JP2005252710A - Image processing apparatus, image processing method, and image processing program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing apparatus capable of improving image quality, while suppressing the decrease in the processing speed. <P>SOLUTION: Whenever each pixel, constituting image data, is converted into data of an expression form depending on the presence or the absence of dot formation, an error diffuser 53 calculates the error value to be diffused from a target pixel to a plurality of pixels, on the basis of the error occurring in the target pixel to which conversion is executed. An error buffer 54 stores an accumulated value in which the error values calculated by the error diffuser 53 are accumulated, regarding each pixel constituting the image data. An edge-determining part 51 determines whether an edge exists between the target pixel and the pixels around the target pixel. A dot-determining part 52 determines the presence or the absence of the dot formation of the target pixel on the basis of the correction data calculated, by using the accumulated value, corresponding to the determination results of the edge-determining part 51 and the gradation value of the target pixel. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image processing apparatus, an image processing method, and an image processing program.

  2. Description of the Related Art Printing apparatuses that form ink dots on a printing medium and print images are widely used as output apparatuses for images formed by computers and images taken by digital cameras. In these printing apparatuses such as an ink jet printer, only the state of whether or not to form dots can be expressed locally, but by controlling the dot formation density appropriately, the gradation is continuous. Images that change over time can be printed. For this reason, in order to print multi-tone image data, it is necessary to control dot formation so that dots are formed at an appropriate density according to the density of the image.

  There is a technique called an error diffusion method as a typical technique for determining the presence or absence of dot formation so that an appropriate dot formation density according to the image density can be obtained. In the error diffusion method, an error caused by the presence / absence of dot formation in a target pixel is diffused and stored in unprocessed pixels around the target pixel, and when determining the presence / absence of dot formation in the unprocessed pixel, This is a method for determining the presence or absence of dot formation so as to eliminate errors diffused from pixels. According to such an error diffusion method, it is possible to reduce the overall error by distributing the error generated when forming the dot of the target pixel to the pixels to be processed thereafter.

  However, when processing is performed by the error diffusion method, there are cases where the dot representation of the portion where the gradation value changes rapidly in the image data becomes unnatural. For example, if an error that occurs in a low gradation region where the dot representation is sparse is diffused to a high gradation region where the dot representation is dense, dot omission occurs at the rising portion of the high gradation region or image distortion occurs. May occur.

  Therefore, various methods have been proposed in order to eliminate the adverse effects of such an error diffusion method. For example, in Patent Document 1, when there is no edge around the pixel of interest to be converted, the threshold for determining the presence or absence of dot formation of the pixel of interest is changed according to the gradation value of the pixel of interest, and the edge exists. In part, an image processing apparatus that fixes a threshold value has been proposed. Thereby, in the high gradation region and the low gradation region, error diffusion in each region can be performed gently, and the edge portion between both regions can be expressed sharply.

Patent Document 2 proposes a configuration in which the error diffusion range is changed according to the gradation value of the pixel to be converted and the presence or absence of dot formation of the pixel. For example, when the tone value of the pixel of interest is small and dots are formed, the error diffusion range is widened, and in other cases, the error diffusion range is narrowed. As a result, when a large negative error occurs in the pixel of interest, it is possible to suppress the formation of dots in a concentrated manner by widening the diffusion range, and to display the boundary between the shades of the image well.
JP-A-8-242374 JP 11-261818 A

  However, if the threshold value is changed according to the gradation value of each pixel constituting the image data and the presence / absence of an edge, or the error diffusion matrix is changed according to the gradation value of each pixel and the presence / absence of dot formation, It takes time to perform error diffusion processing of pixels. In either case, it is necessary to select an optimal threshold value and error diffusion matrix according to the density of pixels and the density gradient between surrounding pixels. However, in order to obtain high image quality, the processing becomes complicated and takes time. In some cases, it was not always possible to obtain good image quality.

  SUMMARY An advantage of some aspects of the invention is that it provides an image processing apparatus, an image processing method, and an image processing program capable of improving image quality while suppressing a decrease in processing speed.

  In order to achieve the above object, the present invention provides an image processing apparatus for converting multi-tone image data composed of a plurality of pixels into data in an expression format depending on the presence or absence of dot formation. An error diffusion unit that calculates an error value to be diffused from the target pixel to a plurality of pixels based on an error that occurs in the target pixel that performs the conversion each time the pixel is converted into data in an expression format based on the presence or absence of dot formation; For each pixel constituting the image data, there is an edge between a diffusion error storage unit that stores a cumulative value obtained by accumulating error values calculated by the error diffusion unit, and the pixel of interest and pixels around the pixel of interest. Based on the edge determination unit that determines whether or not it exists, the correction data calculated using the cumulative value according to the determination result of the edge determination unit, and the gradation value of the pixel of interest And determining the dot determination section whether or not to form dots eye pixels, further comprising a a gist.

  Further, an image processing method of the present invention corresponding to the image processing apparatus described above is an image processing method for converting multi-gradation image data composed of a plurality of pixels into data in an expression format depending on the presence or absence of dot formation. Each time each pixel constituting the image data is converted into data in an expression format based on the presence or absence of dot formation, an error value for diffusing from the target pixel to a plurality of pixels is calculated based on an error generated in the target pixel that performs the conversion. An error diffusion step, a diffusion error storage step for storing a cumulative value in which the error value calculated by the error diffusion step is accumulated for each pixel constituting the image data, and the pixel of interest and the periphery of the pixel of interest An edge determination step for determining whether an edge exists between the pixel and a cumulative value of the pixel of interest according to a determination result of the edge determination step There a correction data was calculated, based on the tone value of the noted pixel, it is summarized in that with a dot judgment step of judging whether or not to form dots of the target pixel.

