JP2020017892A - Image processing apparatus, image forming apparatus, image processing method, and program - Google Patents

Abstract

To efficiently detect a dot area.SOLUTION: An image processing apparatus according to the present invention includes generating means for generating halftone image data used for image formation from multi-valued input image data by using a threshold matrix, acquisition means for acquiring the halftone image data generated by the generation means with a specific bit width, comparison means for comparing the upper M bits of the acquired image data with the specific bit width with its lower M bits (M is an integer), and first determination means for determining whether the obtained image data with the specific bit width forms a halftone dot region on the basis of the comparison result by the comparison means.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a technique for generating halftone image data in a color image forming apparatus in accordance with input multi-tone image data and detecting an area of the halftone image data.

  2. Description of the Related Art As an image recording method used in an image forming apparatus such as a printer or a copying machine, an electrophotographic method in which a latent image is formed on a photoreceptor using a laser beam and developed with a charged color material (hereinafter, referred to as toner). It has been known. An image formed by the developed toner is transferred to a recording medium and fixed by using heat and pressure to be recorded.

  In a printing technique using such an electrophotographic method, there is an image processing technique for discriminating a halftone area and a non-halftone area in the input image data by performing pattern matching on input image data to the apparatus. . Patent Literature 1 discloses a technique for detecting, when a region including a target pixel and neighboring pixels adjacent to the target pixel in input image data conforms to a halftone dot pattern, the region is detected as a halftone dot.

JP 2017-85238 A

  However, the technique of Patent Document 1 has a problem that the processing load of the general-purpose processor becomes too large depending on the magnitude of the processing amount of pattern matching in proportion to the magnitude of the resolution. Further, there is a problem in that if the pattern matching is realized by a hardware circuit, the circuit scale becomes large and the cost is increased.

  The present invention has been made in view of at least one of the above-described problems, and has as its object to efficiently detect a dot area.

  In order to solve the above problem, an image forming apparatus according to an aspect of the present invention includes: a generating unit configured to generate halftone image data used for image formation from multi-valued input image data using a threshold matrix; Acquiring means for acquiring the generated halftone image data with a specific bit width; comparing means for comparing upper M bits of the acquired image data with the specific bit width with lower M bits (M is an integer); A first determination unit configured to determine whether or not the acquired image data having the specific bit width forms a halftone dot region based on a comparison result by the comparison unit.

  According to the present invention, a halftone dot region can be detected efficiently.

Block diagram of image forming apparatus Explanatory diagram of an image forming apparatus Block diagram of image processing for printing The figure which shows an example of a threshold value matrix Diagram showing an example of a memory configuration Flowchart showing an example of the procedure for deriving the area of a halftone dot area The figure for explaining the example of the area derivation processing procedure of the halftone dot area Diagram showing an example of an area derivation processing code of a halftone dot region The figure which shows an example of a threshold value matrix Block diagram of image processing for printing The figure which shows an example of a threshold value matrix The figure which shows an example of a threshold value matrix

  Prior to the description of the embodiment of the present invention, a process of converting image data input to an image forming apparatus and performing recording will be described.

  The image data input to the image forming apparatus is multi-tone image data including a halftone, but in the above-described electrophotographic method, the multi-tone image data is converted into binary data by a pseudo-tone method using a dither method. The image data is converted into image data having a low gradation and recorded. The binary image data is data that uses a repetitive pattern of small dots called halftone dots and expresses the gradation by the magnitude of the point. At this time, non-halftone characters and lines are not converted to halftone dots and are not patterned.

  In the dither method, a halftone image data (halftone image data) is obtained by using a threshold matrix, acquiring a threshold from the threshold matrix according to the pixel position of the multi-tone image data, and comparing the threshold with each pixel. Also called). In the threshold value matrix, threshold values are arranged so that halftone dots are periodically formed at a certain angle. Generally, in a color image forming apparatus, a threshold matrix is designed such that halftone dots after halftone processing have different screen angles for each toner.

  In particular, in an image forming apparatus using an electrophotographic method, when a halftone dot is small, toner hardly adheres to a recording medium, and when the heat and pressure used for the fixing are not sufficient, a low density region with a small halftone dot is reproduced. There is a possibility that the properties and graininess are deteriorated. Therefore, it is determined whether or not the area is a region where the halftone dot is small, and the fixing temperature is controlled (adjusted) according to the determination result.

  In addition, different image processing may be performed between a dot area and a non-dot area such as a character or a line. For example, when performing line width control to increase or decrease the drawing width of a line or a character, a process of adding a dot to increase the width or thinning out the dot to reduce the width is considered. However, if these processes are performed on the halftone dot area, the halftone image density changes when added, or when the halftone image density is deleted, the halftone image density changes slightly. There is a possibility that an image change different from the above may occur. For this reason, it is necessary to determine whether or not the line width control for dot addition or deletion is a non-halftone area and to perform the determined non-halftone area.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the components described in this embodiment are merely examples, and are not intended to limit the scope of the present invention thereto. Further, all combinations of the components described in the embodiments are not necessarily essential to the means for solving the problems, and within the scope of the present invention described in the claims, Various modifications and changes are possible.

[Embodiment 1]
In this embodiment, a case will be described in which the present invention is applied to a digital multifunction peripheral having general functions such as copying, printing, and faxing. FIG. 1 is a block diagram schematically illustrating an example of the image forming apparatus according to the present embodiment. The image forming apparatus 10 includes a scanner unit 101, a controller 102, a printer unit 103, and an operation unit 104.

  The scanner unit 101 is a functional unit that performs a document reading process. The controller 102 has a memory 105 therein and is a functional unit that controls each functional unit in the image forming apparatus 10 such as the scanner unit 101, the printer unit 103, and the operation unit 104. The controller 102 performs image processing on the image data read by the scanner unit 101 and stores the processed image data in the memory 105 as image data for printing.

