JP5434799B2 - Image coding apparatus, image coding method, and image coding program - Google Patents

Description

  The present invention relates to an image encoding device, an image encoding method, and an image encoding program, and in particular, an image encoding device that encodes multilevel image data, an image encoding method that is executed by the image encoding device, and an image The present invention relates to an image encoding program for causing a computer to execute an encoding method.

  Conventionally, as a technique for encoding multi-value image data, a block traction technique is known in which a quantization coefficient is determined and quantized in units of blocks. However, if the multi-valued image data includes a halftone dot area, the post-encoding is caused by a shift between a halftone line included in the halftone dot area and a block that is a processing unit of the encoding process by the block traction technique There is a problem that moire noise occurs in the image.

  In order to deal with this problem, Japanese Patent Application Laid-Open No. 11-252374 (Patent Document 1) and Japanese Patent Application Laid-Open No. 11-252375 (Patent Document 2) describe that the quantization coefficient is more than the block that is a unit of encoding. A technique for determining and quantizing with a large block is described.

On the other hand, for the photo area in which the photo is represented, the quantization coefficient is determined by the block unit, and the quantization coefficient is determined by a block having a larger size than the block that is the encoding unit, and compared with the quantization. Therefore, it is preferable because the image quality is hardly deteriorated.
JP-A-11-252374 JP 11-252375 A

  The present invention has been made to solve the above-described problems. One of the objects of the present invention is to reduce noise generated in a halftone dot attribute region when quantizing and to reduce halftone dot attributes. It is an object of the present invention to provide an image encoding apparatus capable of reducing deterioration in image quality in a non-region.

  Another object of the present invention is an image encoding method capable of reducing noise generated in a halftone dot attribute area during quantization and reducing image quality deterioration in a non-halftone attribute area. Is to provide.

  Still another object of the present invention is to perform image coding capable of reducing noise generated in a halftone dot attribute area during quantization and reducing image quality degradation in a non-halftone area. Is to provide a program.

  In order to achieve the above-described object, according to one aspect of the present invention, an image encoding device is detected with block extraction means for extracting a first block of a plurality of pixels to be encoded from multi-value image data. Attribute detection means for detecting the attribute of the first block, first quantization coefficient calculation means for calculating the first quantization coefficient from all the pixel data included in the first block, and a plurality of pixels including the pixels of the first block Using second quantization coefficient calculation means for calculating the second quantization coefficient from all pixel data included in the second block of pixels, and using either the calculated first quantization coefficient or second quantization coefficient Quantizing means for quantizing each pixel data of the first block into code data having a gradation number smaller than the gradation number of the multi-valued image data, the quantization means having a detected attribute of halftone dots For the attribute of , And quantized using the second quantization coefficient, the detected attribute if not an attribute of the halftone dots is quantized using a first quantization coefficient.

  According to this aspect, when the attribute of the first block is a halftone dot attribute, the second quantization coefficient determined from all the pixel data included in the second block of the plurality of pixels including the pixel of the first block is obtained. When the plurality of pixels of the first block are quantized and the attribute of the first block is not the halftone dot attribute, the first quantization coefficient determined from all the pixel data included in the pixels of the first block is used. In use, the plurality of pixels of the first block are quantized. For this reason, image code that can reduce noise generated when quantizing a halftone dot attribute region and reduce image quality degradation when quantizing a non-halftone region A device can be provided.

  Preferably, when the detected attribute is a halftone dot attribute, the quantization means is further provided that the dynamic range of all pixel data included in the first block is equal to or greater than a predetermined threshold value. If the dynamic range of all the pixel data included in the first block is not equal to or greater than a predetermined threshold even if the attribute is quantized using the second quantization coefficient and the detected attribute is a halftone dot attribute, Quantize using one quantization coefficient.

  According to this aspect, when the attribute of the first block is a halftone dot attribute, if the dynamic range of the first block is equal to or greater than a threshold value, the plurality of pixels of the first block are quantized using the second quantization coefficient. If it is not greater than or equal to the threshold value, the plurality of pixels of the first block are quantized using the first quantization coefficient. For this reason, it is possible to reduce deterioration in image quality of a block having a relatively small dynamic range. As a result, it is possible to reduce deterioration of the image quality in the halftone dot attribute region.

