JP2008085599A - Image processor, image processing method, and printing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the size of a table for use in half tone processing. <P>SOLUTION: Image processing ASIC 155 of a printer 100 performs half tone processing with respect to each picture element constituting image data by referring to in order an encode table ET, a predecode table DT1, a post decode table DT2 and a sequence value table ST stored in a RAM 152 to determine dot arrangement formed on a printing medium. Herein, although in the encode table ET an encode value and a gradation value are stored in pairs or in units of block numbers, data capacity thereof is previously reduced up to a capacity less than data capacity in response to a product of an encode value less than the maximum value which 5 bit encode value can take and the number of blocks. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for printing an image by forming dots on a print medium.

  2. Description of the Related Art Printing apparatuses that form dots on a printing medium and print an image are widely used as apparatuses that output an image created by a computer, an image taken by a digital camera, or the like. In such a printing apparatus, a technique such as a systematic dither method, a density pattern method, and a method of arranging a plurality of types of dots having different sizes (Japanese Patent No. 3292104) is developed to perform high-quality printing at extremely high speed. A technique capable of achieving this is proposed by the applicant of the present application (see, for example, Patent Document 1).

JP 2006-14036 A

  In the image processing technique described in Patent Document 1, the global dither matrix is divided into blocks having 4 × 2 thresholds, and which block in the dither matrix each pixel of the input image corresponds to. On the basis of various tables such as the multi-value conversion table described in FIG. 34 of Patent Document 1, the conversion table for intermediate data in FIG. 38, the sequence value matrix in FIG. Halftone processing is performed on the image.

  In order to increase the processing speed in such a conventional technique, it is necessary to previously develop and store the various tables described above from storage means such as ROM into RAM. This is because the access speed of the RAM is generally faster than that of the ROM. However, for example, if the halftone processing described above is to be realized by a single printing apparatus, the mounted RAM is often low in capacity, and in such a case, it is difficult to develop all the tables in the RAM. Thus, there has been a problem that the speed cannot be increased. To deal with these problems, it is conceivable to increase the RAM mounting capacity, but this has been a cause of cost increase and is not reasonable.

  In consideration of the above-described problems that occur when halftone processing is performed with reference to various tables, the problem to be solved by the present invention is to reduce the size of the table used in halftone processing.

Based on the above problems, the image processing apparatus of the present invention is configured as follows. That is,
An image processing apparatus that determines the arrangement of dots to be formed on a printing medium when printing image data based on a dither matrix in which a threshold value for determining whether or not to generate dots is recorded according to the gradation value of the image data Because
The dither matrix is divided into blocks each composed of a predetermined number of threshold values, and the gradation values are sequentially set to the threshold values in a range that the gradation values can take for each block identifier uniquely assigned to each block. Encoding table storage means for storing an encoding table in which each gradation value that generates each encoding value is recorded in association with each encoding value representing the arrangement of dots in the block obtained by comparison;
Image data input means for inputting the image data;
Based on the position of each pixel constituting the input image data and the position of each block existing in the dither matrix, the block identifier corresponding to each pixel is discriminated, and the block identifier and the level of each pixel are distinguished. Encoding means for obtaining the encoding value corresponding to the gradation value of each pixel by referring to the encoding table based on the tone value;
Decoding means for determining the arrangement of dots generated in the block corresponding to the position of each pixel according to the encoded value obtained by the encoding means and the discriminated block identifier;
The encoding table storage means represents each encoding value with a predetermined number of bits, and the encoding table is a product of an encoding value less than the maximum encoding value that can be expressed with the number of bits and the number of blocks. The gist is that the data is stored in a capacity less than the corresponding data capacity.

  In the image processing apparatus of the present invention, by referring to the encoding table stored in the encoding storage unit, the gradation value of the image is converted into the encoding value, and the dot arrangement is determined based on this encoding value, thereby Halftone processing is performed on each pixel constituting the data. In the encode table used in the halftone process, an encode value and a gradation value are stored in association with each block identifier. In the present invention, the data capacity of the encoding table is limited to a capacity equal to or less than the data capacity corresponding to the product of the encoding value less than the maximum encoding value that can be taken by the encoding value represented by the predetermined number of bits and the number of blocks. It was decided to. Therefore, since the entire range that the encoding value can take is not recorded in the encoding table, the data capacity of the encoding table can be reduced.

In the image processing apparatus having the above configuration,
The encoding table storage means stores the encoding table as a set of a plurality of elements in which the gradation value is recorded, and for any block up to the maximum value of the encoding value that can be expressed by the number of bits. In the case where the gradation value is not substantially recorded, at least a part of the element in which the gradation value is not substantially recorded in the encoding table is deleted in advance. It may be remembered.

  With such a configuration, the gradation value is substantially associated when the gradation value is not substantially recorded for any block up to the maximum encoding value that can be expressed by a predetermined number of bits. Since at least a part of the elements that have not been deleted are deleted, the size of the encoding table itself can be reduced. Note that “an element in which a gradation value is not substantially recorded” refers to an element associated with a meaningless value as a gradation value, such as “0” or “NULL”. However, some “0” s have meanings as gradation values, and such gradation values are included in “elements where gradation values are not substantially recorded”. There is no.

In the image processing apparatus having the above configuration,
The encoding table storage means includes, in addition to the elements that are not substantially associated with the gradation values for the block that is associated with the largest number of gradation values among the blocks represented by the block identifiers, The elements of other blocks corresponding to the above may be deleted from the encoding table in advance and the encoding table may be stored.

  With such a configuration, the data capacity of other blocks can be reduced in accordance with the data capacity required by the block associated with the largest number of gradation values. Therefore, it is possible to easily reduce the size of the encoding table itself.

In the image processing apparatus having the above configuration,
The encoding table storage means may store all the elements that are not substantially associated with the gradation values from the encoding table in advance and store the encoding table.

  With such a configuration, all elements that are not substantially associated with gradation values are deleted, so that the data capacity of the encoding table can be further reduced.

In the image processing apparatus having the above configuration,
The encoding table storage means stores the block identifier, the encoded value, and the gradation value in a continuous address space with the encoding table from which all of the elements not substantially associated with the gradation value are deleted. It is good also as what memorize | stores in the format which matched.

  In such a configuration, since all elements that are not substantially associated with gradation values are deleted and all data is stored in a continuous address space, an encoding table is efficiently stored. Is possible.

In the image processing apparatus having the above configuration,
The encoding table storage means may separately store an address table indicating an address corresponding to each block identifier in the address space.

  With such a configuration, data corresponding to each block can be easily read from the encoding table by referring to the address table.

In the image processing apparatus having the above configuration,
The encoding value is set according to the number of changes in the arrangement of the dots according to the threshold value in each block when the gradation value is increased from the minimum value to the maximum value for each block. Can be.

  With such a configuration, the gradation value of the pixel can be converted into the number of changes in which the arrangement of dots in the block changes, so that the gradation value can be converted into less data.

In the image processing apparatus having the above configuration,
Limit value storage means for storing a limit value that limits in advance the upper limit of the range that the encoded value can take,
The encoding table storage unit may store the encoding table by reducing the range of the encoding value in advance according to the limit value for a block whose number of changes exceeds the limit value.

  With such a configuration, the maximum encoding value can be limited, so that the data capacity of the encoding table can be further reduced.

In the image processing apparatus having the above configuration,
The encoding table storage means, for a block in which the number of changes exceeds the limit value, from the encode value on the darkest gradation value side by the number of differences between the limit value and the maximum value of the encoded value of the block, The reduction may be performed by thinning out the encoded value and the gradation value associated with the encoded value.

  With such a configuration, since the gradation value is thinned out from the encoding value on the darkest gradation value side, that is, the encoding value where many dots are formed, the image quality can be reduced by reducing the number of encoding values. It is possible to suppress the influence.

In the image processing apparatus having the above configuration,
The encoding table storage means may adjust the gradation values remaining after the thinning.

  With such a configuration, by adjusting the remaining gradation value, it is possible to prevent the dot arrangement from changing unnaturally as the encoded value is reduced.

In the image processing apparatus having the above configuration,
The encoding table storage means stores the average value of the gradation value deleted by the thinning and the gradation value adjacent to the gradation value before the deletion and remaining by the thinning. The adjustment may be performed by replacing with a gradation value.

  With such a configuration, it is possible to easily adjust the gradation value only by calculating the average value.

In the image processing apparatus having the above configuration,
The encoding means inputs the pixels constituting the image data for each line, and temporarily stores the block identifier range corresponding to the pixels in the line by partially inputting from the encoding table With means,
The encoding unit may obtain the encoding value using a partial encoding table held in the temporary storage unit.

  With such a configuration, it is possible to efficiently perform the encoding process for each line by referring to the partial encoding table stored in the temporary storage unit.

In the image processing apparatus having the above configuration,
The encoding means may recover the deleted element in the encoding table and input the partial encoding table to the temporary storage means.

  With such a configuration, the data capacity of the partial encoding table input to the temporary storage means can be handled uniformly, so that the data in the temporary storage means can be easily managed.

In the image processing apparatus having the above configuration,
The encoding unit may input the partial encoding table to the temporary storage unit without recovering the deleted element in the encoding table.

  With such a configuration, a partial encoding table with a reduced data capacity is also input to the temporary storage means, so that the storage capacity of the temporary storage means can be reduced.

In the image processing apparatus having the above configuration,
Furthermore, for each of the block identifiers, a decoding table storage means for storing a decoding table in which the arrangement of the dots generated in the block according to the encoding value is recorded in association with the encoding value.
The decoding unit may determine the dot arrangement based on the encoded value obtained by the encoding unit and the discriminated block identifier with reference to the decoding table.

  With such a configuration, the dot arrangement can be easily determined from the encoded value and the block identifier by referring to the decode table stored in advance in the decode table storage means.

In the image processing apparatus having the above configuration,
The decoding table is
For each block identifier, a predecode table representing a correspondence relationship between the encoded value and an index value representing the number of dots generated in the block;
A post-decoding table representing the correspondence between the index value and the number of dots generated in the block;
An order value table representing the correspondence between the number of dots and the position and order of dots generated in the block;
The decoding means may determine the arrangement of the dots in accordance with the encoded value and the block identifier by sequentially referring to the predecode table, the postdecode table, and the order value table. .

  With such a configuration, it is not necessary to have a one-to-one correspondence of dot arrangement with each encode value in the decode table, so that the storage capacity of the decode table can be reduced. The decoding means directly refers to the encoding table and the dither matrix that is the generation source of the decoding table instead of using the order value table, so that the position and order of the dots generated in the block can be determined each time decoding processing is performed. It may be what you want.

In the image processing apparatus having the above configuration,
The decoding table storage means represents each encoded value with a predetermined number of bits, and the predecoding table is a product of an encoded value less than the maximum encoded value that can be expressed with the number of bits and the number of blocks. It is good also as what is memorize | stored with the capacity | capacitance below the data capacity according to.

  With such a configuration, the storage capacity of the predecode table can be reduced by a method similar to that of the encode table.

  In addition to the configuration as the image processing apparatus described above, the present invention can be configured as an image processing method, a printing apparatus, and a computer program. Such a computer program may be recorded on a computer-readable recording medium. As the recording medium, for example, various media such as a flexible disk, a CD-ROM, a DVD-ROM, a magneto-optical disk, a memory card, and a hard disk can be used.

