JPH09322162A - Image compression method and image expansion method - Google Patents

Abstract

(57) Abstract: A block coding compression method and a decompression method for coding gradation image data used at a printer, a copying machine, etc. that handle gradation images at a high compression rate. When one block of image data has the same data in all blocks, one representative value a1 as gradation information and a fixed value a0 meaning that the data in the block are all equal Turn into. In addition, the block code is generated with the fixed value a0 and the representative value a1 indicating the exceptional processing and the continuous length a2 while the gradation pixels having the same value are not limited to one block range and are continuous.
In this way, the compression rate can be further increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image compression method and image expansion / contraction method for multi-gradation pixels used in printers, copying machines and the like.

[0002]

2. Description of the Related Art In recent years, office equipment such as printers and copiers have been spreading to individual users including various offices. Further, in these devices, it is required to improve the image quality and handle color images. Particularly in the case of a color image, the amount of data becomes enormous, and an increase in memory, and thus an increase in price, is an unavoidable problem. In order to realize the low price, it is necessary to compress the image data while maintaining the high image quality and reduce the memory required for storing the image.

Conventionally, there is a technique called block coding as a method of compressing information of a gradation image. This method will be described with reference to FIGS. 1 and 12 to 17. FIG. 1 is an enlarged view of a part of the image. In FIG. 1, the pixel 1 is the minimum unit that constitutes an image. One pixel handles 256 gradations with 8-bit data, and has a value of 0 to 255. The block 2 in which 4 × 4 (total number of pixels 16) neighboring pixels are collected is taken, and processing is performed for each block.

FIG. 12 shows an actual example of a certain block 2, and FIG. 15 is a flow chart showing a procedure for block-encoding the block 2. First, the average value of block 2 to be block-encoded is calculated, and this is set as the intra-block average value m (step S1). The average value m in the block in FIG. 12 is m = 122 by rounding off fractional numbers. Next, the average value in the block is compared with each pixel of the block 2, and the pixel having a value less than the average value in the block is “0”,
A pixel array pattern is formed by flag bits that set pixels having a value equal to or greater than the average value within the block to "1" (step S2). A pixel array pattern corresponding to block 2 in FIG. 12 is shown in FIG. Next, a1 is calculated as the average value a0 of the pixel set whose pixel array pattern is "0" and a1 of the pixel set whose pixel array pattern is "1", respectively, and is set as the block representative value (step S3). In block 2 of FIG. 12, the block representative values are a0 = 34 (22h) and a1 = 162 (A2h).
It is. Note that h indicates hexadecimal notation. The final block code of this block 2 is 8 as shown in FIG.
Two representative values (a0, a1), which are bit data,
It is configured by one 16-bit pixel array pattern (Bitmap) in which FIG. 13 is serially arranged. In the example of FIG. 12, the code is as shown in FIG.

FIG. 16 is a flowchart of a decompression process for restoring this block code as actual image data.
Shown in The block representative value a0 contained in the block code of FIG. 14 is applied to the pixel which is "0" in the pixel array pattern, and the representative value a1 is applied to the pixel which is "1" in the pixel array pattern. This is performed for all the pixels in one block. FIG. 17 shows a decompressed image obtained by decompressing the block code of FIG.

Generally, when one pixel is represented by q-bit data and the number of pixels in one block is represented by N, the amount of data in one block of the original image is q × N bits, and the image data after block coding is N. Bit pixel array pattern and 2
It is 2q + N bits, which is a combination of block representative values of × q bits. Therefore, the compression rate of the image data by the block coding is qN / (2q + N). In the example shown in FIG.
Since 1 pixel q = 8 bits and 1 block N = 16,
The compression rate is 4.

[0007]

Generally, when the types of images are roughly divided into two, it is possible to distinguish between images composed of characters and figures and natural images such as photographs and paintings. In the former case, most of the data is a solid image in which the data values in the block are the same, and even if there is a change in gradation, the change is significantly large, and in the latter case, the gradation between neighboring pixels is large. Changes are always, but often small. However, in the conventional block coding as described above, the compression code of one block is always expressed by two representative values and one bit map data, and when the data in one block are all the same, that is, a solid image. Even in such a case, similar coding compression was performed. Therefore, one representative value has two representative values even for a sufficient block, and the representative value data is encoded as it is for an image with a small gradation change, so that the compression rate is poor. .