  Preferably, the first block further includes dynamic range calculation means for calculating the dynamic range of the first block and each of the plurality of peripheral blocks located around the first block, and the quantization means is included in the first block. When the dynamic range of all pixel data is equal to or greater than a predetermined threshold, the value obtained by subtracting the dynamic range of the first block from the maximum dynamic range of each of the plurality of peripheral blocks is equal to or greater than the second threshold. Under certain conditions, the second quantization coefficient is used for quantization, and even if the dynamic range of all the pixel data included in the first block is equal to or greater than a predetermined threshold value, The dynamic range of the first block is subtracted from the maximum value of the dynamic range. There is quantized using a first quantization coefficient if the second threshold value or more.

  According to this aspect, if the value obtained by subtracting the dynamic range of the first block from the maximum value of the dynamic range of each of the plurality of peripheral blocks is equal to or greater than the second threshold, the first block is used using the second quantization coefficient. The plurality of pixels of the first block are quantized, and if not equal to or greater than the second threshold, the plurality of pixels of the first block are quantized using the first quantization coefficient. For this reason, even if the block has a relatively small dynamic range, the plurality of pixels in the first block are quantized using the second quantization coefficient only in a block having a relatively large difference from the dynamic range of the surrounding blocks. It becomes. As a result, it is possible to further reduce the deterioration of the image quality in the halftone dot attribute region.

  According to another aspect of the present invention, an image encoding method extracts a first block of a plurality of pixels to be encoded from multilevel image data, and detects an attribute of the extracted first block. A step of detecting an attribute of the detected first block; a step of calculating a first quantization coefficient from all pixel data included in the first block; and a step of calculating a plurality of pixels including pixels of the first block. A step of calculating a second quantization coefficient from all the pixel data included in the two blocks, and, when the detected attribute is a halftone dot attribute, each pixel data of the first block is converted into a gradation of multi-value image data. A step of quantizing the code data having the number of gradations smaller than the number using the second quantization coefficient, and if the detected attribute is not a halftone dot attribute, each pixel data of the first block is converted into a multi-valued image; A number of gradations smaller than the gradation number of the code data over data, comprising the steps of quantized using a first quantization coefficient, a.

  According to this aspect, there is provided an image encoding method capable of reducing noise generated in a halftone dot attribute region during quantization and reducing image quality deterioration in a non-halftone dot region. can do.

  According to still another aspect of the present invention, an image encoding program extracts a first block of a plurality of pixels to be encoded from multilevel image data, and detects an attribute of the extracted first block A step of calculating a first quantization coefficient from all the pixel data included in the first block, and a second quantum from all the pixel data included in the second block of the plurality of pixels including the pixels of the first block. A step of calculating a quantization coefficient, and when the detected attribute is a halftone dot attribute, the second quantization coefficient is used to convert each pixel data of the first block to a scale smaller than the number of gradations of the multivalued image data. A step of quantizing the code data of the logarithm, and if the detected attribute is a halftone dot attribute, each pixel data of the first block is encoded data having a gradation number smaller than the gradation number of the multi-value image data Quantizing using the second quantization coefficient, and if the detected attribute is not a halftone dot attribute, each pixel data of the first block is represented by a number of gradations smaller than the number of gradations of the multivalued image data. And causing the computer to execute a step of quantizing the code data using the first quantization coefficient.

  According to this aspect, there is provided an image encoding program capable of reducing noise generated in a halftone dot attribute area during quantization and reducing image quality degradation in a non-halftone area. can do.