  The printer unit 103 is a functional unit that forms an image visualized on a recording medium such as paper in accordance with print setting conditions set by the operation unit 104, which will be described in detail later, from print image data read from the memory 105. . The operation unit 104 is a functional unit that sets various printing conditions for image data read by the scanner unit 101.

  The image forming apparatus 10 is connected via a network 106 to a server 107 for managing image data, a personal computer (PC) 108 for instructing the image forming apparatus 10 to execute printing, and the like.

  2A and 2B are explanatory views of the image forming apparatus of the present embodiment. FIG. 2A shows a cross section of the image forming apparatus, FIG. 2B shows an upper surface of a document table, and FIG. 3 shows the side of the platen. This image forming apparatus has functions of copy, printer, and facsimile. The image forming apparatus includes a scanner 201, a document feeder (DF) 202, and a print recording printer 213 having a four-color drum.

  First, a reading operation mainly performed by the scanner 201 will be described.

  When the original is set on the platen 207 and reading is performed, the user sets the original on the platen 207 and closes the DF 202. Then, after the open / close sensor 224 detects that the document table 207 has been closed, the light reflection type document size detection sensors 226 to 230 in the housing of the scanner 201 detect the document size of the set document. . The light source 210 irradiates the document with the size detection as a starting point, and a CCD (charge-coupled device) 231 receives reflected light from the document via the reflection plate 211 and the lens 212 to read an image.

  The controller 102 (FIG. 1) of the image forming apparatus converts the image data read by the CCD 231 into a digital signal, performs image processing for a scanner, and converts the image data into image data for printing. To be stored. The image data for printing at this time is composed of three color signals of red (R), green (G), and blue (B).

  When a document is set on the DF 202 and read, the user places the document face up on the tray of the document setting unit 203 of the DF 202. Then, the document presence / absence sensor 204 detects that the document has been set, and in response thereto, the document feed roller 205 and the transport belt 206 rotate to transport the document, and the document is placed at a predetermined position on the document table 207. Is set. Thereafter, the image is read in the same manner as the reading on the document table 207, and the obtained image data for printing is stored in the memory 105 in the controller 102.

  When the reading is completed, the transport belt 206 rotates again, and the original is sent to the right side in the cross-sectional view of the image forming apparatus in FIG. 2A, and is sent to the original discharge tray 209 via the transport roller 208 on the discharge side. The document is ejected. When there are a plurality of originals, the originals are ejected and conveyed to the right side in the cross-sectional view of the image forming apparatus from the original platen 207, and from the left side in the cross-sectional view of the image forming apparatus via the original feeding roller 205 at the same time. The next original is fed, and the reading of the next original is continuously performed. The above is the operation of the scanner 201.

  Next, a printing operation mainly performed by the printer 213 will be described.

  The print image data once stored in the memory 105 in the controller 102 is again stored in the memory 105 after the print image processing described later is performed in the controller 102 again. Then, after performing a process of separating a halftone dot region and a non-halftone dot region from the print image data stored again in the memory 105, the area of the halftone dot region in a specific density range is counted, The area count value of the halftone dot region is stored in the memory 105 together with the image data. Thereafter, the image data is transferred to the printer 213 in synchronization with the image transfer request from the printer 213. At this time, the above-described area count value is simultaneously notified to a fixing device 220 described later.

  In the printer 213, the laser recording unit converts the laser light into recording laser light corresponding to each of the four colors of yellow (Y), magenta (M), cyan (C), and black (K). Then, the recording laser light is applied to the photoconductors 214 of each color to form an electrostatic latent image on each photoconductor 214.

  Then, the printer 213 performs toner development on each photoconductor 214 with the toner supplied from the toner cartridge 215, and the toner image visualized on each photoconductor 214 is primarily transferred to the intermediate transfer belt 219. The intermediate transfer belt 219 rotates clockwise in FIG. 2A, and when the recording medium fed from the paper cassette 216 through the paper feed conveyance path 217 reaches the secondary transfer position 218, the intermediate transfer belt 219 is rotated. From 219, the toner image is transferred to the recording medium.

  The recording medium on which the image has been transferred is fixed with toner by pressure and heat in a fixing device 220, and is conveyed through a paper discharge conveyance path. Then, the recording medium is transferred to a center tray 221 for face-down discharge or a face-up discharge. The sheet is discharged to the side tray 222.

  The amount of heat required for this fixing is controlled in accordance with the area count value of the halftone dot region in the specific density range described above, and the fixing process is performed with the minimum necessary amount of heat according to the image.

  The flapper 223 is for switching the transport path for switching these paper discharge ports. In the case of double-sided printing, after the recording medium has passed through the fixing device 220, the flapper 223 switches the conveyance path, and then switches back to send the recording medium downward, and the secondary transfer is performed again via the double-sided printing paper conveyance path 225. The sheet is fed to the position 218, and double-sided printing is performed.

  Next, the above-described image processing for printing will be described in detail with reference to FIG. FIG. 3 is a block diagram showing image processing for printing. The image processing unit 301 includes a color conversion processing unit 302, a gamma correction unit 303, and a halftone processing unit 304.

  The image processing unit 301 is a functional unit that performs image processing for printing in the controller 102. The image data for printing once recorded in the memory 105 in the controller 102 is multi-gradation data composed of 8-bit pixels, and has 256 gradation colors in a pixel value range of 0 to 255 per pixel. Have a number.

  The color conversion processing unit 302 is a functional unit that converts the image data for printing input from the memory 105 for each pixel from three colors of RGB to four colors of CMYK corresponding to the colors of toner. The color conversion processing unit 302 generally converts the data into density data in a direction in which the larger the RGB value, the brighter and the larger the CMYK value, the higher the density. The image data color-converted by the color conversion processing unit 302 is output to the gamma correction unit 303.

