JP4315027B2 - Image background removal technology - Google Patents

Description

  The present invention relates to a technique for removing background noise contained in an image.

  When copying with a scanner / printer / copy multifunction machine or an image processing system including a scanner and a printer, image data acquired by reading an image original with the scanner is output by the printer.

  An image original is usually created by outputting an original image on paper as a printing medium. When the brightness of the paper is not sufficiently high, the image data obtained by reading the image original with the scanner may have a light gray color (hereinafter referred to as “background noise image”) in the background of the area representing the original image. . Therefore, when image data read by a scanner is output by a printer, a paper area that is originally white may not become white due to a background noise image. Such a background noise image can be suppressed by, for example, image processing that adjusts the contrast and level of the entire image data to make the color of the background noise image white. In general, the image processing for removing the background noise image is called “background removal”.

JP 2000-253250 A

  However, if the background is removed by adjusting the contrast or level of the entire image data, the color of the area that is not background noise changes due to image processing, so the image of the area that is not background noise may be output in an unnatural color. is there.

  The present invention has been made to solve the above-described conventional problems, and an object of the present invention is to provide a technique for performing background removal while suppressing the influence on the color of an area that is not background noise.

  In order to achieve at least a part of the above object, the color conversion apparatus of the present invention is a second device that represents the amount of a plurality of inks used for printing the first color system color represented by the input image data. A color conversion device that converts a color component value of the first color system from a color component value of the first color system, and a color conversion lookup table that generates a color component value of the second color system, A color conversion unit that generates an output value that is a color component value of the second color system from an input value that is a color component value of the first color system by referring to a color conversion lookup table; The color conversion unit has a high brightness mode for increasing the brightness of the color represented by the output value when the color represented by the input value is a color in a predetermined range including an achromatic color. It is characterized by.

  According to this configuration, a predetermined range of colors including an achromatic color can be output as a color with higher brightness, so that the background noise region is output as a color with higher brightness. On the other hand, since a change in a color that is not within a predetermined range including an achromatic color is suppressed, it is possible to suppress an image in an area that is not background noise from being output in an unnatural color. Therefore, the background can be removed while suppressing the influence on the color of the region that is not background noise.

  The color conversion unit may further include a reference mode for generating an output value representing a color whose brightness is lower than a color represented by the output value in the high brightness mode.

  According to this configuration, a change in the color of the entire image can be suppressed.

  The color conversion unit generates a high lightness input value representing a color whose lightness is higher than a color represented by the input value in the high lightness mode, and uses the high lightness input value to generate the color conversion lookup table. It is also possible to obtain an output value representing a color with high brightness by referring to.

  According to this configuration, since the same color conversion lookup table can be used in different modes, the storage capacity required to hold the color conversion lookup table can be reduced.

  The color in the predetermined range may be a color whose brightness represented by the input value is not less than a first threshold value.

  According to this configuration, it is possible to suppress the color change in the low brightness area.

  In the high brightness mode, the color conversion unit further includes the input value when the brightness of the color represented by the input value is equal to or less than a second threshold having a value equal to or less than the first threshold. A low lightness input value representing a color having a lower lightness than the color represented by is generated, and an output value representing a low lightness color is obtained by referring to the color conversion lookup table using the low lightness input value It is good also as what to do.

  According to this configuration, the color of a region such as a black character with low brightness is output as a color with lower brightness, so that the quality of the output image can be improved.

  The color conversion unit generates an output value representing a color having a high lightness by adjusting an output value obtained by referring to the color conversion lookup table from the input value in the high lightness mode. It is good.

  Also with this configuration, it is possible to remove the background while suppressing the influence on the color of the region that is not background noise.

  The color conversion device further represents a color conversion source lookup table for generating the color component value of the second color system from the color component value of the first color system, and the color in the predetermined range. A lookup table generation unit that generates the color conversion lookup table by modifying the output value of the color conversion original lookup table so as to increase the brightness of the output value relative to the input value.

  According to this configuration, since the generated color conversion lookup table can be used, the image processing time can be shortened.

  The color in the predetermined range may be a color whose brightness represented by the input value is not less than a first threshold value.

  Also with this configuration, it is possible to suppress the color change in the low brightness area.

  In the high brightness mode, the look-up table generation unit further includes the predetermined value when the brightness of the color represented by the input value is not more than a second threshold having a value not more than the first threshold. The output value of the color conversion source lookup table may be modified so that the brightness of the output value with respect to the input value representing the color in the range is reduced.