1 is a perspective view showing an appearance of an MFP. 2 is a block diagram illustrating an example of a hardware configuration of an MFP. FIG. It is a block diagram which shows an example of the function of CPU. It is a figure for demonstrating a 1st block and a 2nd block. It is a flowchart which shows an example of the flow of an image encoding process. It is a figure which shows an example of a detailed structure of the judgment part which CPU in a 1st modification has. It is a flowchart which shows an example of the flow of the image coding process in a 1st modification. It is a figure which shows an example of a detailed structure of the judgment part which CPU in a 2nd modification has. It is a figure which shows the relationship between a 1st block and a surrounding block. It is a flowchart which shows an example of the flow of the image coding process in a 2nd modification.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

  In the following, an MFP (Multi Function Peripheral) will be described as an example of an image encoding device. FIG. 1 is a perspective view showing the appearance of the MFP. Referring to FIG. 1, MFP 100 includes an operation panel 9, an automatic document feeder (ADF) 10, a document reading unit 20, an image forming unit 30, and a paper feeding unit 40.

  The ADF 10 automatically conveys a plurality of documents set on the document feeding tray 11 one by one to a predetermined document reading position set on the platen glass of the document reading unit 20, and the document reading unit 20 The document on which the document image is read is discharged onto a document discharge tray. The document reading unit 20 includes a light source that emits light to the document conveyed to the document reading position and a photoelectric conversion element that receives light reflected from the document, and scans a document image corresponding to the size of the document. The photoelectric conversion element converts the received light into image data, which is an electrical signal, and stores it in a memory or outputs it to the image forming unit 30.

  The image forming unit 30 forms an image by a known electrophotographic method. The image forming unit 30 performs various types of data processing such as shading correction on the image data input from the document reading unit 20, and the image data after the data processing is processed. Based on this, an image is formed on the sheet conveyed by the sheet feeding unit 40. The paper feed unit 40 conveys the paper stored in the paper feed tray to the image forming unit 30.

  FIG. 2 is a block diagram illustrating an example of a hardware configuration of the MFP. Referring to FIG. 2, MFP 100 includes a main circuit 101, an ADF 10, a document reading unit 20, an image forming unit 30, a paper feeding unit 40, and an operation panel 9 as a user interface.

  The main circuit 101 includes a CPU 111, a communication interface (I / F) unit 112, a ROM (Read Only Memory) 113, a RAM (Random Access Memory) 114, an EEPROM (Electronically Erasable and Programmable ROM) 115, and a large capacity. A hard disk drive (HDD) 116 as a storage device, a facsimile unit 117, a network I / F 118, and an external storage device 119 on which a CD-ROM (Compact Disc-Read Only Memory) 119A is mounted. CPU 111 is connected to ADF 10, document reading unit 20, image forming unit 30, paper feed unit 40, and operation panel 9, and controls the entire MFP 100.

  The ROM 113 stores a program executed by the CPU 111 and data necessary for executing the program. The RAM 114 is used as a work area when the CPU 111 executes a program.

  Communication I / F unit 112 is an interface for connecting MFP 100 to another apparatus using a serial communication cable. The connection form may be wired or wireless.

  The facsimile unit 117 is connected to a public switched telephone network (PSTN) and transmits facsimile data to the PSTN or receives facsimile data from the PSTN. The facsimile unit 117 stores the received facsimile data in the HDD 116 or outputs it to the image forming unit 30. The image forming unit 30 prints the facsimile data received by the facsimile unit 117 on a sheet. Further, the facsimile unit 117 converts the data stored in the HDD 116 into facsimile data, and transmits the facsimile data to a facsimile apparatus connected to the PSTN.

  A network I / F 118 is an interface for connecting the MFP 100 to a network. The CPU 111 can communicate with a computer connected to the Internet via the network I / F 118.

  The external storage device 119 is loaded with a CD-ROM 119A. The CPU 111 can access the CD-ROM 119A via the external storage device 119, and can load a program stored in the CD-ROM 119A into the RAM 114 and execute it. Note that the program executed by the CPU 111 is not limited to the program stored in the CD-ROM 119A, and may be a program stored in another storage medium, a program stored in the HDD 116, or a network I / O. It may be a program written in the HDD 116 by another computer connected to the network via the F118.