  The gamma correction unit 303 is a functional unit that performs gamma correction on image data input from the color conversion processing unit 302 for each of CMYK colors. The image data gamma-corrected by the gamma correction unit 303 is output to the halftone processing unit 304.

  The halftone processing unit 304 performs halftone processing (described later) for each of CMYK colors on the image data input from the gamma correction unit 303, and outputs a 1-bit halftone image that can be printed by the printer 213 from a pixel value of 8 bits. This is a functional unit that converts the data into data and stores the data in the memory 105.

  The controller 102 (FIG. 1) includes a CPU (Central Processing Unit) 110. The CPU 110 controls the entire operation of the image processing unit 301 by reading and executing the control program stored in the memory 105. The memory 105 is also used as a work area of the CPU 110 at the same time. In the memory 105, a threshold matrix described later is also stored for each color of the toner corresponding to CMYK. In the present embodiment, the halftone processing unit 304 outputs 1-bit halftone image data to the printer unit 103. However, when the printer unit performs multi-value printing using PWM control or the like, the halftone processing unit 304 outputs 1-bit halftone image data. It is not limited to bits. In this case, the halftone processing unit 304 uses a multi-valued threshold matrix to generate multi-valued halftone image data. For example, if 16 levels of shading can be expressed by PWM control, 4-bit multi-valued halftone dot image data or the like can be adopted.

  Thereafter, the CPU 110 in the controller 102 executes a process of counting the area of the halftone dot region in a specific density range excluding the halftone dot 100% and the halftone dot 100% from the halftone image data in the memory 105. For each of the four CMYK image data on the memory 105, it is determined whether or not a dot area is represented in units of 32 pixels. The density of 32 pixels determined to represent a dot area is estimated, and the sum of CMYK is calculated. It is determined whether or not the density is within a predetermined range, and the area (the number of pixels) is counted. The counted area is stored in the memory together with the image data.

  Next, the operation of the halftone processing unit 304 of the image processing unit 301, the threshold matrix used in the halftone processing, and the fixing temperature will be described. Note that the fixing temperature is a predetermined temperature that is the minimum required to stably fix the toner on the recording medium.

  Usually, a different threshold matrix is used for each color in order to have different halftone dot angles for each of cyan, magenta, yellow, and black. In this embodiment, one of the colors will be described as an example.

  The matrix is a matrix in which halftone cells composed of a plurality of thresholds are repeatedly arranged. The halftone cell corresponds to a sub-matrix including 8 × 4 = 32 pixels. The halftone processing unit 304 binarizes each pixel of the image data into two gradations having values of 0 to 1. Specifically, the halftone processing unit 304 reads one threshold value from a predetermined position in the matrix for each pixel. The halftone processing unit 304 performs a binarization by comparing the read threshold value with the pixel value and outputting 1 when the pixel value exceeds the threshold value and outputting 0 when the pixel value is equal to or less than the threshold value. . In other words, a pixel having a larger value and a value of a dark pixel has a higher probability of exceeding the threshold value, an area for outputting 1 increases, and it is possible to express an image with a larger area, and conversely, a smaller value is used. A pixel having a thin pixel value does not easily exceed the threshold value, and the area of 0 meaning white becomes large. The threshold matrix described in the present embodiment uses a method called a sub-matrix in which a plurality of halftone cells having different thresholds are combined in order to obtain a sufficient number of gradations (256 gradations).

  FIGS. 4A to 4D are diagrams illustrating an example of a binary threshold matrix applied to a predetermined color. 4 (a) to 4 (d) show threshold matrices composed of the same numerical values, respectively, and show a state where colors (gray) are attached to numerical values below the numerical values with different numerical values as boundaries. I have. That is, FIGS. 4A to 4D show examples in which binarization is performed at different densities. One halftone cell of each of the threshold matrices in FIGS. 4A to 4D includes 8 × 4 = 32 thresholds, and eight types of halftone cells having different combinations of thresholds are regularly arranged. They are arranged in a row. That is, the threshold matrix has eight sub-matrices of 4 × 2 cells. The entire threshold matrix has a total of 256 arrangements of 8 rows and 32 rows, and is repeatedly used so that the threshold matrices are laid out in tiles at the pixel positions of the image data.

  In the threshold matrix shown in FIG. 4A, halftone dots are formed in the Bayer order of halftone cells (sub-matrices) 401, 403, 406, 408, 402, 404, 405, and 407 in order from the low density area. In this way, the threshold matrix is divided into 4 × 2 = 8 sub-matrices in a grid pattern. Since the threshold matrix in which the halftone cells are arranged is repeatedly used on the image, a halftone cell 401 of another threshold matrix is repeated on the right side of the halftone cell 404 of a certain threshold matrix. Further, the dot cell 401 is similarly repeated below the dot cell 407 shifted by two dot cells (sub-matrices) in the main scanning direction of the dot cell 401 of a certain threshold matrix. The reason why the Bayer order is used for the arrangement of the sub-matrices is that the layout of dots in this arrangement increases the period of the entire halftone dot, making it difficult to visually recognize the structure and texture of the halftone dot.

  When the threshold matrix is composed of the sub-matrices shown in FIG. 4, since the halftone cell is composed of eight pixels in width, a repeated pattern is generated in units of eight pixels in the main scanning direction.

  In FIG. 4A, the entire surface covering the threshold value matrix is colored (gray) in a state after the binarization of the density range of the density 8, that is, at a portion below the threshold value 8. In this state, the dots of the halftone cells included in the eight sub-matrices are lit as one isolated dot. Subsequently, in FIG. 4B, a color (gray) is similarly attached to a portion less than the threshold value 32, to a portion less than 64 in FIG. 4C, and to a portion less than 192 in FIG. 4D.

