JP2009212800A - Image compression apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image compression apparatus capable of compression without damaging a density difference in an intermediate color region even when both the intermediate color region and a line drawing region with a large density difference are present in the same block. <P>SOLUTION: A limited tone range (example; 64-192) narrower than a tone range (256 tones) that image data can take is set in a setting part 13, a maximum value/minimum value detection part 14 detects the maximum value and the minimum value within the limited tone range of a tone value in one block, a threshold generation part 15 divides the tone range having them at both ends into a plurality of regions, and a judgement part 16 judges whether or not the tone values of respective pixels are within the limited tone range. An encoding part 17 encodes the pixel whose tone value exceeds the upper end value of the limited tone range to a first code, the pixel whose tone value is less than the lower end value of the limited tone range to a second code, and the pixel within the limited tone range to the code corresponding to the region to which the tone value of the pixel belongs among the plurality of previously divided regions. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image compression apparatus that compresses image data, and more particularly to an image compression apparatus that compresses an image to be compressed in blocks of a predetermined number of pixels.

  In an apparatus such as a digital multi-function peripheral, when image data in bitmap format is stored in a memory or a hard disk device, the image data is compressed and stored in order to reduce a necessary storage capacity.

  One of the still image compression methods is a BTC (Block Truncation Coding) compression method. In BTC compression, an image to be compressed is divided into blocks each having a predetermined number of pixels (for example, 4 pixels × 4 pixels). Then, for each block, the maximum and minimum values (information indicating the distribution range) of the gradation values of the pixels belonging to the block, and the gradation values of each pixel in the block within the distribution range are converted into the original image data. Compressed data represented by code data normalized (quantized) with a smaller number of gradations than the number of gradations is generated.

  At the time of decompression, a representative value for each value of the code data is determined from the distribution range of gradation values indicated by the maximum value and the minimum value included in the compressed data, and each code data included in the compressed data is an image of the corresponding representative value. It is designed to restore the image by converting it to data.

  For example, FIG. 7 shows the original image data 100 for one block of 4 pixels × 4 pixels in which each pixel is represented by 8-bit 256 gradations (in the figure, the gradation value is expressed in hexadecimal). A case where BTC compression is performed on bit (4 gradations) code data is illustrated. The original image data has a data amount of 16 bytes per block.

  At the time of compression, the distribution range having the maximum value A0h and the minimum value 40h of the gradation value in the block as both ends is equally divided into four, and the gradation value D is a pixel whose maximum value (A0h) ≧ D> threshold value 3 (88h). Indicates code data “11”, pixels with threshold 3 (88h) ≧ D> threshold 2 (70h) indicate code data “10”, and pixels with threshold 2 (70h) ≧ D> threshold 1 (58h) indicate code data “11”. 01 ”, a pixel having a threshold value 1 (58h) ≧ D ≧ minimum value (40h) is encoded into code data“ 00 ”. The compressed data 101 is composed of code data for these 16 pixels, a maximum value A0h, and a minimum value 40h, and the data amount is 6 bytes.

  At the time of expansion, the maximum value A0h indicated by the compressed data 101 indicates the representative value 4 corresponding to the code data “11”, and the minimum value 40h indicates the representative value 1 corresponding to the code data “00”. Of the two boundary values that equally divide the range (A0h to 40h) into three, the larger 80h corresponds to the representative value 3 corresponding to the code data “10”, and the smaller 60h corresponds to the code data “01”. Set to representative value 2. Each code data included in the compressed data 101 is converted into a representative value corresponding to the value of the code data, so that the image data 102 for one block is expanded.

  In the BTC compression, the gradation value distribution range in the block is normalized to the number of gradations corresponding to the number of bits of the code data. Therefore, as the gradation value distribution range in the block becomes wider, the original scale of each pixel is increased. There is a possibility that an error between the tone value and the gradation value after compression / expansion becomes large.

  Therefore, it is determined whether each block corresponds to one of a flat part having almost no density change, a quasi-flat part having a binary density distribution, and an edge part having a strong density change, and a flat part or a quasi-flat part. There is an image processing apparatus that improves image quality and compression rate by selecting an optimal encoding method according to an edge portion (see, for example, Patent Document 1).

