JP2011000829A - Image forming apparatus, image forming method, and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain superior printing images with reduced density fluctuation by providing a method for correcting the density fluctuation in real time in multipath printing.SOLUTION: Original printing data used for printing in each path are obtained. The sum value in the range where the density value to be corrected are set in advance of the image shown by respective original printing data is obtained using the original data used in each path from the first path to the n-th path (n>1). The density value in the range where the density value is corrected for the image printed on a recording medium in each path from a first path to n-th path (n>1) is obtained before printing in the (n+1)th path. The differential density value between the obtained density value and the calculated density value is obtained and the original printing data used for printing is corrected using the differential density value before printing in the (n+1)th path. The printing in the (n+1) path is carried out using the corrected printing data.

Description

  The present invention relates to an image forming apparatus, an image forming method, and a program for performing recording while reducing variation in recording characteristics among a plurality of recording elements in a recording head and density unevenness of print data caused by paper feeding unevenness of the recording apparatus. About. In particular, the present invention relates to an image forming apparatus, an image forming method, and a program for performing upper recording on a recording medium by a multi-scan method.

  As an example of an apparatus using a recording head provided with a plurality of recording elements, an ink jet recording apparatus using a recording head provided with a plurality of ink discharge ports has been known. In such an apparatus, the size and position of dots formed by ink vary due to variations in ink ejection amount and variations in ink ejection direction, and density unevenness may occur in a printed image.

  In order to correct such density unevenness, there is a multi-pass printing or multi-pass recording method that uses an inkjet recording head having a plurality of ejection openings and complements one line of image data by a plurality of scans (scans or passes). Proposed. In addition, after printing the test pattern, the printed test pattern is scanned to detect density unevenness, and correction data is generated based on the detected density unevenness. A method of reducing density unevenness by performing printing while performing correction using this correction data is also proposed in Patent Document 1.

JP-A-2-286341

  As the demand for higher image quality of printing is increasing, the above problems can be cleared and density unevenness can be suppressed with the conventional method alone by reducing the size of the print droplets and increasing the print resolution. It is gone. In particular, it is difficult to correct the density unevenness caused by the paper feed error of the printing paper and the density unevenness caused by the accidental error even by using the correction data based on the test pattern generated in advance. .

  The present invention provides a method of correcting density unevenness in real time in multipass printing, and makes it possible to obtain a good printed image with reduced density unevenness.

In order to achieve the object of the present invention, for example, an image forming apparatus of the present invention comprises the following arrangement. That is,
An image forming apparatus that performs printing on a recording medium by a multi-pass method,
Means for obtaining original print data used for printing in each pass;
Using the original print data used in each pass from the first pass to the nth (n> 1) pass, among the images indicated by the respective original print data, it is within a preset density value correction target area. A calculation means for obtaining a total value of density values;
Before printing the (n + 1) th pass, the density value in the density value correction target area of the image printed on the recording medium in each pass from the first pass to the nth (n> 1) pass. Obtaining means for obtaining
Means for obtaining a difference density value between the density value acquired by the acquisition means and the density value calculated by the calculation means before printing of the (n + 1) th pass;
Correction means for correcting original print data used in the printing using the difference density value before printing of the (n + 1) th pass;
An image forming apparatus comprising: a printing unit that performs printing of the (n + 1) th pass using the print data corrected by the correcting unit.

  A method for correcting density unevenness in real time in multi-pass printing is provided, and a good print image with reduced density unevenness can be obtained.

Block diagram of an image forming apparatus according to first to sixth embodiments Arrangement explanatory diagram of recording medium, head, and sensor according to first embodiment Overall block diagram according to the first embodiment Block diagram of processing for each color according to the first embodiment Arrangement explanatory diagram of nozzles and sensors according to the first embodiment Arrangement explanatory diagram of nozzles and sensors according to the second embodiment Explanatory drawing of arrangement of nozzle and sensor according to third embodiment Explanatory drawing regarding gradation reduction using error diffusion according to the sixth embodiment Nozzle and sensor arrangement explanatory diagram according to the fourth embodiment Arrangement explanatory diagram of nozzles and sensors according to the second embodiment The block diagram containing the sensor switching means based on 1st Embodiment Block diagram of processing for each color according to the second embodiment Block diagram of processing for each color according to the second embodiment Block diagram of processing for each color according to the second embodiment Arrangement explanatory diagram of nozzles and sensors according to the first embodiment Arrangement explanatory diagram of nozzles and sensors according to the second embodiment Block diagram of processing for each color according to the fourth embodiment Block diagram of processing for each color according to the fourth embodiment Flow chart according to the first embodiment Example of correction gain according to the fourth embodiment Detailed arrangement explanatory diagram of nozzles and sensors according to the third embodiment Block diagram of processing for each color according to the fourth embodiment Block diagram of processing for each color according to the fourth embodiment

Embodiments of the present invention will be described below with reference to the drawings. However, the scope of the invention is not limited to the following examples.
[First Embodiment]
In FIG. 1, an example in which an image photographed by the digital camera 30 is directly sent to the printer 10 and printed will be described. The image stored in the personal computer 20 may be sent to the printer 10 via the USB device interface 30. First, the type of recording medium on which image data is printed is detected. A CPU 100 that reads information on a recording medium by a recording medium sensor (not shown) for detecting the type of the recording medium (not shown) set in the printer engine 180 and operates according to a program stored in the ROM 110. Determines the type of the recording medium. Several proposals have been made for sensors for detecting the type of recording medium, such as a method of projecting light of a specific wavelength and reading the reflected light. Here, since the method for reading the recording medium itself is not proposed, the details are omitted. Image data photographed by the digital camera 30 is stored in a memory (not shown) in the digital camera 30 as a JPEG image. The digital camera 30 is connected to the USB host interface 140 of the printer 10 by a connection cable. The captured image stored in the memory of the digital camera 30 is temporarily stored in the RAM 120 in the printer 10 via the USB host interface 140. Since the image data received from the digital camera 30 is a JPEG image, the compressed image is decompressed using the CPU 100 to form image data, and stored in the RAM 120. Based on this image data, print data for printing by the print head in the printer 10 is generated. The image data stored in the RAM 120 is subjected to color conversion, binarization processing, and the like by the image processing unit 150, converted into print data (dot data) for printing, and further to support multi-pass printing. Perform path splitting. This part will be described in detail later. The data that has been converted into the print data and further divided into passes is passed to the print control unit 160 and sent to the print head of the printer engine 180 in accordance with the drive order of the print head. Then, in synchronization with the mechanical control unit 170 that controls the motor and mechanical part of the printer engine 180 and the printer engine 180 controlled thereby, the ejection pulse is generated by the print control unit 160 to eject ink droplets and record them. An image is formed on the medium (not shown).