  The storage medium for storing the program is not limited to the CD-ROM 119A, but an optical disc (MO (Magnetic Optical Disc) / MD (Mini Disc) / DVD (Digital Versatile Disc)), IC card, optical card, mask ROM, A semiconductor memory such as an EPROM (Erasable Programmable ROM) or an EEPROM (Electrically Erasable and Programmable ROM) may be used.

  The program here includes not only a program directly executable by the CPU 111 but also a source program, a compressed program, an encrypted program, and the like.

  FIG. 3 is a block diagram illustrating an example of functions of the CPU. The functions of the CPU 111 shown in FIG. 3 are formed in the CPU 111 when the CPU 111 executes an image encoding program. Referring to FIG. 3, CPU 111 extracts a block from image data, a block extraction unit 51, a determination unit 53 that determines whether a predetermined condition is satisfied for each block, and an attribute of an area included in the image data. An attribute discriminating unit 59, a quantizing factor calculating unit 55 for calculating a quantizing factor, and a quantizing unit 57 for quantizing the image data based on the quantizing factor.

  Each of the block extraction unit 51, the determination unit 53, the attribute determination unit 59, the quantization coefficient calculation unit 55, and the quantization unit 57 receives image data output from the document reading unit 20 when the document reading unit 20 reads the document. Is done.

  The block extraction unit 51 divides the image data input from the document reading unit 20 into a plurality of blocks having a predetermined size, and sequentially extracts one to be processed from the plurality of blocks. Here, the block extracted by the block extraction unit 51 as a processing target is referred to as a first block, and the first block is composed of 16 pixels in which 4 pixels are arranged vertically and horizontally. The block extraction unit 51 outputs the first block extracted from the image data to the determination unit 53, the quantization coefficient calculation unit 55, and the quantization unit 57.

  The attribute discriminating unit 59 divides the image data into a plurality of areas and discriminates image attributes for each area. The size of the area into which the attribute determination unit 59 divides the image data is not particularly limited, and may be the same as or different from the first block. Image attributes include a halftone dot attribute representing a halftone dot, a photo attribute representing a photograph, a character attribute representing a character, and a graphic attribute representing a graphic. Since the technique for discriminating image attributes is well known, detailed description thereof will not be repeated here. The attribute determination unit 59 outputs the attribute determined for each area to the determination unit 53.

  The determination unit 53 receives the first block from the block extraction unit 51 and the attribute for each area of the image data from the attribute determination unit 59. The determination unit 53 includes an attribute comparison unit 61. The attribute comparison unit 61 determines whether the attribute of the first block is a halftone dot based on the attribute for each region input from the attribute determination unit 59. If the attribute of the first block is a halftone dot, the attribute comparison unit 61 outputs an extended calculation instruction to the quantization coefficient calculation unit 55. If the attribute of the first block is not a halftone dot, the attribute comparison unit 61 Output extended calculation instructions.

  The quantization coefficient calculation unit 55 includes a non-expansion calculation unit 71 and an expansion calculation unit 73. In response to the input of the non-extended calculation instruction from the determination unit 53, the non-extended calculation unit 71 is based on the values of a plurality of pixels included in the first block input from the block extraction unit 51 in the image data. A quantization coefficient is calculated. The quantization coefficient is a threshold value used for quantization. The non-expanded calculation unit 71 outputs the calculated quantization coefficient to the quantization unit 57. Since the quantization coefficient calculation method is well known, the description thereof will not be repeated here. For example, the technique described in Japanese Patent Application Laid-Open No. 11-252374 or Japanese Patent Application Laid-Open No. 11-252375 described above can be used.

  On the other hand, the extension calculation unit 73 is included in the second block including a plurality of pixels included in the first block and pixels around the first block in response to the input of the extension calculation instruction from the determination unit 53. A quantization coefficient is calculated based on the plurality of pixels. The extension calculation unit 73 outputs the calculated quantization coefficient to the quantization unit 57.