  Also with this configuration, the contrast of the image can be increased.

  The present invention can be realized in various modes. For example, an image processing method and an image processing apparatus, a printing method and a printing apparatus, a computer program for realizing the functions of these methods or apparatuses, and the like The present invention can be realized in the form of a recording medium on which a computer program is recorded.

Next, the best mode for carrying out the present invention will be described in the following order based on examples.
A. First embodiment:
B. Second embodiment:
C. Tone curve variations:
D. Variations:

A. First embodiment:
FIG. 1 is an explanatory diagram showing a configuration of a scanner printer copier 100 (hereinafter referred to as “SPC 100”) as an embodiment of the present invention. The SPC 100 includes a scanner unit 110, a printer unit 130, and a control unit 120 that controls the scanner unit 110 and the printer unit 130.

  The scanner unit 110 includes a light source for irradiating the image original with light and a CCD for converting the intensity of reflected light from the image original into an electric signal. The output signal of the CCD is converted into a digital signal by an A / D converter, thereby generating original image data.

  The printer unit 130 is an ink jet printer that forms an image by discharging a plurality of colors of ink onto a printing medium such as paper. The printer unit 130 ejects ink onto the print medium according to the print data supplied from the control unit 120. On the print medium, a color image is formed by forming dots (ink dots) of a color corresponding to the amount of ejected ink.

  The control unit 120 includes a control execution unit 200, an operation panel 300, and an I / F unit 310 that is an interface with an external computer. The control execution unit 200 is configured as a microcomputer including a CPU, ROM, RAM, and the like. The control execution unit 200 includes an image data acquisition unit 210 and a print data generation unit 220. The functions of these units 210 and 220 are realized by a computer program recorded in the ROM being executed by the CPU.

  The image data acquisition unit 210 performs image processing on the original image data supplied from the scanner unit 110, thereby generating image data (RGB image data) represented in the RGB color system. The generated RGB image data is supplied to the print data generation unit 220 or the I / F unit 310 according to the operation mode of the SPC 100.

  The print data generation unit 220 is supplied with RGB image data from the image data acquisition unit 210 or the I / F unit 310 according to the operation mode of the SPC 100. The print data generation unit 220 generates print data representing ink amounts for each of a plurality of colors used for printing in the printer unit 130 in accordance with the supplied RGB image data. This print data can be said to be image data expressed in a color system corresponding to the ink amount.

  The operation panel 300 includes an operation button for acquiring an input from a user, and a display device for displaying an operation mode, a set value, and the like of the SPC 100. The user can change the operation mode and setting value of the SPC 100 by pressing the operation button while viewing the setting content displayed on the display device.

  FIG. 2 is an explanatory diagram showing how the SPC 100 is set by operating the operation panel 300. The operation panel 300 includes operation buttons such as a copy button CB and a start button SB, and a display device DP.

  FIG. 2A shows that the copy button CB is pressed by the user and the copy mode is selected as the operation mode of the SPC 100. In the display device DP, characters of “copy setting” indicating that various settings in the copy mode can be changed, setting items such as “copy”, “number of sheets”, “magnification”, and setting of each setting item The value is displayed.

  FIG. 2B shows a state where the menu button MB is pressed by the user and the copy mode setting is changed. When the menu button MB is pressed, a triangle indicating that the setting value can be changed is displayed on the left and right of the setting value to be changed. The setting item to be changed is selected by pressing the selection button XB up or down. The setting value of the selected setting item is changed by the user pressing the left or right of the selection button XB.

  FIG. 2C shows how the setting of “copy”, which is the type of copy, is changed among the setting items of the copy mode. In the example of FIG. 2C, the setting value of the “copy” mode is changed from the “photo” mode to the “clear background” mode. After the setting value of each setting item is changed in this way, when the user presses the OK button OB, the changed setting value is supplied to the control execution unit 200 (FIG. 1). The control execution unit 200 stores the supplied setting value in a temporary storage device such as a RAM.

  Execution of copying is started by the user pressing the start button SB. When the start button SB is pressed, the image data acquisition unit 210 (FIG. 1) acquires RGB image data representing the image of the image document placed on the document table of the scanner unit 110 (FIG. 1). The acquired RGB image data is converted into print data by the print data generation unit 220 and supplied to the printer unit 130.