  According to the above-described image processing apparatus and image processing method, the image is added to the gradation value of the target pixel when performing dot determination of the target pixel depending on whether or not an edge exists around the target pixel to be converted. The correction data value to be changed is changed. For this reason, when there is an edge around the pixel of interest, the error value diffused from the area having a large difference in gradation value is not used in the correction data so that the dot formation of the pixel of interest is not caused. It can suppress becoming natural. At the same time, since an error diffused from a region having a large difference in gradation value is not added to an error value generated when forming a dot of the target pixel, an unnatural error value is diffused from the target pixel to surrounding pixels. Can be suppressed. When there is no edge around the pixel of interest, smooth dot expression is obtained by performing dot determination using the accumulated value in which the error value diffused from the surrounding pixels is accumulated in the pixel of interest. Can do. Further, since the image quality can be improved only by adjusting the correction data calculated using the cumulative value of the pixel of interest, the processing can be performed in a relatively short time. Therefore, it is possible to improve the image quality while suppressing a decrease in processing speed.

  In the above-described image processing apparatus, when the edge determination unit determines that no edge exists, the dot determination unit adds the accumulated value of the pixel of interest to the correction data, and the edge determination unit If it is determined that the pixel is present, a value smaller than the accumulated value of the target pixel may be added to the correction data.

  According to this, when there is no edge around the pixel of interest, it is possible to obtain a dot representation that reflects an error diffused from the surrounding pixels. In addition, when there is an edge, an area having a large difference in gradation value is obtained by increasing the specific gravity of the gradation value of the pixel of interest by decreasing the ratio of the correction data in the data used for dot determination. Thus, the influence of the error value diffused from the dot can be reduced, and the dot expression with the emphasized edge can be obtained.

  In the image processing apparatus described above, the diffusion error storage unit includes a horizontal buffer for storing a cumulative value in which error values diffused in the horizontal direction of the image data are accumulated, and a vertical direction of the image data. A vertical direction buffer for storing a cumulative value in which error values to be diffused are accumulated, and the edge determination unit determines at least one of the horizontal direction and the vertical direction with respect to the target pixel. It is determined whether or not there is an edge in the direction, and when the dot determination unit determines that no edge exists in the determination direction, the diffusion error storage unit is included in the correction data. When the edge determination unit determines that an edge exists, a buffer for that direction is added to the correction data. The stored cumulative value of the target pixel may be not added.

  According to this, it is determined whether or not an edge exists in at least one of the determination directions of the horizontal direction and the vertical direction with respect to the target pixel, and the accumulation stored in the buffer for the direction according to the determination result Whether the value is added to the correction data is changed. For this reason, it is possible to realize dot expression in which edges are emphasized in the determination direction, change correction data relatively easily, and suppress a decrease in processing speed. Note that the vertical direction with respect to the pixel of interest includes a vertical direction and an oblique direction with respect to the pixel of interest, and a direction that is not horizontal with respect to the pixel of interest.

  Furthermore, in the above-described image processing apparatus, the diffusion error storage unit includes a horizontal buffer for storing a cumulative value in which error values diffused in the horizontal direction of the image data are accumulated, and a vertical direction of the image data. A vertical direction buffer for storing a cumulative value in which error values to be diffused are accumulated, and whether the edge determination unit has an edge in the horizontal direction or the vertical direction with respect to the target pixel. When the edge determination unit determines that no edge exists in the horizontal direction and the vertical direction, the dot determination unit stores the correction data in the buffer for that direction. When the accumulated value of the target pixel is added and the edge determination unit determines that an edge exists, the correction data stored in the buffer for the direction is stored in the correction data. It may not adding the accumulated value of eye pixels.

  According to this, it is determined whether or not there is an edge in the horizontal direction and the vertical direction with respect to the pixel of interest, and the cumulative value stored in the buffer for that direction is added to the correction data according to the determination result. No or not changed. For this reason, it is possible to realize dot expression in which edges are emphasized in the horizontal direction and the vertical direction of image data, and it is possible to change correction data relatively easily, and to suppress a decrease in processing speed. Note that the vertical direction with respect to the pixel of interest includes a vertical direction and an oblique direction with respect to the pixel of interest, and a direction that is not horizontal with respect to the pixel of interest.

In the above-described image processing device, the edge determination unit may determine whether an edge exists between the target pixel and the processed pixel.
According to this, when the region of interest has a large difference in gradation value, more appropriate dot determination can be performed on the pixel of interest, and the error value diffused from the pixel of interest can be set to a more appropriate value. .

Furthermore, in the above-described image processing apparatus, the edge determination unit may determine whether an edge exists between the target pixel and an unprocessed pixel.
According to this, in the case where an unprocessed pixel is a region having a large difference in gradation value with respect to the target pixel, an error value diffused from the target pixel to the region can be set to a more appropriate value.

  In the image processing apparatus described above, the edge determination unit selects whether to perform edge determination between the target pixel and the processed pixel, or to perform edge determination between the target pixel and the unprocessed pixel. It may be possible.

  According to this, by selecting whether the pixel of interest and the edge determination pixel are the processed pixels or the unprocessed pixels, the image quality and the processing speed more suitable for the image data to be converted can be obtained.

  Further, in the above-described image processing apparatus, the edge determination unit has edges in the horizontal direction and the vertical direction between the target pixel and the processed pixel and between the target pixel and the unprocessed pixel. It is good also as determining whether to do.