JP-A-6-121175

  As in the device disclosed in Patent Document 1, in the technique of optimizing the encoding method for each block, if a region with a strong density change and a region with little density change are mixed in one block, the density change There is a problem that the coding method suitable for the intense region is selected, and the density of the region having almost no density change is lost.

  For example, if the same block contains both images (intermediate color areas) such as photographs and pictures with many intermediate colors that have almost no density change and images (line drawing areas) such as high-contrast text, the density of the intermediate color areas after compression Will disappear. FIG. 8 shows an example. The shaded portion is a neutral color region, and the shaded portion is a high-contrast line drawing region. When this block image is decompressed by BTC compression to 2 bits (4 gradations) per pixel, the subtle density difference in the intermediate color area of the original image disappears. For example, the pixel a and the pixel b have the gradation value 60h and the gradation value 70h in the original image data 110, but in the image data 112 after compression / expansion, both are expanded to the representative value 55h and the density difference is lost. ing.

  In order to eliminate this drawback, there is a method of increasing the number of gradations of each pixel at the time of compression (increasing the number of bits of code data to 3 bits per pixel, for example), but the density difference between the intermediate color area and the line drawing area When is large, the effect is small. FIG. 9 shows an example in which the same original image data 110 as in FIG. 8 is compressed and expanded into code data of 3 bits for each pixel. In the expanded image data 115, both the pixel a and the pixel b are expanded to the representative value 6Ch, and the density difference is lost.

  The present invention is intended to solve the above-described problem. Even when both the intermediate color area and the line drawing area having a large density difference are included in the same block, the density difference of the intermediate color area is not impaired. An object of the present invention is to provide a compressible image compression apparatus.

  The gist of the present invention for achieving the object lies in the inventions of the following items.

[1] a dividing unit that divides an image to be compressed into blocks each having a predetermined number of pixels;
A setting unit in which a limited gradation range that forms a part of a gradation range that the image can take is set;
For each block obtained by dividing by the dividing unit, a detecting unit for detecting the maximum value and the minimum value of the gradation values of the pixels belonging to the block within the limited gradation range;
For each block obtained by dividing by the dividing unit, a region setting unit that divides a first range having both ends of the maximum value and the minimum value detected by the detection unit for the block into a plurality of regions;
For each pixel of each block obtained by dividing by the dividing unit, when the gradation value of the pixel is larger than the upper limit value of the restricted gradation range, first code data is assigned, and the gradation value is If the lower limit value of the limit gradation range is smaller than the lower limit value, the second code data is allocated. If the gradation value is within the limit gradation range, the area setting unit sets the gradation value of the block to which the pixel belongs. An encoding unit that assigns code data indicating which of the plurality of divided areas belongs to; and
For each block obtained by the division by the division unit, compressed data including information indicating the first range related to the block and code data allocated by the encoding unit to each pixel belonging to the block is generated. An output unit to
An image compression apparatus comprising:

  In the above invention, among the pixels belonging to one block, the first code data is fixed to pixels whose gradation value exceeds the upper limit value of the limited gradation range, and the second code data is fixed to pixels less than the lower limit value. To assign. Further, only the pixels within the limited gradation range are targeted, and the maximum and minimum values of the gradation value are detected, and each pixel within the limited gradation range has the maximum value and the minimum value as both ends. The gradation range (first range) is quantized so as to be expressed by a predetermined plurality of gradations, and is encoded into code data having a value different from that of the first and second code data. As a result, when the line drawing area and the intermediate color area are mixed in one block, the line drawing area is determined to be outside the limited gradation range, and a large number of gradations can be assigned to the intermediate color area for encoding. Compression can be performed without impairing the gradation difference in the intermediate color area.

[2] The upper limit value of the limited gradation range is smaller than the maximum gradation value that the image can take,
The lower limit value of the limited gradation range is larger than the minimum gradation value that the image can take,
The image compression apparatus according to [1], wherein

  According to the invention, the limited gradation range is set to a range excluding near the maximum value and the minimum value of the gradation range that can be taken by the image. That is, the limited gradation range is a gradation range excluding a very dark area and a very thin area. As a result, the line drawing area can be removed, and the image area excluding the line drawing can be quantized by giving a larger number of gradations.