  FIG. 3 is a block diagram illustrating a configuration of the print data generation unit 220 in the first embodiment. The print data generation unit 220 includes a resolution conversion unit 222, a color conversion unit 224, a halftone processing unit 226, a data arrangement unit 228, a lookup table selection unit 230, and a plurality of color conversion lookup tables 232. I have.

  The resolution conversion unit 222 converts the resolution of the input RGB image data R, G, and B into a printing resolution. The print resolution image data R ′, G ′, and B ′ output from the resolution converter 222 is image data in which the color of each pixel is represented by an RGB value. That is, the print resolution image data R ′, G ′, and B ′ are image data represented by the RGB color system.

  The color conversion unit 224 includes a color adjustment unit 240. The color adjustment unit 240 is supplied with color adjustment setting values determined according to various parameters stored in the control execution unit 200 (FIG. 1). When the “copy” mode is set to the “clear background” mode as in the example of FIG. 2C, the color adjustment unit 240 prints the print resolution image data R′G based on the supplied color adjustment setting value. Internal image data rgb is generated from 'B'. The conversion from the print resolution image data R′G′B ′ to the internal image data rgb by color adjustment will be described later. On the other hand, when the “copy” mode is set to the “photo” mode for reproducing the color of the image original more accurately, the color adjustment by the color adjusting unit 240 is not performed.

  The color conversion unit 224 converts the internal image data rgb into ink amount data CMYK using one color conversion lookup table 232. A series of processes performed by the color conversion unit 224 is a process of converting the print resolution image data R′G′B ′ or rgb represented by the RGB color system into image data CMYK represented by the CMYK color system. Equivalent to.

  A plurality of color conversion lookup tables 232 suitable for each of a plurality of printing modes are stored in a ROM provided in the control execution unit 200 (FIG. 1). The look-up table selection unit 230 selects one table suitable for the print mode used at the time of actual printing from among the plurality of color conversion look-up tables 232. The color conversion unit 224 performs color conversion using the selected table. In the present embodiment, the color conversion lookup table 232 is selected according to two parameters of the print medium and the print resolution.

  The color conversion lookup table 232 outputs ink amounts CMYK corresponding to input points that are specific values of RGB values input to the color conversion unit 224. The color conversion unit 224 outputs an ink amount CMYK from the RGB values by performing an interpolation operation on the input RGB values as necessary. The input points of the color conversion lookup table 232 are often set at regular intervals in the range of the RGB component values, and such input points are also called grid points.

  The halftone processing unit 226 generates dot formation data Dc, Dm, Dy, and Dk indicating the dot formation state for each print pixel by executing halftone processing for each ink. The data arrangement unit 228 arranges these dot formation data Dc, Dm, Dy, Dk and outputs them as print data PD.

  FIG. 4 is a flowchart showing a procedure for generating the internal image data rgb by the color adjustment unit 240. FIG. 5 is an explanatory diagram showing the result of the color adjustment process. In the routine of FIG. 4, each pixel of the print resolution image data R′G′B ′ is sequentially analyzed (hereinafter, the pixel to be analyzed at this time is referred to as “target pixel”), and the color of the target pixel is adjusted. To generate internal image data rgb.

  FIG. 5A shows print resolution image data GD1 supplied to the color adjustment unit 240. The image data GD1 is generated by reading an image original including black characters (corresponding to the black characters LTR of the image data GD1) and photographs (corresponding to the photographs PHT of the image data GD1) by the scanner unit 110. Print resolution image data. The image data GD1 includes a background noise image PCL representing the color of the paper (ground color) in addition to the black characters LTR and the photograph PHT which are images included in the image original.

  In step S100, the color adjustment unit 240 acquires an RGB value that is color information of the target pixel. In step S102, the color adjustment unit 240 calculates a saturation value S from the acquired RGB values. The saturation value S is obtained using, for example, the following formulas (1) to (3).

C ₁ = 0.7 × R−0.59 × G−0.11 × B (1)
C ₂ = −0.3 × R−0.59 × G + 0.89 × B (2)
S = (C ₁ ² + C ₂ ² ) ^1/2 (3)

  In this embodiment, the saturation value S is obtained by calculation according to the equations (1) to (3). Instead, a table having saturation values S corresponding to RGB values. It is good also as what is calculated | required using.