  In FIG. 1, the image processing unit 150 has been described as performing binarization processing. However, this is for lowering the gradation for printing an input image, and is limited to binarization. Not to do. For example, N-value conversion (N is an integer of 2 or more) is included for printing using dark and light ink, for setting the size of ink droplets, for setting large, medium, and small, and for reducing the amount of data. 1 shows an example in which the CPU 100 determines the type of the recording medium set in the printer by reading the recording medium with a recording medium sensor (not shown) arranged in the printer engine 180. There is also a method for selecting the type of the recording medium in the operation on the printer main body or the digital camera. In the present invention, since the generation of print data is controlled by the print density read by the sensor, the same kind of effect can be obtained for both the detection and selection of the type of the recording medium.

FIG. 2 shows the relationship between the inkjet head, the sensor, and the print medium. In the present embodiment, the sensor 230 will be described as an RGB color sensor. However, as a sensor, there are a CMY complementary color sensor, a monochrome sensor, and the like, and the present invention can be similarly applied.
In FIG. 2A, 220_C is an inkjet head having a plurality of ejection nozzles, and is a cyan head. 220_M is a magenta head, 220_Y is a yellow head, and 220_Bk is a black head. The carriage 210 equipped with the inkjet heads 220_C, 220_M, 220_Y, 220_Bk, and the sensor 230 scans the recording medium 200 in the main scanning direction (from the thin arrow to the right). During this scanning, ink droplets are ejected from the ejection nozzles of the inkjet heads 200_X to perform printing. When the main scanning is finished and printing in one scanning is finished, the recording medium 200 is conveyed in the sub-scanning direction (from the bottom to the top of the thick arrow) by a printer mechanism (not shown), and the recording medium is moved to the next main scanning position. Set 200. In the present invention, in order to perform multi-pass printing in which printing is performed by scanning the printing area a plurality of times, the transport amount of the recording medium 200 at one time is smaller than the nozzle width of the inkjet head 220_X. For example, when 4-pass printing is performed, 1/4 of the nozzle width of the inkjet head 220_X is transported for each scan of the carriage 210. In FIG. 2A, the sensor 230 that detects the printing state of the recording medium is located on the upstream side of the inkjet head 220_X with respect to the main scanning direction. By arranging it upstream, it is possible to acquire an image printed on the recording medium in each pass from the first pass to the n-th pass in scanning when performing the printing of the (n + 1) th pass. As described above, by arranging the sensor 230 on the upstream side, it is possible to detect the state of printing by the previous pass (scanning) when multipass printing is performed. That is, it is possible to detect density unevenness due to the ejection characteristics of the inkjet head (variation in ink ejection amount and variation in ink ejection direction) and variation in the conveyance amount of the recording medium 200 by the printer mechanism. In the present embodiment, print data generation by the inkjet head 220_X is controlled based on the print state detected by the sensor 230 disposed on the upstream side. (Details will be described later in the explanation of the configuration diagram)
FIG. 2B shows the position of the sensor when the main scanning direction is from the right to the left of the arrow. The sensor 231 that detects the printing state of the recording medium is located on the upstream side of the inkjet head 220_X with respect to the main scanning direction. In this manner, by disposing the sensor 231 on the upstream side, it is possible to detect density unevenness as in FIG. 2A, and printing by the ink-jet head 220_X is performed by the sensor 231 as in FIG. Control data generation.

  FIG. 2C is a diagram showing the arrangement of sensors when printing is performed by bidirectionally scanning the carriage. Reference numeral 232 denotes a sensor arranged on the upstream side of the inkjet head 220_X when the carriage is scanned rightward, and reference numeral 233 denotes a sensor arranged on the upstream side of the inkjet head 220_X when the carriage is scanned leftward. In this arrangement example, the sensor 232 and the sensor 233 are arranged on both sides of the inkjet head 220 when the carriage 250 is scanned bidirectionally, that is, when printing is performed by bidirectional scanning in the right direction and the left direction. is there. With this arrangement, when bidirectional printing is performed, the sensors 232 and 233 can be arranged upstream of the inkjet head 220 in either direction, and in either case of right-handed printing or left-handed printing. It is possible to perform similar control and processing.

  FIG. 3 is a block diagram showing details of the image processing unit 150. In FIG. 3, an input image 320 to be printed is an RGB signal, and is converted into a CMY signal for printing by an ink jet printer by a color conversion unit 330. Further, the RGB signal detected by the sensor 340 for detecting the printing state is also converted into a CMY signal by the color conversion unit 350. The sensor 340 represents the sensors 230, 231, 232, and 233 in FIG. In the case of bidirectional printing, a signal detected by one of the sensors 232 or 233 upstream in the main scanning direction is used. The color conversion unit 350 performs color conversion to CMY in consideration of the RGB color filter characteristics of the sensor 340, the characteristics of the light source irradiated to the detection area of the sensor 340, and the characteristics of the ink that performs printing. The cyan 335_C, magenta 335_M, and yellow 335_Y signals that have undergone color conversion from the input image 320 enter the print data generation units 370_C, 370_M, and 370_Y for each color together with the sensor 340 signals that have been converted to CMY by the color conversion unit 350. . The print data generation unit 370_X performs binarization or N-value conversion (N is an integer equal to or greater than 2) to generate print data in order to perform printing with an inkjet head. At this time, control is performed by the detection signal of the sensor 340 converted by the color conversion unit 350. After the print data is generated by the print data generation unit 370_X, the print control units 380_C, 380_M, and 380_Y for each color perform print control on the inkjet head and the printer mechanism, and form an image on the recording medium. Go.

  The processing of the image forming apparatus according to the present embodiment will be briefly described with reference to FIG. First, the color conversion unit 330 acquires an input image (S3001). As an input image, an image to be printed in each scan may be acquired from a memory such as the RAM 120. Subsequently, the color conversion unit 330 converts the input image into CMY to obtain a print image (S3002). Next, the color conversion unit 330 inputs the print image to the multiplier 420_X. Then, the multiplier 420_X calculates an input density that is a print density (original print data) in each pass (S3003). Further, the multiplier 620_X calculates the total value of the input densities in the previous pass for each pass and sets it as the target density (S3004). The sensor 340 acquires the density value of the image already printed in the print area of each pass as the measured density (S3005). The adder 630_X calculates the difference between the target density and the measured density as a density difference (difference density value) (S3006). The adder 640_X corrects the input density using the density difference (S3007). The gradation reduction unit 650_X reduces the gradation of the corrected input density (image data after correction) (S3008) and stores it in the recording image memory 460_X. Using the image in the recorded image memory 460_X, the print control unit 160 controls the print head to perform printing (S3009). Here, the image forming apparatus divides the input image into print images for each pass. However, the input density and the target density after being divided into each pass may be input. In this case, the image forming apparatus according to the present embodiment may start processing from the measured density acquisition processing in step S3005.