[3] The setting unit inputs density information indicating a density level of the image to be compressed, and changes the limited gradation range according to the input density information [1] or The image compression apparatus according to [2].

  In the above invention, since the limited gradation range is changed according to the density level of the image to be compressed, it is possible to perform appropriate compression according to the characteristics of the image.

  The image compression apparatus according to the present invention compresses image data without losing the density difference between the intermediate color areas even when both the intermediate color area and the line drawing area with a large density difference are included in the same block. be able to.

  Hereinafter, various embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows the configuration of an image compression apparatus 10 according to the first embodiment of the present invention. The image compression apparatus 10 has a function of inputting an image to be compressed (image data), dividing the image into blocks each having a predetermined number of pixels, and outputting compressed data compressed for each block.

  The image compression apparatus 10 is used, for example, in compression processing when compressing and storing image data of an input document in a hard disk device or the like in a multi-function peripheral having a document copy function, a print function, and the like.

  The image compression apparatus 10 is configured by a hardware circuit such as a sequencer or a logic circuit incorporating a microprogram. Note that the present invention may be realized by executing a predetermined program in a computer device including a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like.

  The compression method used in the image compression apparatus 10 is based on BTC compression. The image data to be compressed input to the image compression apparatus 10 is image data in a bitmap format in which one pixel is expressed by 256 tones with a depth of 8 bits. The image compression apparatus 10 generates compressed data in which the gradation of each pixel is normalized and expressed as code data of N bits (N is an integer equal to or greater than 3, here “3”).

  The image compression apparatus 10 includes an input unit 11, a division unit 12, a setting unit 13, a maximum / minimum value detection unit 14, a threshold generation unit 15, a determination unit 16, an encoding unit 17, and an output unit 18. And is configured.

  The input unit 11 receives input of image data to be compressed. The dividing unit 12 divides the image data to be compressed input from the input unit 11 into blocks each having a predetermined number of pixels. Here, as shown in FIG. 2, the image G is divided into 4 × 4 pixel blocks BL with the upper left corner as the base point. Each time the dividing unit 12 divides one block, the image data of the block is sent to the maximum / minimum value detecting unit 14, the determining unit 16, and the encoding unit 17.

  The setting unit 13 sets a limited gradation range that forms a part of a gradation range (here, 0 to 255 gradations) that the image data input from the input unit 11 can take, and a CPU (Central Processing) (not shown). Unit), and the function of storing the set limited gradation range is achieved. Here, the upper limit value A and the lower limit value B of the limited gradation range are received and stored. The setting unit 13 sends the upper limit value A and the lower limit value B of the set limited gradation range to the maximum / minimum value detection unit 14 and the determination unit 16.

  The limited gradation range is set to a gradation range excluding a very dark area and a very thin area. In other words, when the line drawing area and the intermediate color area are mixed and there is a large density difference between the density of the intermediate color area and the density of the line drawing area, the intermediate color area and the line drawing area are set to be inside and outside the restricted gradation range. The limited gradation range is set so that it can be separated. For example, an intermediate 80% portion excluding 10% on the maximum gradation side and 10% on the minimum gradation side of the gradation range that the image can take is set as the limited gradation range.

  In addition, the gradation range that can be taken by the image data G to be compressed is divided by the number of types M (“8”) that can be expressed by N bits (3 bits), which is the data amount of code data per pixel. If the obtained size Q is set to (M−2) times or less, the quantization target range per gradation when the restricted gradation range is expressed by (M−2) gradations can be obtained. When the entire gradation range is expressed by M gradations, the quantization target range per gradation can be made equal to or less.

  For each block obtained by dividing by the dividing unit 12, the maximum value / minimum value detecting unit 14 falls within the limited gradation range of the gradation values of pixels belonging to the block (the processing target block input from the dividing unit 12). Detect maximum and minimum values at. For example, the gradation value D of each pixel of the block to be processed is compared with the upper end value A and the lower end value B input from the setting unit 13 to extract pixels that satisfy the upper end value A ≧ D ≧ lower end value B, The extracted gradation values D of the pixels are sequentially compared to detect the maximum and minimum gradation values in the extracted pixels. The detection result 21 is sent to the threshold value generator 15 and the output unit 18.