  According to this, even if the target pixel and the pixel whose edge is determined are the processed pixel and the unprocessed pixel, the target pixel is not processed even when the target pixel becomes a region having a large difference in gradation value. Since dot determination and error diffusion are performed in consideration of the case where a pixel has a large difference in tone value, image quality can be improved.

  The image processing program of the present invention that realizes the above-described image processing method is such that the control unit performs processing for converting multi-gradation image data composed of a plurality of pixels into data in an expression format depending on the presence or absence of dot formation. This is an image processing program to be executed, and each time the control unit converts each pixel constituting the image data into data in an expression format based on the presence or absence of dot formation, an error that occurs in the target pixel that performs the conversion An error diffusion process for calculating an error value to be diffused from the pixel of interest to a plurality of pixels based on the same, and a cumulative value in which the error value calculated by the error diffusion process is accumulated for each pixel constituting the image data A diffusion error storage process, an edge determination process for determining whether or not an edge exists between the target pixel and pixels around the target pixel, and a determination of the edge determination process In order to execute a dot determination process for determining whether or not a dot is formed on the target pixel based on correction data calculated using the accumulated value of the target pixel according to a result and a gradation value of the target pixel belongs to.

  According to this, if the various functions described above are realized, dot determination that reflects the edges around the pixel of interest is made in a relatively simple manner, and image quality is improved while suppressing a decrease in processing speed. Can do.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the drawings.
(Device configuration)
FIG. 1 is a diagram illustrating an appearance of a printer as an image processing apparatus according to the present embodiment. 1A is a perspective view of the printer as viewed from the front, and FIG. 1B is a schematic side view of the printer.

  The printer 1 is an ink jet printer provided with a control panel 3 functioning as a user interface (UI) on the upper right side of the main body 2. By operating the control panel 3, the user operates various instructions such as a print instruction. Can be provided to the printer 1. A paper supply device 4 is provided on the back side of the main body 2, and the paper set in the sheet feeder 4 a or the roll paper set in the roll paper support portion 4 b is fed into the main body 2 (hereinafter, referred to as “sheet feeding device”). In this embodiment, a print medium such as paper or roll paper is simply referred to as “paper”). A printing mechanism (not shown) is provided below the cover 5 provided at the center of the main body 2, and a sheet 6 printed by the operation of the printing mechanism is discharged at the lower front portion of the main body 2. It is discharged from the mouth 7.

  The control panel 3 includes a display panel 8 and an operation panel 9. The display panel 8 includes a display screen 8a such as a liquid crystal monitor, and the display screen 8a is, for example, various message screens such as a screen for setting a paper size, a paper type, a layout, a photo selection, a number of copies, and the like. Is displayed. The operation panel 9 includes a plurality of operation switches such as a power switch 9a and a print start switch 9b.

  On the front right side of the main body 2 of the printer 1 is provided a slot 11 in which a memory card 10 such as a storage medium can be mounted. An image read from the memory card 10 is displayed on the upper portion of the main body 2 of the printer 1. A display device 12 is provided for display.

  As shown in FIG. 1B, USB ports 21 and 22 are provided on the lower side of the main body 2 of the printer 1. The USB port 21 connects, for example, the printer 1 and a personal computer (hereinafter referred to as “PC”) 23 as a host device via a cable 24.

  In this case, the printer 1 functions as a peripheral device of the PC 23, and the user operates the PC 23 and stores it in a storage medium (for example, MO disk) inserted into an external storage device (for example, MO) (not shown) connected to the PC 23. The printer 1 can be instructed to print the image that has been displayed. On the other hand, the USB port 22 connects, for example, the printer 1 and the digital camera 25 via a cable 26.

(Control part)
FIG. 2 is a block diagram illustrating the configuration of the printer 1. The same components as those in FIG. 1 will be described with the same reference numerals.

  The printer 1 of this embodiment functions as a peripheral device of the PC 23 when connected to the PC 23, and functions as a stand-alone printer when not connected to the PC 23. In other words, the printer 1 is not connected to the PC 23 but can be connected to the digital camera 25 to perform printing (so-called direct printing). For example, the printer 1 is connected to the digital camera 25 via a USB interface. In this case, the user is a USB host and the counterpart device is a USB device.

The printer 1 includes a first RAM 31, a ROM 32, a second RAM 33, a paper detection sensor 34, a printer head 35, and a CPU built-in ASIC 36.
The first RAM 31 stores various storage media and data (image file etc.) read directly from the digital camera 25. The ROM 32 stores programs necessary for the CPU 41 to be described later to execute processing and various application programs. In the present embodiment, the ROM 32 also stores an image processing program for the CPU 41 to execute halftone processing described later.

  The second RAM 33 is used as a working memory when the CPU 41 executes processing. For example, the second RAM 33 is used to store a print job received from the digital camera 25, or temporarily stores data (print image data or the like) generated at the later-described image processing stage. Or used as a buffer for

  The paper detection sensor 34 detects the size of the paper fed into the printer 1 (hereinafter referred to as “paper size”) and the type of paper such as plain paper or dedicated paper (hereinafter referred to as “paper type”) by an optical method or the like. To detect.

  The printer head 35 is provided at the lower end of a carriage (not shown) that reciprocates in the main scanning direction (direction perpendicular to the paper feed direction), and is provided on the surface facing the paper (print medium) conveyance path for each color. It has many nozzles. The printer head 35 ejects ink droplets of each color from the nozzles corresponding to the respective colors while reciprocating in the main scanning direction based on the control of the head control circuit 42 described later. An image is formed on. In the present embodiment, the main scanning direction of the printer head 35 is the horizontal direction in the image data, and the paper feed direction (sub-scanning direction) is the vertical direction in the image data.

