JP2003125217A - Image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processor capable of suppressing deviation in the gradation reproducibility without unbalancing color balance even when a screen pattern and an output form are revised. SOLUTION: An RGB signal of an input image is converted into an L*a*b* signal and its background is corrected by using an INLUT 30 set to a multi- value mode being one of output forms of an image output section 16. A color space conversion section 32 comprises; a DLUT 44A being a multi-value mode use three-dimensional lookup table; a binary mode use three-dimensional lookup table DLUT 44B; and a selector 42 to select the DLUT 44A or DLUT 44B according to output form data 40 denoting an output form set by a user's operation or the like. A lightness correction section 38 includes a lightness correction LUT 39 corresponding to the binary mode and rewrites table data so that the DLUT 44B has characteristics in response to the binary mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus, and more particularly to an image processing apparatus capable of detecting and removing background density of a read document.

[0002]

2. Description of the Related Art In image processing apparatuses such as copiers and fax machines,
When the read image is output as it is, the optimum output may not be obtained due to, for example, the background density of the document. In such a case, in order to obtain an optimum output without being affected by the background density, there is a method in which the background density of the paper is detected, and the background is removed by image processing to output.

When performing image processing, for example, an RGB signal which is an image input signal is converted into a device-independent L ^* a ^* b ^* signal by a three-dimensional LUT (look-up table), and this L ^* a ^* b ^* signal is converted. ^* Image processing is performed on the signal, converted into a YMCK signal by the three-dimensional LUT, and output to the image output unit.

The image output unit outputs the read image in a set output format, for example, a screen pattern (screen frequency) and a halftone processing mode (binary mode, multivalued mode).
However, when the output format is changed, the gradation reproducibility of the same image, especially the gradation reproducibility of the highlight part, may be different. For example, even if a low-tone halftone image is output in the binary mode or the multi-valued mode, if the reproduction start point (threshold for removing the background) is lowered in accordance with the multi-valued mode, the binary mode is used. In the case of output, the pixels are likely to stand out.

In order to absorb this deviation in gradation reproducibility, gradation correction is performed on the YMCK signal, or as described in Japanese Patent Application Laid-Open No. 10-200771,
It is conceivable to change the value of YMCK of the dimension LUT.

[0006]

However, the above-mentioned conventional technique has a problem that the color balance (hue) may be lost because the value of YMCK is directly changed.

The present invention has been made in view of the above facts, and provides an image processing apparatus capable of suppressing deviation in gradation reproducibility without degrading color balance even when a screen pattern or an output format is changed. The purpose is to provide.

[0008]

In order to achieve the above object, the invention according to claim 1 uses a first color signal corresponding to a first color system for an input image as a second color system. A plurality of three-dimensional look-up tables of conversion relations corresponding to each of the image output units for converting the corresponding second color signal and outputting the input image, and the background density of the input image Detecting means for detecting from the first color signal;
The detected background density and a predetermined output format among the plurality of output formats of the image output unit are set based on
A background correction lookup table that corrects the background level of the input image, and an output image that corresponds to an output format different from the predetermined output format and has the background level corrected in the predetermined output format. A brightness correction lookup table for correcting an input image having a background level correction according to a format, and setting the brightness correction lookup table based on the detected background density, and at least a part of the first color signal Is calculated by the brightness correction lookup table, the second color signal corresponding to the corrected first color signal is obtained from the three-dimensional lookup table corresponding to the different output format, and the obtained 3D look-up table corresponding to the different output formats so that the second color signal corresponds to the uncorrected first color signal. Characterized by comprising changing means for changing the Le, the.

The three-dimensional look-up table is a first color signal corresponding to the first color system, such as L ^* a ^* , for an input image input by an image input unit such as a scanner ^.
The b ^* signal is converted into a second color signal corresponding to the second color system, for example, a YMCK signal. A plurality of these three-dimensional look-up tables are provided, and these are three-dimensional look-up tables having a conversion relationship corresponding to each of a plurality of output formats of the image output unit for outputting the input image.

The output format includes at least one of a screen pattern (screen ruling etc.) and a halftone processing method (binary or multi-valued) of the image output section, as set forth in claim 3, for example.

That is, a plurality of three-dimensional lookup tables are provided according to the output format of the image output unit.

The detecting means detects the background density of the input image from the first color signal of the input image. The background correction look-up table is set based on the background density detected by the detection means and a predetermined output format among a plurality of output formats of the image output unit. This background correction lookup table corrects the background level of the input image.