The operation of one color of the print data generation unit 370_X in FIG. 3 will be described in detail with reference to FIG. 4 showing details of the print data generation unit 370_X. The description in FIG. 4 will be described using four-pass printing as an example. Needless to say, the same applies to multi-pass printing other than 4-pass printing.
With the configuration of FIG. 4, the print state on the medium by the previous scan is detected by a sensor, the difference (density error) between the detected print density and the calculated target output density is obtained, and the next print data is generated. Density correction is performed on the image. First, the multiplier 420_X that calculates the print density for each pass acquires the print image signal 400 converted to each ink color, and multiplies the coefficients (k1, k2, k3, k4) acquired from the pass division table 410. Then, the print density of each pass is determined. Each coefficient of k1, k2, k3, and k4 indicates a division ratio. Each coefficient is
0 ≦ k ≦ 1 (i: 1,2,3,4)
k1 + k2 + k3 + k4 = 1
It is. In the case of 4-pass printing, k1, k2, k3, and k4 may each be 0.25, or the printing ratio of the subsequent passes is increased by decreasing the printing ratio of the first pass (k1, k2, k3, and k4 are 0.1, 0.2, 0.3, and 0.4), respectively. By determining this coefficient, pass division can be performed at an arbitrary density ratio. That is, for the first pass print data generation, the multiplier 420_1 multiplies the print image signal 400 and the coefficient k1 to determine the print density, and the gradation reduction unit 650_1 generates the print data, and the first pass print data is generated. Store in the recorded image memory 460_1.

  For the second and subsequent passes, the multiplier 420_X calculates the print density in each pass, and at the same time, the multiplier 620_X calculates the target output density from the previous scan. That is, when printing in the second pass, the multiplier 620_1 multiplies the print image signal 400 by the print ratio (k1) in the first pass to obtain the target output density in the first pass. On the other hand, the detection signal of the sensor 340 is color-converted to CMY by the color conversion unit 350 and further converted to the detection density by the density conversion unit 600. The detected density in the second pass, that is, the detected print density in the first pass is compared with the calculated target output density in the first pass, and an adder (subtracter) is calculated together with the output of the multiplier 620_1 to calculate a difference. Device) is input to 630_1. The difference between the detected density and the target output density calculated by the adder 630_1 is added to the print density of the second pass calculated by the multiplier 420_1 by the adder 640_2. Using the print density of the second pass corrected by the difference between the target output density of the first pass and the detected print density, the gradation reduction unit 650_2 generates print data and generates a second pass recorded image. And stored in the second pass recorded image memory 460_2.

  Similarly, for the printing of the third pass, the printing density of the third pass is calculated by the multiplier 420_3, and the total target output density of the first and second passes already printed is calculated by the coefficient (k1 + k2). Used by the multiplier 620_2 to calculate. The adder 630_2 calculates the difference between the target output density after the second pass printing and the detected density, and the adder 640_3 adds the difference to the print density of the third pass. The gradation reduction unit 650_3 generates print data from the print density calculated by the adder 640_3 and stores the print data in the third pass recorded image memory 460_3 as a third pass recorded image. The print density in the fourth pass is the same except that the coefficient used by the multiplier 420_4 is k4 and the coefficient used by the multiplier 620_3 is k1 + k2 + k3. Based on the print data stored in the record image memory 460_X, the print control unit 160 drives the inkjet head to form an image on a recording medium.

  In describing this embodiment, the block diagram of the entire printing process is first shown in FIG. 3, and the process after color separation into ink colors CMY is described with reference to FIG. In this embodiment, the print state is detected by the sensor, the print density of the print result of the previous scan is detected, the density error is calculated, and the print is performed so that the density error is corrected in the print of the next scan. Data is being generated. For this reason, instead of converting the detection signal detected by the sensor into CMY ink color, color conversion to the ideal CMY system for image formation is performed, and the density for this ideal CMY color space is converted. An error may be calculated and the print data may be corrected. As a result, due to the relationship between the ink and the recording medium, it is possible to correct the difference in coloring due to the difference in the recording medium with respect to the calculated color conversion and improve the coloring characteristics.

  FIG. 5 is a diagram showing the arrangement of the inkjet head and sensor shown in FIG. 2 in more detail with respect to the detection area by the sensor.

  FIG. 5A is a diagram showing the position of the sensor when the main scanning direction of the carriage 210 on which the inkjet head and the sensor 550 are mounted is the direction from left to right. FIG. 5B is a diagram showing the arrangement of sensors when printing is performed by bidirectionally scanning the carriage 210. A thick arrow shown in the upper direction in the figure indicates the conveyance direction of the recording medium. The main scanning direction is indicated by a thin arrow in each figure. Similar to the description of FIG. 4, the description of FIG. 5 will be described using four-pass printing as an example. Needless to say, the same concept applies to multi-pass printing other than 4-pass printing.

  With reference to FIG. 5, when 4-pass printing is performed, which part of the nozzle array is responsible for data for printing what number of passes will be described. The following description applies similarly to the respective nozzle arrays of 220_Bk (for black), 220_Y (for yellow), 220_M (for magenta), and 220_C (for cyan). Therefore, the state of division for each pass in four-pass printing will be described with the nozzle row for any one color being the nozzle rows of 510 to 540. When the recording medium is conveyed in the direction indicated by the thick arrow, each nozzle row is divided into four for each pass as indicated by 510-540. The nozzle group indicated by 510 prints the first pass, the nozzle group indicated by 520 prints the second pass, the nozzle group indicated by 530 prints the third pass, and the group indicated by 540 prints the fourth pass print data.

  In this embodiment, when generating print data for each of the multi-pass divided passes (second pass and after), the target output density to be printed in the previous pass and the actual print density detected by the sensor This is to correct the difference (that is, the density error). Therefore, it is not necessary to detect the print state and to dispose a sensor for generating the print data for the first pass. Therefore, as shown in FIG. 5A, when unidirectional printing is performed by scanning the carriage from the left side to the right side in the drawing, a sensor arrangement such as 550 is sufficient. In this case, the sensor is located on the upstream side in the scanning direction of the carriage with respect to the inkjet head that performs printing, and does not exist in the region corresponding to the first pass. As shown in FIG. 5B, the positions of the sensors when printing is performed by bidirectionally scanning the carriage are the positions indicated by 552 and 553. That is, in each of the scanning directions from the left side to the right side and from the right side to the left side in the figure, the sensor is arranged to be upstream of the ink jet head, and is arranged upstream according to the scanning direction. Switch the sensor for use.

  As another example, FIG. 15 shows the position of the sensor when three-pass printing is performed. Similarly to FIG. 5, FIG. 15A shows an example of a carriage that performs unidirectional printing, and FIG. 15B shows an example of a carriage that performs bidirectional printing. Reference numerals 1550 in FIG. 15A and 1552, 1553 in FIG. 15B denote sensors for detecting the printing state on the printing medium. The difference from FIG. 5 is that the division number of each head is 3 because the division number of multipath is 3. Compared to the fact that the sensor detection area required for four-pass printing is 3/4 of the head length because the print area for the first pass has increased, the required sensor 1550, 1552, 1553 detection area is the head. It can be seen that it is 2/3 of the length, which is shorter than that for 4-pass printing.