  The threshold value generation unit 15 sets a threshold value for dividing a range (first range) having both the maximum value and the minimum value input from the maximum value / minimum value detection unit 14 into (M−2) areas. Generate. Here, five threshold values 1 to 5 for dividing the first range into six equal parts are generated. It is assumed that threshold value 1, threshold value 2, threshold value 3,... In order from the smaller threshold value. Each generated threshold value is sent to the encoding unit 17.

  The determination unit 16 determines, for each pixel in each block obtained by the division unit 12, whether the gradation value of the pixel is within the limited gradation range. Specifically, the gradation value D of each pixel of the block to be processed is sequentially input from the dividing unit 12 in a predetermined order (for example, the number order given to each pixel in the block BL in FIG. 2). In order of input, the gradation value D of the pixel is compared with the upper limit value A and the lower limit value B of the limited gradation range input from the setting unit 13. When the upper end value A ≧ D ≧ the lower end value B, the first signal indicating the limit gradation range is satisfied. When D> the upper end value A, the second signal indicating that the upper end value is exceeded, and D <the lower end value B In this case, a third signal indicating less than the lower end value is output to the encoding unit 17.

  For each pixel of each block obtained by dividing by the dividing unit 12, the encoding unit 17 encodes the first value when the gradation value D of the pixel is larger than the upper end value A of the limited gradation range. Data is assigned, and if the gradation value D is smaller than the lower limit value B of the restricted gradation range, code data of the second value is assigned. Further, when the gradation value D is within the limited gradation range, the gradation value D of the pixel has the maximum value and the minimum value detected by the maximum value / minimum value detection unit 14 for the block to which the pixel belongs. It is determined which of the (M−2) areas obtained by dividing the first range by the plurality of threshold values generated by the threshold value generation unit 15 for the same block, and The code data having a uniquely indicated value is assigned to the pixel.

  The gradation value (image data) D of the same pixel as the pixel input from the dividing unit 12 to the determining unit 16 is sequentially input to the encoding unit 17. The encoding unit 17 sequentially assigns code data to the input pixels.

  Specifically, when the gradation value D (image data) of the pixel to be processed is input from the dividing unit 12 to the encoding unit 17, the determination result in the determination unit 16 for the same pixel is input from the determination unit 16. The When the second signal is input from the determination unit 16, the encoding unit 17 assigns 3-bit code data “111” to the pixel to be processed. When the third signal is input from the determination unit 16 at the same time when the gradation value D (image data) of the pixel to be processed is input from the dividing unit 12, the code data “ 000 "is assigned.

  The determination unit 16 outputs a signal indicating whether the signal is within the restricted gradation range or outside the restricted gradation range to the encoding unit 17 without distinguishing between the second signal and the third signal. Arbitrary input from the threshold generation unit 15 on whether the tone value of the corresponding image is higher than the upper limit value A or lower than the lower limit value B on the encoding unit 17 side when a signal indicating out of the adjustment range is received This threshold value may be compared with the gradation value D of the pixel. That is, it can be determined that the upper limit value A is exceeded when the gradation value of the pixel is outside the limit gradation range and exceeds any threshold value, and is less than the lower limit value B when it is less than the threshold value.

  When the gradation value D (image data) of the pixel to be processed is input from the dividing unit 12 and the first signal (signal indicating the limited gradation range) is input from the determination unit 16 at the same time, The gradation value D of the pixel is compared with the thresholds 1 to 5 input from the threshold generation unit 15, and the code data is assigned to the processing target pixel as follows according to the comparison result.

If D> threshold 5, code data “110”
If threshold 5 ≧ D> threshold 4, code data “101”
If threshold 4 ≧ D> threshold 3, code data “100”
If threshold 3 ≧ D> threshold 2, code data “011”
If threshold 2 ≧ D> threshold 1, code data “010”
If the threshold value 1 ≧ D, the code data “001”
The encoding unit 17 sequentially outputs the code data allocated in this way to the output unit 18.

  The output unit 18 includes, for each block obtained by dividing by the dividing unit 12, information indicating the first range related to the block and code data assigned by the encoding unit 17 to each pixel belonging to the block. Generate and output compressed data. That is, the maximum value and the minimum value (information indicating the first range) related to the processing target block input from the maximum value / minimum value detection unit 14 and the pixels of the block sequentially input from the encoding unit 17 are assigned. The compressed data composed of the coded data for 16 pixels is generated and output for each block.