The changing means corrects the background level-corrected input image corresponding to the output format different from the predetermined output format and having the predetermined output format to the background level-corrected input image according to the different output format. It is equipped with a brightness correction lookup table. The background correction lookup table corresponds to a predetermined output format among a plurality of output formats. Therefore, when the output format of the image output unit is changed to an output format different from the predetermined output format, the background correction table is not changed. In that case, the gradation reproducibility and the like deteriorate. Therefore, the brightness correction lookup table enables correction of the background-level-corrected input image in the predetermined output format into the background-level-corrected input image in the changed output format.

This brightness correction lookup table is set based on the detected background density. Further, since the brightness correction lookup table corrects only the brightness, the color balance is not lost.

When the output format of the image output section is set to a different output format from the predetermined output format, the changing means corrects the first color signal with the brightness correction lookup table, A second color signal corresponding to the corrected first color signal is obtained from the three-dimensional lookup table corresponding to the different output format. And
The three-dimensional lookup table corresponding to the different output format is changed so that the obtained second color signal corresponds to the uncorrected first color signal.

As described above, even when the look-up table for background correction is set for a predetermined output format, 3 corresponding to an output format different from the predetermined output format is used.
Since the dimensional lookup table is changed, it is possible to suppress the deviation in gradation reproducibility even when the output format is changed. Further, since only the brightness is corrected, it is possible to suppress the deviation of the gradation reproducibility without degrading the color balance.

According to a second aspect of the present invention, for the input image, the first color signal corresponding to the first color system is converted into the second color signal corresponding to the second color system, and the input image is converted into the second color signal. Based on a plurality of three-dimensional lookup tables of conversion relations corresponding to each of a plurality of output formats of the image output section for outputting an image, and a predetermined output format of the plurality of output formats of the image output section A background correction lookup table that is set and corrects the background level of the input image, and in an image processing apparatus that corresponds to an output format different from the predetermined output format, and that is based on the predetermined output format. At least the first color signal by the brightness correction lookup table for correcting the background level corrected input image into the background level corrected input image in the different output format. Part is corrected, the second color signal corresponding to the corrected first color signal is obtained from the three-dimensional lookup table corresponding to the different output format, and the obtained second color signal is The three-dimensional look-up table corresponding to the different output format is previously changed by the changing means for changing the three-dimensional look-up table corresponding to the different output format so as to correspond to the first color signal before correction. It is characterized by

According to the present invention, the three-dimensional look-up tables corresponding to different output formats are previously changed, that is, corrected by the changing means, so that the processing can be performed quickly and the apparatus can be simplified. can do.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION (First Embodiment) The first embodiment of the present invention will be described below. FIG. 1 shows a schematic block diagram of an image processing apparatus 10 according to this embodiment.

As shown in FIG. 1, the image processing apparatus 10 includes
Image input unit 12, image processing unit 14, and image output unit 16
It is composed of.

The image input unit 12 includes a CCD (charge coupled device) sensor 18, an A / D converter 20,
And a correction unit 22.

The CCD sensor 18 optically reads a document, and the read image is, for example, R, G, B.
Each color signal of 8 bits is decomposed and output. The A / D converter 20 converts the RGB signal of the image read by the CCD sensor 18 into a digital signal. Correction unit 22
Performs correction such as shading correction on the image data.

The image processing section 14 is composed of a color space conversion section 24, a scaling section 26, a filter 28, an INLUT 30, a color space conversion section 32, a gradation correction section 34, a background density detection section 36, and a lightness correction section 38. Has been done.

The color space conversion unit 24 is a color signal corresponding to the first color system of the RGB signals output from the image input unit 12, and is independent of the device, for example, L ^* a ^* b of 8 bits each.
^* Convert to signal. This conversion is performed by, for example, a three-dimensional LUT.

The enlargement / reduction unit 26 enlarges or reduces the image data according to the set enlargement ratio or reduction ratio.

The filter 28 uses the MT for image data.
Image processing such as F correction, sharpening processing, and smoothing is performed.