  FIG. 11 is a diagram for explaining sensor switching means when a plurality of sensors are arranged when printing is performed by bidirectionally scanning the carriage. 3 is different from the configuration diagram of FIG. 3 described above in that a first sensor 552 and a second sensor 553 exist instead of the sensor 340, and the selection circuit selects the first sensor and the second sensor signal. 1100 is added. As described with reference to FIG. 5B, when a plurality of sensors are switched and used in accordance with the moving direction of the carriage, a sensor selection circuit is required. Detection signals are input to the selection circuit 1100 in FIG. 11 from a plurality of sensors 552 and 553 (same as the sensors 552 and 553 in FIG. 5B). When printing is performed by scanning in both directions as shown in FIG. 5B, the two sensors alternately read the printing state immediately before printing and the printing state immediately after printing depending on the movement direction of the carriage. The selection circuit 1100 acquires carriage movement direction information, and selects a detection signal from the sensor 552 when scanning is from left to right in FIG. When the carriage movement direction information is scanning from right to left in FIG. 5B, a detection signal from the sensor 553 is selected.

  In the present embodiment, the configuration in which the sensor is arranged on the upstream side in the main scanning direction with respect to the inkjet head has been described. Conversely, a mode in which the sensor is disposed on the downstream side of the inkjet head will be briefly described. When the sensor is arranged on the downstream side of the ink jet head, the print state recorded by the ink jet head is detected immediately after recording. As a result, there is an advantage that only the characteristics of the ink-jet head (discharge amount variation and discharge direction variation) can be directly detected. However, when the multi-pass printing is repeated, the next pass is printed with respect to the state where the recording medium is conveyed by the printer mechanism. When the sensor is arranged on the downstream side, the variation in the conveyance amount of the recording medium cannot be detected, and as a result, the density error (density unevenness) due to the conveyance error cannot be corrected. Further, the required capacity of the storage means for temporarily storing the detected print state and the storage means (band memory) for temporarily storing the generated print data increases.

  As described above, in the present embodiment, a sensor is arranged on the upstream side in the main scanning direction with respect to the ink jet head, and the printing state recorded in the preceding scanning immediately before the scanning is detected. This makes it possible to detect density unevenness such as streaks due to a transport error of the recording medium by the printer mechanism and variations in the nozzles of the inkjet head (e.g., variation in ejection amount and variation in ejection direction) immediately before scanning. It is possible to perform optimum dot generation control corresponding to the detection value, and as a result, it is possible to reduce density unevenness. Further, immediately before scanning, it is possible to detect the print state and generate print data, transfer the generated print data to the ink jet head, and reduce the capacity of the storage means for storing the print data. There is also an effect. In addition, it is not necessary to detect the printing state immediately before the first pass recording, and the sensor circuit is minimized so that the size of the sensor circuit is minimized. It is possible to reduce the cost.

[Second Embodiment]
In the first embodiment, the print state immediately before recording in the second to fourth passes is detected by the sensor. Incidentally, the print data is generated by being divided for each pass, as described in FIG. 4 of the first embodiment. For the recording print data generation after the second pass, the print state by the previous scan read from the sensor is detected, the density error with the target output density is calculated based on the detection result, and the print data is generated by correcting the density Was. However, similarly to the data generation in the first pass, it is also possible to generate print data without using the print state of the previous scan read from the sensor. Adding the sensor itself and the peripheral circuits and control circuits of the sensor leads to an increase in cost. Therefore, it is possible to realize high-quality printing while reducing costs by arranging sensors to perform density correction only for the path that has the highest effect of density unevenness reduction by density correction based on sensor detection. .

  For example, when performing 4-pass multi-pass printing, although depending on the input density, the dot density becomes higher in the third pass and the fourth pass, and the effect of density correction may be reduced. On the other hand, the density correction in the second pass has a high density correction effect because the dot density is still low and density unevenness is conspicuous. In the second embodiment, only when generating print data of a partial region (second pass as an example) of each multi-pass divided pass region for a print region of an inkjet head having a plurality of nozzles, The print state by the previous scan read from the sensor is detected. Based on the detection result, a density error from the target output density is calculated, density correction is performed, and print data is generated.

  FIG. 6 is a diagram showing the arrangement of the ink jet head and the sensor when the pass for generating print data using the print state detected by the sensor is limited to the second pass only. FIG. 6A shows the position of the sensor when the main scanning direction is from left to right. Each reference numeral is the same as in FIG. 5A, but in FIG. FIG. 6B is a diagram showing the arrangement of sensors when printing is performed by bidirectionally scanning the carriage. The reference numerals in FIG. 6B are also the same as those in FIG. 5B, but the sensors are denoted by 652,653.

  Similar to the description of FIG. 5, the description of FIG. 6 will be described using four-pass printing as an example. Needless to say, the same concept applies to multi-pass printing other than 4-pass printing. In addition, although the path using the sensor is described as the second path, the sensor may be used for paths other than the first path. The print data handled by the nozzle arrays 510 to 540 is the same as that shown in FIG.

  In the present embodiment, only when the print data of the second pass divided by the multi-pass is generated, the print state by the previous scan (first pass) read by the sensor is detected. A density error between the detection result and the target output density (in the first pass) is calculated, and density correction is performed to generate print data. When generating print data for the first, third, and fourth passes, no print state is detected, so no sensor is arranged. FIG. 6A is a diagram showing the position of the sensor when the main scanning direction is from the right to the left of the arrow. A sensor 651 that detects the printing state of the recording medium is located upstream of the inkjet head 220_X with respect to the main scanning direction. FIG. 6B is a diagram showing the arrangement of sensors when printing is performed by bidirectionally scanning the carriage. A sensor 652 is arranged on the upstream side of the inkjet head 220_X when the carriage is scanned rightward. Reference numeral 653 denotes a sensor arranged on the upstream side of the inkjet head 220_X when the carriage is scanned leftward.

  The processing flow in this embodiment is the same as that in the first embodiment, but the differences will be described in detail with reference to FIG. The difference from the configuration of FIG. 4 according to the first embodiment is that a density correction selection unit 1210 is added. The density correction selection unit 1210 is a processing method in print data generation after the second pass, that is, uses the sensor to detect the print state by the previous scan, corrects the density error, and generates print data. Select whether to generate print data without using it. For this purpose, the density correction selection unit 1210 outputs signals sel2, sel3, and sel4.

  In print data generation in the second pass, according to the signal sel2 output from the density correction selection unit 1210, the selection circuit 1200_2 prints the density corrected print image (output of the adder 640_2) and the print image without density correction (of the multiplier 420_2). Output). Similarly, also in the third and fourth pass print data generation, the selection circuits 1200_3 and 1200_4 output the signal sel3 output from the density correction selection unit 1210 as one of the density corrected print image and the density uncorrected print image. , Select according to sel4.