  The CPU built-in ASIC 36 includes a CPU 41 as a control unit, a head control circuit 42, an image processing circuit 43, a memory interface circuit (hereinafter referred to as “memory I / F”) 44, a USB host circuit 45, a host interface circuit (hereinafter referred to as “host I / F”). F ”) 46 and a card interface circuit (hereinafter referred to as“ card I / F ”) 47, which are connected to each other via an internal bus 48. The internal bus 48 is also connected to the first RAM 31, ROM 32, paper detection sensor 34, and control panel 3 described above.

  The CPU 41 reads out a program stored in the ROM 32 and executes a process according to the program, thereby comprehensively controlling the printer 1. For example, the CPU 41 controls color conversion processing, halftone processing, and microweave processing in an image processing circuit 43 described later, and carriage motor drive control in the head control circuit 42.

  The image processing circuit 43 has functions as a color conversion unit, a halftoning unit, and a microweave unit. More specifically, the color conversion unit performs color conversion processing on RGB multi-gradation image data stored in the first RAM 31, for example, cyan (C), magenta (M), yellow (Y), Multi-tone image data of seven colors of black (K), light cyan (LC), light magenta (LM), and light yellow (LY) is generated and stored in the second RAM 33.

  The halftoning unit performs halftone processing on the image data after the color conversion processing, generates print image data (color plane) that represents multi-tone image data by binarization processing for each color, It is stored in the second RAM 33. That is, the image data obtained by the color conversion process is data having 256 gradations that take values from gradation value 0 to gradation value 255, but the printer 1 determines whether or not to form dots on the paper 6. Only the state can be taken. Therefore, the halftoning unit converts image data having 256 gradations into data indicated by the presence or absence of dot formation.

  The microweave unit rearranges the raster of each color plane after the halftone process based on the number of nozzles, the number of head scans, the paper feed amount, etc., generates print data, and stores it in the second RAM 33.

  The head control circuit 42 controls driving of a carriage motor that reciprocates the printer head 35, and whether or not ink droplets are ejected based on print data of each color generated by the image processing circuit 43, and ejected ink droplets. The amount of control is controlled.

  The USB host circuit 45 is connected to a digital camera 25 or the like serving as a USB device via the cable 26, and performs bidirectional data communication with the digital camera 25 in accordance with the USB standard based on the control of the CPU 41. Image data to be printed is acquired from the digital camera 25.

  The host I / F 46 is connected to the PC 23 via the cable 24, and performs bidirectional data communication with the PC 23 based on the control of the CPU 41, and acquires image data to be printed from the PC 23, for example.

  The card I / F 47 functions as an interface circuit when the memory card 10 is inserted into the slot 11 and converts the image data stored in the memory card 10 into data that can be processed inside the printer 1 under the control of the CPU 41. To do.

(Overview of error diffusion processing)
Next, an error diffusion process executed by the halftoning unit included in the image processing circuit 43 described above will be described.

  The halftoning unit 50 shown in FIG. 3 converts each pixel constituting the image data into data in an expression format depending on whether or not dots are formed. The halftoning unit 50 includes an edge determination unit 51 that determines whether or not there is an edge around the pixel of interest that performs this conversion, a dot determination unit 52 that determines whether or not a pixel of interest has been formed, An error diffusion unit 53 that calculates an error value to be diffused to the plurality of pixels. The accumulated value obtained by accumulating the error values calculated by the error diffusion unit 53 for each pixel constituting the image data is stored in an error buffer 54 as a diffusion error storage unit (diffusion error storage step).

  The error buffer 54 stores a horizontal buffer 54a that stores a cumulative value dx in which only the error value diffused in the horizontal direction is accumulated in each pixel, and only an error value diffused in the vertical direction in each pixel. A vertical buffer 54b for storing the accumulated value dy. The error buffer 54 is configured by using a partial area of the second RAM 33.

  The edge determination unit 51 determines whether or not an edge exists between the pixel of interest for which conversion processing is performed and pixels around the pixel of interest (edge determination step). For example, the gradation value d * of the pixel of interest indicated by “*” in FIG. 4 is compared with the gradation values of the pixels around the pixel of interest indicated by hatching in FIG. If the difference is larger than the predetermined threshold th1, it is determined that an edge is present. If the difference is equal to or smaller than the predetermined threshold th1, it is determined that there is no edge, and the result is stored in the second RAM 33.

  As shown in FIG. 4, the edge of the target pixel in the horizontal direction can be determined by comparing the target pixel with a pixel adjacent to the left or right of the target pixel. When compared with the pixel on the left, the edge between the processed pixels is determined, and when compared with the pixel on the right, the edge between the unprocessed pixels is determined. Further, by comparing the pixel of interest with a pixel adjacent to either the upper or lower side, the edge determination in the vertical direction of the pixel of interest can be performed. When compared with the pixel immediately above, the edge between the processed pixels is determined, and when compared with the pixel immediately below, the edge between the unprocessed pixels is determined.

  In this embodiment, it is determined whether or not there is an edge in the horizontal direction and the vertical direction with respect to the target pixel, and the horizontal edge determination is performed between the target pixel and the left adjacent pixel, Vertical edge determination between the pixel of interest and the pixel immediately below is performed.

  The dot determination unit 52 determines the presence / absence of dot formation of the target pixel based on the correction data dc corresponding to the determination result of the edge determination unit 51 and the gradation value d * of the target pixel (dot determination step). The correction data dc is calculated using the accumulated value dx stored in the horizontal buffer 54a and the accumulated value dy stored in the vertical buffer 54b.