  As described above, in the state where the halftone dots are small, the toner hardly adheres to the recording medium, and in particular, reproduction of halftone dots in a low density region requires heat and pressure for sufficient fixing. In the case of this threshold matrix, specifically, the entire surface covering this threshold matrix intends to express the density by 32 isolated dots of one dot in the density area of density 32 (FIG. 4B). However, due to the small size of the halftone dot, a stable expression of this density requires a sufficient fixing temperature.

  The density gradually increases, and after the density range of 64 (FIG. 4C), the halftone dots are arranged in a line, and no isolated dots are present. For this reason, the heat required for fixing is sufficient at a lower temperature than in the low-density region expressed only by the isolated dots.

  As the density further increases, the absolute amount of toner increases, and the heat required for fixing the toner to fuse on paper (recording medium surface) increases. For example, after the density range of 192 (FIG. 4D), halftone dots are not isolated and are stable, but the amount of heat required for fixing increases due to an increase in the total amount of toner, and a sufficiently high fixing is performed. Requires temperature. That is, in the case of FIGS. 4A to 4D, the state of FIG. 4C can be fixed with the lowest amount of heat.

  Next, referring to FIGS. 5 and 6, the CPU 110 in the controller 102 counts the area of a halftone dot area having a density equal to or less than a specific density using the halftone image data on the memory 105 (FIG. 1). Will be described in detail. Note that this series of processing is performed by reading a program executable by a computer, which describes the following procedure, from a storage such as a ROM onto a memory 105 such as a RAM, and then executing the program by the CPU 110. .

  The binary CMYK image data converted by executing the halftone processing is laid out on a memory starting from a different address for each color. FIG. 5 shows the state of the memory space. In FIG. 5, for example, cyan image data is laid out in the same size on the memory with an offset of “0x00200000” for magenta, yellow, and black in the same manner with the address “0xFF000000” at the top. Starting from each address, data of the image size is stored in one bit per pixel, that is, in 32 bits per 32 pixels. When the address is incremented, an image 32 pixels ahead can be accessed. If the image shown in FIG. 4C is stored, a data string 501 as shown in FIG. 5 exists on the memory. When the binary image data converted by executing the halftone processing is cut out in units of 32 bits, it can be seen that the upper 24 bits and the lower 24 bits completely match. A program operating on the CPU in the controller 102 accesses data in units of 32 bits for each color (in units of one row in the figure), determines whether or not the 32 pixels represent a halftone dot area, and specifies the density. The operation (processing) will be described with reference to FIGS.

  FIG. 6 is a flowchart illustrating an example of an area deriving process procedure of a halftone dot region by the image forming apparatus according to the present embodiment. FIG. 7 is a diagram for explaining an example of an area deriving process procedure of a halftone dot region. FIG. 7 schematically shows the processing executed in each step of S601 to S604 in FIG. In addition, for ease of explanation, this flowchart describes one specific color as an example. However, the CMYK four colors also count, and the density is counted by the sum of the four colors. In the following, the symbol “S” in the description of the flowchart indicates a step.

  First, in step S601, the image data to be processed is accessed to acquire 32-bit (specific bit width) data at the location indicated by the memory address.

  Subsequently, in S602, a value shifted to the right by N bits (a value shifted by N bits to the lower bits) with respect to the 32-bit data acquired in S601 is obtained (obtained). That is, data of upper M bits (= 32 bits−lower N bits) is obtained (M is an integer). The value of N (N is an integer) indicates the cycle of the threshold matrix used in the halftone processing, and is 8 in the case of the image on which the halftone processing using the threshold matrix described in FIG. Becomes The value of N differs depending on the threshold value matrix used. Since different dot periods are usually used for each color of CMYK, different values are used for different colors.

  Subsequently, in S603, a value obtained by masking the upper bits of the 32-bit data obtained in S601 with N bits is obtained. That is, data of lower M bits (= 32 bits−higher N bits) is obtained (M is an integer). The value of N (N is an integer) is also the cycle of the threshold matrix, as in S602, and is 8 in the case of the threshold matrix of FIG.

  Subsequently, in S604, it is determined whether or not the value obtained in S602 matches the value obtained in S603. That is, the upper M bits and the lower M bits in the 32 bits acquired in S601 are compared, and it is determined whether or not the comparison result matches. Here, if the comparison results match, this data has a strong correlation in units of N pixels, and if N = 8, it is highly likely that the data is halftone image data using the threshold matrix shown in FIG. Can be On the other hand, when the comparison result does not match, it is considered that the data is not subjected to the halftone dot processing and represents a binarized area of a character area or the like. If it is determined that the comparison results match (S604: YES), the process proceeds to S605, and if it is determined that the comparison results do not match (S604: NO), the processes of S605 to S607 are skipped and S608 is performed. Proceed to.

  In step S605, the density of the determination point (the data acquired in step S601) is derived. For example, by multiplying the number of 1s out of 32 bits, the density at the determination point is derived. In the case of the data string 501 as shown in FIG. 5, since there are eight 1s in 32 bits, this density is 8/32.

  In step S606, based on the density obtained in step S605, it is determined whether the density is within a required density range for a high fixing temperature. That is, it is determined whether or not the density obtained in S605 is within a necessary density range for controlling the fixing device to a high fixing temperature. As described with reference to FIGS. 4A and 4B, in a low-density region, a halftone dot is isolated, so that a sufficient temperature for fixing is required. That is, a sufficient fixing temperature is required. On the other hand, in the high-density region as described with reference to FIG. 4D, the amount of toner used increases, so that the temperature required for fixing increases.

  Here, by performing a threshold process of determining whether the density is lower than the threshold LOW or higher than another threshold HI (threshold HI> threshold LOW), it is determined whether the area is the high-temperature fixing area. . When the density is 0, it is an area where the toner is not used and the fixing is unnecessary, and is lower than the threshold LOW, but is not regarded as a high-temperature fixing area. If it is determined that it is within the threshold range (S606: YES), that is, if it is determined that a high fixing temperature is required, the process proceeds to S607. When it is determined that the temperature is outside the threshold range (S606: NO), that is, when it is determined that the high fixing temperature is unnecessary, the process of S607 for incrementing the area counter is skipped and S608 is performed. Proceed to.