  In step S104, the color adjustment unit 240 determines whether the saturation value S of the target pixel is equal to or less than a predetermined saturation threshold St. If the saturation value S of the target pixel is equal to or less than the saturation threshold St, that is, if the color of the target pixel is in a predetermined range including an achromatic color, control is transferred to step S106. On the other hand, when the saturation value S of the target pixel is larger than the saturation threshold value St, control is transferred to step S108, and the color of the target pixel is not changed. In the example of FIG. 5, the color of the PHT, which is a photograph area in the image data GD1, is not changed because the saturation value S is greater than the saturation threshold St.

  The saturation threshold value St is, for example, the RGB image data (color sample image data) output from the image data acquisition unit 210 when a test chart having a different color area such as a color sample is read as an image original. Can be determined based on. Specifically, when tone conversion is performed on the color sample image data (described later), a color region in which a hue change is allowed is determined, and a saturation value S calculated from RGB values representing the color of the region. Can be set as the saturation threshold St.

  The saturation threshold value St can be obtained, for example, by storing the value obtained as described above in a ROM or a non-volatile RAM included in the control execution unit 200 and reading it out as necessary. Moreover, it is good also as what a user sets as needed using a test chart.

  In step S106, the color adjusting unit 240 adjusts the color of the target pixel by rewriting the three color component values of RGB of the target pixel. The color adjustment of the pixel of interest is performed by gradation conversion in which the color adjustment unit 240 corrects each RGB color component value according to gradation conversion characteristics (tone curve).

FIG. 6 is a tone curve TC used for color adjustment of the target pixel in the first embodiment. The horizontal axis of FIG. 6 indicates the input value of the color component (input color component value I _X ), and the vertical axis of FIG. 6 indicates the output value of the color component when gradation conversion is performed on the input color component value I _X. (Output color component value O _x ). The tone curve TC is expressed by the following equations (4) to (6) using the intermediate reference value I _M and the highlight reference value I _H. A broken line SC in FIG. 6 indicates a reference tone curve when color adjustment is not performed (O _X = I _X ).

O _X = I _X 0 ≦ I _X <I _M (4)
O _X = Δ (I _X −I _M ) + I _M I _M ≦ I _X <I _H (5)
However, Δ = (255−I _M ) / (I _H −I _M )
O _X = 255 I _H ≦ I _X ≦ 255 (6)

Thus, in the tone curve TC, when the input color component value I _X is smaller than the intermediate reference value I _M , the output color component value O _X is the same value as the input color component value I _X (O _X = I _X ). Is output. On the other hand, when the input color component value I _X is in the range between the intermediate reference value I _M and the highlight reference value I _H , the tone curve slope Δ is larger than 1, so that the output color component value O _X is input. A value larger than the color component value I _X is output. When the input color component value I _X is equal to or higher than the highlight reference value I _H , the maximum value (255) of the output color component value O _X is output as the output color component value O _X.

The tone curve TC of FIG. 6 is depicted as having an intermediate reference value I _M of 127 and a highlight reference value I _H of 230. These intermediate reference value I _M and highlight reference value I _H Can be set to an appropriate value such that the color of the background noise image PCL is converted to white. Such intermediate reference value I _M and highlight reference value I _H can be determined based on, for example, RGB image data (blank image data) output by the image data acquisition unit 210 when a blank sheet is read. Specifically, a value slightly smaller than the minimum RGB value of the blank image data is set as the highlight reference value I _{H so} that the slope Δ of the tone curve does not exceed a predetermined value (for example, 1.5). Is set to the intermediate reference value I _M.

These reference values I _M and I _H are acquired by storing predetermined values in the ROM or non-volatile RAM included in the control execution unit 200 and reading them out as necessary, as with the saturation threshold value St. be able to. The user may be able to change the settings of these reference values I _M and I _H as necessary.

If RGB component values of the pixel of interest is greater than the intermediate reference value I _M (high brightness), by performing tone conversion using the tone curve TC, the color of the target pixel is changed to a higher color brightness . In particular, when the RGB color component values of the target pixel are larger than the highlight reference value I _H , the color of the target pixel is changed to white (R = G = B = 255). On the other hand, when the brightness of the pixel of interest is low, the color component value of the pixel of interest does not change, so the color of the pixel of interest is not changed. Thus, when the lightness of the color of the target pixel is higher than a predetermined threshold (R = G = B = I _M ), the lightness of the color of the target pixel is increased by tone conversion using the tone curve TC.