The INLUT 30 corrects the background level of the image data based on the background density detected by the background density detecting section 36. That is, the background is removed. INLUT3
0 is constituted by a look-up table having characteristics as shown by the dotted line in FIG. 3, for example. As shown in FIG.
The LUT 30 converts the input L ^* into L ^* ', a ^* ,
b ^* is output as it is, for example. When the input L ^* is less than or equal to the threshold TH1 ('32' in FIG. 3) which is the reproduction start point set based on the detected background density, the INLUT 30 sets the output L ^* 'to' 0 '. To As a result, the so-called background fogging phenomenon can be suppressed.
Further, since only the lightness is corrected, the fog phenomenon can be suppressed without destroying the color balance such as hue and saturation.

The color space conversion unit 32 is configured to include a DLUT (direct lookup table) and has L ^* a ^* b.
^{* The} signal is converted into a YMCK signal which is a color signal corresponding to the second color system for image output.

The tone correction unit 34 performs tone correction according to the image output unit 16. That is, the gradation correction is performed so as to cancel out the adverse effects of the device characteristics of the image output unit 16 (bad gray balance, gradation non-linearity, etc.).

As described above, when the output format of the image output unit 16 such as the screen pattern and the halftone processing mode (binary mode, multivalued mode) is changed, the gradation reproducibility, especially the highlight part The gradation reproducibility may be different. Therefore, in order to obtain the optimum output in each output format, the INLUT 30 and the DLUT included in the color space conversion unit 32 may be provided corresponding to each output format. However, it is not preferable to provide the INLUT 30 separately according to the output format because of cost reasons, and there is a case where the INLUT 30 has to be shared.

In the present embodiment, a case will be described in which the INLUT 30 is common and the color space conversion unit 32 is provided with DLUTs for the binary mode and the multivalued mode, as an example.

As shown in FIG. 2, the color space conversion unit 32 is
DL, which is a three-dimensional lookup table for multi-valued mode
D according to the UT 44A, the DLUT 44B which is a three-dimensional lookup table for the binary mode, and the output format data 40 indicating the output format set by the user's operation or the like.
It is composed of a selector 42 for switching to the LUT 44A or the DLUT 44B. Further, the gradation correction unit 34
Is composed of a gradation correction unit 34A for multi-value mode and a gradation correction unit 34B for binary mode. Gradation correction unit 34
A and the gradation correction unit 34B perform gradation correction suitable for the multi-value mode and the binary mode, respectively.

Here, the INLUT 30 is set for the multi-valued mode. That is, the characteristic indicated by the dotted line in FIG. 3 is the characteristic for the multi-valued mode. Therefore, when the output format is the binary mode, the L ^* a ^* b ^* signals are sent to the DLUT44.
If the BMC is converted to the YMCK signal as it is, the gradation reproducibility is deteriorated. Therefore, the DLUT 44B is reconfigured based on the brightness correction unit 38.

The brightness correction section 38 includes an LUT similar to the INLUT 30 shown by the dotted line in FIG. 3, and a brightness correction LUT 39 corresponding to the binary mode having the characteristic shown by the solid line in FIG. The threshold TH2, which is the reproduction start point in the brightness correction LUT 39, is set according to the detected background density, for example. Brightness correction unit 38 for correcting the original L ^*, before being input to INLUT30 from L ^* 'inputted to DLUT44B the L ^* INLUT3
0 is obtained from the LUT similar to 0, and L ^* "corresponding to L ^* is obtained from the brightness correction LUT. Then, the obtained L ^* ",
The value of YMCK corresponding to a ^* and b ^* is obtained from the DLUT 44B, and the DLUT 44B is rewritten with this value of YMCK.

For example, when the output value from the INLUT 30 is (L ^* 'a ^* ', b ^* ') = (48,16,16), L ^* before passing through the INLUT 30 is as shown in FIG.
It is '60'. Then, L ^* is '60' L ^* "at the time of the
Is 32 from the brightness correction LUT. Therefore,
The value of YMCK corresponding to (L ^* ", a ^* ', b ^* ') = (32, 16, 16) is obtained from the DLUT 44B, and the DLUT 44B is rewritten with the obtained value of YMCK. Thereby, the output from the INLUT 30 If the value is (L ^* 'a ^* ',
If b ^* ') = (48,16,16),
The value of YMCK corresponding to (L ^* ″, a ^* ′, b ^* ′) = (32, 16, 16) is output to the gradation correction unit 34B.
Then, the gradation correction unit 34B performs gradation correction suitable for the binary mode, and then outputs the image to the image output unit 16.