  In step S607, an area counter for high-temperature fixing, which requires a high fixing temperature, is incremented.

  In subsequent S608, the address of the memory is advanced to enable access to the subsequent 32-bit data, and it is determined whether the end of the image data has been reached (S609). If not, the process returns to S601.

  By performing these processes up to the end of the image data, it is possible to derive the area of the high-temperature fixing required in the image data. If this is represented by a code simulating a C program, the description is as shown in FIG. 8, and as an operator, addition, bit shift, mask, and two if statements can be described. It is a very simple code as a halftone dot area deriving processing program (software), and it is possible to derive the area of high temperature fixing required in image data at high speed.

  As described above, the CPU 110 executes the processing from S602 to S605 on each of the CMYK image data. Further, the CPU 110 derives the area of the high-temperature fixing required in the image data by executing the processing of S606 to S607.

  Based on the halftone processing as shown in FIG. 4 and the periodicity of the halftone image subjected to the halftone processing, simple processing in 32-bit units is performed according to the flowchart of FIG. The point area can be detected. That is, the dot area can be detected efficiently.

  The area of the high-temperature fixing required thus obtained is stored in a memory, and is used for controlling the temperature of the fixing device 220 which is a target when the image is fixed on a recording medium such as paper. Specifically, if this area is large, it is assumed that there are many regions where halftone dots are isolated. Therefore, when fixing the toner on the recording medium, the fixing temperature is set high so that the bonding force between the toner and the recording medium can be kept strong. This suppresses a decrease in fixing quality. On the other hand, if this area is small, it is assumed that there are few regions where the halftone dots are isolated, and thus the fixing temperature is set low. Thereby, the power consumption required for fixing is reduced. That is, the fixing temperature is adjusted according to the ratio of the area of the high-temperature fixing. Note that the fixing device 220 performs temperature adjustment based on the basis weight of the recording medium and the like (temperature of a recording medium having a large basis weight is set to a high temperature, and recording medium of a small basis weight is set to a low temperature) and temperature adjustment based on an area of a halftone dot region Are controlled appropriately so that an appropriate target temperature is obtained. In the present embodiment, an example has been described in which all pixels (lines) of image data are processed to derive the area for fixing at high temperature. However, lines of image data are thinned out according to required accuracy. Can also be processed. For example, with respect to the temperature of the fixing device described in the present embodiment, it is not always necessary to accurately determine the area occupied by all image data in pixel units, and it is possible to control the temperature by using sampled image data. is there. Such processing simplifies the processing and further reduces the processing time.

  In this embodiment, the case where the processor that performs the arithmetic processing is a 32-bit CPU is described, and the most efficient 32-bit data access is described as an example. However, the present invention is not limited to this. For example, when a 64-bit CPU is used, data access can be performed in 64-bit units. In this case, the restriction on the period of the dither matrix can be relaxed. In the case where the arithmetic operation is performed using hardware such as an ASIC or an FPGA, the bit access width can be appropriately customized according to the cycle.

  In the present embodiment, in S604 in FIG. 6, it is determined that the image data is halftone image data on the condition that the upper M bits and the lower M bits are completely matched. However, the present invention is not limited to this. Even when there is a slight difference between the upper order and the lower order, if the correlation is high, it can be determined that the image data is dot image data. For example, FIG. 9A is a diagram showing an example of a case where halftone processing is performed on an input image in the density range 68 using the threshold matrix of FIG. In this case, in the first line 901 and the fifth line 902, although the values of the upper 24 bits and the lower 24 bits do not completely match, there is a sufficient correlation. For example, if some bits do not match and all bits do not completely match, but it is recognized that the correlation is high, the process may proceed to the density derivation process in S605.

  More specifically, the upper M / 2 bits and the lower M / 2 bits obtained by executing the processing of S602 and S603 on a bit string (image data of a specific bit width) obtained by sampling only even-numbered pixels in the horizontal direction (main scanning direction). The comparison may be performed using M / 2 bits. Alternatively, comparison may be performed using data obtained by performing the processing of S602 and S603 on a bit string (image data having a specific bit width) in which only odd-numbered pixels in the horizontal direction (main scanning direction) are masked. Good. In this case, as shown in FIG. 9B, the bit causing the mismatch is removed (see the first line and the fifth line), and the upper bit and the lower bit match. This makes use of the fact that the thresholds constituting the threshold matrix of the present embodiment are arranged so as to increase as they move in the horizontal direction (lower side) from the reference point.

  The process of allowing the mismatch of some of the bits can be realized by adding one bit operation to the code shown in FIG. Specifically, this can be realized by taking the logical product of the 32-bit pixel row acquired in S601 and “0x55555555”. When the above operation is performed, the bit string corresponding to the odd-numbered pixel becomes zero. That is, in the calculations after S602, the bit calculation of the odd-numbered pixels can be set as “don't care”. As a result, the accuracy of the determination can be increased while suppressing the calculation cost.

  Further, the upper M / 2 bits and the lower M / 2 bits obtained by performing the processing of S602 and S603 on a bit string (image data of a specific bit width) obtained by sampling only odd pixels in the horizontal direction (main scanning direction). The comparison may be performed using bits. Alternatively, comparison may be performed using data obtained by performing the processing of S602 and S603 on a bit string (image data having a specific bit width) in which only the even-numbered pixels in the horizontal direction (main scanning direction) are masked. Good.

  Further, the correlation can be derived by another algorithm. For example, in image data in which odd or even bit strings are masked with image data having a specific bit width, the Hamming distance between upper bits and lower bits is derived, and when the derived Hamming distance is equal to or less than a predetermined distance, the correlation is determined. It can be determined to be high.