In the example of FIG. 5, among the images PCL and LTR of the image data GD1 whose saturation value S is equal to or less than the saturation threshold St, the background noise image PCL is an image with high brightness. Therefore, the background noise image PCL is color-adjusted so that the brightness is higher. The background noise image PCL is converted into a white image PCLa as shown in FIG. 5B because the highlight reference value I _H is set as described above. On the other hand, since the black character LTR has low brightness, the color is not changed, and the same image LTR is obtained before and after the color adjustment.

  In step S <b> 108, the color adjustment unit 240 determines whether or not the color adjustment processing for all pixels of the print resolution image data R′G′B ′ has been completed. When it is determined that the color adjustment for all the pixels has been completed, this color adjustment routine ends. On the other hand, if there is a pixel that has not been subjected to color adjustment processing, the target pixel is set as an unprocessed pixel, and control returns to step S100. Then, the color adjustment unit 240 repeatedly executes steps S100 to S108 until the color adjustment of all pixels is completed.

  In this way, the color adjustment unit 240 does not change the color of the pixels of the high-saturation region like a photograph and the low-lightness region like a character in the print resolution image data R′G′B ′. Therefore, these areas in the internal image data rgb are the same image as the print resolution image data R′G′B ′. On the other hand, since the ground color region with low saturation and high lightness becomes white by color adjustment, the image of the ground color region does not appear in the internal image data rgb.

  Since the color conversion unit 224 of the first embodiment performs color conversion of the internal image data rgb generated by the color adjustment unit 240, the color conversion unit 224 is white (with respect to the ground color region of the print resolution image data R′G′B ′). Ink amount data CMYK representing C = M = Y = K = 0) is output. On the other hand, since the colors of areas such as photographs and characters are not changed by the color adjustment unit 240, ink amount data CMYK suitable for printing is output to these areas. As a result, the color of the paper can be output as white with an ink amount of 0 while suppressing changes in the color of photographs and characters, so that the output image when the input image generated by reading the image original is output. Quality is improved.

  In the color conversion unit 224 of the first embodiment, the color adjustment by the color adjustment unit 240 is the same as the color conversion lookup table 232 used in the mode in which color adjustment is not performed (“photo” mode). The color conversion lookup table 232 can be used in a color adjustment mode (“background refresh” mode). Therefore, it is not necessary to prepare a plurality of color conversion lookup tables 232 for each mode, so that the capacity of the storage device mounted on the SPC 100 can be reduced.

B. Second embodiment:
FIG. 7 is a block diagram showing the configuration of the print data generation unit 220a in the second embodiment. The print data generation unit 220a according to the second embodiment is provided with a lookup table generation unit 242 in place of the lookup table selection unit 230 (FIG. 3) and the color adjustment unit 240 (FIG. 3). It is different from the example. The lookup table generation unit 242 selects one table suitable for the print mode used at the time of actual printing from the plurality of color conversion source lookup tables 232a. The lookup table generation unit 242 generates a color conversion lookup table LUT supplied from the selected color conversion source lookup table 232a to the color conversion unit 224a. The generation of the color conversion lookup table LUT will be described later. The color conversion unit 224a converts the print resolution image data R′G′B ′ into ink amount data CMYK using the color conversion lookup table LUT generated by the lookup table generation unit 242. Other configurations are almost the same as those of the first embodiment.

  The color conversion lookup table LUT is generated by the lookup table generation unit 242 when the set value of the “copy” mode is changed to the “clear background” mode as in the example of FIG. On the other hand, when the setting value of the “copy” mode is not changed and remains in the “photo” mode, the color conversion unit 224a selects one table selected from the plurality of color conversion original lookup tables 232a. The print resolution image data R′G′B ′ is converted into ink amount data CMYK.

  FIG. 8 is a flowchart showing a procedure for generating a color conversion lookup table LUT by the lookup table generator 242 in the second embodiment. In the routine of FIG. 8, each grid point of the color conversion lookup table LUT is sequentially analyzed (the grid point to be analyzed at this time is referred to as “target grid point”), and the color conversion source lookup is performed on the target grid point. Ink amount data (hereinafter also referred to as “color conversion value”) determined according to the table 232a and the color adjustment setting value is set.

  FIG. 9 is an explanatory diagram showing a state of the process of generating the color conversion lookup table LUT in the second embodiment. The numerical values in parentheses attached to the grid points P1 to P5 represent the color component values in the RGB color system of the grid points P1 to P5.