  The quantization unit 57 quantizes each of the pixel values of the plurality of images included in the first block of the image data using the quantization coefficient input from the quantization coefficient calculation unit 55 and encodes the image data Output data. The quantization unit 57 encodes the quantization coefficient used for the quantization among the normal quantization coefficient and the extended quantization coefficient and the coded data of each quantized pixel into the plurality of pixels included in the first block. Output as data. For example, the output destination may be either the EEPROM 115 or the HDD 116.

  FIG. 4 is a diagram for explaining the first block and the second block. Referring to FIG. 4, the first block 203 includes 16 pixels in which the pixel of interest 201 is arranged at the upper left, and 4 pixels are arranged in the vertical direction and 4 pixels are arranged in the horizontal direction from the pixel of interest 201. The second block 205 includes a first block 203 and 20 pixels adjacent to the first block 203. The values of the 16 pixels included in the first block 203 are the quantization coefficient value determined based on the values of the 16 pixels included in the first block 203, and the 36 pixels included in the second block 205. Quantization is performed using any one of quantization coefficient values determined based on the values of.

  FIG. 5 is a flowchart illustrating an example of the flow of the image encoding process. The image encoding process is a process executed by the CPU 111 when the CPU 111 executes an image encoding program stored in any of the ROM 113, the EEPROM 115, the HDD 116, and the CD-ROM 119A. Referring to FIG. 5, when image data is input from document reading unit 20, CPU 111 divides the input image data into a plurality of blocks, and one of the divided blocks is processed. A first block is selected (step S01). In the next step S02, it is determined whether or not the attribute of the first block is a halftone dot. If the attribute of the first block is a halftone dot, the process proceeds to step S03; otherwise, the process proceeds to step S05.

  In step S03, an extended quantization coefficient is determined. The extended quantization coefficient is determined based on the values of all pixels included in the second block including all of the pixels included in the first block and pixels around the first block. In the next step S04, based on the extended quantization coefficient determined in step S03, the values of a plurality of pixels included in the first block are quantized, and the process proceeds to step S07.

  On the other hand, in step S05, a normal quantization coefficient is determined. The normal quantization coefficient is determined based on all the values of the pixels included in the first block. In the next step S06, the values of the plurality of pixels included in the first block are quantized based on the normal quantization coefficient determined in step S05, and the process proceeds to step S07.

  In step S07, it is determined whether or not there is a next block not to be processed. If there is a first block not to be processed, the process returns to step S01. If not, the process ends.

  Quantize the first block whose attribute is included in a halftone dot region using an extended quantization coefficient, and quantize the first block included in a region whose attribute is not a halftone dot using a normal quantization coefficient To do.

  Since the extended quantization coefficient is determined based on the values of a plurality of pixels included in the second block that is larger than the first block, the extended quantization coefficient is the periphery of the first block compared to the normal quantization coefficient. This value is close to the quantization coefficient of the block. For this reason, when quantizing with the extended quantization coefficient, the value of the pixel after quantization between adjacent blocks becomes an approximate value as compared with the case of quantizing with the normal quantization coefficient.

  In the halftone dot region, the period in which the halftone dots determined by the number of lines and the gradation of the halftone dots are arranged may coincide with the period in which the blocks are arranged. If they are different, moire noise is generated. Since the range of the pixels that define the quantization coefficient is different depending on whether the attribute of the first block is a halftone dot or not, the cycle in which the halftone dots determined by the number of lines of the halftone dots and the gradation are arranged is as follows. Even when the first block coincides with the arrangement period, the probability of occurrence of moire noise can be reduced. As a result, it is possible to reduce the probability of occurrence of moire noise in an area where the attribute is a halftone dot, and it is possible to reduce deterioration in image quality in an area where the attribute is not a halftone dot.

<First Modification>
In the above-described embodiment, if the attribute of the first block is a halftone dot, the quantization is performed using the extended quantization coefficient, and if the attribute of the first block is not the halftone dot, the quantization is performed using the normal quantization coefficient. I made it. In the first modification, a condition for performing quantization using an extended quantization coefficient is added in order to prevent deterioration of image quality in the area where the attribute is a halftone dot. The MFP 100 according to the first modification is obtained by modifying the determination unit 53 among the functions of the CPU 111 illustrated in FIG. 3 to form a determination unit 53A.