[Embodiment 2]
In the first embodiment, the method of detecting a halftone dot region at high speed from halftone image data including a threshold matrix having eight sub-matrices has been described. In this method, the detection condition is that the sub-matrix to be configured has a periodic structure in the horizontal direction, and all the horizontally connected sub-matrices need to have a density range in which 1 is output in the same manner. Therefore, in the present embodiment, a method for minimizing the density range that cannot be detected will be described. The description of the contents common to the first embodiment will be omitted or simplified, and a difference matrix which is a difference will be described below.

  The operation of the halftone processing unit 304 of the image processing unit 301 and the threshold matrix used in the halftone processing will be described with reference to FIG.

  FIG. 10A is a threshold value matrix composed of the same numerical values as in FIG. 4A, and shows a case where the threshold value is set to less than 12. Pixels whose threshold value is less than 12 are colored (gray).

  FIG. 10B and FIG. 10C are threshold matrixes which are features of the present embodiment, and are configured with the same numerical values, respectively. (Gray). That is, FIGS. 10B and 10C show an example in which binarization is performed at different densities. In the present embodiment, a different threshold matrix is used for each color in order to have different halftone dot angles for cyan, magenta, yellow, and black colors. In this embodiment, one of the colors will be described as an example.

  FIG. 10A shows the arrangement of dots in the density region 12 in the threshold matrix shown in FIG. As can be seen from FIG. 10A, eight pixel periods are generated in the main scanning direction in the first line 1001 and the fifth line 1002, and are determined to be halftone areas according to the flow of FIG. However, the third line 1003 and the seventh line 1004 have a period of 16 pixels, and have an undetectable density that is not detected as a halftone dot region despite being a halftone dot region.

  Therefore, as shown in FIGS. 10B and 10C, the sub-matrices are rearranged while being composed of the same halftone cells as those in FIG. 10A. Specifically, the sub-matrices are arranged so that the eight pixel periods, which are halftone dot periods, are immediately arranged in the horizontal direction. FIG. 10B shows the arrangement of dots in the density area 12 as in FIG. 10A, but the same density area as in FIG. There are no lines that cannot be detected in the condition (12). In the threshold value matrix shown in FIG. 10B, halftone dots are formed in the order of halftone cells (sub-matrices) 1011, 1013, 1016, 1018, 1012, 1014, 1015, and 1017 from the low density area. By arranging the halftone cells in this manner, the shortest horizontal period, in this case, eight pixel periods, is formed, and the undetectable density region can be minimized. That is, a plurality of halftone cells (submatrices) are arranged such that halftone dots grow continuously in the data width direction.

  Of course, even in this arrangement, as shown in FIG. 10C, the arrangement of dots colored with numbers less than 64 completely matches that shown in FIG. 4C. Thus, at multiples of 8 where all eight sub-matrices are used equally, those of FIG. 4 and FIGS. 10 (b) and 10 (c) are completely identical.

  After that, the CPU 110 in the controller 102 executes the processing according to the flowchart of FIG. Also in the present embodiment, by executing the processing according to the same flowchart as in the first embodiment, it is possible to derive the area of the high-temperature fixing required in the image data with higher accuracy than in the first embodiment.

  As described above, according to the present embodiment, by performing halftone processing using a threshold matrix composed of sub-matrices as shown in FIGS. With the simple processing, the halftone dot area can be detected at high speed.

  By performing halftone processing using a threshold matrix (threshold table) having such a sub-matrix configuration, the period of the halftone dot pattern is reduced, and the texture arranged visually in the Bayer order is visually reduced. It may be easier to see. Therefore, it is possible to switch the threshold matrix (threshold table) between the density range where the texture is easily visible and the other density ranges. For example, a threshold table having a Bayer-ordered sub-matrix in which the texture is difficult to see in the input of the density range where the detection may fail may be used, and in other places, as shown in FIG. 10B described in the present embodiment. It is also possible to perform halftone processing using a threshold table.

[Embodiment 3]
In the first and second embodiments, halftone processing using a single threshold matrix in image data is premised, and it is detected that only one type of halftone pattern exists in the image data if they have the same color. The method has been described. In this method, different halftone patterns may be formed by using a plurality of types of threshold matrices in the image data according to the characteristics of the image data. Therefore, in the present embodiment, a description will be given of a configuration of a threshold value matrix capable of detecting a halftone dot region by the same detection means even when a plurality of types of halftone patterns coexist. Note that description of contents common to the first and second embodiments will be omitted or simplified, and a threshold matrix which is a difference will be described below.

  In this embodiment, when a print instruction is issued from the PC 108 to the image forming apparatus to execute printing, the controller 102 converts the print data into image data and stores the image data in the memory 105. At this time, the controller 102 converts the vector data including the characters and the like received from the PC 108 into RGB raster image data, and at the same time, attribute data indicating whether or not the pixel is drawn with the character is assigned to each pixel. Generated as bit flags. Then, this flag is stored in the memory 105 together with the image data.

