 6) + (2 * 3) +1
+ 1 = 36, which is equivalent to 64 gradations. In the general case, it is considered that the coding length tends to be smaller in the case of 32 gradations because the change of the pixel value in the comparison limiting unit 23 is less and the effect of the low-pass filter is more likely to be obtained.

The encoding of the image information according to the second embodiment is performed losslessly. Therefore, even when the display resolution of the image receiving terminal 2 is 32 gradations, the decoding unit 16 of the image receiving terminal 2 uses the coding unit of the output of the comparison limiting unit 23 of the image transmitting terminal 1 shown in FIG. Is reproduced as it is. The encoding unit is truncated by 3 bits in the display unit 17, and an image based on the truncation is displayed.

FIG. 13 is a diagram showing an encoding unit after the truncation processing in the display unit 17. This is obtained by simply truncating the output of the comparison limiting unit 23 of the image transmitting terminal 1 shown in FIG. 11 by 3 bits. It should be noted that the same result is obtained when the coding unit shown in FIG. 2 is truncated by 3 bits. Therefore, this case is also lossless.

As described above, according to the second embodiment, the calculation method in the comparison limiting unit 23 of the image transmitting terminal 1 changes depending on the display resolution of the display unit 17 of the image receiving terminal 2. The effect is obtained that the encoding length can be shortened (compression rate is increased) as the display resolution is reduced while maintaining the lossless characteristics.

Embodiment 3 In the first and second embodiments, a case has been described in which all processing is implemented by hardware, but functions equivalent to those can be implemented by software.

That is, a procedure for performing a pre-process of applying a low-pass filter to the image information area of the original image, a value of each pixel before performing a pre-process by the low-pass filter on the image information of the original image, and a pre-process The computer causes the computer to execute a procedure for suppressing the difference between the pixel values after the execution from LSB to m (m <n) bits and a procedure for encoding image information preprocessed by the low-pass filter. , A program for realizing image encoding, and a program for changing the value of m are stored in a computer-readable program recording medium,
A computer processor sequentially reads out and processes the program, thereby realizing a processing function equivalent to that of hardware.

Embodiment 4 Hereinafter, another embodiment of the present invention will be collectively described as a fourth embodiment.
In the first and second embodiments, the DP
The coding unit to be CM is 8 pixels in the horizontal direction, but this may be the number of pixels of the horizontal width of the entire image. In addition, the scan is extended to two dimensions, and at the right end point, the next prediction point is moved to the lower row, and the scanning direction is turned back. When processing is performed to the right end, prediction is continuously performed to the left end of the next row. It is also possible. In addition, such a return scan is performed using blocks such as 8 × 8 (areas in image information).
It can also be done on a unit basis.

Although the description has been given of the case where the horizontal spatial filter 22 is used as the low-pass filter, another method may be used. For example, a weighted average in a 3 × 3 area can be used.

In the second embodiment, the calculation method in the comparison limiting unit 23 is changed depending on the resolution of the image receiving terminal 2. However, instead of changing the calculation method, the image receiving terminal 2 If the resolution has a certain level or more, the processing in the comparison limiting unit 23 may be omitted. For example, when the resolution is 256 gradations, there is essentially no loss in the display unit 17, so that the processing of the comparison limiting unit 23 may be omitted.

[0083]

As described above, according to the present invention, the pre-processing unit that performs pre-processing on the area in the input image information sets the value of the pre-processed pixel to the value of the pre-processed pixel. Since it is configured to limit to a predetermined section including the value, the error of the pixel value after pre-processing with respect to the pixel value before pre-processing is limited, and the reproducibility of the displayed image with respect to the input image is improved. If the display device of the receiving terminal has a low resolution, by restricting the pixel values after the pre-processing within a predetermined section, if the error is limited to such a degree that the error is masked by the low resolution of the display device, the pre-processing is performed. This has the effect of obtaining an image encoding device that can maintain the same image quality as that in the case where the image encoding is not performed.

According to the present invention, the preprocessing unit compares the binary values (n bits) representing the pixel values before and after the preprocessing on a bit-by-bit basis, and the difference falls within the lower m bits (m <n). So that it is possible to improve the reproducibility of the displayed image with respect to the input image,
In particular, when the display device cuts off and displays the lower m bits, there is an effect that an image encoding device capable of displaying the same image as when the preprocessing is not performed is obtained.