  FIG. 3 shows the flow of compression processing by the image compression apparatus 10. Image data to be compressed is input from the input unit 11 (step S101), and the dividing unit 12 sequentially divides the input image into blocks each having a predetermined number of pixels (such as 4 × 4 pixels) (step S102).

  The maximum value / minimum value detection unit 14 determines the maximum gradation value (maximum value) within the limited gradation range given from the setting unit 13 among the pixels belonging to the processing target block given from the division unit 12. The minimum gradation value (minimum value) is detected (step S103).

  The threshold value generation unit 15 equally divides the gradation distribution range (first range) having the maximum value and the minimum value detected by the maximum value / minimum value detection unit 14 at both ends into (M-2) equal parts (here, equal to 6 parts). A threshold value is generated (step S104). Further, the determination unit 16 determines whether or not the gradation value of each pixel belonging to the processing target block is outside the limited gradation range (step S105).

  The encoding unit 17 assigns code data to each pixel based on the determination result of the determination unit 16 and the threshold value generated by the threshold value generation unit 15 and encodes it (step S106). That is, when the gradation value of the pixel to be processed is larger than the upper limit value A of the restricted gradation range, the first code data (3-bit code data “111”) is assigned, and the gradation value is within the restricted gradation range. If the tone value is smaller than the lower end value B, the second code data (3-bit code data “000”) is assigned. If the tone value is within the limit tone range, the tone value and the threshold value generator 15 generate Compared with each of the threshold values 1 to 5, it is determined which of the plurality of regions divided by the threshold values 1 to 5 belongs, and code data (“001”) corresponding to the region is determined. To any one of “110”.

  The output unit 18 generates and outputs compressed data related to the block to be processed from the code data for each pixel assigned by the encoding unit 17 and the maximum value and the minimum value given from the maximum value / minimum value detection unit 14. (Step S107).

  Next, an example of image compression by the image compression apparatus 10 will be described. FIG. 4 shows an example in which the original image data 110 of the same block as in FIG. 9 is compressed by the image compression apparatus 10. In this example, the limited gradation range is set to 20h to DFh. Each pixel in the shaded area is a pixel whose gradation value is within the limited gradation range. Since the maximum value of the gradation values of the pixels within the limited gradation range is 90h and the minimum value is 60h, the range of 60h to 90h (first range) with the maximum value and the minimum value at both ends is divided into 6 equal parts. Threshold value 1 (68h), threshold value 2 (70h), threshold value 3 (78h), threshold value 4 (80h), and threshold value 5 (88h) are generated by the threshold value generation unit 15.

The encoding unit 17 determines that the gradation value D of the pixel to be processed is
-If E0h to FFh, code data "111"
-Code data "110" if in 88h to 90h
-Code data "101" if in 80h to 87h
-Code data "100" if in 78h-7Fh
-Code data “011” in the range from 70h to 77h
-Code data "010" in 68h-6Fh
-If it is in 60h to 67h, code data "001"
-Code data "000" if in 00h to 1Fh
Is assigned to the pixel, and the output unit 18 generates compressed data 51 composed of the code data, maximum value, and minimum value for these 16 pixels.

  The compressed data 51 generated by the image compression apparatus 10 is expanded as follows.

  The maximum value FFh in the gradation range that the image can take (0 to 255 gradations) is set to the representative value 8 for the code data “111”, and is the minimum value in the gradation range that the image can take. 00h is set as the representative value 1 for the code data “000”. Further, the maximum value (90h) included in the compressed data is set to the representative value 7 for the code data “110”, and the minimum value (60h) is set to the representative value 2 for the code data “001”.

  Furthermore, four boundary values (69h, 72h, 7Bh, and so on) for equally dividing the range having the maximum value (90h) and the minimum value (60h) at both ends into five (ie, (M-2) -1). 84h), the smallest boundary value 69h among them is the representative value 3 for the code data “010”, the next smallest boundary value 72h is the representative value 4 for the code data “011”, and the next smallest boundary value 7Bh is The largest boundary value 84h is set to the representative value 6 for the code data “101” and the largest boundary value 84h is set to the representative value 6 for the code data “100”. Then, the code data of each pixel included in the compressed data 51 is converted into representative values 1 to 8 corresponding to the values, so that the image data 55 is expanded.