  FIG. 11 is a block diagram illustrating image processing for printing according to the present embodiment. In FIG. 3, a flag (attribute data) indicating whether or not the pixel represents a character is read from a memory at the same time as raster image data converted from print data from the PC 108, and image processing is performed in accordance with the attribute data. Is different. The image data for printing once stored in the memory in the controller is multi-gradation data composed of 8-bit pixels, and the number of colors of 256 gradations in the range of 0 to 255 as a pixel value per pixel. have. In addition, it also has 1-bit attribute data of 0/1 indicating whether or not each pixel represents a character. For example, the attribute data is defined to be 1 for a pixel constituting a character object, and to 0 otherwise. The image data for printing input from the memory is converted from three colors of RGB into four colors of CMYK corresponding to the colors of the toner in the color conversion processing unit 302 for each pixel. The image data color-converted by the color conversion processing unit 302 is output to the gamma correction unit 1103. Next, the gamma correction unit 1103 performs gamma correction according to characters and non-characters for each color. In this process, two types of gamma correction for characters and gamma correction for non-characters are switched according to attribute information indicating whether or not a character is represented. The image data gamma-corrected by the gamma correction unit 1103 is output to the halftone processing unit 1104. Finally, the halftone processing unit 1104 performs halftone processing on the image data input from the gamma correction unit 1103 using a different threshold matrix (dither matrix) for each CMYK color and each attribute. By the halftone processing, the 8-bit multi-value data is converted into 1-bit halftone image data printable by the printer 213, and is written back (stored) in the memory 105.

  Next, the operation of the halftone processing unit 1104 of the image processing unit 1101 and the threshold matrix used in the halftone processing will be described with reference to FIG. In the present embodiment, a case will be described as an example in which halftone dots having a high screen ruling and halftone dots having a low screen ruling are selectively used in any one of cyan, magenta, yellow, and black in accordance with the attribute.

  The halftone dots usually have a wider period, that is, the lower the number of lines, the more stable the density characteristic and the higher the gradation reproducibility of the image, but the jaggies are likely to occur at the edge portion. Conversely, halftone dots having a narrow cycle and a high number of lines have poor gradation reproducibility, but are less likely to cause jaggies at edge portions. For this reason, halftone dots with a low line count are used in an image such as a photograph or a graphic in which the gradation is important, while halftone dots with a high line count are used in an image composed of many edges such as characters and lines. Is used, and there is a proper use of halftone dots according to the attribute of the image (pixel). In the present embodiment, by executing the processing of S601 to S609 in FIG. 6, a threshold matrix with a matched period is used so that both the high screen ruling and the low screen ruling can be determined with the same shift amount. Will be described.

  FIG. 12 is a diagram illustrating an example of a threshold matrix that forms a halftone dot having a higher screen ruling than the threshold matrix described with reference to FIGS. 10B and 10C. Normally, jaggies are prevented from being generated by applying a threshold matrix forming halftone dots having such a high screen ruling to a character area. For example, in the present embodiment, the threshold matrix is configured such that a strong autocorrelation appears in a four-pixel cycle twice as long as the eight-pixel cycle described with reference to FIG. This halftone cell corresponds to a sub-matrix including 4 × 4 = 16 pixels. One halftone cell of each threshold matrix contains 16 thresholds, and 16 types of halftone cells having different combinations of thresholds are arranged so as to be regularly and continuously arranged. That is, each threshold matrix has 16 4 × 4 sub-matrices. The entire threshold matrix has a total of 256 arrangements of 16 rows and 16 columns, and is repeatedly used so that the threshold matrices are laid out in tiles at the pixel positions of the image data.

  In the threshold matrix shown in FIG. 12, halftone dots are formed in the order of halftone cells (sub-matrices) 1201, 1203, 1211, 1209, 1206, 1208, 1216, 1214, 1202, 1204, and 1212 from the low density area. You. Halftone dots are formed in the order of halftone cells (sub-matrix) 1210, 1205, 1207, 1215, and 1213 toward the high-density area following the halftone cell (sub-matrix) 1212. In this way, the threshold matrix is divided into 4 × 4 = 16 sub-matrices in a grid pattern. Since the threshold matrix in which the halftone cells are arranged is repeatedly used on the image, a halftone cell 1201 of another threshold matrix is repeated on the right side of the halftone cell 1204 of a certain threshold matrix. Similarly, a halftone cell 1201 of another threshold matrix is repeated below a halftone cell 1213 of a certain threshold matrix.

  In FIG. 12, values below the density (threshold) 20 are colored. That is, an example in which binarization is performed with a density indicating 20 is shown. It can be seen from the threshold matrix of FIG. 12 that each line has an eight pixel period.

  After that, the CPU 110 in the controller 102 executes the processing according to the flowchart of FIG. Also in the present embodiment, by executing the processing according to the same flowchart as that of the first embodiment, the image data subjected to the halftone processing using the different types of halftone patterns can be fixed at the high temperature required in the image data. The area can be derived. That is, when a plurality of types of halftone dot patterns coexist, N bits used in the processing of S602 and S603 are determined based on the least common multiple of the period of the pattern in the data width direction. In other words, M bits used in S602 to S604 are determined. Thereby, the process of detecting the halftone dot region is efficiently performed.

  As described above, according to the present embodiment, it is possible to provide a threshold matrix that can be detected by the same detection unit even when a plurality of types of halftone dot patterns coexist in image data. With the simple processing, the halftone dot area can be detected at high speed.

[Other Embodiments]
In the above-described embodiment, the case where the detection result of the halftone dot region is used for controlling the temperature of the fixing device has been described as an example, but the present invention is not limited to this. The present invention can be applied to various kinds of processing that requires extraction of a halftone dot region. For example, when different processing is performed between a halftone dot region and a non-halftone dot region, it can be used to distinguish between a halftone dot region and a non-halftone dot region. Based on the result of the distinction, the present invention can be applied to a process for appropriately controlling the thickness of a non-dot area such as a character or a line.

  According to the present invention, a program that implements one or more functions of the above-described embodiments is supplied to a system or an apparatus via a network or a storage medium, and one or more processors in a computer of the system or the apparatus read and execute the program. Processing can also be realized. Further, it can also be realized by a circuit (for example, an ASIC) that realizes one or more functions.