  When the edge determination unit 51 determines that there is no edge in the horizontal direction of the pixel of interest, the dot determination unit 52 reads the cumulative value dx of the pixel of interest stored in the horizontal buffer 54a and adds it to the correction data dc To do. If the edge determination unit 51 determines that no edge exists in the vertical direction of the pixel of interest, the cumulative value dy of the pixel of interest stored in the vertical buffer 54b is read and added to the correction data dc. Thus, the dot determination unit 52 calculates the correction data dc.

  Then, the dot determination unit 52 adds the correction data dc and the gradation value d * of the pixel of interest, and if the value is larger than the predetermined threshold th2, it determines that a dot is to be formed on the pixel of interest and binarizes it. “1” data is input to the second RAM 33 as data. On the contrary, if the value is equal to or less than the predetermined threshold th2, it is determined that no dot is formed in the pixel of interest, and “0” data is input to the second RAM 33 as binarized data. Note that the value of the threshold th2 at this time is the gradation value expressed in the pixel in which the dot is formed (for example, the gradation value is “255”) and the gradation value expressed in the pixel in which the dot is not formed (for example, the gradation value). The tone value is set to an intermediate tone value from “0”). The threshold th2 is preferably set to a value that can improve image quality based on the gradation value of the pixels constituting the image data.

  According to this, the value of the correction data dc added to the gradation value d * of the target pixel at the time of dot determination of the target pixel depends on whether or not there is an edge around the target pixel to be converted. Be changed. Specifically, for example, if there is no edge in the horizontal direction or the vertical direction of the pixel of interest, the correction data dc is “dx + dy”, and if there is an edge in the horizontal direction and the vertical direction, it is “0”. . Further, if there is an edge only in the horizontal direction, it becomes “dy”, and if it exists only in the vertical direction, it becomes “dx”. As described above, when there is an edge around the pixel of interest, data that has been diffused from a region having a large difference in gradation value is not used as the correction data dc, thereby preventing dot formation of the pixel of interest. It can suppress becoming natural. In addition, when there is no edge around the pixel of interest, smooth dot expression is obtained by performing dot determination using the accumulated values dx and dy of error values diffused from the surrounding pixels to the pixel of interest. Can do.

  The dot determination unit 52 adjusts the correction data dc only by reading the accumulated error values dx and dy diffused in the pixel of interest to improve the image quality. Compared with the case where the threshold value th2 for dot determination is changed according to the value d * or the like, it is possible to determine the presence or absence of dot formation in a relatively short time.

  The error diffusion unit 53 calculates an error value to be diffused from the target pixel to a plurality of pixels based on the error E * generated in the target pixel when the dot determination unit 52 determines the presence / absence of dot formation of the target pixel. Diffusion step).

  If the gradation value expressed by the pixel of interest is “255” when the error diffusion unit 53 forms a dot (when the binarized data in the dot determination unit 52 is “1”), the error E * Is assumed to be “d * + dc-255”. If the gradation value expressed by the pixel of interest is “0” when no dot is formed (when the binarized data in the dot determination unit 52 is “0”), the error E * is set to “d”. * + Dc-0 ”.

  As described above, the error value diffused from the region having a large difference in the gradation value to the error E * generated when the dot of the pixel of interest is formed (for example, the accumulated value dx when the edge exists in the horizontal direction). Therefore, it is possible to suppress the diffusion of an unnatural error from the target pixel to surrounding pixels.

  The error diffusion unit 53 calculates an error value for diffusing the calculated error E * from the pixel of interest according to a predetermined error diffusion matrix as shown in FIG. FIG. 5A shows the weight of the error E * diffused from the target pixel indicated by “*”. For example, the error E * is diffused with a weight of 4/16 to the pixel right next to the pixel of interest, and the error E * is diffused with a weight of 2/16 for the pixel immediately under the pixel of interest.

  Thus, the error value calculated by the error diffusion unit 53 is stored in the error buffer 54 by the CPU 41 calculating a cumulative value for each pixel each time. In the present embodiment, a cumulative value dx in which error values diffused in the horizontal direction of the pixel of interest are accumulated is stored in the horizontal buffer 54a, and an accumulation of error values diffused in the vertical direction of the pixel of interest is accumulated. The value dy is stored in the vertical buffer 54b.

  On the other hand, when error diffusion is performed using the error diffusion matrix shown in FIG. 5A, the target pixel has an error value from the pixels indicated by a to g in FIG. 5B (hereinafter referred to as “pixels a to g”). Has been spread. For the target pixel indicated by “*”, the accumulated value dy of the error value diffused from the pixels a to c is stored in the vertical buffer 54b, and the error value diffused from the pixels d to g. Is stored in the horizontal buffer 54a. In the horizontal buffer 54a and the vertical buffer 54b, accumulated values dx and dy in which error values are accumulated in this way are stored for each pixel constituting the image data.

Here, the error diffusion of the edge portion in the present embodiment will be specifically described.
In the pixel of interest indicated by “*” in FIG. 6A, error values are diffused from the pixels a to c in the vertical direction, and errors diffused from these pixels in the vertical buffer 54b. A cumulative value dy of values is stored. Further, in the target pixel, error values are diffused from the pixels d to g in the horizontal direction, and the accumulated value dx of error values diffused from these pixels is stored in the horizontal buffer 54a. . Then, according to the error diffusion matrix shown in FIG. 5A, errors are diffused from the target pixel to the four pixels d ′ to g ′ in the right direction and to the three pixels a ′ to c ′ in the lower direction in the vertical direction. Is done.