In step S <b> 200, the lookup table generation unit 242 acquires RGB values that are color information of the target lattice point. In step S202, the lookup table generation unit 242 calculates a displacement amount D that represents the distance between the target grid point and the gray axis GRY that represents the achromatic color from the acquired RGB values R, G, and B. Note that the displacement amount D is obtained by using, for example, the RGB values R, G, B of the target lattice point and the RGB values R ₀ , G ₀ , B ₀ of the lattice point on the gray axis GRY closest to the target lattice point. It can be calculated by the following equation (7).

D = max (| R−R ₀ |, | G−G ₀ |, | B−B ₀ |) (7)

  In step S204, the lookup table generation unit 242 determines whether the displacement amount D of the target lattice point is equal to or less than a predetermined displacement amount threshold value Dt. When the displacement amount D of the target lattice point is equal to or less than the displacement amount threshold value Dt, that is, when the color of the target lattice point is a color within a predetermined range including an achromatic color, the control is moved to step S206. On the other hand, when the displacement amount D of the target lattice point is larger than the displacement amount threshold value Dt, the control is moved to step S208. In FIG. 9, lattice points represented by circles are lattice points whose displacement amount D is equal to or less than the displacement amount threshold value Dt. Among these, white circles indicate lattice points on the gray axis GRY, and black circles indicate lattice points outside the gray axis GRY.

  Note that the predetermined displacement amount threshold value Dt can be set to, for example, the displacement amount D calculated from the RGB values of grid points adjacent to the gray axis GRY. Further, the saturation value S in the first embodiment may be used as the displacement amount D, and the displacement amount threshold value Dt may be set to the saturation threshold value St in the first embodiment.

In step S206, the lookup table generation unit 242 performs gradation conversion on the RGB component values of the target lattice point in the same manner as in the first embodiment. In the example of FIG. 9, the RGB component values of the grid points P4 and P5 take values between the intermediate reference value I _M (FIG. 6) and the highlight reference value I _H (FIG. 6). As a result, the RGB component values are changed. Therefore, the RGB component values of the points P4a and P5a are used for setting the color conversion values of the grid points P4 and P5. On the other hand, since the RGB component values of the grid points P1, P2, and P3 are not changed even by gradation conversion, the RGB conversion values of the grid points P1, P2, and P3 are set for the RGB conversion values of the grid points P1, P2, and P3. The component value is used as it is.

  In step S208, the lookup table generation unit 242 sets color conversion values corresponding to the RGB component values subjected to gradation conversion to the target grid point. The noticed grid points whose displacement amount D is larger than the displacement region value Dt and the noticed grid points whose RGB component values are not changed by gradation conversion are color conversion values C0, M0, Y0, K0 of the color conversion original lookup table 232a. Is set as is. Color conversion values CMYK corresponding to the changed RGB component values are set at the target grid points whose RGB component values have been changed by gradation conversion. In this case, since the points represented by the changed RGB component values deviate from the grid points, the color conversion value CMYK is obtained by an interpolation operation using the color conversion original lookup table 232a. In the example of FIG. 9, since the RGB component values at the grid points P4 and P5 are changed by gradation conversion, color conversion values CMYK corresponding to the points P4a and P5a are set at the grid points P4 and P5. On the other hand, the RGB component values of the grid points P1, P2, and P3 are not changed by the gradation conversion, so that the color conversion values C0, M0, Y0, and K0 of the color conversion original lookup table 232a are set.

  As described above, the color conversion lookup table LUT generated by the lookup table generation unit 242 of the second embodiment has the color conversion source lookup table 232a when an input value representing a predetermined range of colors is input. The output value is corrected so that it represents a color with higher brightness.

  In step S210, the lookup table generation unit 242 determines whether the setting of color conversion values for all grid points has been completed. If it is determined that the setting of the color conversion values for all grid points has been completed, this lookup table generation routine ends. On the other hand, if there is a grid point for which no color conversion value has been set, the target grid point is set to an unprocessed grid point, and control returns to step S200. Then, the lookup table generation unit 242 repeatedly executes steps S200 to S210 until the setting of the color conversion values of all grid points is completed.