  In the decompressed image data 55 shown in FIG. 4, there is a gradation difference between the pixel a and the pixel b, and the gradation difference corresponding to the intermediate color area of the image data 110 before compression is reproduced. As described above, the image compression apparatus 10 can perform compression without losing the density difference between the intermediate color areas even when both the intermediate color area and the line drawing area having a large density difference are included in the same block. Is improved.

  Next, a second embodiment of the present invention will be described.

  FIG. 5 shows a configuration of an image compression apparatus 10B according to the second embodiment. The image compression apparatus 10B according to the second embodiment is different from the image compression apparatus 10 according to the first embodiment in that a setting unit 19 is provided instead of the setting unit 13. Other configurations are the same as those of the image compression apparatus 10 according to the first embodiment, and a description thereof will be omitted.

  The setting unit 19 inputs grayscale information indicating the grayscale level of the image to be compressed from the outside, and changes the limited gradation range according to the input grayscale information. For example, as shown in FIG. 5, the image compression apparatus 10B is provided in an apparatus (for example, a copying machine) including a scanner apparatus 60 that optically reads an original, and an original document is detected from an EE sensor 61 included in the scanner apparatus 60. It is configured to receive density information indicating a density level. The scanner device 60 is configured to irradiate a document with light emitted from a light source and receive the reflected light with an image sensor so as to read a document image. The EE sensor 61 follows a path through which the reflected light passes. It is a sensor that is provided and detects the intensity of light.

  When image data obtained by reading a document with the scanner device 60 is compressed with the image compression device 10B, the setting unit 19 reads the document (or the document read with the scanner device 60 based on density information input from the EE sensor 61). Determine whether the background of the manuscript is dark, pale, or standard. If it is determined that the image is dark, the limited gradation range is changed so as to be shifted toward a smaller value than the standard gradation value (initial setting value). As a result, the shading in the dark tone range is less likely to be lost when encoding.

To change the limited gradation range so that the gradation value is smaller,
(1) Lower the lower limit value of the limited gradation range to widen the limited gradation range to the minimum value side.
(2) Shift the whole to the minimum value side without changing the size of the limited gradation range,
(3) Both the size reduction of the limited gradation range and the lower end value are reduced.
and so on. Note that the relationship between the density and the gradation value is such that the higher the density, the smaller the gradation value, and the gradation value of black (maximum density) is 00h.

  If the setting unit 19 determines from the density information input from the EE sensor 61 that the document read by the scanner device 60 (or the background of the document) is light (light), the limit gradation range has a large gradation value. Change it so that it is closer to you. As a result, the shading in the gradation range of light colors is difficult to be lost when encoding.

To change the limit gradation range so that it is offset toward the larger gradation value,
(1) Increasing the upper limit value of the limited gradation range to widen the limited gradation range to the maximum value side.
(2) Shift the whole to the maximum value side without changing the size of the limit gradation range,
(3) Both the size reduction of the limited gradation range and the upper end value are increased.
and so on.

  FIG. 6 shows the flow of compression processing in the image compression apparatus 10B of the second embodiment. Steps having the same processing contents as those in the flowchart of FIG. 3 are given the same step numbers. The process of FIG. 6 differs from the process of FIG. 3 in that S102B is inserted between S102 and S103. That is, the limited gradation range is set and changed based on the density information from the EE sensor 61 (step S102B), and the processing after S103 is performed based on the limited gradation range after the setting change. The setting change of the limited gradation range is performed for each page.

The density information output from the EE sensor 61 indicates the density level in the range of 00h to FFh. On the other hand, the setting unit 19 sets the limited gradation range as follows.
a. When the density information is 00h to 4Fh, the limited gradation range is set to 10h to CFh.
b. When the density information is 50h to AFh, the limited gradation range is set to 20h to DFh.
c. When the density information is B0h to FFh, the limited gradation range is set to 30h to EFh.