10: Image forming apparatus, 101: Scanner unit, 102: Controller, 103: Printer unit, 104: Operation unit, 105: Memory, 106: Network, 107: Server, 108: PC

Claims (20)

Generating means for generating halftone image data used for image formation from multi-valued input image data using a threshold matrix,
Acquisition means for acquiring the halftone image data generated by the generation means with a specific bit width,
Comparing means for comparing upper M bits of the acquired image data of a specific bit width with lower M bits (M is an integer);
An image processing apparatus comprising: a first determination unit configured to determine whether the acquired image data having the specific bit width forms a halftone dot region based on a comparison result obtained by the comparison unit.   The first determination means determines that the acquired image data having the specific bit width constitutes the halftone dot area when the upper M bits and the lower M bits match. The image processing device according to claim 1.   The first determination unit is configured to determine whether upper M / 2 bits and lower M / 2 bits in the image data obtained by removing an odd bit sequence or an even bit sequence from the acquired image data having a specific bit width match, 2. The image processing apparatus according to claim 1, wherein it is determined that the acquired image data having the specific bit width forms the halftone dot area.   The first determination unit is configured to determine the acquired specific data when upper M bits and lower M bits thereof match in image data obtained by masking an odd bit sequence or an even bit sequence with the acquired specific bit width image data. The image processing apparatus according to claim 1, wherein it is determined that image data having a bit width of?   The first determining means derives a Hamming distance between upper M bits and lower M bits in image data obtained by masking an odd bit sequence or an even bit sequence with the acquired image data having a specific bit width, and 2. The image processing apparatus according to claim 1, wherein when is less than or equal to a predetermined distance, it is determined that the acquired image data having the specific bit width forms the halftone dot area. The threshold matrix is composed of a plurality of sub-matrices,
6. The plurality of sub-matrices, wherein a threshold value is set such that a bit string forming the generated halftone image data has a periodicity at least within the specific bit width. The image processing device according to claim 1. The threshold matrix is composed of a plurality of sub-matrices,
The image processing apparatus according to claim 1, wherein the plurality of sub-matrices are arranged such that halftone dots continuously grow in a data width direction. The generating unit generates the halftone image data using a plurality of threshold matrices having different numbers of lines depending on an attribute of an object of the input image data,
Included in the generated halftone image data, each of the regions generated based on the threshold matrix having a different number of lines has the same period in the data width direction, so that the number of lines in the plurality of threshold matrices different from each other is different. The image processing apparatus according to claim 1, wherein a threshold value is arranged. 9. The image processing apparatus according to claim 8, wherein the M bits used in the comparing unit are determined based on a least common multiple of a cycle in a data width direction of the plurality of threshold matrices having different numbers of lines. Generating means for generating halftone image data used for image formation from multi-valued input image data using a threshold matrix,
Acquisition means for acquiring the halftone image data generated by the generation means with a specific bit width,
Comparing means for comparing upper M bits of the acquired image data of a specific bit width with lower M bits (M is an integer);
A first determination unit that determines whether the obtained image data of the specific bit width forms a halftone dot region based on a comparison result by the comparison unit,
Derivation means for deriving the density of the pixel values of a plurality of pixels constituting the image data for the image data, which is determined to constitute the halftone dot region by the first determination means,
A second determination unit that determines whether the concentration derived by the derivation unit is within a predetermined concentration range,
A recording in which toner is transferred based on the generated halftone image in accordance with a ratio of an area determined as being within the predetermined density range by the second determination unit in the generated halftone image data. Adjusting means for adjusting the temperature of the fixing means when fixing the toner on the medium,
An image forming apparatus comprising: A generating step of generating halftone image data used for image formation from multi-valued input image data using a threshold matrix,
An obtaining step of obtaining the halftone image data generated in the generating step with a specific bit width,
A comparing step of comparing upper M bits of the acquired image data of a specific bit width with lower M bits (M is an integer);
A determining step of determining whether or not the obtained image data of the specific bit width forms a halftone dot area based on the comparison result obtained by the comparing step;
An image processing method comprising:   In the determining step, when the upper M bits and the lower M bits match, it is determined that the obtained image data having the specific bit width forms the halftone dot area. Item 12. The image processing method according to Item 11.   In the determining step, when the upper M / 2 bits and the lower M / 2 bits in the image data obtained by removing the odd bit sequence or the even bit sequence from the acquired image data of the specific bit width coincide with each other, the acquired The image processing method according to claim 11, wherein it is determined that image data having a specific bit width forms the halftone dot area.   In the determining step, when the upper M bits match the lower M bits in the image data obtained by masking the odd bit string or the even bit string with the acquired specific bit width image data, the acquired specific bit width The image processing method according to claim 11, wherein it is determined that the image data of (a) constitutes the halftone dot area.   In the determining step, a Hamming distance of upper M bits and lower M bits thereof is derived in image data obtained by masking an odd bit sequence or an even bit sequence with the acquired image data of a specific bit width, and the Hamming distance is a predetermined distance. 12. The image processing method according to claim 11, wherein it is determined that the acquired image data having the specific bit width forms the halftone dot area in the following cases. The threshold matrix is composed of a plurality of sub-matrices,
16. The plurality of sub-matrices, wherein a threshold value is set such that a bit string constituting the generated halftone image data has a periodicity at least within the specific bit width. An image processing method according to claim 1. The threshold matrix is composed of a plurality of sub-matrices,
17. The image processing method according to claim 11, wherein the plurality of sub-matrices are arranged such that halftone dots continuously grow in a data width direction. In the generating step, the halftone image data is generated using a plurality of threshold matrices having different numbers of lines depending on attributes of an object of the input image data,
Included in the generated halftone image data, each of the regions generated based on the threshold matrix having a different number of lines has the same period in the data width direction, so that the number of lines in the plurality of threshold matrices different from each other is different. The image processing method according to claim 11, wherein a threshold value is arranged. 19. The image processing method according to claim 18, wherein the M bits used in the comparing step are determined based on a least common multiple of a cycle in a data width direction of the plurality of threshold matrices having different numbers of lines.   A program for causing a computer to function as the image processing device according to claim 1.

Images