  FIG. 6 is a diagram illustrating an example of a detailed configuration of the determination unit included in the CPU according to the first modification. Referring to FIG. 6, determination unit 53 </ b> A receives the first block from block extraction unit 51 and the attribute for each region of the image data from attribute determination unit 59. The determination unit 53A includes an attribute comparison unit 61 and a DR comparison unit 63. The attribute comparison unit 61 determines whether the attribute of the first block is a halftone dot based on the attribute for each region input from the attribute determination unit 59. If the attribute of the first block is a halftone dot, the attribute comparison unit 61 outputs a comparison instruction to the DR comparison unit 63. If the attribute of the first block is not a halftone dot, the attribute comparison unit 61 outputs a non-extended calculation instruction to the quantization coefficient calculation unit 55 without outputting a comparison instruction to the DR comparison unit 63.

  When a comparison instruction is input from the attribute comparison unit 61, the DR comparison unit 63 compares the dynamic range (DR) of the values of the plurality of pixels included in the first block with a predetermined threshold value T1. The dynamic range (DR) is a value obtained by subtracting the minimum value from the maximum value of a plurality of pixels included in the first block. If the dynamic range (DR) of the first block is equal to or greater than the threshold value T1, the DR comparing unit 63 outputs an extension calculation instruction to the quantization coefficient calculating unit 55, and the dynamic range (DR) of the first block is the threshold value. If it is smaller than T1, a non-expanded calculation instruction is output to the quantization coefficient calculation unit 55.

  FIG. 7 is a flowchart illustrating an example of the flow of the image encoding process in the first modification. The image encoding process is a process executed by the CPU 111 when the CPU 111 executes an image encoding program stored in any of the ROM 113, the EEPROM 115, the HDD 116, and the CD-ROM 119A. Referring to FIG. 7, the difference from the process shown in FIG. 5 is that step S11 and step S12 are added between step S02 and step S03. Other processing is the same as that shown in FIG. 7, and therefore, description thereof will not be repeated here.

  In step S11, the dynamic range of interest (DR) is calculated. The noticed DR is a value obtained by subtracting the minimum value from the maximum value of the plurality of pixels included in the first block selected in step S01. In the next step S12, the attention DR is compared with the threshold value T1. If the noted DR is equal to or greater than threshold value T1, the process proceeds to step S03. If not, the process proceeds to step S05.

  If the dynamic range of the first block included in the halftone dot region is greater than or equal to the threshold T1, the quantization is performed using the extended quantization coefficient. If the dynamic range is smaller than the threshold T1, the normal quantization is performed. Quantize using coefficients. When the dynamic range of the block is equal to or greater than the threshold value T1, the probability of occurrence of moire noise is smaller than when the block is smaller than the threshold value T1. For this reason, when the halftone dot attribute region is quantized, it is possible to reduce the deterioration of the image quality of the halftone line region as much as possible, and to reduce the probability of occurrence of moire noise.

<Second Modification>
In the first modification, the first block whose attribute is included in the halftone dot region and the dynamic range of which is greater than or equal to the threshold T1 is quantized using the extended quantization coefficient, and the dynamic range is The first block smaller than the threshold value T1 is usually quantized using a quantization coefficient. In the second modified example, a condition for performing quantization using an extended quantization coefficient is added in order to further prevent deterioration of image quality compared to the first modified example. The MFP 100 according to the second modification is obtained by modifying the determination unit 53 among the functions of the CPU 111 illustrated in FIG. 3 to form a determination unit 53A.