Hereinafter, in order to further clarify the operations and effects of the present invention described above, embodiments of the present invention will be described based on examples in the following order.
(A) Printer configuration:
(B) Print processing:
(C) Initialization process:
(C-1) Encoding table and predecoding table generation processing:
(C-2) Data amount reduction processing:
(D) Halftone processing:
(D-1) Encoding process:
(D-2) Decoding process:
(E) Effect:
(F) Modification:

(A) Printer configuration:
FIG. 1 is an explanatory diagram showing a printer 100 as an embodiment of the present invention. The printer 100 is a so-called multi-function printer, and is connected to devices such as a scanner 110 for optically reading an image, a memory card slot 120 for inserting a memory card MC on which image data is recorded, and a digital camera. A USB interface 130 and the like are provided. The printer 100 can print an image captured by the scanner 110, an image read from the memory card MC, an image read from the digital camera via the USB interface 130, and the like on the printing paper P. Also, it is possible to print an image input from a personal computer (not shown) connected by a printer cable or the like.

  The printer 100 includes an operation panel 140 for performing various setting operations relating to printing, such as selection of an image to be printed and setting of the type and size of printing paper. A liquid crystal monitor 145 is provided at the center of the operation panel 140. The liquid crystal monitor 145 displays a list of images input from the memory card MC or the like and various graphical interfaces (GUI).

  FIG. 2 is an explanatory diagram showing the internal configuration of the printer 100. As shown in the figure, the printer 100 has a carriage 210 on which an ink cartridge 212 is mounted, a carriage motor 220 that drives the carriage 210 in the main scanning direction, and a printing paper P in the sub-scanning direction as a mechanism for printing on the printing paper P. A paper feed motor 230 and the like are provided.

  An ink cartridge 212 containing cyan (C), light cyan (Lc), magenta (M), light magenta (Lm), yellow (Y), and black (K) inks is mounted on the carriage 210. Yes. The carriage 210 includes a total of six types of ink heads 211 corresponding to these colors. The ink supplied from the ink cartridge 212 to the ink head 211 is ejected onto the printing paper P by driving a piezoelectric element (not shown).

  The printer 100 of this embodiment can form dots of four types on the printing paper P by controlling the voltage waveform applied to the piezo elements of the ink head 211 by the control unit 150. In the following, the four types of dots are referred to as “no dot”, “S dot”, “M dot”, and “L dot” in order of size, including the case where no dot is formed. Each size dot is represented by an ink amount of 1.5 pl (picoliter), 3 pl, and 7 pl, respectively. In this embodiment, four types of dots can be formed, but two types, three types, or five or more types of dots may be formed.

  The carriage 210 is movably held by a sliding shaft 280 installed in parallel with the axial direction of the platen 270. The carriage motor 220 rotates the drive belt 260 in response to a command from the control unit 150 to reciprocate the carriage 210 in parallel with the axial direction of the platen 270, that is, in the main scanning direction. The paper feed motor 230 conveys the printing paper P perpendicular to the axial direction of the platen 270 by rotating the platen 270 according to a command from the control unit 150. That is, the paper feed motor 230 can relatively move the carriage 210 in the sub-scanning direction.

  The printer 100 includes a control unit 150 for controlling operations of the ink head 211, the carriage motor 220, and the paper feed motor 230 described above. The control unit 150 is connected to the scanner 110, the memory card slot 120, the USB interface 130, the operation panel 140, and the liquid crystal monitor 145 shown in FIG.

  The control unit 150 includes a CPU 151, a RAM 152, a ROM 153, and an image processing ASIC 155.

  The ROM 153 stores a control program for generally controlling the operation of the printer 100. The CPU 151 reads this control program into the RAM 152 and executes it when the printer 100 is turned on. The CPU 151 performs print processing, initialization processing, and the like, which will be described later, by executing this control program.

  In addition to the control program, the ROM 153 stores dither matrix data DM and a dot generation amount table DGT that are used in initialization processing described later. The RAM 152 stores various tables generated by this initialization process.

  The image processing ASIC 155 performs color conversion processing and halftone processing on the image data input from the memory card MC or the like, and controls the printing mechanism (ink head 211, carriage motor 220, paper feed motor 230) to perform printing. An integrated circuit for performing. The logic circuit in the image processing ASIC 155 is designed by describing a program for realizing the functions described below in a predetermined hardware description language.

  FIG. 3 is an explanatory diagram showing a detailed configuration of the image processing ASIC 155. As shown in the figure, the image processing ASIC 155 of this embodiment includes a color conversion unit 300, a halftone processing unit 400, and an ink ejection control unit 500.

  The color conversion unit 300 changes the color of image data expressed by a combination of R, G, and B gradation values to a color expressed by a combination of gradation values for each color of ink provided in the carriage 210. It is a circuit to convert. As described above, the printer 100 prints an image using a maximum of six colors of C, M, Y, K, Lc, and Lm. Therefore, the color conversion unit 300 converts the image data expressed by RGB colors into image data expressed by the gradation values of these six colors.

  The color conversion unit 300 performs the color conversion described above by referring to a table called a color conversion table LUT stored in the RAM 152. In this color conversion table LUT, RGB gradation values and CMYKLcLm gradation values are stored in association with each other in advance. By referring to this color conversion table LUT, the color conversion unit 300 can quickly convert image data represented in the RGB format into image data in the CMYKLcLm format (hereinafter referred to as “CMY format”). The image data after color conversion has gradation values of 256 colors (0 to 255) for each color.

  The halftone processing unit 400 is a circuit that inputs the CMY format image data color-converted by the color conversion unit 300 and converts the image data into dot arrangement data representing the arrangement of dots formed on the print medium. .

  The printer 100 can only adjust the size of ink droplets to be ejected to a maximum of four levels, and cannot use CMY format data of 256 gradations for each color as it is. Therefore, the printer 100 expresses a halftone color by converting the CMY format gradation value into dot arrangement data representing the density of dots per unit area by the halftone processing unit 400.

  FIG. 4 is an explanatory diagram showing an example of dot arrangement data. In this figure, when the gradation value of the color (for example, cyan) of the upper left pixel (1, 1) of the CMY format image data is “52”, this is converted into dot arrangement data in the lower left of the figure. It was shown to. As described above, in this embodiment, when the gradation value of one pixel is inputted, S dots, M dots, L dots, etc. are arranged in the dot group of 4 × 2 dots, and the gradation values thereof are arranged. The color represented by is represented on the printing paper. In addition, the dot group mentioned above may be called a "block" below.

  Here, a method for generating dot arrangement data will be briefly described. The arrangement of each dot in the dot arrangement data is a local threshold value group (FIG. 7 (FIG. 7)) extracted from the global dither matrix data DM (see FIG. 7A) according to the position of the pixel to be processed with halftone processing. b)) and the generation amount of dots of each size corresponding to the gradation value TD of the pixel (see FIG. 9). That is, the dot generation amount table DGT shown in FIG. 9 is used to determine the generation amount of dots of each size according to the input gradation value TD, and the generation amount of each dot is compared with the threshold value in the threshold group. The number of dots of each size arranged in one block is obtained. Then, dot arrangement data is generated by arranging the number of dots from a large size dot to a place with a low threshold in the threshold group. In the present embodiment, in order to perform such a series of processes, various tables called an encoding table ET, a predecoding table DT1, a postdecoding table DT2, and an order value table ST are generated in advance, and these tables are generated. By sequentially referencing, dot arrangement data is efficiently generated. Detailed processing contents for generating dot arrangement data will be described later.

  In the present embodiment, even if the gradation values of the pixels in the CMY image are the same, the threshold value group referred to in the dither matrix data DM shown in FIG. Therefore, different dot arrangement data are generated. For example, FIG. 4 shows another pixel (j, 1) having a gradation value of “52” in addition to the upper leftmost pixel of the CMY image. An example of dot arrangement data corresponding to this pixel is shown in the lower right of the figure, but this dot arrangement data has a dot arrangement different from the dot arrangement data shown above. In other words, in the present embodiment, even if the gradation value is the same, the dot arrangement data to be generated is changed according to the position of the pixel to be subjected to halftone processing. It is possible to output an image with higher image quality than the method.

  As described above, in this embodiment, since the unit of the dot group of the output image is 4 × 2, for example, if the resolution of the input image is 360 dpi × 360 dpi, the output resolution output to the printing paper is 1440 dpi. × 720 dpi. The reason why the horizontal resolution of the dot group is higher than the vertical resolution is that the followability of the human eye with respect to gradation changes is more sensitive in the horizontal direction than in the vertical direction. Further, since the control for moving the carriage 210 in the main scanning direction is relatively easier than the control for moving the printing paper P in the sub-scanning direction, the horizontal resolution is set higher than the vertical resolution. Yes. However, of course, the size of the dot group may be 4 × 4 or 2 × 2 having the same aspect ratio other than 4 × 2. In this embodiment, the minimum unit of the constituent elements of the RGB format or CMY format image is referred to as “pixel”, and the minimum unit of the constituent elements of the output image formed on the printing paper P is referred to as “dot”. . Here, an RGB format or CMY format input image is increased in resolution from, for example, 360 dpi × 360 dpi to 1440 dpi × 720 dpi, and a plurality of pixels are grouped into 4 × 2 or 2 × 2 blocks. A pixel representing a plurality of pixels (for example, an average of gradation values of pixels in a block) may be handled as one “pixel”.

  Returning to FIG. The halftone processing unit 400 includes an encoding unit 410 and a decoding unit 420.

  When the halftone processing unit 400 receives CMY format image data from the color conversion unit 300, the halftone processing unit 400 inputs this image data to the encoding unit 410. At this time, the encoding unit 410 inputs CMY images in units of one line.

  The encoding unit 410 represents the gradation value (8 bits) of each pixel constituting the input image data as data of 5 bits for each color with reference to the encoding table ET (see FIG. 10) stored in the RAM 152. The encoded value EV is converted. Such conversion processing is hereinafter referred to as “encoding processing”. The encoding value EV obtained by the encoding process is a value that indirectly indicates what kind of dot arrangement data (see FIG. 4) is to be formed on the printing paper. The term “indirect” is because the encoding value EV is converted into dot arrangement data as a result of conversion based on a corresponding decoding table by a decoding process described later. A method for generating the encoding table ET referred to in the course of the encoding process and a detailed method for the encoding process will be described later.

  Note that the encoding table ET referred to at the time of the encoding process described above is an encoding value of less than “31”, which is the maximum value that the 5-bit encoding value EV can take, by the data amount reduction processing described later. The data is reduced to a capacity equal to or less than the data capacity corresponding to the product of the total number and stored in the RAM 152. Thus, in this embodiment, the amount of use of the RAM 152 is reduced by reducing the data capacity of the relatively large encoding table ET in advance.

  The image processing ASIC 155 includes an SRAM 156 capable of reading and writing at a higher speed than the RAM 152 shown in FIG. In converting the CMY image for one line into the encoded value EV, the encoding unit 410 temporarily inputs only the data necessary for the encoding process for one line from the encoding table ET in the RAM 152 to the SRAM 156. The encoding process is performed by referring to the partial encoding table ET. Although details will be described later, for example, when encoding the uppermost line of the CMY image, block numbers 0 to 127 (the maximum value of the block number is “8191”) in the encoding table ET shown in FIG. Is input (see FIG. 20). In the encoding process for one line, some blocks in the encoding table ET are repeatedly used. Therefore, by inputting only necessary data into the SRAM 156 in the image processing ASIC 155 in advance, the encoding can be efficiently performed. Processing can be performed. In this embodiment, the encoding unit 410 performs the encoding process in units of one line. However, the encoding unit 410 may perform the encoding process in units of two lines or more, or the entire image. .