The present invention has been made in view of such conventional problems, and it is a technical object to further increase the compression rate as compared with the conventional compression technique by block coding.

[0009]

According to the invention of claim 1 of the present application, a plurality of pixels of an image in which each pixel is expressed in multi-gradation is set as one block, and an average value of all the pixels in the block is calculated. By dividing each pixel into a pixel larger than the average value and a pixel smaller than the average value and obtaining the average value, the gradation information and the flag bit indicating whether each pixel is larger or smaller than the average value of all pixels An image compression method characterized by reducing the amount of image information by generating a block code composed of a pixel array pattern and showing an exceptional process when all the pixels in the block have the same value. The feature is that the block code of the block is generated only with a fixed value and a representative value.

According to a second aspect of the present invention, a plurality of pixels of an image in which each pixel is expressed in multiple gradations are set as one block, and an average value of all the pixels in the block is taken to be a pixel larger than the average value. Each pixel is divided into small pixels, each average value is obtained, and this is used as gradation information. The gradation information and a pixel arrangement pattern by a flag bit indicating whether each pixel is larger or smaller than the average value of all pixels are configured. In the image compression method, which is characterized by reducing the amount of image information by generating a block code that is generated, all pixels in the block have the same value, and the value is not limited to one block range and is continuous. In this case, the block code is generated with a fixed value indicating the exceptional process and a representative value and a continuous length of the same pixel value.

The invention of claim 3 of the present application is an image decompression method for discriminating a decompressing block code including an exceptional process of claim 1 and decompressing the block code, which is typical for a block code having a fixed value indicating an exceptional process. Values are applied to all the pixels in the block, and pixels in the multi-tone image are reconstructed by applying the average values to the pixels larger and smaller than the average value to the other block codes according to the pixel arrangement pattern. It is characterized by doing.

The invention according to claim 4 of the present application is an image expansion method for determining and expanding a block code including an exception process according to claim 2, which is performed when a fixed value indicating an exception process is included. A representative value is applied to all the pixels of the block corresponding to the length, and the average values are applied to the other block codes according to the pixel arrangement pattern to the pixels larger and smaller than the average value. It is characterized by reconstructing pixels.

According to the invention of claim 1 of the present application having such a characteristic, the gradation of each pixel is identified by the average value of the blocks, with a plurality of pixels represented by multiple gradations as one block, and The block code is generated by a pixel array pattern indicating the magnitude of the average value and the average value of all pixels.
Then, when all the pixels in the block have the same value, the block code is generated by the fixed value and its representative value. By doing so, in the case of a solid image in which the data values in blocks such as characters and figures are the same, the amount of image information is reduced and the compression efficiency is increased. According to the second aspect of the invention, when the same pixel value continues in a plurality of blocks, the block code indicating the exceptional process is generated by adding the continuous length of the same pixel value. Therefore, when the same pixel value continues in a plurality of blocks, the image compression efficiency can be further improved. According to claims 3 and 4, the block code compressed in this way is restored based on the compression rule to generate the original image.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of the present invention will be described below with reference to FIGS. FIG. 2 is an enlarged view of the original image of the image for explaining the coding compression according to the present invention. 1 in FIG. 2 is a pixel, which is the minimum unit that constitutes an image. 2 for 8-bit data for each color
Assuming that 56 gradations are handled, one pixel 1 has a value of 0-255. As shown by a thick line, a block 2 in which 4 × 4 (total number of pixels 16) neighboring pixels are collected is taken. The present invention is characterized by the encoding by the exceptional processing when all the pixels of the block 2 have the same value, and FIGS. 2 and 3 are diagrams for explaining the encoding. FIG. 2 shows an example in which all the blocks 2 have the same value, and FIG. 3 shows a coding state after the exception processing is performed on FIG. Further, since special rules are required for the conventional block coding for the exception processing of the present invention, examples thereof are shown in FIGS. 4 and 5. FIG. 4 shows an example of encoded data that requires a special rule, and FIG. 5 shows encoded data after applying the special rule to FIG.
FIG. 6 is a flowchart showing the procedure for block-encoding the block 2.