  As illustrated in FIG. 6B, when an edge exists between the target pixel indicated by “* 1” and the pixel adjacent to the left of the target pixel, the edge determination unit 51 determines that an edge exists in the horizontal direction. Then, according to the determination result, the dot determination unit 52 reads the accumulated error value dy from only the vertical buffer 54b, and performs dot determination using the correction data dc as the accumulated value dy. As described above, since the error value diffused from the pixels d to g existing in the region having a large gradation value difference from the target pixel is not used for the dot determination of the target pixel, the dot determination of the target pixel becomes unnatural. Can be suppressed.

  In addition, when calculating the error value to be diffused from the pixel of interest, the error value diffused from the pixels d to g is not taken into account, and therefore, the error value from the region of interest including an error value from a region having a greatly different gradation value is included. It is possible to suppress the diffusion, and it is possible to suppress an unnatural dot expression of a pixel converted after the pixel of interest.

  As described above, when performing edge determination between the target pixel and the processed pixel, if the target pixel is a region having a large difference in gradation value, more appropriate dot determination is performed on the target pixel, and the target pixel is determined. The error value to be diffused from can be set to a more appropriate value.

  On the other hand, as shown in FIG. 6C, when an edge exists between the target pixel indicated by “* 2” and the pixel directly below the target pixel, the edge determination unit 51 determines that an edge exists in the vertical direction. . Then, according to the determination result, the dot determination unit 52 reads the accumulated error value dx from only the horizontal buffer 54a, and performs dot determination using the correction data dc as the accumulated value dx.

  After that, when calculating the error value to be diffused from the pixel of interest, the error value diffused from the pixels a to c is not taken into consideration, and therefore, the error value from the pixel of interest including the error value from the region having greatly different gradation values is included. It is possible to suppress the diffusion, and it is possible to suppress an unnatural dot expression of a pixel converted after the pixel of interest.

  As described above, when edge determination is performed between the target pixel and the unprocessed pixel, if the target pixel and the unprocessed pixel are in a region having a large difference in gradation value, the target pixel is diffused to the region. The error value can be a more appropriate value.

  Therefore, the edge determination is performed between the target pixel and the processed pixel in the horizontal direction, and the edge determination is performed between the target pixel and the unprocessed pixel in the vertical direction. Even when an edge exists, a case where an edge exists between a pixel of interest and an unprocessed pixel is reflected, and a dot expression with a good balance can be achieved in the entire image data.

Next, the flow of error diffusion processing executed by the image processing circuit 43 (halftoning unit 50) will be described.
As shown in FIG. 7, first, the halftoning unit 50 reads a raster of image data (step 100). At this time, in this embodiment, since error diffusion is performed using the error diffusion matrix shown in FIG. 5A, two rasters are read.

  Then, the edge determination unit 51 reads raster pixels (step 110). Next, it is determined whether or not an edge exists between the pixel (target pixel) and a pixel adjacent to the left of the target pixel (step 120). If it is determined that there is no edge between the pixel on the left (NO in step 120), the dot determination unit 52 reads the accumulated value dx from the horizontal buffer 54a and adds it to the correction data dc (step 130). ).

  If it is determined that an edge exists (step 120, YES), then the edge determination unit 51 determines whether an edge exists between the pixel of interest and the pixel immediately below the pixel of interest (step) 140). If it is determined that there is no edge between the pixel immediately below (step 140, NO), the dot determination unit 52 reads the accumulated value dy from the vertical buffer 54b and adds it to the correction data dc (step 150). .

  If it is determined that there is an edge (step 140, YES), then the dot determination unit 52 performs dot determination for the pixel of interest (step 160). Thereafter, the error diffusion unit 53 calculates an error value to be diffused from the pixel of interest (step 170). At this time, the error value diffused in the horizontal direction from the pixel of interest is added to the accumulated value dx stored in the horizontal buffer 54a for each pixel. The error value diffused in the vertical direction is added to the accumulated value dy stored in the vertical buffer 54b for each pixel.

  Next, it is determined whether the pixel of interest is the rightmost pixel in the raster, that is, whether or not conversion processing has been performed for all the pixels constituting the raster being processed (step 180), and if it is not the rightmost pixel (step 180, NO). ), Returning to step 110 and performing a series of subsequent processes.

  If it is the rightmost pixel in the raster (step 180, YES), it is determined whether the raster is at the lower end, that is, whether all rasters constituting the image data have been processed (step 190). If it remains (NO in step 190), the process returns to step 100 and a series of processes are performed. Here, if it is determined that the raster is the lower end raster (step 190, YES), the series of processing ends.

Actually, the image data was printed by the printer 1 described above.
As shown in FIG. 8A, the upper half of the image data is an area with a high gradation value (area where the dot formation is dense), and the lower half is an area with a low gradation value (dot formation). Is a sparse area). FIG. 8B shows an output image by the printer 1 of the present embodiment. FIG. 8C shows an output image by a conventional printer that performs error diffusion processing without performing edge determination on the pixels constituting the image data in the halftoning unit.

  According to a conventional printer, in the lower region, a portion where no dot is formed occurs at a boundary portion (edge portion) with the upper region. That is, a dot formation delay occurred at the rising portion of the lower region.

  However, according to the printer 1 according to the present invention, dots were well formed at the edge portion in the lower region. That is, it was confirmed that the delay of the dot formation rise at the rising portion of the lower region is suppressed.

As described above, according to the present embodiment, the following effects can be obtained.
(1) The correction data dc to be added to the gradation value d * of the target pixel is changed in determining the dot of the target pixel depending on whether or not an edge exists around the target pixel. For this reason, in the case where there is an edge, the error value diffused from the region having greatly different gradation values is not added to the correction data dc so that the dot formation of the pixel of interest becomes unnatural. Can be suppressed. In addition, the error value diffused from the region with greatly different gradation values is not taken into account in the error value diffused from the pixel of interest, so that it is possible to suppress unnatural dot formation for pixels processed after the pixel of interest. can do.