  When the encoding unit 410 converts the CMY gradation value represented by 8 bits into a 5-bit encoded value EV, the encoding unit EV stores the encoded value EV in an intermediate buffer BF secured in a predetermined area of the RAM 152. Thus, in this embodiment, the image data input from the memory card MC or the like can be converted from 8-bit data to 5-bit data and stored in the RAM 152, so that the used capacity of the RAM 152 is reduced, Cost can be reduced.

  The decode unit 420 reads the encode value EV from the intermediate buffer BF, and stores the predecode table DT2 (see FIG. 12), the postdecode table DT2 (see FIG. 8), and the sequence value table ST (see FIG. 7 (FIG. 7). By sequentially referencing d), the encoded value EV is converted into 4 × 2 dot arrangement data as shown in FIG. Such conversion processing is referred to as “decoding processing”. A method for generating each table referred to in the decoding process and details of the decoding process will be described later. Since the decoding process is performed in units of one line as in the encoding process, only necessary data in the predecode table DT1 and the sequence value table ST is input to the SRAM 156.

  When the decode unit 420 converts the encoded value EV into dot arrangement data, the decode unit 420 outputs the dot arrangement data to the ink ejection control unit 500.

  The ink ejection control unit 500 controls the ink head 211, the carriage motor 220, and the paper feed motor 230, and ejects ink droplets onto the printing paper based on the dot arrangement data input from the halftone processing unit 400 to form dots. To do. In the dot arrangement data, the sizes of the dots of the respective sizes are represented by 2 “00 (no dot)”, “01 (S dot)”, “10 (M dot)”, and “11 (L dot)”, respectively. A value represented by bit data (hereinafter referred to as “dot size value”) is set. Therefore, the ink ejection control unit 500 adjusts the voltage waveform applied to the piezo elements in the ink head 211 in accordance with the dot size value, thereby performing dot classification with different sizes. By doing so, the printer 100 of this embodiment performs color printing on the printing paper.

(B) Print processing:
FIG. 5 is a flowchart of print processing executed by the printer 100. This printing process is a process executed by the CPU 151 when a predetermined printing operation is performed by the operation of the operation panel 140 by the user.

  When this printing process is executed, the CPU 151 first performs an initialization process for generating the encode table ET, predecode table DT1, postdecode table DT2, and sequence value table ST shown in FIG. 3 (step S10). ). A detailed description of the initialization process will be described later.

  When the initialization process is completed, the CPU 151 inputs image data designated by the user from the memory card MC or the like (step S20). Then, the input image data is output to the image processing ASIC 155. Then, the image processing ASIC 155 performs color conversion processing (step S30) by the color conversion unit 300 and halftone processing (step S40) by the halftone processing unit 400, and generates dot arrangement data for each pixel of the input image. Is done.

  When the dot arrangement data is generated by the halftone processing unit 400, ink ejection control is performed by the ink ejection control unit 500 (step S50). As a result, a color image is printed on the printing paper. Detailed description of the halftone processing performed by the image processing ASIC 155 will be described later.

(C) Initialization process:
FIG. 6 is a flowchart showing details of the initialization process executed in step S10 of the printing process described above. As described above, this initialization process is a process for generating various tables to be referred to by the halftone processing unit 400 during the halftone process.

  When this initialization process is executed, the CPU 151 first inputs the dither matrix data DM from the ROM 153 to the RAM 152 (step S100), and generates the sequence value table ST based on the dither matrix data DM (step S100). S110). The order value table ST is a table for determining the order in which dots are arranged in a block having 4 × 2 elements in a decoding process described later.

  FIG. 7 is an explanatory diagram showing a data structure of the dither matrix data DM and a method for generating the order value table ST. FIG. 7A shows the data structure of dither matrix data. In the dither matrix data DM of this embodiment, a total of 65536 threshold values from 0 to 65535 are recorded uniformly in an array of horizontal 512 × vertical 128 (illustration of each threshold is omitted). The dither matrix data DM is a table similar to the dither matrix used in the systematic dither method known as the conventional halftone technique. Therefore, also in the illustrated dither matrix data DM, when it is assumed that the image data has gradation values from 0 to 65535, each binarization of the image data is performed so that the dispersibility of black and white dots is increased. A threshold is arranged. As such dither matrix data DM, for example, a matrix having blue noise characteristics can be adopted. In this embodiment, the size of the dither matrix data DM is 512 × 128, but various sizes such as 128 × 32 and 1024 × 256 can be applied.

  In step S110, the order value table ST is generated from the dither matrix data DM. First, the CPU 151 divides the dither matrix data DM into a total of 8192 (= (512/4) × (128/2)) blocks, with 4 × 2 threshold values as one block. Give the block number. In this embodiment, as shown in FIG. 7A, the block number “0” is given to the block located at the upper left of the dither matrix data DM, the block number “1” is given to the block located right next to the block, The block number is given in order to the block at the lower right corner.

  FIG. 7B shows the threshold values recorded in the block with the block number “0” as a threshold value group. FIG. 7C shows sequence value data generated based on this threshold value group. The CPU 151 increases the gradation value from 0 to 65535 at the same time for all the elements in the threshold group shown in FIG. 7B, and the order in which the gradation value exceeds the threshold is from 0 to 7. By recording by value, the sequence value data of FIG. 7C is generated. In the case of FIG. 7B, since the gradation value exceeds the threshold value in the order of 23, 472, 1010, 1322,..., The order value data corresponding to the block number “0” is as shown in FIG. As shown in (c).

  When the CPU 151 generates the sequence value data for all the blocks, it records this for each block number and generates the sequence value table ST. FIG. 7D shows an example of the finally generated sequence value table ST. The CPU 151 stores the generated order value table ST in the RAM 152.

  Returning to FIG. After generating the order value table ST, the CPU 151 next generates a post-decoding table DT2 used in decoding processing to be described later (step S120).

  FIG. 8 is an explanatory diagram showing an example of the post-decode table DT2. As shown in the figure, 16-bit dot number data is recorded in the post-decode table DT2 in association with index values from 0 to 164. The dot number data is data obtained by concatenating eight 2-bit dot size values from large dots to right justification. The 165 dot number data represents all combinations of the number of dots of each size generated in one block.

  Here, the reason why the number of combinations of dots of each size generated in one block falls within 165 will be described. As described above, the size of one dot in the block can take four states of “no dot”, “S dot”, “M dot”, and “L dot”. Accordingly, the combination of these dots within one block is equal to the number of combinations when these four states are selected eight times with allowance for overlap. The number of dot combinations can be obtained by. For this reason, only 165 combinations (0 to 164) appear at most. Therefore, the index value corresponding to the dot number data is a value from 0 to 164.

₄ H ₈ (= _{4 + 8-1} C ₈ ) = 165 (1)
( _N H _r is an operator for determining the number of overlapping combinations when r times are selected from n objects, and _{n Cr} is not allowed to overlap from n objects. (It is an operator for obtaining the number of combinations when selecting r times.)

  The CPU 151 generates the post decode table DT2 shown in FIG. 8 as follows. That is, first, the CPU 151 sets all the numbers of S dots, M dots, and L dots generated in one block to “0”, and this is used as dot number data corresponding to the index value “0”. Record in predecode table DT1. In this case, since the size of all eight dots in one block is “no dot (00)”, the dot number data is “0000000000000”.

  Next, the CPU 151 sequentially increases the number of S dots from 1 to 8, and records the generated number of dots as index number data from 1 to 8. That is, when the number of S dots is 1, the dot number data is “0000000000000001”, and when the number of S dots is 8, “0101010101010101”.

  Subsequently, the CPU 151 sets the number of M size dots to “1”, and increases the number of S size dots from 0 to 7 so that the total number of generated dots is 8 or less. Then, the combination of the number of occurrences of each dot is recorded after the index value “9”.

  Further, the CPU 151 sets the number of M size dots to “2” and increases the number of S size dots from 0 to 6. Then, the number of occurrences is recorded in the predecode table DT1. Similarly, the post-decode table DT2 is generated by changing the number of dots of each size until the number of dots of L size becomes “8”. The CPU 151 stores the post decode table DT2 thus generated in the RAM 152. Note that each dot number data only needs to be assigned a unique index value, and the correspondence relationship is not particularly specified.

  Here, the description returns to FIG. 6 again. When the generation of the post-decoding table DT2 is completed, the CPU 151 inputs a dot generation amount table DGT used for generating an encoding table ET and a pre-decoding table DT1 described later from the ROM 153 to the RAM 152 (step S130).

  FIG. 9 is an explanatory diagram showing an example of the dot generation amount table DGT. As shown in the figure, in the dot generation amount table DGT, the generation amounts of S dots, M dots, and L dots are defined on the vertical axis in accordance with the input gradation values TD shown on the horizontal axis. According to this figure, only S dots are generated when the input gradation value TD is about 0 to 70, and thereafter both S dots and M dots are generated until the input gradation value TD is about 120. I understand. Further, when the input gradation value TD is about 110, L dots are generated in addition to S dots and M dots, and when the input gradation value TD is approximately 180 or later, only L dots are generated. I understand. Note that the dot generation amount shown on the vertical axis takes a value from 0 to 65535 so that it can be compared with each threshold value in the dither matrix data DM shown in FIG.

  FIG. 9 shows changes in the total weight of the ink ejected when these dots are formed on the print medium, along with the generation amounts of S dots, M dots, and L dots. According to the illustrated change in the total ink weight, the total ink weight gradually increases as the input tone value TD increases. The amount of dots generated for each size is set according to the change in the total weight. The dot generation amount table DGT stores a plurality of types of tables in the ROM 153 according to the types of printing paper supported by the printer 100. This is because printing paper has different ink absorptivity and light reflectance depending on the material, and the ink dot generation amount is set according to such characteristics. Accordingly, the CPU 151 inputs a dot generation amount table DGT corresponding to the type from the ROM 153 according to the type of the printing paper P set by the user. By doing so, it becomes possible to perform printing suitable for the selected printing paper P.

  Returning to FIG. When the dot generation amount table DGT is input in step S130, the CPU 151 performs processing for generating the encoding table ET and the predecode table DT1 using the dot generation amount table DGT and the dither matrix data DM input in step S100. This is performed (step S140). Hereinafter, the processing in step S140 is simply referred to as “table generation processing”.

  Since the table generation process is a complicated process, a detailed description will be given later. Here, the data structures of the encode table ET and the predecode table DT1 generated by the table generation process will be described.

  FIG. 10 is an explanatory diagram showing the data structure of the encoding table ET. As shown in the drawing, in this encoding table ET, any gradation value from 0 to 255 is associated with the block number from 0 to 8191 and the 5-bit encoded value EV from 0 to 31. It is recorded. Hereinafter, the gradation value recorded in the encoding table ET is referred to as a “gradation threshold value”. As described above, the encoding table ET includes the encoding unit 410 shown in FIG. 3 that converts the 8-bit gradation value of the CMY image into the 5-bit encoded value according to the position of the pixel, that is, the block number. It is a table for converting into EV.

  Here, a method for converting the gradation value to the encoded value EV using the encoding table ET will be briefly described. First, the encoding unit 410 calculates which block in the dither matrix data DM corresponds to the pixel in the CMY image to be encoded. Then, the row in the encoding table ET corresponding to the block number is referred to, and the gradation threshold value of the row is compared with the gradation value of the pixel in order from the lowest gradation threshold value. As a result of this comparison, the encoded value EV corresponding to the gradation threshold when the gradation threshold referred to in order exceeds the gradation value of the pixel for the first time becomes the encoded encoded value EV. For example, if the block number is “3” and the gradation value of the pixel to be processed is “199”, the encoded value EV is “16” in the case of FIG. That is, in the encoding table ET, the range of gradation values corresponding to each encoded value is recorded by a parameter called a gradation threshold value in association with each encoded value.