Next, the processing flow of the image compression method of this embodiment will be described with reference to these figures. First, in the flowchart of FIG. 6, it is checked in block 2 whether all pixel values are the same (step S1).
If all the pixels have the same value, a block is composed of a fixed value a0 indicating exceptional processing and a pixel value (representative value) a1 of 2 bytes as a special flowchart processing (step S2). The final block code of this block 2 is composed of two representative values which are 8-bit data as shown in FIG. 3A, and specifically, the code is as shown in FIG. 3B. 2 is an example in which all the pixel values in FIG. 2 are 128, and in this case, the block after encoding is shown in FIG.
As shown in, the fixed value indicating the exception handling is a0 = 1,
The pixel value a1 = 128. Here, a0 = 1 is defined as a fixed value indicating exceptional processing. The reason is that when pixels are drawn by a personal computer or a printer, both 0 and 1 show black as pixel data in 256 gradations. Since it is difficult to distinguish visually, it is decided to give 1 a special meaning. When all the pixels in the block have the same value by the above operation, the compression rate of 8 can be realized by expressing the block 2 of 16 bytes by 2 bytes.

Next, the processing when the blocks 2 do not have the same value will be described below. First, the average value of the block 2 to be block-encoded is calculated, and this is set as the intra-block average value m (step S3). By comparing the average value in the block with each pixel in block 2, the pixel having a value less than the average value in the block is set to "0", and the pixel having a value not less than the average value in the block is set to "1". A pixel array pattern is formed (step S4). Next, a1 is calculated as the average value a0 of the pixel set having the pixel array pattern of "0" and a1 of the pixel set having the pixel array pattern of "1", and is set as the block representative value (step S5). Up to this point, the flow of encoding has been the same as that of the conventional technique, but since the pixel value 1 is a fixed value indicating exceptional processing in the present invention, the following special rule must be applied. The block code of this block 2 is composed of two representative values, each of which is 8-bit data, and one pixel array pattern represented by 16 bits, as shown in FIG. The code is as shown in (b). Here, when the value of a0 in FIG. 4 is 1, it is replaced with the value 0 in order to approximate it as a black pixel (steps S6 and S7). That is, the block is re-formed with a0 = 0. Therefore, when decompressing, a0 = 1 always indicates exceptional processing. Finally, as shown in FIG. 5, a block is formed by a0 and a1, and the pixel array pattern (step S8). Similarly, block coding can be realized by repeating the processing of the next block.

Next, the method of decompressing the compressed block will be described. FIG. 7 shows a flowchart of a decompression process for restoring as actual image data. First, it is checked whether the value of a0 is 1 (step S1). When a0 = 1, the block is reproduced by applying the value of a1 to all pixels (step S2). If a0 is not 1, as in the conventional decompression method, the process of applying a0 to pixels that are "0" in the pixel array pattern and applying a1 to pixels that are "1" in the pixel array pattern is performed within the block. Are performed for all pixels (step S3). By repeating the above operation for each block, the pixel can be expanded.

Next, FIG. 8 shows the second embodiment of the present invention.
It will be described with reference to FIGS. In the second embodiment, in the processing in the case where all values in the block are the same, the processing is applied not only to one block but to a plurality of consecutive blocks, and the length is added to the block information as an attribute, It further increases the compression rate. The compression method will be described below with reference to the flowchart of FIG. When all the pixel values in one block are the same value, the continuous length, for example, the number of pixels, is calculated while the same value continues for each block (step S2). In this case, if at least one different value is included in the block, the previous block is applied. Next, when forming the block code after compression, in addition to the fixed value a0 = 1 and the pixel value a1 indicating the exceptional processing, the code is configured by 3 bytes of the continuous pixel number a2 (step S3). Finally, as shown in FIG. 9A, it is represented by a fixed value and a representative value which are each 8-bit data and a length of a continuous pixel of 8 bits. FIG. 8 shows an example of the case where the same value 128 of pixels continues over three blocks,
FIG. 9 (b) shows the block code after compression, and since 48 pixels for three blocks have the same value continuously, a
2 = 48. Note that the processing (steps S4 to S9) when the values in all the blocks are not the same is the same as that in the first embodiment, and is omitted here.