  (2) The error buffer 54 includes a horizontal buffer 54a and a vertical buffer 54b. In the horizontal buffer 54a, an accumulated value dx of error values diffused in the horizontal direction is used for the vertical direction. The buffer 54b stores a cumulative value dy of error values diffused in the vertical direction. For this reason, it is possible to perform edge determination in the horizontal direction and vertical direction of the image data, and to calculate the correction data dc using the accumulated values dx and dy according to the result. Accordingly, the correction data dc is adjusted by taking into account the presence of edges in the horizontal and vertical directions of the image data, the image quality can be improved, and the correction data dc can be changed relatively easily. A reduction in processing speed can be suppressed.

  (3) The edge determination unit 51 performs edge determination between the target pixel and the processed pixel in the horizontal direction, and performs edge determination between the target pixel and the unprocessed pixel in the vertical direction. For this reason, dot determination and error diffusion are performed in consideration of a case where the target pixel is a region having a large difference in gradation value and a case where an unprocessed pixel of the target pixel is a region having a large difference in gradation value. As a result, the image quality can be improved.

Note that the embodiment is not limited to the above, and the following modifications may be employed.
(Modification 1) The edge determination unit 51 may perform edge determination in only one of the horizontal direction and the vertical direction of image data. In this case, if there is an edge, the accumulated error value may not be fetched from the buffer (horizontal buffer 54a or vertical buffer 54b) in the direction in which the edge is determined. As a result, the error diffusion process with the edge in the determination direction is performed, so that the image quality can be improved to some extent, and the conversion process can be speeded up as compared with the case where the edge is added in both directions.

(Modification 2) When the edge determination unit 51 determines an edge, the threshold th1 for comparing with the difference between the gradation value of the pixel of interest and the gradation value of the pixel around the pixel of interest is set to any value. Good.
(Modification 3) The pixel for which the edge is determined with respect to the target pixel is not limited to the pixel on the left side or the pixel directly below the target pixel. For example, as shown in FIG. 9A, the edge between the target pixel indicated by “* 3” and the right adjacent pixel is determined, and if there is an edge between the right adjacent pixel of the target pixel, the pixel d The error value diffused from ~ g may not be added to the dot determination. Further, as shown in FIG. 9B, the edge between the pixel of interest indicated by “* 4” and the pixel immediately above is determined, and if there is an edge between the pixel immediately above the pixel of interest and the pixel a The error value diffused from ~ c may not be added to the dot determination.

(Variation 4) The pixel whose edge is determined for the pixel of interest may be only a processed pixel or only an unprocessed pixel.
(Modification 5) The pixel that performs edge determination on the pixel of interest may be changed to either a processed pixel or an unprocessed pixel. For example, until the target pixel passes the edge, the edge determination is performed with the unprocessed pixel (see FIGS. 6C and 9A), and when the target pixel passes the edge, the edge determination is performed with the processed pixel. This may be performed (see FIG. 6B and FIG. 9B). Then, if there is an edge between the target pixel and the unprocessed pixel, the error diffused from the target pixel to the unprocessed pixel beyond the edge is adjusted, and if the target pixel exceeds the edge, the error is diffused from the processed pixel. The error reading is adjusted. For this reason, the image quality of the edge portion can be improved.

(Modification 6) When the edge around the pixel of interest is determined, the comparison with the pixel of interest does not necessarily have to be one pixel, and the edge may be determined by comparing with a plurality of pixels.
(Modification 7) When the dot determination unit 52 performs dot determination, the threshold th2 for comparison with the value obtained by adding the gradation value d * of the target pixel and the correction data dc may be set to any value. .

(Modification 8) The error diffusion matrix used when the error diffusion unit 53 diffuses an error may have any form.
(Modification 9) The error diffusion matrix used when the error diffusion unit 53 diffuses an error may be changed according to a predetermined condition. For example, when determining an edge between a target pixel and an unprocessed pixel, the error diffusion matrix may be changed so that the error is not diffused in the direction in which the edge exists. In this case, as shown in FIG. 10A, when an edge exists in the horizontal direction with respect to the target pixel indicated by “* 5”, the error is not diffused to the pixels d ′ to g ′. . As shown in FIG. 10B, when an edge exists in the vertical direction with respect to the target pixel indicated by “* 6”, the error is not diffused to the pixels a ′ to c ′.

  (Modification 10) The diffusion error storage unit does not have the horizontal buffer 54a and the vertical buffer 54b, and stores the accumulated error value diffused in each pixel regardless of the direction. Good. In this case, the correction data dc can be calculated, for example, by reducing the cumulative value of error values stored in the diffusion error storage unit to a predetermined ratio.

(Modification 11) The control panel 3 may be operated to determine whether the edge between the target pixel and the processed pixel is determined, or whether the edge between the target pixel and the unprocessed pixel is determined.
(Modification 12) Not only the second RAM 33 but also a predetermined area of the first RAM 31 may be used as the error buffer 54. Then, the memory area of the printer 1 can be used effectively.

  (Modification 13) Although the case where the image processing apparatus according to the present invention is applied to the printer 1 has been described, it may be applied to image processing or the like in a liquid crystal display device.