  FIG. 11 is an explanatory diagram schematically illustrating an example of a change in the encode value EV acquired from the encode table ET according to the gradation value of the CMY image. FIG. 11 shows an example of changes in the encoding value EV according to the gradation value for five blocks having block numbers “X1” to “X5”. For convenience of illustration, the change in the encoded value is drawn with the vertical axis shifted for each block. As shown in the figure, when the gradation value increases in each block, the value of the encoded value EV also increases, but the mode of increase and the number of changes (number of steps) differ from block to block. The encoding value EV in each block is a value that indirectly indicates dot arrangement data (see FIG. 4). Therefore, even if there are pixels having the same gradation value in the CMY image, if the positions of the pixels are different, the applied block numbers are different, and different encoded values EV are acquired. As a result, differently arranged dots are formed at positions on the print medium corresponding to these pixels.

  FIG. 12 is an explanatory diagram showing the data structure of the predecode table DT1. As shown in the figure, in the predecode table DT1, the index value of the postdecode table DT2 shown in FIG. 8 is recorded in association with the block number and the encode value EV. The decoding unit 420 shown in FIG. 3 reads the encoded value EV stored in the intermediate buffer BF during the decoding process, and converts the encoded value EV into an index value by referring to the post-decoding table DT2.

  Here, the contents of the decoding process will be briefly described. When the decode unit 420 refers to the predecode table DT1 shown in FIG. 12 and converts the encode value EV into an index value, the decode unit 420 refers to the postdecode table DT2 in FIG. 8 to obtain dot number data corresponding to the index value. The dot arrangement data shown in FIG. 4 is generated based on the acquired dot number data and the order value table ST shown in FIG. In this way, the decoding unit 420 can generate dot arrangement data based on the encoded value EV. A detailed description of the decoding process will be described later.

  Returning to FIG. 6, after completing the table generation process in step S140, the CPU 151 next executes a data amount reduction process for reducing the data amount of the encode table ET shown in FIG. 10 (step S150). As described above, the encoding table ET is a table that is dynamically generated based on the dither matrix data DM and the dot generation amount table DGT, and the contents thereof may change for each printing process. This is because the dot generation amount table DGT has different contents depending on the type of printing paper, and the contents of the dither matrix data DM may change due to firmware update or the like. Therefore, the range of the encoding value EV recorded in the encoding table ET may change, and there is a case where an unreferenced area exists in the table. For example, in the encode table ET shown in FIG. 10, although the encode value EV is prepared from “0” to “31”, the gradation threshold is actually recorded in the encode value EV. Is up to 20. Therefore, the gradation threshold value with the encode value EV of 21 or later is not referred to in any block in the encoding process, and becomes a useless area where the encode value EV and the gradation threshold value do not substantially correspond to each other. Therefore, in this embodiment, in the data amount reduction process in step S150, a process for deleting at least a part of such a useless area (element) is performed. By doing so, it is possible to reduce the data amount of the encoding table ET stored in the RAM 152. Details of the data amount reduction process will be described later.

  The flow of the initialization process in the present embodiment has been described above. Hereinafter, the details of the table generation process in step S140 and the data amount reduction process in step S150 will be described.

(C-1) Encoding table and predecoding table generation processing:
FIG. 13 is a flowchart showing details of the table generation process (encode table and predecode table generation process) executed in step S140 of FIG. When this process is executed, the CPU 151 first prepares an encode table ET and a predecode table DT1 in which all elements are set to “0” in the RAM 152 (step S200).

  Specifically, as shown in FIG. 10, in the encoding table ET, an 8-bit gradation threshold value from 0 to 255 is recorded in each element for the range where the encoding value EV is from 0 to 31, It is stored for 8192 blocks. Therefore, a capacity of 256 K bytes (= 32 × 8 bits × 8192), which is the product of these, is prepared in the RAM 152 for the encode table ET. On the other hand, for the predecode table DT1, as shown in FIG. 12, an 8-bit index value from 0 to 164 is recorded in each element in the range where the encoded value is from 0 to 31. 8192 items are stored. Accordingly, the RAM 152 has a capacity of 256 Kbytes for the predecode table DT1, similarly to the encode table ET.

  When the encoding table ET and the predecode table DT1 in which all elements are set to “0” are prepared, the CPU 151 then sets the block number N to be currently processed to “0” (step S210), and thereafter As a variable used in the above process, the encoding value EV is set to “0”, and the number of dots of each size (S dot, M dot, L dot) generated in the current block is set to “0” (step S220). . Further, the current input gradation value TD is set to “0” (step S230).

  Subsequently, the CPU 151 executes a dot number counting process for determining the number of dots of each size generated in the current block based on the input gradation value TD set in step S220 (step S240).

  FIG. 14 is a flowchart showing details of the dot number counting process executed in step S240. In this process, first, the CPU 151 obtains the dot generation amount of each size corresponding to the input tone value TD set in step S230 from the dot generation amount table DGT (see FIG. 9) (step S400).

  When acquiring the dot generation amount of each size, the CPU 151 acquires a threshold value group corresponding to the current block number N from the dither matrix data DM (see FIG. 7). Then, the count number (dotL, dotM, dotS) of each size dot to be counted is set to “0”, and the total count number n of all dots is set to “0” (step S410).

  When the count number and the total count number are set, the CPU 151 reads the nth lowest threshold value from the threshold value group according to the current total count number n. Then, it is determined whether or not the L dot generation amount acquired in step S400 is larger than this threshold (step S420). As a result of this determination, if the amount of L dots generated is larger than the nth lowest threshold (step S420: Yes), the L dot count number dotL is incremented (step S430). That is, in this case, the number of L dots generated in the 4 × 2 dot arrangement data is increased by one.

  When the L dot count number dotL is incremented, the CPU 151 determines whether or not the current total count number n is “7” (step S440). As a result, if the total count number n is “7”, it means that all the threshold values in the threshold value group have been referred to, and it can be determined that the counting of the number of dots has been completed. On the other hand, if the total count number n is not “7” in step S440, the total count number n is incremented (step S450) in order to continue the comparison between the next lower threshold and the dot generation amount (step S450). Return to S420.

  If the amount of L dots generated is equal to or less than the nth lowest threshold value in step S420 (step S420: No), the CPU 151 next generates M size and L size dots obtained from the dot generation amount table DGT. It is determined whether or not the accumulated value of the amount has exceeded the nth lowest threshold value (step S460). As a result, when it is determined that the accumulated value of the M size and L size dot generation amounts is larger than the nth lowest threshold value (step S460: Yes), the M dot count number dotM is incremented (step S470). ). That is, in this case, the number of M dots arranged in the 4 × 2 dot arrangement data is increased by one.

  When the M dot count number dotM is incremented, the CPU 151 shifts the processing to step S440 described above. As a result, if the total count number n has not reached “7”, the total count number n is incremented (step S450), and the process returns to step S420.

  If it is determined in step S460 that the accumulated value of the M dot and L dot dot generation amounts is equal to or less than the nth lowest threshold value (step S460: No), the CPU 151 then causes the dot generation amount table DGT. It is determined whether or not the accumulated value of the dot generation amount of S dots, M dots, and L dots obtained from (1) exceeds the nth lowest threshold value (step S480). If it is determined that the threshold value is larger than the nth lowest threshold value (step S480: Yes), the S dot count number dotS is incremented (step S490). That is, in this case, the number of S dots arranged in the 4 × 2 dot arrangement data is increased by one.

  When the S dot count is incremented, the CPU 151 shifts the processing to the above-described step S440. In this way, if the total count number n has not reached “7”, the total count number n is incremented by 1 (step S450), and the process returns to step S420.

  If it is determined in step S480 that the cumulative amount of S, M, and L dot generation amounts is equal to or less than the nth lowest threshold (step S480: No), or in step S440, the total When it is determined that the count number is “7” and all the eight threshold values have been referenced (step S440: Yes), the CPU 151 finally sets the number of dots of each size to the value counted so far. Confirm (step S500).

  FIG. 15 is an explanatory diagram showing a specific example of the dot number counting process described above. As shown in FIG. 15A, here, in the dot generation amount table DGT, the dot generation amount of L dots corresponding to the current input tone value TD is “409”, the M dot is “1550”, and the S dot. Is “1304”, and the threshold value group corresponding to the current block is the threshold value group shown in FIG.

  In this case, first, in step S420 of FIG. 14, the lowest threshold “23” in the threshold group is compared with the dot generation amount “409” of L dots. Then, since the dot generation amount “409” is larger than the threshold value “23”, “Yes” is determined in step S420, and the count number of L dots is incremented to “1” in step S430. . Then, since the total count number n is incremented in step S450, in the subsequent step S420, the second lowest threshold “472” is compared with the L dot generation amount “409”. In this case, since the dot generation amount “409” is smaller than the threshold value “472”, “No” is determined in step S420, and the process proceeds to step S460. At this time, the count number of L dots is fixed at “1” as shown in FIG.

  In step S460, the accumulated value “1959 (= 409 + 1550)” of the L dot generation amount and the M dot generation amount is compared with the second lowest threshold “472”. As a result, since the accumulated value “1959” of the dot generation amount of L dots and M dots is larger than the threshold value “472”, “Yes” is determined in step S460, and the count number of M dots is determined in step S470. Incremented to “1”. Then, the next threshold “1010” is further referred to. However, even in this case, since the accumulated value “1959” of the dot generation amount is still larger than the threshold value “1010”, the M size is counted until the seventh lowest threshold value “2240” is eventually referred to. The number is incremented. As a result, the M size count is fixed at “5” as shown in FIG.

  When the seventh lowest threshold value “2240” is referred to, “No” is determined in step S460. Next, in step S480, the cumulative value “L”, “M”, and “S” dot generation amount “ 3263 (= 409 + 1550 + 1304) ”and the seventh lowest threshold value“ 2240 ”are compared. As a result, since the accumulated value “3263” of the dot generation amount is larger than the seventh lowest threshold value, “Yes” is determined in step S480, and the S dot count is incremented by “1” in step S490. become. Then, the highest threshold “3262” is referred to next.

  The accumulated value “3263” of the dot generation amount of L dots, M dots, and S dots is still larger than the eighth threshold value “3262”. Therefore, in step S480, it is determined again as “Yes”, and in step S490, the count number of S size dots is incremented to “2” as shown in FIG. Then, since the current total count number n is “7”, it is determined as “Yes” in step S440, and the count number of dots of each size is determined. That is, in the example shown above, there are one L dot, five M dots, and two S dots.

  As described above with reference to FIG. 15, in the dot number counting process described above, the number of L dots generated is first determined, and then the number of M size and S size dots is determined. When the number of occurrences of each size is determined, the dot generation amount of each size is added and compared with a threshold value. As a result, as shown in FIG. The number of dots is a number that allows dots of each size to be arranged in order from L dots without overlapping each other in the block.