  In the case of printing in the second pass in FIG. 6A, when printing is performed by moving from the left to the right, which is the main scanning direction, a sensor is disposed at a position 650. In FIG. 6B, at the time of printing in the second pass, when printing is performed in both directions in the main scanning direction, sensors are arranged as 652 and 653. In order to perform correction in the second pass, the density correction selection unit 1210 instructs to select a print image whose density has been corrected by sel2. The selection circuit 1200_2 selects a print image whose density has been corrected (the output of the adder 640_2) according to sel2. In the present embodiment, since density correction is not performed in the third and fourth passes, the density correction selection unit 1210 instructs to select a print image that has not been density corrected by sel3 and sel4 in the third and fourth passes. . That is, the selection circuits 1200_3 and 1200_4 select print images (outputs from the multipliers 420_3,420_4) that are not subjected to density correction.

  In general, a printer supports a plurality of printing conditions. That is, the number of passes differs depending on the printing conditions, and the pass for density correction also differs. In order to support printing under these multiple printing conditions, switching by a selection circuit is required. If the printing conditions are specified and the density correction is performed based on the printing state detected by the sensor only in a specific pass, the system for performing the density correction and the system for not performing the density correction can be systematically without using such a selection circuit. Needless to say, it can be fixed. For this reason, a selection circuit is provided in a path for switching whether or not density correction is performed according to printing conditions, but a selection circuit may not be provided in a path that can be fixed as a path for which density correction is performed or not.

  An example in which means for switching print data generation processing after performing density correction and processing for generating print data without performing density correction using another configuration will be described with reference to FIG. In the configuration of FIG. 13, the operation of the density correction selection unit 1210 is the same as that in the configuration of FIG. In the case of the configuration of FIG. 13, according to the selection signal sel2 output from the selection circuit 1300_2, one of the two input signals of the density error signal output from the adder 630_1 and the correction value “0” is output. That is, when performing density correction, the density correction selection unit 1210 instructs the selection circuit 1300_2 by the selection signal sel2 to select the density error signal output from the adder 630_1. When the density correction is not performed, the selection signal sel2 instructs to output “0”. Similarly, the density correction selection unit 1210 instructs the selection circuits 1300_3 and 1300_4 to select the density error signal output from the adders 630_2 and 630_3 and the correction value “0” by using the selection signals sel3 and sel4. .

  As shown in FIG. 6A, when the sensor 651 is arranged to detect an area corresponding to the second pass of four-pass printing, the print detected by the sensor when generating the second-pass print data. Print density error is calculated from the status. Then, the selection circuit 1300_2 selects this density error signal (the output of the adder 630_1) in accordance with an instruction from the density correction selection unit 1210. Since the density correction is not performed in the other print data generations in the third and fourth passes, the selection circuits 1300_3 and 1300_4 select the correction value “0”. Similarly, as shown in FIG. 10A, when the sensor 1050 is arranged so as to detect the areas corresponding to the second pass and the third pass of the 4-pass printing, the printing of the second pass and the third pass is performed. The print density error is calculated from the print state detected by the sensor when generating data. Then, the selection circuits 1300_2 and 1300_3 select this density error signal (the outputs of the adders 630_1 and 630_2). Since density correction is not performed in the fourth pass print data generation, the selection circuit 1300_4 selects the correction value '0'. In this way, the selection circuit 1300_X selects the density error signal or the correction value “0” and outputs it to the adder 640_X. The adder 640_X adds the output of the selection circuit 1300_X to the pass-divided print images (outputs of the multipliers 420_2, 420_3, and 420_4), and inputs them to the gradation reduction units 650_2, 650_3, and 650_4.

An example in which the means for switching between the two processes described with reference to FIG. 13 is implemented with another configuration will be described with reference to FIG. In the configuration of FIG. 14, the selection circuit 1400_X uses the selection signal from the density correction selection unit 1210 to correct the density of the print image of each pass and then perform density correction on the print data in which the gradation is reduced and the print image of each pass. Without selecting the print data, the gradation is reduced. The gradation reduction units 1450_2 to 1450_4 added in the configuration of FIG. 14 directly acquire the output of the multiplier 420_X that performs pass division without performing density correction using a sensor, and perform gradation reduction. In print data generation after the second pass, two types of print data are generated simultaneously. One is print data that has been reduced in gradation using the print image (output of adders 640_2, 640_3, 640_4) whose density has been corrected according to the print state detected by the sensor, and is output from the gradation reduction unit 650_X. The The other is print data that has been reduced in gradation using a print image (output of multipliers 420_2, 420_3,420_4) that is not subjected to density correction according to the printing state detected by the sensor, and is output from the gradation reduction unit 1450_X. Is done. The selection circuit 1400_X acquires these two types of print data, selects one of them according to the selection signal selX that is the output of the density correction selection unit 1210, and stores it in the recording image memory 460_X.

  For example, in the second pass, the selection circuit 1400_2, when performing density correction, print data (output of the gradation reduction unit 650_2) obtained by reducing the gradation of the density-corrected print image (output of the adder 640_2). This is selected and stored in the second pass recorded image memory 460_1. When density correction is not performed, print data (output of the gradation reduction unit 1450_2) obtained by reducing the gradation of the print image without the density correction (output of the multiplier 420_2) is selected, and the second pass recorded image is selected. Store in memory 460_1. The same applies to the third pass and the fourth pass.

  For a path where no sensor is arranged, the selection circuit 1400_X needs to use a print image that has not been subjected to density correction. However, the selection circuit 1400_X may select either of the paths for calculating the density error from the target output density based on the detection result of the sensor and correcting the density to generate the print data. In this way, it is possible to intentionally select whether or not to perform density correction of the print image using the print state detected by the sensor. For this reason, it is possible not only to switch to the area where the sensor is not arranged, but also to control whether or not density correction is performed for the area where the sensor is arranged. That is, the density correction is performed by switching the selection circuit 1400_X only in a region where density correction is necessary (that is, a region where density unevenness is conspicuous). In this way, it is possible to reduce the density unevenness by correcting the density only for the effective area, or by correcting the density for a specific pass under various printing conditions.

  The configuration in the case where the sensor is provided in the second pass during four-pass printing has been described above, but another case will be described. In the configuration of FIG. 10, when generating print data of the second pass and the third pass that are divided into multi-passes during four-pass printing, the print state by the previous scan is detected by the sensor. Based on the detection result, a density error from the target output density is calculated, density correction is performed, and print data is generated. The configuration in FIG. 10 is the same as that in FIG. 6, and in FIG. 6, 1050, 1052, and 1053 are sensors for detecting the print state on the print medium. Hereinafter, the operation will be described with reference to FIG. 12, but the operation can be performed with other configurations as shown in FIGS. Regarding the first pass print data generation, it is not necessary to detect the print state and it is not necessary to arrange a sensor. Similarly, in the fourth pass, it is not necessary to detect the print state since no density correction is performed. In FIG. 10A, when printing is performed by moving from the left to the right in the main scanning direction, the position of 1050 Place the sensor in With this arrangement, the density can be detected by the sensor in the second and third passes. The density correction selection unit 1210 controls the selection signals sel2 and sel3 in FIG. 12 so as to use the print image (output of the adders 640_2 and 640_3 in FIG. 12) whose density has been corrected according to the detected printing state. In FIG. 10B, when printing is performed with the main scanning direction being bidirectional, sensors are arranged at positions 1052 and 1053. Then, in order to perform correction in the second pass and the third pass, the selection circuits 1200_2 and 1200_3 cause the density-corrected print images (adders 640_2) by the selection signals sel2 and sel3 in FIG. 12 issued by the density correction selection unit 1210. , 640_3 output). As shown in FIG. 10, since no sensor is arranged at a position corresponding to the fourth pass, density correction is not performed in the fourth pass. That is, a print image (output of the multiplier 420_4) whose density is not corrected is selected by sel4 in FIG.