1A and 1B are schematic views of a printer according to an embodiment of the present invention, in which FIG. 1A is a perspective view of the printer as viewed from the front, and FIG. FIG. 2 is a block diagram illustrating a configuration of a printer. The block diagram which shows the structure of a halftoning part. Explanatory drawing of the surrounding pixel with respect to the focused pixel. (A) is explanatory drawing of an error diffusion matrix, (b) is explanatory drawing of the pixel which diffuses an error to the attention pixel. (A)-(c) is explanatory drawing of the pixel which diffuses an error to the attention pixel, and the pixel which diffuses an error from the attention pixel. 10 is a flowchart for explaining error diffusion processing. (A) is multi-tone image data, and (b) and (c) are output images of (a). (A), (b) is explanatory drawing of a modification. (A), (b) is explanatory drawing of a modification.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Printer as image processing apparatus, 51 ... Edge determination part, 52 ... Dot determination part, 53 ... Error diffusion part, 54 ... Error buffer as diffusion error storage part, 54a ... Horizontal direction buffer, 54b ... Vertical direction Buffer, dc ... correction data, dx ... accumulated value, dy ... accumulated value, E * ... error.

Claims (10)

In an image processing device that converts multi-gradation image data composed of a plurality of pixels into data in an expression format depending on the presence or absence of dot formation,
Each time each pixel constituting the image data is converted into data in an expression format based on the presence or absence of dot formation, an error value for diffusing from the target pixel to a plurality of pixels is calculated based on an error generated in the target pixel that performs the conversion. An error diffusion unit to
For each pixel constituting the image data, a diffusion error storage unit that stores a cumulative value obtained by accumulating error values calculated by the error diffusion unit;
An edge determination unit that determines whether an edge exists between the target pixel and pixels around the target pixel;
A dot determination unit that determines the presence or absence of dot formation of the pixel of interest based on the correction data calculated using the cumulative value according to the determination result of the edge determination unit and the gradation value of the pixel of interest;
An image processing apparatus.   The image processing apparatus according to claim 1, wherein when the edge determination unit determines that no edge exists, the dot determination unit adds a cumulative value of the pixel of interest to the correction data, and the edge determination unit An image processing apparatus that adds a value smaller than a cumulative value of the pixel of interest to the correction data when the determination unit determines that an edge is present. 3. The image processing apparatus according to claim 1, wherein the diffusion error storage unit stores a cumulative value in which an error value diffused in a horizontal direction of the image data is accumulated, and the image data. A vertical buffer for storing a cumulative value in which error values diffused in the vertical direction are accumulated,
The edge determination unit determines whether or not an edge exists in at least one of the horizontal direction and the vertical direction with respect to the target pixel;
The dot determination unit adds the accumulated value of the pixel of interest stored in the diffusion error storage unit to the correction data when the edge determination unit determines that no edge exists in the determination direction. An image processing apparatus that does not add the cumulative value of the pixel of interest stored in the buffer for the direction to the correction data when the edge determination unit determines that an edge exists. 3. The image processing apparatus according to claim 1, wherein the diffusion error storage unit stores a cumulative value in which an error value diffused in a horizontal direction of the image data is accumulated, and the image data. A vertical buffer for storing a cumulative value in which error values diffused in the vertical direction are accumulated,
The edge determination unit determines whether an edge exists in the horizontal direction and the vertical direction with respect to the target pixel,
The dot determination unit, when the edge determination unit determines that no edge exists in the horizontal direction and the vertical direction, the accumulated value of the target pixel stored in the buffer for the direction in the correction data When the edge determination unit determines that an edge exists, the image processing apparatus does not add the cumulative value of the pixel of interest stored in the buffer for the direction to the correction data.   5. The image processing apparatus according to claim 1, wherein the edge determination unit determines whether an edge exists between the target pixel and a processed pixel. 6.   5. The image processing apparatus according to claim 1, wherein the edge determination unit determines whether an edge exists between the target pixel and an unprocessed pixel. 6. The image processing apparatus according to any one of claims 1 to 4, wherein the edge determination unit includes:
An image processing apparatus capable of selecting whether an edge determination is performed between the target pixel and a processed pixel, or whether an edge determination is performed between the target pixel and an unprocessed pixel.   5. The image processing apparatus according to claim 1, wherein whether or not an edge exists between the target pixel and the processed pixel and between the target pixel and the unprocessed pixel is determined. An image processing apparatus for determining. In an image processing method for converting multi-tone image data composed of a plurality of pixels into data in an expression format depending on the presence or absence of dot formation,
Each time each pixel constituting the image data is converted into data in an expression format based on the presence or absence of dot formation, an error value for diffusing from the target pixel to a plurality of pixels is calculated based on an error generated in the target pixel that performs the conversion. An error diffusion step to
A diffusion error storage step for storing a cumulative value obtained by accumulating the error value calculated by the error diffusion step for each pixel constituting the image data;
An edge determination step for determining whether or not an edge exists between the target pixel and pixels around the target pixel;
A dot determination step for determining the presence or absence of dot formation of the target pixel based on the correction data calculated using the cumulative value of the target pixel according to the determination result of the edge determination step and the gradation value of the target pixel When,
An image processing method comprising: An image processing program for the control unit to execute processing for converting multi-gradation image data composed of a plurality of pixels into data in an expression format depending on the presence or absence of dot formation,
The control unit is
Each time each pixel constituting the image data is converted into data in an expression format based on the presence or absence of dot formation, an error value for diffusing from the target pixel to a plurality of pixels is calculated based on an error generated in the target pixel that performs the conversion. Error diffusion processing to
For each pixel constituting the image data, a diffusion error storage process for storing a cumulative value in which error values calculated by the error diffusion process are accumulated;
Edge determination processing for determining whether or not an edge exists between the target pixel and pixels around the target pixel;
Dot determination process for determining the presence or absence of dot formation of the target pixel based on the correction data calculated using the cumulative value of the target pixel according to the determination result of the edge determination process and the gradation value of the target pixel When,
An image processing program for executing

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