  Here, another specific example of the dot count processing will be considered. For example, it is assumed that the dot generation amounts of L dots and M dots corresponding to the current input gradation value TD are both “0” and the dot generation amount of S dots is “1500”. Further, it is assumed that the threshold value group corresponding to the current block is the threshold value group shown in FIG. Then, in step S420 and step S460 of the flowchart shown in FIG. 14, the generation amount of L dots and the generation amount of M dots are “0”, which is smaller than the lowest threshold value “23”, and therefore “No” is determined. Then, the process proceeds to step S480. In step S480, when the fifth lowest threshold “1659” exceeding the dot generation amount “1500” is referred to, in step S480, “No” is determined, and the S dot count is “5”. To confirm, and the counting process ends. That is, in this embodiment, since the comparison is performed in the order from the lowest threshold value, it is not necessary to refer to the next threshold value when the threshold value exceeding the cumulative value of the dot generation amount of all sizes is referred. Accordingly, the dot count processing for each size is speeded up, and as a result, the initialization processing can be performed at high speed.

  Returning to FIG. When step S240 in FIG. 13, that is, the dot count process shown in FIG. 14 is executed and the number of dots of each size is determined, the CPU 151 then determines the number of dots of each size in the previous input tone value TD. On the other hand, it is determined whether any of the number of dots of each size in the current input tone value TD has changed (step S250). For example, if there were 2 S dots, 5 M dots, and 1 L dot in the previous count process, 2 S dots, 4 M dots, and L in the current count process If the number of dots is two, the count numbers of the M dots and the L dots have changed, so in Step S250, it is determined that there is a change (Yes).

  If it is determined in step S250 that there is a change (step S250: Yes), the number of dots of each size corresponding to the current block number N and encoded value EV is updated to the value counted in step S240 (step S240). Step S260). Then, the current input gradation value TD is set as the gradation threshold value in the element corresponding to the current block number N and the encoded value EV of the encoding table ET prepared in step S200 (step S270).

  After setting the gradation threshold value in the encoding table ET, the CPU 151 refers to the post-decoding table DT2 (see FIG. 8) created in advance, and the same number as the number of dots of each size updated in step S260. An index value having the number of dots is obtained. For example, if it is determined in step S260 that there are 0 S dots, 2 M dots, and 6 L dots, the obtained index value is “160” with reference to FIG. When the index value is acquired in this way, the CPU 151 sets the index value to the element corresponding to the current block number N and the encoded value EV of the predecode table DT1 prepared in step S200 (step S280).

  When the recording of the gradation threshold value in the encode table ET and the recording of the index value in the predecode table DT1 are completed, the CPU 151 increments the current encode value EV (step S290). That is, in this step S290, the encoded value EV changes every time it is determined in step S250 that the number of dots of any size has changed. In other words, the encoded value EV changes each time the mode of dots generated in one block changes.

  After incrementing the encoding value EV in step S290 or if it is determined in step S250 that the number of dots of any size has not changed (step S250: No), the CPU 151 It is determined whether or not the input gradation value TD exceeds “255” (step S300). As a result, if the input tone value TD does not exceed “255” (step S300: No), the current input tone value TD is incremented (step S310), and the process returns to step S240. By doing so, it is possible to record the correspondence relationship between the input tone value TD and the encode value EV in the encode table ET while changing the input tone value TD from 0 to 255 for the current block. At the same time, the correspondence between the encoded value EV and the index value can be recorded in the predecode table DT1.

  On the other hand, if the current input tone value TD exceeds “255” in step S300 (step S300: Yes), it is determined whether the above-described processing is completed for all blocks (step S320). If not (step S320: No), after incrementing the current block number N (step S340), the process returns to step S220. In this way, for all the blocks, the correspondence relationship between the input gradation value TD and the encode value EV can be recorded in the encode table ET, and the correspondence relationship between the encode value and the index value can be recorded in the predecode table DT1. If the processing described above has been completed for all blocks (step S320: Yes), the table generation processing is terminated, and the processing proceeds to step S150 (data amount reduction processing) in FIG.

  The details of the table generation process shown in step S140 of FIG. 6 have been described above. The CPU 151 can generate the encoding table ET shown in FIG. 10 and the predecoding table DT1 shown in FIG. 12 by executing this table generation processing.

(C-2) Data amount reduction processing:
FIG. 16 is a flowchart of the data amount reduction process executed in step S150 of FIG. This data amount reduction process is a process for reducing the data amount of the encoding table ET generated by the table generation process described above.

  When this process is executed, the CPU 151 first refers to the encoding table ET (FIG. 10) stored in the RAM 152 (step S600) and acquires the maximum number of steps (step S610). The maximum number of steps means the largest number of steps (the number of steps) of all blocks. In other words, the maximum number of gradation threshold values is associated except for “0” as an initial value. This is the range of the encoded value EV that the given block has. In the encoding table ET shown in FIG. 10, the maximum number of steps is “21” of block number 3.

  When the maximum number of steps is acquired, the CPU 151 then secures a storage area necessary for storing the encoding table ET after the data amount reduction (hereinafter referred to as “compression encoding table ETC”) in the RAM 152 (step S620). Specifically, the required storage capacity is a value obtained by multiplying the maximum number of steps, the data capacity (8 bits) of each gradation threshold value, and the number of blocks. In the example of FIG. 10, the required storage capacity is 168 Kbytes (= 8192 × 21 × 8 bits). In the experiments by the inventors of the present application, the maximum number of steps is often about “24”. In this case, the required storage capacity is 192 Kbytes (= 8192 × 24 × 8 bits). .

  When the necessary storage capacity is secured, the CPU 151 copies each gradation threshold value from the encoding table ET already stored in the RAM 152 to the area secured in step S620 (step S630). Finally, the encoding table ET before the data amount reduction is deleted from the RAM 152 (step S640). By doing so, elements corresponding to encoded values exceeding the maximum number of steps are deleted for all blocks, and the size of the encoded table ET is reduced.

  FIG. 17 is an explanatory diagram showing an example of the compression encoding table ETC generated by the data amount reduction processing. If this compression encoding table ETC is compared with the encoding table ET before the data amount reduction shown in FIG. 10, the gradation threshold value “0” recorded in the element having the maximum number of steps or more is deleted for each block. Obviously, the amount of data is reduced. In this embodiment, the data amount reduction process can reduce the data amount of the encoding table ET by about 30%.

  The entire content of the initialization process executed in step S10 in FIG. 5 has been described above. Hereinafter, detailed processing contents of the halftone processing executed using the various tables generated by the initialization processing will be described.

(D) Halftone processing:
FIG. 18 is a flowchart of the halftone process executed in step S40 of the printing process shown in FIG. This halftone process sequentially refers to each table (encode table ET, predecode table DT1, postdecode table DT2, order value table ST) generated in the initialization process described above for each pixel of the CMY image. This is a process for generating dot arrangement data.

  When the image processing ASIC 155 first executes the halftone process, the tone value of each color (C, M, Y, K, Lc, and Lm) of the pixel to be processed using the encoding unit 410 shown in FIG. Encoding processing for converting each into an encoded value EV is performed (step S800).

(D-1) Encoding process:
FIG. 19 is a flowchart showing details of the encoding process. This encoding process is a process for converting the gradation value of the input pixel into the encoded value EV.

  When this encoding process is executed, the encoding unit 410 first inputs a CMY image from the color conversion unit 300 for each line, and acquires a gradation value GV in the X direction (right direction) pixel by pixel. (Step S802).

  When obtaining the gradation value GV, the encode unit 410 calculates the block number to which the pixel belongs (step S804). Here, in this embodiment, as shown in FIG. 4, one gradation value GV is expressed by 4 × 2 dots. These 4 × 2 dots correspond to 4 × 2 elements of the dither matrix DM shown in FIG. 7, that is, one block. Therefore, if the position of the pixel to be processed in the CMY image is known, the position of the block in the dither matrix DM corresponding to the position of the pixel can be easily obtained.

  FIG. 20 is a diagram illustrating the concept of the block number calculation method. The dither matrix data DM shown in FIG. 7A has a total of 8192 blocks, 128 blocks in the horizontal direction and 64 blocks in the vertical direction. Therefore, one piece of input image data is divided in units of 128 pixels × 64 pixels, and block numbers are calculated so that block numbers from 0 to 8191 are allocated to each pixel in the unit. That is, as shown in FIG. 20, the block number is calculated on the assumption that the dither matrix DM is arranged in a tile in one piece of input image data.

  Specifically, when the coordinates of the current pixel to be encoded are (X, Y), the Y coordinate of the current pixel is “0”, and the X coordinate is changed from “0” to “127”. The block numbers from “0” to “127” are allocated in order. After that, when the Y coordinate does not change and the X coordinate of the current pixel becomes “128”, the block number becomes “0” again. Thus, after the block number is repeatedly applied from “0” to “127” with respect to the X direction of the image, the block number when the current pixel coordinates become (0, 1). Becomes “128”. After that, while the Y coordinate is “1”, block numbers from “128” to “255” are repeatedly allocated. That is, the block number N can be obtained by the following equation (2), where the coordinates of the target pixel to be encoded are (X, Y). However, in the following formula (2), “%” is an operator for calculating a remainder.

N = (X% 128) + (Y% 64) * 128 (2)

  FIG. 21 is an explanatory diagram showing the concept of another calculation method of block numbers. In the block number calculation example shown in FIG. 20, the block number is calculated on the assumption that a plurality of dither matrices DM are arranged in a tile shape in one image data. On the other hand, in FIG. 21, among the dither matrix data DM arranged in a tile shape, the even-numbered dither matrix data DM is arranged by shifting 64 pixels in the X direction with respect to the odd-numbered dither matrix data DM. . Thus, if the dither matrix DM is shifted and arranged in the image, it is possible to suppress the tendency of the repetitive pattern of the dither matrix DM from appearing in the output image, so that the image quality can be improved.

  In this embodiment, since the encoding unit 410 inputs CMY images in units of lines, referring to FIGS. 20 and 21, the block numbers 0 to 127 are repeatedly applied to the first line. It will be. Therefore, in the present embodiment, when the encoding unit 410 executes the encoding process, only a necessary part of the compression encoding table ETC is input to the SRAM 156 in advance according to the input Y coordinate of the line. By doing so, it is not necessary to read the encoding table ET to the RAM 152 having a lower access speed than the SRAM 156 every time the gradation value of each pixel is acquired, and the processing can be speeded up. In this embodiment, it is assumed that a part of the compression encoding table ETC is input to the SRAM 156 while the data capacity is reduced. By doing so, the storage capacity required by the SRAM 156 can be reduced.

  When the calculation of the block number is completed in step S804 of FIG. 19, the encoding unit 410 initializes the encoding value EV to be obtained to “0” (step S806). Then, referring to the compression encoding table ETC shown in FIG. 17 (actually, the partial compression encoding table ETC read into the SRAM 156), it corresponds to the block number and the current encoding value EV calculated in step S804. The gradation threshold value GVt to be acquired is acquired (step S808).

  Upon obtaining the gradation threshold GVt, the encoding unit 410 compares the gradation threshold GVt with the gradation value GV of the pixel to be processed input in step S802, and this gradation value GV is equal to or greater than the gradation threshold GVt. Is determined (step S810). As a result of this determination, if the gradation value GV is equal to or greater than the gradation threshold GVt (step S810: Yes), “1” is added to the current encoded value EV (step S812), and the encoded value EV after the addition is It is determined whether the maximum value EVmax has been reached (step S814). The maximum value EVmax is a value obtained by subtracting “1” from the maximum number of steps described above (for example, “20” when referring to the encoding table of FIG. 17). If the encode value EV is equal to or greater than the maximum value EVmax (step S814: Yes), the encode value EV is fixed at the maximum value EVmax at that time (step S816).