According to the present invention, since high-frequency component removal processing is performed on image information as preprocessing, high-frequency components of an image are cut off, the encoding length of a bit stream is reduced, and compression is performed. This has the effect of improving the rate and reducing the amount of data.

According to the present invention, the predetermined section in the pre-processing can be changed by setting, so that when various image receiving terminals are connected to the communication path, depending on the display resolution of the image receiving terminal, the comparison restricting unit may be used. Since the calculation method changes, there is an effect that the encoding length can be shortened as the display resolution decreases, and the compression ratio can be improved, while maintaining the lossless characteristics.

According to the present invention, an image transmitting terminal connected to an image receiving terminal via a communication path includes a pre-processing unit and an encoding unit, and the value of each pixel after pre-processing is set to a value before the pre-processing. The pre-processing unit performs processing to limit the value within a predetermined section including the value of each pixel, and the processing to encode the output image of the pre-processing unit is performed by the encoding unit. The error with respect to the pixel value before the pre-processing of the value is limited, and the reproducibility of the displayed image with respect to the input image is improved. Particularly, when the display device of the image receiving terminal has a low resolution, the pre-processing is performed within a predetermined section. By limiting the pixel value of the image, if the error is limited to the extent that it is erased by the resolution reduction by the display device of the image receiving terminal, an image that can maintain the same image quality as when the preprocessing is not performed Got a communication system There is that effect.

According to the present invention, the binary numbers (n bits) representing the pixel values before and after the pre-processing are compared for each bit, and the difference is limited so as to be within the lower m bits (m <n). Since the processing is configured to be performed by the pre-processing unit in the image transmitting terminal, it is possible to improve the reproducibility of the displayed image with respect to the input image. In particular, the display device of the image receiving terminal cuts off the lower m bits. In the case of displaying, there is an effect that an image communication system capable of displaying exactly the same image as when no pre-processing is performed is obtained.

According to the present invention, the image transmitting terminal is configured to execute the pre-processing by the pre-processing section only when the display resolution sent from the image receiving terminal via the communication path is low. Therefore, even when various image receiving terminals are connected, communication can be performed in accordance with the display resolution, and there is an effect that the flexibility of the system is improved.

According to the present invention, the image transmission terminal is configured to change the predetermined section in the pre-processing based on the information of the display resolution sent from the image reception terminal via the communication path. When the image receiving terminal is connected, communication according to the display resolution can be performed, and the flexibility of the system is improved.

According to the present invention, pre-processing for removing high-frequency components of an input image, processing for limiting the difference between bits of a binary number representing the value of each pixel before and after the pre-processing to be within the lower m bits, And a program for executing each procedure of the encoding process is stored in the program recording medium, so that the same image quality as when the original image is directly displayed on a low-resolution image receiving terminal is obtained. There is an effect that can be realized by software.

According to the present invention, since the program for changing the value of m is stored in the program recording medium, various image receiving terminals are connected to the communication path, and display is performed while maintaining lossless characteristics. There is an effect that image communication that can improve the compression ratio as the resolution becomes smaller can be realized by software.

[Brief description of the drawings]

FIG. 1 is a view illustrating a first embodiment and a second embodiment of the present invention;
1 is a block diagram illustrating a configuration of an image communication system according to the first embodiment.

FIG. 2 is a diagram illustrating an example of a coding unit of image information of an input original image according to the first embodiment.

FIG. 3 is a diagram illustrating an example of a coding unit output from a horizontal spatial filter according to the first embodiment.

FIG. 4 is a diagram illustrating an example of a coding unit output from a comparison limiting unit according to the first embodiment.

FIG. 5 is a diagram illustrating an encoding table referred to when an encoding unit according to Embodiment 1 encodes a difference between pixels.

FIG. 6 is a diagram showing an intermediate calculation result at the time of encoding by the encoding unit according to the first embodiment.

FIG. 7 is a diagram illustrating an example of a bit stream output from an encoding unit according to Embodiment 1.

FIG. 8 is a diagram illustrating an intermediate calculation result when the coding unit of the original image according to the first embodiment is directly input to the coding unit.

FIG. 9 is a diagram showing an encoding unit after truncation processing in the display unit according to the first embodiment.

FIG. 10 is a diagram illustrating a result when the output of the horizontal spatial filter in the first embodiment is truncated by 2 bits.