Next, a method of expanding the compressed block will be described. FIG. 11 shows a flowchart of a decompression process for restoring as actual image data. First, it is checked whether or not the value of a0 is 1 (step S1). If a0 = 1, the value of a1 is applied to all pixels by the number of pixels a2 to reproduce the block (step S2). Note that the process (step S3) when a0 = 1 is not the same as that in the first embodiment, and will be omitted here. By repeating the above operation for each block, the pixel can be expanded. The number of pixels may be directly set to a2 for the continuous length, and
It may be 2.

[0020]

As described in detail above, according to the invention of claim 1 of the present application, when all the data in one block of image data are the same, one representative value and a fixed value indicating an exception process. By representing by, it is possible to provide an image compression method with a high compression rate for image data.

According to the second aspect of the present invention, when the multi-gradation pixels have the same value in succession, in addition to the fixed value indicating the exceptional processing and the representative value during the continuous not limited to one block range, By generating block codes with continuous lengths, the compression ratio can be further increased.

According to the inventions of claims 3 and 4, claim 1,
The original image can be restored by decompressing the image compressed by the second invention based on the respective exception rules.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a block that performs block coding according to the present invention.

FIG. 2 is an explanatory diagram showing a block code before compression according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a compressed block code according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a compressed block code according to the first embodiment of the present invention.

FIG. 5 is an explanatory block diagram after compression in the first embodiment of the present invention.

FIG. 6 is a flowchart of a compression process according to the first embodiment of the present invention.

FIG. 7 is a flowchart of decompression processing according to the first embodiment of the present invention.

FIG. 8 is an explanatory diagram of image data before processing according to the second embodiment of the present invention.

FIG. 9 is an explanatory diagram showing a block code after compression processing according to the second embodiment of the present invention.

FIG. 10 is a flowchart of a compression process according to the second embodiment of the present invention.

FIG. 11 is a flowchart of decompression processing according to the second embodiment of the present invention.

FIG. 12 is an explanatory diagram of image data before compression processing in conventional block coding.

FIG. 13 is an explanatory diagram of bitmap data in conventional block encoding.

FIG. 14 is an explanatory diagram of a block code after compression in conventional block coding.

FIG. 15 is a flowchart of a conventional block coding compression process.

FIG. 16 is a flowchart of decompression processing of conventional block coding.

FIG. 17 is an explanatory diagram of image data after decompression processing of conventional block encoding.

[Explanation of symbols]

 1 pixel 2 blocks

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Claims (4)

[Claims] 1. A plurality of pixels of an image in which each pixel is expressed in multiple gradations are set as one block, an average value of all the pixels in the block is taken, and each pixel is divided into a pixel larger than the average value and a pixel smaller than the average value. An average value is obtained and used as gradation information, and a block code composed of the gradation information and a pixel array pattern of flag bits indicating whether each pixel is larger or smaller than the average value of all pixels is generated. By that,
In an image compression method characterized by reducing the amount of image information, when all the pixels in the block are all the same value,
An image compression method characterized in that a block code of the block is generated only with a fixed value and a representative value indicating exceptional processing. 2. A plurality of pixels of an image in which each pixel is expressed in multiple gradations are set as one block, an average value of all pixels in the block is taken, and each pixel is divided into a pixel larger than the average value and a pixel smaller than the average value. An average value is obtained and used as gradation information, and a block code composed of the gradation information and a pixel array pattern of flag bits indicating whether each pixel is larger or smaller than the average value of all pixels is generated. By that,
In an image compression method characterized by reducing the amount of image information, when all pixels in the block have the same value and the values are not limited to one block range and are continuous, a fixed value indicating exceptional processing and An image compression method characterized in that a block code is generated by a representative value and a continuous length of the same pixel value. 3. An image decompression method for discriminating and decompressing a block code including an exceptional process according to claim 1, wherein a block code having a fixed value indicating the exceptional process has a representative value of all of the blocks. While applying to the pixel,
An image decompression method characterized by reconstructing the pixels of a multi-tone image by applying the respective average values to pixels larger than the average value and pixels smaller than the average value for other block codes depending on the pixel arrangement pattern. 4. An image decompression method for discriminating and decompressing a block code including an exceptional process according to claim 2, wherein a block corresponding to a length of a block when a fixed value indicating the exceptional process is included. It is possible to reconstruct the pixels of a multi-tone image by applying representative values to all pixels and applying other averages to other block codes depending on the pixel arrangement pattern for pixels larger than the average value and smaller than the average value. Characteristic image expansion method.