  If it is determined in step S814 that the encoded value EV is less than the maximum value EVmax (step S814: No), the encoding unit 410 returns the process to step S808, and the level corresponding to the encoded encoded value EV after addition. The gradation threshold GVt is acquired, and the gradation threshold GVt and the gradation value GV are compared again. As a result of repeating the comparison, if the gradation value GV is less than the gradation threshold value GVt for the first time in step S810 (step S810: No), the value of the encoded value EV at that time is the encoded value EV of the encoded result. (Step S816).

  According to the encoding process described above, the gradation value of the input pixel is converted into a 5-bit encoded value according to the block to which the pixel belongs by referring to the encoding table ET shown in FIG. Can do. The encoding unit 410 performs this encoding process for all pixels of the CMY image.

  In the encoding process described above, a part of the compression encoding table ETC is input to the SRAM 156 with the data amount reduced. On the other hand, regarding the block input to the SRAM 156, the data amount may be recovered to the original format shown in FIG. 10 and input. With such a configuration, the format of the encoding table input to the SRAM 156 is uniform, so that the memory management of the SRAM 156 can be easily performed.

  Returning to FIG. When the encoding unit 410 converts the gradation value into the encoded value EV by the encoding process described above, the value is buffered in the intermediate buffer BF in the RAM 152 (step S820). As a result of the encoding process described above, the intermediate buffer BF stores not the 8-bit gradation value but the 5-bit encoded value EV, so that the storage capacity of the RAM 152 can be greatly reduced. .

  When the encode value EV is buffered in the intermediate buffer BF by the encode unit 410, the decode unit 420 then executes a decoding process for generating the dot arrangement data shown in FIG. 4 based on the encode value EV ( Step 840). This decoding process is executed by the decoding unit 420 when at least one encoded value EV is buffered without waiting for the encoded value EV to be buffered for all the pixels constituting the image data. That is, the encoding process and the decoding process are executed simultaneously in parallel.

(D-2) Decoding process:
FIG. 22 is a detailed flowchart of the decoding process. When this decoding process is executed, the decoding unit 420 first reads the encoded value EV stored in the intermediate buffer BF (step S822). Also in this decoding process, the encoded value EV corresponding to one line is read from the intermediate buffer BF in units of lines.

  After reading the encoded value EV, the decoding unit 420 calculates the block number to which the pixel that is the source of the encoded value EV to be processed belongs (step S824). Since the encoded value EV is buffered in the intermediate buffer BF in the encoded order, if the encoded value EV is read in this order, the block number corresponding to the encoded value EV is also automatically calculated by the above-described calculation method. Can be calculated. In the decoding process, only the portion corresponding to the block number necessary for the decoding process is input to the SRAM 156 in advance from the predecode table DT1 and the sequence value table ST stored in the RAM 152.

  When the block number is calculated, the decode unit 420 refers to the predecode table DT1 shown in FIG. 12 (actually, the partial predecode table DT1 input to the SRAM 156), and obtained in step S822. An index value corresponding to the encoded value EV and the block number obtained in step S824 is acquired (step S826). Further, by referring to the postdecode table DT2, dot number data corresponding to this index value is acquired (step S828). As for this post-decode table DT2, the entire table is input to the SRAM 156 in advance. This is because the post-decoding table DT2 has a relatively small capacity (165 × 16 bits = 330 bytes) and does not impose the capacity of the SRAM 156. When the dot number data is acquired from the post-decode table DT2, the decode unit 420 executes a dot arrangement process for arranging dots of each size in the block (step S830).

  FIG. 23 is a detailed flowchart of the dot arrangement process. FIG. 24 is an explanatory diagram schematically showing a dot placement process by this dot placement processing. As shown in FIG. 23, when this dot arrangement process is executed, the decoding unit 420 first stores the sequence value data (see FIG. 7C) corresponding to the current block partially in the SRAM 156. Obtained from the order value table ST (see FIG. 7D) (step S900). Then, a dot size value “00” representing “no dot” is set and initialized for all elements in the block in which dots are to be arranged (step S910), and the position in the block from which dots are to be arranged in the future. Is set to the position “a” shown in FIG. 24A (upper left position) (step S920).

  When the position where the dot is to be arranged is set, the decoding unit 420 acquires the order value corresponding to the current dot position from the order value data (step S930). FIG. 24B shows an example of order value data input from the order value table ST. In the illustrated example, since the order value corresponding to the current position “a” is “5”, this value “5” is acquired in step S930.

  Next, the decode unit 420 acquires the dot size value corresponding to the order value acquired in step S930 from the dot number data acquired from the post-decode table DT2 (step S940). Specifically, every two bits from the lower bit side of the dot number data are counted by the order value acquired in step S930, and the dot size value recorded at that position is acquired. FIG. 24C shows an example of dot number data acquired from the post-decode table DT2. In this figure, the dot size value corresponding to the order value “5” is the sixth dot size value “01” counted from the lower bits of the dot size data. The dot size value “01” represents an S size dot.

  When the dot size value is acquired, the CPU 151 sets the acquired dot size value at the current position in the block (step S950). That is, as shown in FIG. 24D, if the current position is “a”, the dot size value “01” acquired from the dot number data in FIG. 24C is set at this position. To do.

  When the dot size value is set at the current position, the CPU 151 determines whether or not the dot size value has been set for all positions (step S960). As a result, if the setting of dot size values has been completed for all positions (step S960: Yes), the dot arrangement process is terminated. On the other hand, if not finished, the current position in the block is moved in the order of a, b, c, d,... (See FIG. 24A) (step S970), and the process goes to step S930. By returning, dot size values are set for other positions.

  When the dot arrangement process described above is completed for all the encoded values EV buffered in the intermediate buffer BF, the decoding process by the decoding unit 420 is completed. Each dot arrangement data generated by this decoding process is delivered to the ink ejection control unit 500 each time it is generated, and is used for dot formation by the ink head 211. In this embodiment, the dots are arranged by referring to the order value table. However, the corresponding blocks in the dither matrix data DM are directly referred to, and dots are generated in order from the element having the lowest threshold value. Thus, dot arrangement may be performed.

(E) Effect:
According to the present embodiment described above, the following effects are produced. That is, when the generation result of the dot arrangement data shown in FIG. 24D is compared with the 4 × 2 order value data shown in FIG. It can be seen that they are arranged at positions with low order values without duplication. A low order value means that the threshold value corresponding to the order value is low (see FIG. 7B). In the dither matrix data DM shown in FIG. 7A, the threshold value is arranged with better dispersion as the value is lower due to the nature of the dither pattern in the systematic dither method. Is arranged so as to avoid the specified threshold location. For this reason, in the dot arrangement data shown in FIG. 24D, the dots having the largest size are preferentially arranged at a lower order value, that is, at a lower threshold location. The sex will be well placed. Therefore, according to the dot arrangement method in the present embodiment, the dispersibility of large dots that are conspicuous on the printing paper can be maximized, so that the image quality of the output image can be greatly improved.

  In the present embodiment, if a matrix having blue noise characteristics that can disperse dots satisfactorily is used as the dither matrix data DM, there is no special regularity as to which block in the matrix has what gradation. Thus, as shown in FIG. 11, the values that the encoding value can take differ from block to block. Therefore, in the conventional density pattern method and the like, since multi-value conversion is uniformly performed according to the gradation value, there is a possibility that the substantial number of gradations may be reduced. While halftone processing is performed in units of 4 × 2 dot blocks, the dot generation pattern changes variously depending on the position of the block in the dither matrix, which is significantly greater than the conventional multi-value method. In addition, the image quality of the output image can be improved.

  In this embodiment, since various tables used for the halftone process are generated based on the dither matrix data DM and the dot generation amount table DGT, the arrangement of the threshold values in the dither matrix data DM is appropriately optimized. As a result, the image quality of the output image can be improved relatively easily. That is, if a table structure that can perform halftone processing corresponding to the position of each block is used for each block as in this embodiment, a uniform conversion table used in conventional multi-value conversion is used. The image quality can be improved more flexibly.

  Further, according to the halftone process of the present embodiment, the halftone process for the CMY image is performed by sequentially referring to the encode table ET, the predecode table DT1, the postdecode table DT2, and the order value table ST. Is completed. Therefore, unlike other halftone processes such as the error diffusion method, it is possible to realize an extremely high speed halftone process without performing a complicated operation for distributing errors.

  In the present embodiment, the halftone process is divided into two stages of an encoding process and a decoding process, and the encoded value EV generated as a result of the encoding process is temporarily stored in the intermediate buffer BF in the RAM 152. To do. Therefore, even if it takes time to form dots by the printing mechanism after the decoding process, the encoding value EV obtained as a result of the encoding process is buffered in the RAM 152, so that only the encoding process is completed at an early stage. Can be made. As a result, the CPU 151 can be quickly released from the processing related to printing. Further, in this intermediate buffer BF, CMY image data having a data capacity of 8 bits (255 gradations) for each color is stored by being reduced to an encoded value EV of 5 bits for each color by the encoding process. The storage capacity can be greatly reduced, and the cost can be reduced.

  Furthermore, in this embodiment, the amount of data is reduced by deleting in advance elements in which the gradation threshold value is not substantially recorded in the encoding table ET used for the encoding process. Therefore, the storage capacity of the RAM 152 can be further reduced. Further, since the RAM 152 stores the encode table ET with a reduced data capacity, the transfer amount can be reduced when transferring a part of the data to the SRAM 156. Therefore, the time required for data transfer is reduced, and the halftone process can be speeded up.

  In the above-described embodiment, as illustrated in FIG. 5, it has been described that the initialization process is executed every time the printing process is executed. However, the initialization process may be executed at other timings. Good. For example, it may be executed when the printer 100 is turned on, or may be executed when the user changes the setting of the printing paper or the printing mode.

(F) Modification:
As mentioned above, although the Example of this invention was described, it cannot be overemphasized that this invention is not limited to such an Example, and can take a various structure in the range which does not deviate from the meaning. For example, a function realized by hardware by the image processing ASIC 155 may be realized by software by execution of a predetermined program by the CPU 151. In addition, the following modifications are possible.

(F-1) First modification:
In the above embodiment, after the encoding table ET shown in FIG. 10 is created, the data amount is reduced according to the maximum number of steps of the encoding table ET (see FIG. 16). On the other hand, in this modification, after the encoding table ET is created, the number of steps is set so that the number of steps of the encoding value EV of each block (the range of the encoding value EV) falls within a preset number of steps. It is assumed that the number of steps to be reduced is reduced. According to this modification, for example, when the storage capacity that can be secured in the RAM 152 is limited, the capacity of the encode table ET can be reliably reduced.

  FIG. 25 is a flowchart of the step number reduction process in this modification. This process is a process executed in the initialization process shown in FIG. 6 after the table generation process (step S140) and prior to the data amount reduction process (step S150).

  When this step number reduction process is executed, the CPU 151 first obtains a step number limit value preset in the ROM 153 (hereinafter referred to as “step number limit value LS”) (step S1000). The block number N of the block for which the restriction is first performed is set to “0” (step S1010).

  When the block number is set, the CPU 151 refers to the encode table ET stored in the RAM 152 (step S1020), and acquires the step number SN of the block (step S1030). Then, the step number SN is compared with the step number limit value LS to determine whether the step number SN is larger than the step number limit value LS (step S1040). As a result of this determination, if the step number SN is larger than the step number limit value LS (step S1040: Yes), the block step number SN is reduced to the step number limit value LS by thinning out the large gradation threshold of the block. The thinning process to be reduced is performed (step S1050). Details of the thinning process will be described later. In step S1040, if the step number SN is smaller than the step number limit value LS (step S1040: No), this thinning process is not performed.