  Similarly, an example in which sensors are arranged in the areas of the second pass and the third pass in the case of performing multi-pass division printing with 6-pass printing will be described with reference to FIG. In the configuration of FIG. 16, when generating the second pass and third pass print data during 6-pass printing, the print state by the previous scan read by the sensor is detected. Based on the detection result, a density error from the target output density is calculated, density correction is performed, and print data is generated. In the drawing, the same parts as those in FIG. 10 are omitted, but reference numerals 1650, 1562, and 1653 denote sensors for detecting the printing state on the printing medium. Regarding the first pass print data generation, it is not necessary to detect the print state and it is not necessary to arrange a sensor. Similarly, regarding the print data generation from the fourth pass to the sixth pass, it is not necessary to detect the print state and it is not necessary to arrange a sensor. In FIG. 16A, when printing is performed by moving from the left to the right in the main scanning direction, the gradation is reduced using the density-corrected print image in the second and third passes. In the other first and fourth to sixth passes, the gradation is reduced from the print image obtained by the pass division without performing density correction. In FIG. 16B, when printing is performed with the main scanning direction being bidirectional, sensors are arranged at positions 1562 and 1653.

  Next, FIG. 21 shows the relationship between the detection area by the sensor and the number of path divisions. 2101 shows a state of nozzle division at the time of 4-pass printing, and 2102 shows a state of nozzle division at the time of 6-pass printing. Reference numeral 2103 denotes a detection region of the sensor. When the sensors are arranged as shown in FIG. 21, it is possible to perform density correction for the second and third passes during 4-pass printing and for the third and fourth passes during 6-pass printing. In multi-scan printing, it is necessary to support printing with different number of pass divisions depending on printing conditions. By arranging the sensor as shown in FIG. 21 and setting the detection area by the sensor, it is possible to perform density correction of print data by the sensor and reduce density unevenness corresponding to various path divisions. . Furthermore, it is possible to reduce density unevenness at low cost by arranging sensors so that density correction is not performed on all paths but density correction is performed on highly effective paths. .

<Third Embodiment>
In the third embodiment, a sensor arrangement method different from the above embodiment will be described. In the present embodiment, description will be given by taking unidirectional printing as an example. In the case of bidirectional printing, sensors are arranged on both the left and right sides of the head, but the detection area in the vertical direction of the sensor is the same regardless of whether the sensor is on one side or both sides. Therefore, even in the case of bidirectional printing, it is possible to detect the printing state and perform density correction in the same manner as in the case of unidirectional printing. The description will be made assuming that the sensor for detecting the printing state is disposed upstream of the inkjet head in the scanning direction.

  FIG. 7 is a diagram showing in detail the relationship among one of the inkjet heads 220_Bk, 220_Y, 220_M, and 220_C in FIG. 6, the position of the sensor, and the detection area of the sensor. Reference numerals 510 to 540 denote areas in charge of printing of each pass in the head as described with reference to FIG. When 4-pass printing is performed, a head region having a plurality of ejection openings is divided into four. 510 indicates an area for printing the first pass, 520 indicates an area for printing the second pass, 530 indicates an area for printing the third pass, and 540 indicates an area for printing the fourth pass.

  Reference numeral 720 denotes a detection area of the sensor for generating data of the second pass in the present embodiment. The region 720 is divided into three regions 710, 711, and 712. Consider a state when the area 720 is scanned in the second pass, that is, the area 720 is not yet printed in the second pass. At this time, 710 is a blank area where the second pass will be printed, 711 is a blank area which will not be printed in the second pass, and 712 is an area which has already been printed in the first pass.

  As shown in FIG. 4 of the first embodiment, the detection signal obtained by the sensor detecting the printing state is color-converted to CMY, and then converted to the detected density by the density conversion unit 600. At this time, the density conversion unit may filter the detection signal that detects the printing state. In order to perform filter processing such as 3 * 3 or 5 * 5, a detection signal that detects a printing state of a point of interest and a reference area in the vicinity thereof is necessary. Further, in order to compare the print state detection signal acquired by the sensor with the print image 400, it is necessary to match the respective resolutions. For example, the density conversion unit 600 may perform resolution conversion on the print state detection signal. However, for example, when performing resolution conversion by the bicubic method, a detection signal that detects the density of 4 points * 4 points around the point of interest is necessary. As described above, in order for the density conversion unit to perform resolution conversion processing and filter processing on the detection signal from the sensor, it is necessary to detect a printing state in a wider range than the target pixel region where density correction is actually performed. According to the configuration of FIG. 7, it is possible to detect a printing state in an area wider than the printing area of the second pass. Even when print data is generated by performing density correction when print data other than the second pass is generated, if a sensor having a size capable of detecting print data in a wider area than the print area is used according to the same concept, the density correction is performed. The density unevenness at the end of the region can be further reduced.

<Fourth Embodiment>
In the first to third embodiments, when the print data of each pass after multi-pass division is generated, the print state by the previous scan read by the sensor is detected. Then, a density error with respect to the target output density is calculated based on the detection result, and density correction is performed to generate print data. On the other hand, density unevenness such as streaks due to mechanical paper feeding errors and variations in the nozzles of the inkjet head (e.g., variation in ejection amount and variation in ejection direction) is likely to occur at the boundary between each pass. For example, streaky unevenness occurs at the boundary of the path due to the variation in the conveyance amount of the recording medium by the printer mechanism. Further, as a characteristic of the ink jet head, the variation in the ejection direction of the nozzles at the upper end portion and the lower end portion of the head is larger than that at the center portion, and the positional deviation of the ink dots at the nozzle end portion is large. Considering this, when generating print data for each pass, the boundary portion of the pass is set as a density value correction target region. A great effect can be obtained by detecting the printing state only within the density value correction target region and performing density correction based on the detection result.