  The lookup table LUT of the second embodiment generated in this way represents a color with higher brightness when the brightness of the color represented by the grid point is high when grid points near the gray axis GRY are input. Output the output value. When the lightness represented by the grid points near the gray axis GRY is equal to or greater than a predetermined value, an output value representing white (C = M = Y = K = 0) is output. For this reason, when color conversion is performed using a color conversion look-up table LUT for a color with low saturation and high lightness, such as a ground color, the ink amount CMYK becomes almost zero.

  Also in the second embodiment, as in the first embodiment, in the ground color area, the ink amount is output as white with almost 0, and the non-ground color area is output in an appropriate color. Therefore, in the second embodiment as well, since the color of the paper can be output as white while suppressing the change in the color of the photograph or character, the output image when outputting the image generated by reading the image original The quality is improved. Also in the second embodiment, since the same color conversion source lookup table 232a can be used in a plurality of modes, the storage capacity required to hold the color conversion source lookup table 232a can be reduced.

  The second embodiment is preferable to the first embodiment in that the color adjustment processing for each pixel is not performed, and thus the time required for the processing for improving the quality of the output image can be shortened. On the other hand, since the first embodiment performs color adjustment for each pixel, it is preferable to the second embodiment in that the value of the ink amount for the ground color pixels can be set to zero.

  In the second embodiment, the brightness of the color represented by the output value is increased by adjusting the output value set to the grid point. However, the output value generated using the color conversion original lookup table 232a It is good also as what raises the brightness of the color which an output value represents by adjusting. For example, when the color represented by the input value is in a predetermined range including an achromatic color, the output ink amount may be reduced based on a predetermined conversion characteristic.

C. Tone curve variations:
FIG. 10 shows a modification of the tone curve used for gradation conversion. In the above embodiments, although performing tone conversion using the tone curve TC shown in FIG. 6, the tone curve used for the gradation conversion, even more input color component value I _X larger output color component value O _X Can be output. For example, tone curves TCa, TCb, TCc as shown in FIG. 10 can be used for gradation conversion. Even using these tone curves TCa, TCb, TCc, gradation conversion can be performed to increase the brightness.

C1. Variation of tone curve 1:
The tone curve TCa is different from the tone curve TC of FIG. 6 in that the output color component value O _X is set to 0 when the input color component value I _X is equal to or less than the shadow reference value I _L. By using the tone curve TCa, a predetermined range of colors including an achromatic color is black (R = G = B = 0) when the lightness is equal to or lower than a predetermined lightness threshold (R = G = B = I _L ). ). Since this black color is formed by the maximum amount of ink dots of black ink (K), the quality of the black character portion is improved when an input image including black characters is printed.

C2. Tone curve modification 2:
The tone curve TCb is different from the tone curve TC of FIG. 6 in that the input color component value I _X is a smooth curve in the entire range from 0 to 255. In the tone curve TCb, since the output color component value I _X is larger than the input color component value I _{X in} the entire range of the input color component value I _X , color adjustment is performed so that the brightness is high regardless of the brightness level. . When the tone curve TCb is used for gradation conversion, the gradation change at the highlight reference value I _H becomes smooth, so that the quality of the image in the region where the brightness changes continuously is improved.

C3. Variation 3 of the tone curve:
The tone curve TCc is different from the tone curve TCb in that the output color component value O _X is smaller than the input color component value I _{X in} the range of the input color component value I _X from 0 to the intermediate reference value I _M. . Also by this tone curve TCc, the gradation change at the highlight reference value I _H becomes smooth, so that the quality of the image in the region where the brightness changes continuously is improved. Also by using a tone curve TCc, the input color component value I _X is output color component value I _X is smaller than the input color component value I _X The smaller intermediate reference value I _M, brightness predetermined brightness threshold value ( R = G = B = I _x ) or less, it is possible to perform gradation conversion that lowers the brightness. Therefore, the quality of the black character portion is improved when an input image including a black character with low brightness is printed.

D. Variations:
In addition, this invention is not restricted to the said Example and embodiment, It can implement in a various aspect in the range which does not deviate from the summary, For example, the following deformation | transformation is also possible.

D1. Modification 1:
In each of the above embodiments, the color conversion units 224 and 224a improve the quality of the output image by using either the color adjustment unit 240 or the lookup table generation unit 242, but for the improvement of the quality of the output image, It is only necessary that the brightness of the color represented by the output value can be increased with respect to the input value representing a predetermined range of colors including an achromatic color. For example, the color conversion unit performs color conversion on an input value representing a color whose lightness is higher than a predetermined value by using the color conversion unit 224 of the first embodiment, and an input representing a color whose lightness is lower than the predetermined value. The value may be subjected to color conversion using the color conversion unit 224a of the second embodiment. In this way, it is possible to perform more appropriate color adjustment in a high brightness area where the effect of background removal is high, and to shorten the image processing time for a low brightness area.