  In step S1040, when it is determined that the step number SN is smaller than the step number limit value LS, or after the thinning process is completed in step S1050, the CPU 151 completes the above-described process for all blocks. Is determined (step S1060). As a result of the determination, if all the blocks have not been completed, “1” is added to the block number (step S1070), and the process returns to step S600. By doing so, it is possible to perform a thinning process on each block in the encoding table ET as necessary.

FIG. 26 is an explanatory diagram showing the processing contents of the above-described thinning processing. This figure shows an example in which the thinning process is performed for a block having a step number limit value LS “16” and a step number SN “19” (encoding value EV is 0 to 18). FIG. 26A shows the encoding table ET for the block number “4” as a representative.

  When the thinning process is started, the CPU 151 first increases the grayscale threshold by the number of differences between the step number SN and the step number limit value LS (three in the case of FIG. 26), as shown in FIG. Are deleted every other threshold value (the hatched portion in FIG. 26A). Then, the encoded value EV that has taken a value from 0 to 18 before deletion takes a value from 0 to 15, and the step number SN has been changed to “16”. The reason why the gradation threshold value is thinned out from the large gradation threshold value is that a large amount of ink is ejected in a portion where the gradation value is large, and therefore the number of changes in the dot arrangement pattern slightly decreases in that portion. Even if this is the case, the effect is not so noticeable, and the effect on the image quality is negligible.

  When the gradation threshold value is deleted, the CPU 151 next performs a process of adjusting the gradation threshold value remaining after the deletion. Specifically, as shown in FIG. 26 (c), the average value of the gradation threshold value to be deleted and the gradation threshold value that has remained adjacent to the gradation threshold value is calculated as a new value. Replace with the gradation threshold. FIG. 26 shows an example in which the gradation threshold “224” remaining after deletion is replaced with an average value “229” of this gradation threshold and the deleted gradation threshold “234”. In this way, if the gradation threshold value is adjusted, the degree of change in the gradation threshold value becomes smoother than simply thinning out the value, so that it is possible to suppress deterioration in image quality.

  Here, the description returns to FIG. As described above, when the encoding table ET is thinned out, the range that the encoding value EV can take varies for each block. Accordingly, in conjunction with this, the CPU 151 thins out the predecode table DT1 shown in FIG. 12 as well (step S1080). The thinning-out of the predecode table DT1 is completed by deleting the index value corresponding to the encoded value EV to be deleted in the encoding table ET and moving the remaining index value to the left for each block. By doing so, the encoding table ET and the predecoding table DT1 have the same range of encoding values EV for each block. Note that the index value is a value uniquely assigned to each dot number data defined in the post-decoding table DT2, and thus the value is not adjusted.

  When the step number reduction process described above is completed, the CPU 151 executes the data amount reduction process of step S150 in accordance with the initialization process shown in FIG. By doing so, it is possible to reduce the number of steps of each block and further reduce the data amount of the encoding table ET.

(F-2) Second modification:
In the data amount reduction processing of the above embodiment, as described with reference to FIG. 16, the maximum number of steps is obtained from the encoding table ET, and a necessary area is secured in the RAM 152 as a two-dimensional table according to the maximum number of steps. To do. Therefore, in the compression encoding table ETC generated by reducing the data amount, as shown in FIG. 17, the gradation threshold value is recorded for the block corresponding to the maximum number of steps without excess or deficiency (FIG. 17). In the example, block number 4), the gradation threshold value of “0” still remains for the other blocks. Therefore, in this modification, processing for reducing the data amount is performed for such a gradation threshold.

  FIG. 27 is an explanatory diagram showing an example of the compression encoding table ETC2 created in the table generation processing of the present modification described later. In the compression encoding table ETC2, as shown in the figure, a block number, an encoding value, and a gradation threshold are associated with a unique address. This compression encoding table ETC2 removes the portion of the gradation threshold value “0” existing in the element having an encoded value of 1 or more from the encoding table ET shown in FIG. 10, and uses the remaining gradation threshold value as its block number. Along with the encoding value, it is continuously associated with consecutive unique addresses. If such a compression encoding table ETC2 is created, the data amount can be further reduced as compared with the compression encoding table ETC in the embodiment shown in FIG.

  In this modification, when referring to the compression encoding table ETC2 of FIG. 27, the head address of each block is prepared as a separate table so that an arbitrary block can be easily accessed. FIG. 28 shows such an address table AT. As shown in the figure, in this address table AT, the head address of the block is recorded in association with the block number of each block. By referring to this address table AT, the gradation threshold value of the target block can be easily obtained in the above-described encoding process.

  FIG. 29 is a flowchart of the table creation process for generating the compression encoding table ETC of the present modification shown in FIG. 27 and the address table AT shown in FIG. Since most of the table creation processing is the same as the processing content of the table generation processing in the embodiment shown in FIG. 13, different portions will be described below. In the table generation process of the present modification shown in FIG. 29, processing contents different from those in FIG. 13 are indicated by “b” at the end of the step number.

  When this table creation processing is executed, first, the CPU 151 prepares a compression encoding table ETC2 in which all elements are “0” and a predecoding table DT1 in the RAM 152 (step S200b). The capacity of the compression encoding table ETC2 prepared by such processing is set to a maximum capacity that can record an 8-bit gradation threshold value with encoding values “0” to “31” for all blocks.

  When the compression encoding table ETC2 and the predecode table DT1 are prepared, the CPU 151 sets the block number N to be processed at present to “0” and sets the address counter C to “0” (step S210b). This address counter C is a counter used for recording the head address of each block in the address table AT shown in FIG.

  Subsequently, the CPU 151 sets the value of the current address counter C as the head address corresponding to the current block (that is, block number 0) in the address table (step S215b). By doing so, the head address of the block having the block number “0” is set to the address “0”.

  Hereinafter, the process from step S220 to step S260 is the same process as the process shown in FIG. That is, as a variable used in the subsequent processing, the encode value EV is set to “0”, and the number of dots of each size (S dot, M dot, L dot) generated in the current block is set to “0” ( Step S220). Then, the current input gradation value TD is set to “0” (step S230). Further, the CPU 151 performs a dot number count process for determining the number of dots generated in the current block based on the current input gradation value TD (step S240), and if any of the dot numbers of each size changes. (Step S250), the number of dots of each size corresponding to the current block number N and encoded value EV is updated to the value counted in Step S240 (Step S260).

  In the above embodiment, thereafter, the current input gradation value TD is set as the gradation threshold value in the element corresponding to the current block number N and the encoding value EV of the encoding table ET. On the other hand, in the present modification, the current input gradation value TD is set as a gradation threshold value at the address C of the compression encoding table ETC2, and the current block number N and the encoding value EV are set (step). S270b). Then, as in the embodiment, an index value is set in the predecode table DT1 (step S280). When the index value is set in the predecode table DT1, the CPU 151 increments the current encoded value EV as in the embodiment. At this time, in this modified example, the address counter C is also incremented simultaneously (step S290b). .

  Hereinafter, the processing from step S300 to step S340 is the same as that of the embodiment, and the description thereof will be omitted. With the above processing, the block number, the encoded value, and the gradation threshold value are recorded in association with each address while the address counter C is incremented by one. Finally, the CPU 151 deletes an area in which the block number, the encoded value, and the gradation threshold value all remain “0” from the compression encoding table ETC2 in which the maximum capacity is secured in advance. By doing so, the compression encoding table ETC2 of the present modification shown in FIG. 27 is generated. In this modification, the data amount reduction process in step S150 of the initialization process shown in FIG. 6 is not executed. This is because the amount of data is reduced at the same time by the table creation processing of the present modification described above.

  According to the second modification described above, all elements in the encoding table ET that are not substantially associated with gradation values can be deleted. Therefore, compared with the compression encoding table ETC generated in the embodiment. Furthermore, it is possible to generate the compression encoding table ETC2 having a small data capacity. In this modification, the address table AT indicating the head address of each block is generated. However, other than this, for example, a table in which the address corresponding to the head position of each raster is known, or reading to the SRAM 156 is performed. By managing a table in which the head address of the unit is known, a pointer indicating the position of the address at which the previous reading is completed, necessary data can be easily read from the compression encoding table ETC2.

(F-3) Third modification:
In the above embodiment, the data capacity of the encoding table ET is reduced. On the other hand, by executing the same process as the data amount reduction process shown in FIG. 16 for the predecode table DT1 shown in FIG. 12, not only for the encode table ET but also for the predecode table. It is possible to reduce the data capacity. In addition, the data format of the compression encoding table ETC2 shown in FIG. 27 in the second modification can also be applied to the data format of the predecode table DT1.

(F-4) Fourth modification:
In the above-described embodiment, the dot arrangement data is generated from the encoded value EV by referring to the three tables of the predecode table DT1, the postdecode table DT2, and the order value table ST during the decoding process. However, it is also possible to directly convert the encoded value EV into dot arrangement data by integrating these tables into one.

  FIG. 30 is an explanatory diagram showing an example of a table (hereinafter referred to as “direct decode table DDT”) from which dot arrangement data can be obtained directly from the encoding value EV and the block number. In the direct decode table DDT, the dot arrangement data shown in FIG. 4 is stored in association with the encode value EV and the block number. The dot arrangement data stored in each element of the direct decode table DDT are all converted into dot arrangement data based on the index values stored in the predecode table DT1 based on the post decode table DT2 and the sequence value table ST. It is nothing but data. That is, after the predecode table DT1 is created according to the above embodiment, the direct decode table DDT shown in FIG. 30 is generated by performing the decode process of FIG. 22 for all index values in the predecode table DT1. Can do. As described above, if the direct decoding table DDT is generated in advance, the decoding process is completed by referring to only one table, so that the halftone process can be performed at a higher speed.

(F-5) Fifth modification:
In the above-described embodiment, the printer 100 is assumed to be of a multifunction machine type, and performs a single operation from input of image data to halftone processing and ink ejection control. On the other hand, a computer may be connected to the printer 100, and the computer may execute input of image data and a part of halftone processing, that is, encoding processing. In such a configuration, the initialization process and the halftone process described in the embodiments can be realized by installing a predetermined program in the computer. When the printer 100 receives the encoded value EV from the computer via a printer cable or the like, the printer 100 performs a decoding process according to the encoded value EV, controls the printing mechanism, and performs printing on the printing paper.

  As described above, when the encoding process is executed by the computer, image data having a data size of 8 bits (255 gradations) for each color of C, M, Y, and K is encoded with 5 bits. Since it is converted into EV, the amount of communication between the computer and the printer 100 can be reduced. As a result, the printing speed can be improved as a whole.

  Further, when the processing speed in the printer 100 is more problematic than the data communication speed, not only the encoding process but also the decoding process can be performed by a computer. In this way, the processing load on the printer 100 can be reduced, and the overall printing speed can be improved. In addition, if the computer executes both the encoding process and the decoding process in this way, the hardware required for the halftone process becomes unnecessary, and thus the manufacturing cost of the printer 100 can be reduced. .