  In the fourth embodiment, a method of arranging a sensor only at a boundary portion of a path will be described. FIG. 9 shows an example in which sensors are arranged at positions 910 to 940. The sensor 910 is disposed at the boundary between the first path and the second path. The sensor 920 is at the boundary between the second path and the third path, the sensor 930 is at the boundary between the third path and the fourth path, and the sensor 940 is at the boundary part of the fourth path opposite to the third path. Placed in. The processing flow in this embodiment is the same as that in the first embodiment, but the differences will be described in detail with reference to FIG. For the area where data is generated by arranging the sensor and correcting the density according to the printing status detected by the sensor, the selection circuit 2200_X selects the output of the adder 640_X that corrects the density error and prints the print data Store in memory 460_X. For an area where no sensor is arranged, that is, an area where data is generated by a print image without density correction, the selection circuit 2200_X selects the output of the multiplier 420_X that performs path division and stores the print data in the recording image memory 460_X. Further, using the sensor 910 disposed at the boundary between the first pass and the second pass, the density correction can be performed on the print data for the area where the sensor in the first pass is disposed. In order to perform density correction in the first pass, an adder 640_1 that performs density correction on the output of the multiplier 420_1 that performs path division in the first pass and a selection circuit 2200_1 are arranged. The operation is the same as in the second to fourth passes, and the output of the adder 640_1 is selected for the area where the density correction by the sensor is performed, and the output of the multiplier 420_1 is selected for the area where the density correction is not performed. Each selection of the selection circuit 2200_X is performed according to an instruction from the density correction selection unit 1210, specifically according to sel1, sel2, sel3, sel4 output from the density correction selection unit 1210.

  Since the area originally printed in the first pass is not printed before the first pass, it is not necessary to perform density correction. However, as described above, due to variations in the nozzle characteristics of the inkjet head and the conveyance amount of the recording medium, dots printed by the previous scan are formed on the first pass side of the boundary between the first pass and the second pass. Sometimes it protrudes. The density correction may be performed in the first pass by detecting the protruding dots with a sensor. In this way, density unevenness can be reduced.

  In the case of the present embodiment, since a sensor is added only to the boundary part of the path in the same path area, a boundary part between the density correction area and the non-density correction area occurs, which may deteriorate the image quality. In order to smoothly connect the density correction area and the non-density correction area, a method of reducing the correction amount of the boundary area is conceivable. The configuration in this case will be described with reference to FIG. The configuration of FIG. 18 is obtained by adding multipliers 1800_1 to 1800_4 for controlling the correction amount when performing density correction to the configuration of FIG. Multipliers 1800_1 to 1800_4 calculate the correction amount by multiplying the density error by the correction gain. The maximum value of the correction gain is “1” and the minimum value is “0”, and the gain can be changed according to each position in the detection region of the sensor.

  The relationship between the sensor detection area and the correction gain will be described with reference to FIG. In FIG. 20, 2000 represents a sensor, and 2001 to 2003 represent correction gains. For example, the density error may be set as the correction amount for the vicinity of the center of the sensor, and the correction amount may be set to 0 for the end portion of the sensor. Therefore, the gain is set to “1” near the center of the sensor, and the gain is set to “0” at the end of the sensor. Further, by increasing the gain stepwise from the sensor end to the center of the sensor, the correction amount of the sensor peripheral portion can be decreased stepwise. In this way, it is possible to smoothly connect the steps at the boundary between the density correction area and the density correction area. The gain curve shown in the figure is an example, and is not limited to the one shown here, and an optimum curve may be selected according to the characteristics of the nozzle. Also, the correction gain can be determined by a path division method. Information indicating the correction gain may be stored in, for example, the path division 610 and read by the multiplier 1800_X as appropriate.

  In FIG. 18, the form using the selection circuit 2200_X that selects print data obtained by performing density correction and print data that has not been density corrected has been described. FIG. 17 shows an embodiment in which the selection circuits 2200_1, 2200_2, 2200_3, and 2200_4 are not used. As already described, in this embodiment, the correction amount is controlled by controlling the correction gain signal for controlling the correction amount. In the configuration of FIG. 17, the gain is controlled for the area where the sensor is arranged, and the area where the sensor is not arranged and density correction is not performed (the area above and below the sensor 2000 in FIG. 20). , "0" is output as the gain. When the correction gain signal is “0”, the output of the multiplier 1800_X that calculates the correction amount is always “0”, and density correction is not performed. That is, in a region where no sensor is arranged and density correction is not performed, or a sensor is arranged but density correction is not performed due to the number of passes or other factors, the correction gain is set to “0” and density correction is performed. Can be disabled. In this case, for example, the density correction selection unit 1210 determines whether or not to perform density correction by using information indicating the correction gain stored in its own memory, sensor arrangement information, print control information, etc. Control is sufficient.

  As described above, the density unevenness at the pass boundary portion can be effectively reduced by arranging the sensor at the pass boundary portion of the multi-pass printing. According to this configuration, since it is sufficient to arrange a small sensor, it is possible to minimize the cost increase associated with the addition of the sensor itself and the control circuit. In the present embodiment, as shown in FIG. 9, sensors 910, 920, 930, and 940 are arranged at the boundary portions of the paths when performing multi-pass printing. The present embodiment is not limited to the form in which a plurality of sensors are arranged in this way, and can also be realized by arranging one sensor having a long detection area as in the first embodiment. In this case, the output of the density correction selection unit is set and selected so that control is applied only to the part that needs to be controlled / corrected to generate print data (in this embodiment, the boundary part). Switching may be performed by a circuit.

<Fifth Embodiment>
In the first to fourth embodiments of the present invention, when performing bi-directional printing, the sensor is arranged on both sides so that the sensor is located upstream of the print head regardless of which direction is scanned. The configuration using only one sensor in the scanning has been described. However, a configuration in which the sensor is arranged only on one side of the recording head is also conceivable. In this case, density correction can be performed only in scanning in one direction, and the number of passes in which density correction can be performed is halved. However, it is still possible to effectively detect density unevenness and perform optimum dot generation control corresponding to this. According to this configuration, the cost increase due to the addition of the sensor and the peripheral circuit can be minimized.

  The processing of this embodiment will be described with reference to FIG. 13 according to the second embodiment. In the second embodiment, the density correction selection unit 1210 controls the selection of 1300_X by outputting selection signals sel2, sel3, and sel4 according to whether or not density correction is performed. In this embodiment, density correction is performed when printing is performed in a scanning direction in which there is a sensor upstream of the head with respect to the carriage movement direction. Further, when printing is performed in a scanning direction in which there is a sensor downstream of the head with respect to the carriage movement direction, density correction is not performed. When density correction is performed, the density correction selection unit 1210 causes the selection circuit 1300_X to select a density error (output of the adder 630_X) calculated based on the detection result of the sensor. When density correction is not performed, the density correction selection unit 1210 causes the selection circuit 1300_X to select “0”. With the above processing, it is possible to perform density correction during bidirectional printing with only one sensor mounted on one side of the carriage.

<Sixth Embodiment>
In the first to fifth embodiments, a print state immediately before printing is detected by a sensor, and print data is generated after density correction is performed on a print image based on the detection result. In the sixth embodiment, a method for controlling the dot generation position of print data based on the detected print state when reducing the gradation of the print image instead of correcting the density of the print image will be described.