D2. Modification 2:
In each of the embodiments described above, image data represented in the RGB color system is input to the print data generation units 220 and 220a. However, any data that can represent a color image is represented in any color system. The image data may be input. As a color system representing image data, for example, an L * a * b * color system or a YCbCr color system can be used.

D3. Modification 3:
In each of the above-described embodiments, the printer unit 130 uses four types of CMYK inks. However, instead of this, arbitrary plural types of inks may be used. In this case, the color conversion lookup table 232, the LUT or the color conversion original lookup table 232a has a plurality of types of ink amounts with respect to the color system input of the image data supplied to the print data generation units 220 and 220a. It is only necessary to output a color conversion value in the corresponding color system.

D4. Modification 4:
In each of the embodiments described above, the print data generation units 220 and 220a are applied to the SPC 100. However, the print data generation units 220 and 220a can be applied to any processing apparatus that generates print data. In this case, for example, the print data generation units 220 and 220a may be incorporated in a printer driver that operates on a personal computer.

Explanatory drawing which shows the structure of SPC as one Example of this invention. Explanatory drawing which shows the mode of the setting of SPC by operation of an operation panel. FIG. 3 is a block diagram illustrating a configuration of a print data generation unit in the first embodiment. 6 is a flowchart showing a procedure for generating internal image data rgb by a color adjustment unit. Explanatory drawing which shows the processing result of the image data by a color adjustment part. FIG. 3 is a tone curve used for color adjustment of a target pixel in the first embodiment. FIG. The block diagram which shows the structure of the print data generation part in 2nd Example. 12 is a flowchart illustrating a procedure for generating a color conversion lookup table LUT by a lookup table generation unit according to the second embodiment. Explanatory drawing which shows the mode of the production | generation process of the color conversion lookup table by the lookup table production | generation part in 2nd Example. A modification of the tone curve.

Explanation of symbols

100 ... SPC
DESCRIPTION OF SYMBOLS 110 ... Scanner unit 120 ... Control unit 130 ... Printer unit 200 ... Control execution part 210 ... Image data acquisition part 220,220a ... Print data generation part 222 ... Resolution conversion part 224,224a ... Color conversion part 226 ... Halftone processing part 228 ... Data arrangement section 230 ... Look-up table selection section 232 ... Color conversion lookup table 232a ... Color conversion source lookup table 240 ... Color adjustment section 242 ... Look-up table generation section 300 ... Operation panel 310 ... I / F section CB ... Copy button DP ... Display device LUT ... Color conversion lookup table MB ... Menu button SB ... Start button XB ... Select button

Claims (2)

A color conversion device that converts a color of a first color system represented by input image data into a color of a second color system representing a plurality of ink amounts used for printing,
A color conversion original lookup table associating the color component values of the second color system with the grid points that are the color component values of the first color system;
A lookup table generation unit that generates a color conversion lookup table for associating the color component values of the second color system with the grid points from the color conversion source lookup table;
Depending on the mode, by referring to either the color conversion source look-up table or the color conversion look-up table, the second table can be obtained from the input value which is the color component value of the first color system. A color conversion unit that generates an output value that is a color component value of the color system;
With
The color conversion unit, when the input value is out of the grid point, the second color specification associated with the grid point in the color conversion original lookup table or the color conversion lookup table. Generating the output value by interpolating the colors of the system;
The lookup table generator is
For the grid point of the color conversion lookup table representing a predetermined range of colors including an achromatic color, the lightness is higher than the color represented by the grid point as the color component value of the associated second color system. An output value generated by referring to the color conversion source lookup table by the color conversion unit using a high brightness color component value representing a high color as an input value;
For the grid point of the color conversion lookup table representing the color outside the predetermined range, the corresponding grid point of the color conversion source lookup table is used as the color component value of the second color system to be associated. A color conversion device that generates the color conversion lookup table by setting a color component value of a second color system. A color conversion apparatus according to claim 1 Symbol placement,
The color conversion source lookup table is a color conversion device selected from a plurality of color conversion source lookup tables.

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