1 is an explanatory diagram illustrating a printer 100 as an embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating an internal configuration of a printer 100. FIG. It is explanatory drawing which shows the detailed structure of the image processing ASIC155. It is explanatory drawing which shows an example of dot arrangement data. It is a flowchart of a printing process. It is a flowchart which shows the detail of an initialization process. It is explanatory drawing which shows the data structure of the dither matrix data DM, and the production | generation method of the order value table ST. It is explanatory drawing which shows an example of postdecoding table DT2. It is explanatory drawing which shows an example of the dot generation amount table DGT. It is explanatory drawing which shows the data structure of the encoding table ET. It is explanatory drawing which shows the example of a change of the encoding value EV typically. It is explanatory drawing which shows the data structure of predecode table DT1. It is a flowchart which shows the detail of a table production | generation process. It is a flowchart which shows the detail of a dot number count process. It is explanatory drawing which shows the specific example of a dot number count process. It is a flowchart of a data amount reduction process. It is explanatory drawing which shows an example of the compression encoding table ETC. It is a flowchart of a halftone process. It is a flowchart showing the detail of an encoding process. It is description which shows the concept of the calculation method of a block number. It is explanatory drawing which shows the concept of the other calculation method of a block number. It is a detailed flowchart of a decoding process. It is a detailed flowchart of a dot arrangement process. It is explanatory drawing which shows the arrangement | positioning process of a dot typically. It is a flowchart of the step number reduction process in a 1st modification. It is explanatory drawing which shows the processing content of the thinning process in a 1st modification. It is explanatory drawing which shows an example of the compression encoding table ETC2 in a 2nd modification. It is explanatory drawing which shows an example of address table AT in a 2nd modification. It is a flowchart of the table creation process in a 2nd modification. It is explanatory drawing which shows an example of the direct decoding table DDT.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Printer 110 ... Scanner 120 ... Memory card slot 140 ... Operation panel 145 ... Liquid crystal monitor 150 ... Control unit 151 ... CPU
152 ... RAM
153 ... ROM
156 ... SRAM
210 ... carriage 211 ... ink head 212 ... ink cartridge 220 ... carriage motor 230 ... motor 260 ... drive belt 270 ... platen 280 ... sliding shaft 300 ... color conversion unit 400 ... halftone processing unit 410 ... encoding unit 420 ... decoding unit 500 ... Ink ejection control unit BF ... Intermediate buffer DM ... Dither matrix data DGT ... Dot generation amount table AT ... Address table LUT ... Color conversion table ET ... Encoding table ETC ... Compression encoding table DT1 ... Predecoding table DT2 ... Post decoding table ST ... Order value table DDT ... Direct decode table

Claims (21)

An image processing apparatus that determines the arrangement of dots to be formed on a printing medium when printing image data based on a dither matrix in which a threshold value for determining whether or not to generate dots is recorded according to the gradation value of the image data Because
The dither matrix is divided into blocks each composed of a predetermined number of threshold values, and the gradation values are sequentially set to the threshold values in a range that the gradation values can take for each block identifier uniquely assigned to each block. Encoding table storage means for storing an encoding table in which each gradation value that generates each encoding value is recorded in association with each encoding value representing the arrangement of dots in the block obtained by comparison;
Image data input means for inputting the image data;
Based on the position of each pixel constituting the input image data and the position of each block existing in the dither matrix, the block identifier corresponding to each pixel is discriminated, and the block identifier and the level of each pixel are distinguished. Encoding means for obtaining the encoding value corresponding to the gradation value of each pixel by referring to the encoding table based on the tone value;
Decoding means for determining the arrangement of dots generated in the block corresponding to the position of each pixel according to the encoded value obtained by the encoding means and the discriminated block identifier;
The encoding table storage means represents each encoding value with a predetermined number of bits, and the encoding table is a product of an encoding value less than the maximum encoding value that can be expressed with the number of bits and the number of blocks. An image processing device that stores data with a capacity less than the corresponding data capacity. The image processing apparatus according to claim 1,
The encoding table storage means stores the encoding table as a set of a plurality of elements in which the gradation value is recorded, and for any block up to the maximum value of the encoding value that can be expressed by the number of bits. In the case where the gradation value is not substantially recorded, at least a part of the element in which the gradation value is not substantially recorded in the encoding table is deleted in advance. Stored image processing device. The image processing apparatus according to claim 2,
The encoding table storage means includes, in addition to the elements that are not substantially associated with the gradation values for the block that is associated with the largest number of gradation values among the blocks represented by the block identifiers, An image processing apparatus in which elements of other blocks corresponding to are previously deleted from the encoding table and the encoding table is stored. The image processing apparatus according to claim 2,
The image processing apparatus, wherein the encoding table storage unit deletes all elements that are not substantially associated with the gradation value from the encoding table in advance and stores the encoding table. The image processing apparatus according to claim 4,
The encoding table storage means stores the block identifier, the encoded value, and the gradation value in a continuous address space with the encoding table from which all of the elements not substantially associated with the gradation value are deleted. Is stored in a format in which these are associated with each other. The image processing apparatus according to claim 5,
The image processing apparatus, wherein the encoding table storage unit separately stores an address table indicating an address corresponding to each block identifier in the address space. An image processing apparatus according to any one of claims 2 to 6,
The encoding value is set according to the number of changes in the arrangement of the dots according to the threshold value in each block when the gradation value is increased from the minimum value to the maximum value for each block. An image processing device. The image processing apparatus according to claim 7,
Limit value storage means for storing a limit value that limits in advance the upper limit of the range that the encoded value can take,
The image processing apparatus, wherein the encoding table storage unit stores the encoding table by reducing a range of the encoding value in advance according to the limit value for a block whose change number exceeds the limit value. The image processing apparatus according to claim 8,
The encoding table storage means, for the block whose change number exceeds the limit value, from the encoded value on the darkest gradation value side by the number of differences between the limit value and the maximum value of the encoded value of the block, An image processing apparatus that performs the reduction by thinning out an encoded value and the gradation value associated with the encoded value. The image processing apparatus according to claim 9,
The image processing apparatus, wherein the encoding table storage unit adjusts a gradation value remaining after the thinning. The image processing apparatus according to claim 10,
The encoding table storage means stores the average value of the gradation value deleted by the thinning and the gradation value adjacent to the gradation value before the deletion and remaining by the thinning. An image processing apparatus that performs the adjustment by replacing with a gradation value. An image processing apparatus according to any one of claims 2 to 11,
The encoding means inputs the pixels constituting the image data for each line, and temporarily stores the block identifier range corresponding to the pixels in the line by partially inputting from the encoding table With means,
The image processing apparatus, wherein the encoding means obtains the encoded value using a partial encoding table held in the temporary storage means. The image processing apparatus according to claim 12,
The image processing apparatus, wherein the encoding unit recovers the deleted element in the encoding table and inputs the partial encoding table to the temporary storage unit. The image processing apparatus according to claim 12,
The image processing apparatus, wherein the encoding unit inputs the partial encoding table to the temporary storage unit without recovering the deleted element in the encoding table. The image processing apparatus according to any one of claims 1 to 14,
Furthermore, for each of the block identifiers, a decoding table storage means for storing a decoding table in which the arrangement of the dots generated in the block according to the encoding value is recorded in association with the encoding value.
The image processing apparatus, wherein the decoding unit determines the dot arrangement based on the encoded value obtained by the encoding unit and the discriminated block identifier with reference to the decoding table. The image processing apparatus according to claim 15, wherein
The decoding table is
For each block identifier, a predecode table representing a correspondence relationship between the encoded value and an index value representing the number of dots generated in the block;
A post-decoding table representing the correspondence between the index value and the number of dots generated in the block;
An order value table representing the correspondence between the number of dots and the position and order of dots generated in the block;
The image processing apparatus, wherein the decoding unit determines the dot arrangement according to the encoded value and the block identifier by sequentially referring to the predecode table, the postdecode table, and the order value table. The image processing apparatus according to claim 16,
The decoding table storage means represents each encoded value with a predetermined number of bits, and the predecoding table is a product of an encoded value less than the maximum encoded value that can be expressed with the number of bits and the number of blocks. An image processing device that stores data with a capacity equal to or less than the data capacity. An image processing method for determining an arrangement of dots to be formed on a print medium when printing image data based on a dither matrix in which a threshold value for determining whether or not to generate dots is recorded according to a gradation value of image data Because
The dither matrix is divided into blocks each composed of a predetermined number of threshold values, and the gradation values are sequentially set to the threshold values in a range that the gradation values can take for each block identifier uniquely assigned to each block. An encoding table storage step of storing an encoding table in which each gradation value that generates each encoding value is recorded in association with each encoding value representing the arrangement of dots in the block obtained by comparison;
An image data input step for inputting the image data;
Based on the position of each pixel constituting the input image data and the position of each block existing in the dither matrix, the block identifier corresponding to each pixel is discriminated, and the block identifier and the level of each pixel are distinguished. An encoding step of referring to the encoding table based on a tone value and obtaining the encoding value corresponding to a gradation value of each pixel;
A decoding step of determining the arrangement of dots generated in the block corresponding to the position of each pixel according to the encoded value obtained by the encoding means and the distinguished block identifier;
In the encoding table storing step, each encoding value is expressed by a predetermined number of bits, and the encoding table is calculated by multiplying an encoding value less than a maximum encoding value that can be expressed by the number of bits by the number of blocks. An image processing method for storing data with a capacity less than the corresponding data capacity. A printing apparatus that performs printing by determining the arrangement of dots to be formed on a printing medium based on a dither matrix in which thresholds for determining whether or not to generate dots according to the gradation value of image data are recorded. And
The dither matrix is divided into blocks each composed of a predetermined number of threshold values, and the gradation values are sequentially set to the threshold values in a range that the gradation values can take for each block identifier uniquely assigned to each block. Encoding table storage means for storing an encoding table in which each gradation value that generates each encoding value is recorded in association with each encoding value representing the arrangement of dots in the block obtained by comparison;
Image data input means for inputting the image data;
Based on the position of each pixel constituting the input image data and the position of each block existing in the dither matrix, the block identifier corresponding to each pixel is discriminated, and the block identifier and the level of each pixel are distinguished. Encoding means for obtaining the encoding value corresponding to the gradation value of each pixel by referring to the encoding table based on the tone value;
Decoding means for determining the arrangement of dots generated in the block corresponding to the position of each pixel, in accordance with the encoded value obtained by the encoding means and the distinguished block identifier;
Dot forming means for forming the dots on the print medium based on the determined dot arrangement;
The encoding table storage means represents each encoding value with a predetermined number of bits, and the encoding table is a product of an encoding value less than the maximum encoding value that can be expressed with the number of bits and the number of blocks. A printing device that stores data with a capacity less than the corresponding data capacity. A computer program that determines the arrangement of dots to be formed on a print medium when printing image data based on a dither matrix in which thresholds for determining whether or not dots are generated according to the tone value of the image data are recorded. There,
The dither matrix is divided into blocks each composed of a predetermined number of threshold values, and the gradation values are sequentially set to the threshold values in a range that the gradation values can take for each block identifier uniquely assigned to each block. An encoding table storage function for storing an encoding table in which each gradation value that generates each encoding value is recorded in association with each encoding value representing the arrangement of dots in the block obtained by comparison;
An image data input function for inputting the image data;
Based on the position of each pixel constituting the input image data and the position of each block existing in the dither matrix, the block identifier corresponding to each pixel is discriminated, and the block identifier and the level of each pixel are distinguished. An encoding function for obtaining the encoding value corresponding to the gradation value of each pixel by referring to the encoding table based on the tone value;
A computer program that causes a computer to realize a decoding function that determines the arrangement of dots generated in a block corresponding to the position of each pixel according to the encoded value obtained by the encoding means and the distinguished block identifier And
The encoding table storage function represents each encoding value by a predetermined number of bits, and the encoding table is calculated by multiplying an encoding value less than the maximum encoding value that can be expressed by the number of bits by the number of blocks. A computer program that stores data with a capacity that is less than the corresponding data capacity.   A computer-readable recording medium on which the computer program according to claim 20 is recorded.

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