FIG. 11 is a diagram illustrating an example of an output from a comparison limiting unit when a display resolution is 32 gradations according to the second embodiment.

FIG. 12 is a diagram showing an intermediate calculation result at the time of encoding by the encoding unit according to the second embodiment.

FIG. 13 is a diagram illustrating an encoding unit after a truncation process in the display unit according to the second embodiment.

[Explanation of symbols]

Reference Signs List 1 image transmission terminal, 2 image reception terminal, 3 communication network (communication path), 11 image input unit, 12 pre-processing unit, 13 encoding unit, 14 transmission-side communication unit, 15 reception-side communication unit, 1
6 decoding unit, 17 display unit, 21 original image storage unit, 2
2 horizontal spatial filter (low-pass filter), 23 comparison limiting unit, 31 image encoding device.

 ──────────────────────────────────────────────────続 き Continued on front page F term (reference) 5C059 KK00 MA02 MA23 MD02 ME01 SS06 SS20 TA69 TC45 UA02 5C077 LL19 MP01 PP04 PP20 PP46 PP47 PQ08 PQ20 RR06 RR11 RR21 SS07 TT09 5C078 AA04 BA57 CA01 DA00 DA01 BA02 BA11 BA01 BC02 BC18 BD02 BD03

Claims (14)

[Claims] A pre-processing unit that performs pre-processing on an input image and limits a value of a pixel after the pre-processing to a predetermined section including a value of a pixel before the pre-processing; And an encoding unit for encoding an output image from the image encoding apparatus. 2. The image according to claim 1, wherein the preprocessing unit limits a difference between a value of each pixel before the preprocessing and a value of each pixel after the preprocessing to a maximum allowable value. Encoding device. 3. The pre-processing unit compares, for each bit, an n-bit binary number representing a value of each pixel before pre-processing and an n-bit binary number representing a value of each pixel after the pre-processing. 2. The image coding apparatus according to claim 1, wherein the value after the pre-processing is limited so that a difference between the pre-processing and the lower-order bit is within the lower m bits. 4. The image encoding apparatus according to claim 1, wherein the pre-processing unit performs a process of removing a high-frequency component from the input image. 5. The encoding apparatus according to claim 1, wherein the encoding unit performs an encoding process using DPCM on the output image from the preprocessing unit. Image coding device. 6. The image code according to claim 1, wherein the encoding unit performs a discrete cosine transform process on the output image from the preprocessing unit. Device. 7. The image encoding apparatus according to claim 1, wherein the pre-processing unit is capable of changing a predetermined section in the pre-processing by setting. 8. An image transmitting terminal that encodes and transmits an input image, an image receiving terminal that receives and decodes an image encoded by the image transmitting terminal, and displays a decoded image, In an image communication system including an image transmission terminal and a communication path for transmitting an image transmitted and received by the image reception terminal, the image transmission terminal performs pre-processing on the input image, and performs pixel processing after the pre-processing. And a coding unit for coding an output image from the pre-processing unit. Image communication system. 9. The image according to claim 8, wherein the preprocessing unit limits a difference between a value of each pixel before the preprocessing and a value of each pixel after the preprocessing to a maximum allowable value. Communications system. 10. The pre-processing unit compares, for each bit, an n-bit binary number representing a value of each pixel before pre-processing and an n-bit binary number representing a value of each pixel after the pre-processing. 9. The image communication system according to claim 8, wherein the value after the pre-processing is limited so that a difference between the values falls within the lower m bits. 11. An image receiving terminal transmits information on a display resolution to an image transmitting terminal via a communication path, and the image transmitting terminal performs preprocessing only when the display resolution is low. The image communication system according to any one of claims 8 to 10, wherein: 12. An image receiving terminal transmits information of a display resolution to an image transmitting terminal via a communication path, and the image transmitting terminal transmits a predetermined resolution in a preprocessing of a preprocessing unit based on the information of the display resolution. The image communication system according to any one of claims 8 to 10, wherein the section is changed. 13. A procedure for performing pre-processing for removing high-frequency components from an input image, and a difference between lower and upper bits of an n-bit binary number representing a value of each pixel before and after the pre-processing is lower m bits. A method for limiting the value of each pixel after the pre-processing so as to fit within the range, and a procedure for coding an image limited to fit within the lower m bits. A computer-readable program recording medium storing a program to be executed. 14. The computer-readable program recording medium according to claim 13, wherein a program for changing the value of m is stored.