  FIG. 8 is a diagram illustrating an example of a detailed configuration of the determination unit included in the CPU according to the second modification. Referring to FIG. 8, determination unit 53 </ b> A in the second modification includes an attribute comparison unit 61, a DR comparison unit 63 </ b> A, and a peripheral block comparison unit 65. The attribute comparison unit 61 is the same as described above. Similarly to the DR comparison unit 63 described above, the DR comparison unit 63A compares the dynamic range (DR) of the values of the plurality of pixels included in the first block with the threshold value T1. If the dynamic range (DR) of the first block is equal to or greater than the threshold value T1, the DR comparing unit 63A outputs a peripheral block comparison instruction to the peripheral block comparing unit 65, and the dynamic range (DR) of the first block is the threshold value. If it is smaller than T1, a non-expanded calculation instruction is output to the quantization coefficient calculation unit 55.

  The peripheral block comparison unit 65 compares a value obtained by subtracting the dynamic range of the first block from the maximum value of the dynamic range of each of the plurality of peripheral blocks arranged around the first block with a predetermined threshold value T2. . If the value obtained by subtracting the dynamic range of the first block from the maximum value of the dynamic ranges of the plurality of peripheral blocks is equal to or greater than the threshold value T2, an extended calculation instruction is output to the quantization coefficient calculation unit 55. A non-extended calculation instruction is output to the quantization coefficient calculation unit 55.

  FIG. 9 is a diagram illustrating the relationship between the first block and the peripheral blocks. Referring to FIG. 9, peripheral blocks DR00, DR01, DR02, DR10, DR12, DR20, DR21, DR22 have the same size as first block DR11 and are arranged around first block DR11.

  FIG. 10 is a flowchart illustrating an example of the flow of the image encoding process in the second modification. The difference from the image encoding process in the first modification shown in FIG. 7 is that step S21 to step S23 are added between step S12 and step S03. The other processing is the same as that shown in FIGS. 5 and 7, and therefore description thereof will not be repeated here.

  In step S21, the dynamic range of the peripheral blocks for the first block is acquired. Then, the maximum value of the dynamic range of the peripheral block is determined as the maximum peripheral DR (step S22). The absolute value of the value obtained by subtracting the target DR from the maximum peripheral DR determined in step S22 is compared with the threshold value T2 (step S23). If the absolute value of the value obtained by subtracting the attention DR from the maximum peripheral DR is equal to or greater than threshold value T2, the process proceeds to step S03. If not, the process proceeds to step S05.

  The MFP 100 according to the second modification has a difference between the target DR and the maximum value of the dynamic range of the peripheral blocks (maximum peripheral DR) even if the dynamic range (target DR) of the first block is equal to or greater than the threshold T1. Only when the absolute value is equal to or greater than the threshold value T2, quantization is performed using the extended quantization coefficient. If the absolute value of the difference between the target DR and the maximum value of the dynamic range of the peripheral block (maximum peripheral DR) is not greater than or equal to the threshold value T2, the normal quantization coefficient of the first block and the normal quantization coefficient of the peripheral block are The probability of approximation is high. For this reason, when the quantization is performed using the normal quantization coefficient, the probability of occurrence of moire noise is small. On the other hand, when the quantization is performed using the normal quantization coefficient, the image quality is less deteriorated than when the quantization is performed using the extended quantization threshold. Therefore, when the halftone dot attribute region is quantized, it is possible to reduce the deterioration of image quality as much as possible and to reduce the probability of occurrence of moire noise.

  In the above-described embodiment, MFP 100 has been described as an example of an image encoding device. However, an image encoding method for causing MFP 100 to execute the processing illustrated in FIG. 5 or FIG. It goes without saying that the invention can be understood as an image encoding program.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

<Appendix>
(1) Storage means for storing the quantization coefficient used for the quantization and the code data of each pixel quantized by the quantization means as encoded data of a plurality of pixels included in the first block, The image encoding device according to claim 1, further comprising:

DESCRIPTION OF SYMBOLS 10 Automatic document feeder, 20 Document reading part, 30 Image formation part, 40 Paper feed part, 51 Block extraction part, 53,53A Judgment part, 55 Quantization coefficient calculation part, 57 Quantization part, 59 Attribute discrimination | determination part, 61 Attribute comparison unit, 63, 63A DR comparison unit, 65 Peripheral block comparison unit, 71 Non-extended calculation unit, 73 Extended calculation unit, 100 MFP, 101 main circuit, 111 CPU, 112 Communication I / F unit, 113 ROM, 114 RAM 115 EEPROM, 116 HDD, 117 facsimile unit, 118 network I / F, 119 external storage device, 119A CD-ROM.