The operation will be described in detail with reference to FIG. 23 by extracting one color from the print data generation unit 370_X in FIG. The description in FIG. 23 will be made using four-pass printing as an example. Needless to say, the same applies to multi-pass printing other than 4-pass printing.
The print signal converted into each ink color is input to a multiplier 2420_X that calculates the print density for each pass, and multiplied by the coefficients (k1, k2, k3, k4) read from the pass division table 2410, The print density for each pass is determined. The generation of print data for each pass is performed in the same manner as in the first embodiment. For example, for the first pass, the multiplier 420_1 multiplies the print image 2400 for each ink color and the coefficient k1 to determine the print density of the first pass. The first pass print data is generated by the first pass tone reduction unit 2450_1 with the tone reduced, and the detection signal of the sensor is used at that time. The generated first pass print data is stored in the first pass recorded image memory 2460_1. The same applies to the other paths. The print data control unit 2440 inputs a control signal to the gradation reduction unit 2450_X based on the print state detection signal from the sensor. This process will be described later.

  A gradation reduction using error diffusion will be described as an example of the gradation reduction unit 2450_X in FIG. 23 with reference to FIG. FIG. 8 is a block diagram showing error diffusion processing. 2500 is an input image signal for reducing gradation (output of the multiplier 2420_X in FIG. 23), and 2505 is a control signal for controlling gradation reduction (output of the print data control unit 2440 in FIG. 23). . The adder 2510 adds the quantization error signal 2575 to the input image 2500, and generates an input image signal 2515 to which the quantization error is added. The quantizer 2530 quantizes the input image signal 2515 including the error using the threshold value generated by the threshold value generation unit 2520, and generates an output signal 2535 with reduced gradation. The inverse quantizer 2550 performs inverse quantization on the output signal 2535 with reduced gradation using the evaluation value 2540. The adder 2560 calculates an error resulting from the quantization of the input image signal 2515 including the error, and generates a quantization error signal 2565. 2570 is a diffusion / collection unit that diffuses or collects the quantization error signal 2565, 2575 is an error signal that has been diffused or collected, and 2580 is an error buffer.

  Usually, the threshold value generated by the threshold value generation unit 2520 is fixed (may be changed to correct a texture or a dot generation delay). In the present embodiment, the threshold generation unit 2520 acquires control data from the print data control unit 2440 in FIG. That is, it is possible to control the data generation in the error diffusion process by changing the threshold value in the error diffusion process according to the printing state detected by the sensor 2340.

  In the present embodiment, the threshold value is changed by detecting the printing state up to scanning before printing in the current scanning in multi-pass printing. In this way, control is performed so that the position of the newly generated dot is generated at a position away from the already printed dot position (the density is made uniform). Specifically, according to the detection signal of the sensor, for the positions where dots have already been generated and the positions where the dots have been concentrated and the density has increased, the threshold for performing quantization is changed to be high so that dot generation is suppressed. To control. On the other hand, in an area where dots are not generated and an area where the print density is low, control is performed so as to promote dot generation by changing the threshold value for quantization to a low value. With this threshold control, it is possible to improve the dispersibility of dots between passes in multipass printing. In this method, the threshold value is changed in the gradation reduction by the error diffusion method. Therefore, the dot generation position can be controlled without changing the dot generation rate for the image signal shown in FIG. 23 in which the pass density is determined by the pass division coefficient and the print density for each pass is determined.

  Regarding the first pass print data generation of the gradation reduction unit shown in FIG. 8, since the print area in the first pass is not normally printed, the detection signal from the sensor is constant. The control signal is also constant. In the first place, it is possible to have no sensor for the first pass. Accordingly, the threshold value generated by the threshold value generation unit 2520 is also fixed (or changed to correct the texture and dot generation delay), and normal quantization is performed.

  The processing based on error diffusion processing has been described for the gradation reduction unit according to the present embodiment, but the gradation reduction method is not limited to error diffusion processing. Even in the method using a dither matrix, it is possible to control the generation of print data by controlling the threshold value of the matrix as in the error diffusion processing of the present embodiment.

  As described above, when correction is performed using a sensor, density unevenness can be reduced and image quality can be improved not only by density correction but also by controlling the dot generation position. The first to fifth embodiments have been described as embodiments for performing density correction. However, it goes without saying that the first to fifth embodiments also apply to the embodiment in which the dot generation position is controlled as in the sixth embodiment. For this reason, the same description is omitted.

<Other embodiments>
The present invention can also realize processing equivalent to that of the first to fourth embodiments with a computer program. In this case, each of the components including FIG. 1 may be functioned by a function or a subroutine executed by the CPU. In general, the computer program is stored in a computer-readable storage medium such as a CD-ROM. The program can be executed by setting it in a reading device (such as a CD-ROM drive) of the computer and copying or installing it in the system. Therefore, it is obvious that such a computer-readable storage medium is also within the scope of the present invention.

Claims (5)

An image forming apparatus that performs printing on a recording medium by a multi-pass method,
Means for obtaining original print data used for printing in each pass;
Using the original print data used in each pass from the first pass to the nth (n> 1) pass, among the images indicated by the respective original print data, it is within a preset density value correction target area. A calculation means for obtaining a total value of density values;
Acquisition means for acquiring the density value in the density value correction target area of the image printed on the recording medium in each pass from the first pass to the n-th pass before printing the (n + 1) pass. When,
Means for obtaining a difference density value between the density value acquired by the acquisition means and the density value calculated by the calculation means before printing of the (n + 1) th pass;
Correction means for correcting original print data used in the printing using the difference density value before printing of the (n + 1) th pass;
An image forming apparatus comprising: a printing unit that performs printing of the (n + 1) th pass using the print data corrected by the correcting unit.   The image forming apparatus according to claim 1, wherein the acquisition unit acquires a density value by a sensor provided in the print head every time printing is performed in each pass.   In the scanning when performing the printing of the (n + 1) th pass, the acquisition unit includes a density value correction target region in an image printed on the recording medium in each pass from the first pass to the nth pass. The image forming apparatus according to claim 2, wherein a density value is acquired. A method performed by an image forming apparatus that performs printing on a recording medium by a multipass method,
Acquiring original print data used for printing in each pass;
Using the original print data used in each pass from the first pass to the nth (n> 1) pass, among the images indicated by the respective original print data, it is within a preset density value correction target area. A calculation process for obtaining a total value of concentration values;
An acquisition step of acquiring a density value in the density value correction target area of an image printed on the recording medium in each pass from the first pass to the n-th pass before printing the (n + 1) pass. When,
A step of obtaining a difference density value between the density value acquired in the acquisition step and the density value calculated in the calculation step before printing of the (n + 1) th pass;
A correction step of correcting original print data used in the printing by using the difference density value before printing of the (n + 1) th pass;
An image forming method comprising: a printing step of performing printing in the (n + 1) th pass using the print data corrected in the correction step.   The computer program for functioning a computer as each means which the image forming apparatus of any one of Claims 1 thru | or 3 has.

Images