  As described above, by changing either or both of the size of the limited gradation range and the center position according to the density level of the original or the background of the original detected by the EE sensor 61 or the like, the density of the entire original or the paper Characteristics can be taken into account, and compression with less image quality deterioration can be realized.

  The embodiment of the present invention has been described with reference to the drawings. However, the specific configuration is not limited to that shown in the embodiment, and there are changes and additions within the scope of the present invention. Are also included in the present invention.

  For example, an example is shown in which each pixel is converted into 3-bit code data at the time of compression, but the code data per pixel may be 3 bits or more. However, since compression is performed, the number of bits is smaller than the number of gradations (bits) of the original image data.

  In the second embodiment, the limit gradation range is set and changed based on the density information output from the EE sensor 61. For example, a histogram of gradation values in one piece of image data is created. The limit gradation range may be set and changed according to the distribution state of gradation values. In short, the density information may be information that can determine whether the image to be compressed is a dark original or a light original as a whole.

  The image data to be compressed is not limited to that read by the scanner device 60 or the like, and may be arbitrary image data such as image data created by an information processing device such as a personal computer.

  In addition, the size of one block is not limited to that exemplified in the embodiment.

1 is a block diagram showing a schematic configuration of an image compression apparatus according to a first embodiment of the present invention. It is explanatory drawing which shows a mode that the image of compression object is divided | segmented into a block. It is a flowchart which shows the compression process which the image compression apparatus which concerns on the 1st Embodiment of this invention performs. It is explanatory drawing which illustrated various data at the time of compressing and decompress | decompressing with the image compression apparatus the block in which a neutral color area | region and a line drawing area | region are mixed. It is a block diagram which shows schematic structure of the image compression apparatus which concerns on the 2nd Embodiment of this invention, and its peripheral device. It is a flowchart which shows the compression process which the image compression apparatus which concerns on the 2nd Embodiment of this invention performs. It is explanatory drawing which illustrated various data at the time of compressing and decompress | decompressing the image to the code data of 1 pixel 2 bits by the conventional BTC compression. It is explanatory drawing which illustrated various data at the time of compressing and decompress | decompressing the image of the block in which a neutral color area | region and a line drawing area mixed by the conventional BTC compression to the code data of 1 pixel 2 bits. It is explanatory drawing which illustrated various data at the time of compressing and decompress | decompressing the image of the block in which a neutral color area | region and a line drawing area mixed by the conventional BTC compression to the code data of 1 pixel 3 bits.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 10B ... Image compression apparatus 11 ... Input part 12 ... Division | segmentation part 13 ... Setting part 14 ... Maximum value minimum value detection part 15 ... Threshold value generation part 16 ... Determination part 17 ... Encoding part 18 ... Output part 19 ... Setting part 21 ... Detection result 60 ... Scanner device 61 ... EE sensor

Claims (3)

A dividing unit that divides an image to be compressed into blocks each having a predetermined number of pixels;
A setting unit in which a limited gradation range that forms a part of a gradation range that the image can take is set;
For each block obtained by dividing by the dividing unit, a detecting unit for detecting the maximum value and the minimum value of the gradation values of the pixels belonging to the block within the limited gradation range;
For each block obtained by dividing by the dividing unit, a region setting unit that divides a first range having both ends of the maximum value and the minimum value detected by the detection unit for the block into a plurality of regions;
For each pixel of each block obtained by dividing by the dividing unit, when the gradation value of the pixel is larger than the upper limit value of the restricted gradation range, first code data is assigned, and the gradation value is If the lower limit value of the limit gradation range is smaller than the lower limit value, the second code data is allocated. If the gradation value is within the limit gradation range, the area setting unit sets the gradation value of the block to which the pixel belongs. An encoding unit that assigns code data indicating which of the plurality of divided areas belongs to; and
For each block obtained by the division by the division unit, compressed data including information indicating the first range related to the block and code data allocated by the encoding unit to each pixel belonging to the block is generated. An output unit to
An image compression apparatus comprising: The upper limit value of the limited gradation range is smaller than the maximum gradation value that the image can take,
The lower limit value of the limited gradation range is larger than the minimum gradation value that the image can take,
The image compression apparatus according to claim 1. The said setting part inputs the shading information which shows the shading level of the said image to be compressed, and changes the said limitation gradation range according to the inputted shading information. Image compression device.

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