Claims (5)

Block extraction means for extracting a first block of a plurality of pixels to be encoded from multi-valued image data;
Attribute detecting means for detecting an attribute of the detected first block;
First quantization coefficient calculating means for calculating a first quantization coefficient from all the pixel data included in the first block;
Second quantization coefficient calculating means for calculating a second quantization coefficient from all pixel data included in a second block of a plurality of pixels including the pixels of the first block;
Using either the calculated first quantization coefficient or the second quantization coefficient, each pixel data of the first block is quantized into code data having a gradation number smaller than the gradation number of the multivalued image data. A quantization means for converting to
The quantization means performs quantization using the second quantization coefficient when the detected attribute is a halftone dot attribute, and when the detected attribute is not a halftone dot attribute, the first quantization is performed. An image encoding apparatus that performs quantization using coefficients.   When the detected attribute is a halftone dot attribute, the quantization means is further provided that the dynamic range of all pixel data included in the first block is equal to or greater than a predetermined threshold. Quantization is performed using the second quantization coefficient, and even if the detected attribute is a halftone dot attribute, the dynamic range of all pixel data included in the first block is not greater than or equal to a predetermined threshold value. The image coding apparatus according to claim 1, wherein quantization is performed using the first quantization coefficient. Dynamic range calculating means for calculating the dynamic range of the first block and the dynamic range of each of a plurality of peripheral blocks located around the first block;
When the dynamic range of all the pixel data included in the first block is equal to or greater than a predetermined threshold value, the quantization means determines the first block from the maximum value of the dynamic range of each of the plurality of peripheral blocks. On the condition that the value obtained by subtracting the dynamic range is equal to or greater than the second threshold value, the dynamic range of all the pixel data included in the first block is preliminarily quantized using the second quantization coefficient. If the value obtained by subtracting the dynamic range of the first block from the maximum value of the dynamic range of each of the plurality of peripheral blocks is not equal to or greater than a second threshold even if it is equal to or greater than a predetermined threshold, the first quantum The image coding apparatus according to claim 2, wherein quantization is performed using a quantization coefficient. Extracting a first block of a plurality of pixels to be encoded from multi-valued image data;
Detecting the attribute of the extracted first block;
Detecting the attribute of the detected first block;
Calculating a first quantization coefficient from all pixel data included in the first block;
Calculating a second quantization coefficient from all pixel data included in a second block of a plurality of pixels including the pixels of the first block;
When the detected attribute is a halftone dot attribute, the second quantization coefficient is used for each pixel data of the first block as code data having a gradation number smaller than the gradation number of the multi-valued image data. The step of quantizing
When the detected attribute is not a halftone dot attribute, the first quantization coefficient is used for each pixel data of the first block as code data having a number of gradations smaller than the number of gradations of multi-valued image data. And a step of quantizing the image. Extracting a first block of a plurality of pixels to be encoded from multi-valued image data;
Detecting the attribute of the extracted first block;
Calculating a first quantization coefficient from all pixel data included in the first block;
Calculating a second quantization coefficient from all pixel data included in a second block of a plurality of pixels including the pixels of the first block;
When the detected attribute is a halftone dot attribute, each pixel data of the first block is coded data having a number of gradations smaller than the number of gradations of multi-valued image data using the second quantization coefficient. A step of quantizing to
When the detected attribute is a halftone dot attribute, the second quantization coefficient is used for each pixel data of the first block as code data having a gradation number smaller than the gradation number of the multi-valued image data. The step of quantizing
When the detected attribute is not a halftone dot attribute, the first quantization coefficient is used for each pixel data of the first block as code data having a number of gradations smaller than the number of gradations of multi-valued image data. And an image encoding program for causing a computer to execute the quantization step.

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