Thus, even if the INLUT 30 is set for the multi-value mode, the DLUT 44B for the binary mode is used.
Is rewritten to the table suitable for the binary mode by the brightness correction unit 38, the INLUT 30 can be shared, and the optimum output can be obtained even when the multilevel mode or the binary mode is switched.

When the INLUT 30 is set for the binary mode, the DLUT 44B for the multivalued mode is used.
May be rewritten by the brightness correction unit 38 into a table suitable for the multi-valued mode.

Next, as the operation of this embodiment, the operation of the image processing unit 14 will be described with reference to the flowchart shown in FIG. Here, a case where the binary mode is selected as the output format will be described.

In step 100, the color space conversion unit 24 performs color space conversion processing. That is, the RGB signals output from the image input unit 12 are color signals corresponding to the first color system, and are converted into, for example, 8-bit L ^* a ^* b ^* signals that are device-independent.

In the next step 102, the scaling unit 26
At, the scaling process is performed. That is, the image data is enlarged or reduced according to the enlargement ratio or the reduction ratio set by the user's operation or the like.

In the next step 104, the filter processing is performed in the filter 28. That is, image processing such as MTF correction, sharpening processing, and smoothing is performed on the image data.

In the next step 106, the INLUT 30
Thus, the background correction process is performed. That is, the background level of the image data is corrected based on the background density detected by the background density detecting unit 36. Specifically, as a characteristic shown by a dotted line in FIG. 3, to convert the L ^* input to the L ^* ', a ^*, b ^* is output as it is, for example. This allows
It is possible to suppress the so-called fog phenomenon in the background portion. Further, since only the lightness is corrected, the fog phenomenon can be suppressed without disturbing the color balance such as hue and saturation.

In the next step 108, the brightness correction section 38
The table data of the DLUT 44B is rewritten by. That is, the lookup table showing the characteristics of the INLUT 30 shown by the dotted line in FIG. 3 is used to find the L ^* before being input to the INLUT 30 from the L ^* ′ input to the DLUT 44B, and the L ^* ″ corresponding to this L ^* is shown. It is obtained from the brightness correction LUT showing the characteristic shown by the solid line of 3. Then, the value of YMCK corresponding to the obtained L ^* ″, a ^* ′, b ^* ′ is DL.
Obtained from UT44B, DLUT44 with this YMCK value
Rewrite B. Note that rewriting of the table data, all L ^*, a ^*, b ^* may be performed on, but of the predetermined range L ^*, a ^*, may be performed for b ^* only.

In the next step 110, the color space conversion unit 3
2, the color space conversion process is performed. Here, since the output format data 40 indicates the binary mode, the selector 42 selects the DLUT 44B. Therefore, the data output from INLUT30 is DLU.
Input to T44B. The DLUT 44B converts the input data into YMCK values in the table rewritten by the brightness correction unit 38 and outputs the YMCK values to the gradation correction unit 34B.
In this way, in addition to the highlight cut by the INLUT 30, the brightness correction suitable for the binary mode is performed.

In the next step 112, the gradation correction unit 34
Gradation correction suitable for the binary mode is performed by B, and the image data is output to the image output unit.

Thus, even if the INLUT 30 is set for the multi-valued mode, the DLUT 44B for the binary mode is used.
Is rewritten to the table suitable for the binary mode by the brightness correction unit 38, the INLUT 30 can be shared, and the optimum output can be obtained even when the multilevel mode or the binary mode is switched.

The process of step 108, that is, D
The table data of the LUT 44B may be rewritten in advance and the color conversion may be performed by the corrected DLUT 44B from the beginning. As a result, the processing can be performed quickly, the brightness correction unit 38 can be omitted, and the apparatus can be simplified.

(Second Embodiment) Next, a second embodiment will be described. In the second embodiment, the INLUT 30 is omitted, and the INLUT 3 is replaced by the DLUTs 44A and 44B.
A case of performing the same correction as the background correction by 0 will be described. The same parts as those in the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 5 shows a schematic configuration of an image processing apparatus 10 'according to this embodiment. The image processing apparatus 10 'is
The NLUT 30 is omitted, and as shown in FIG.
The table data of the LUT 44A is rewritten by the lightness correction unit 38A, and the table data of the DLUT 44B is rewritten by the lightness correction unit 38B. Brightness correction unit 38
A includes a brightness correction lookup table 39A having characteristics suitable for the multi-valued mode as shown in FIG. 7, for example.
As shown in FIG. 7, when the input L ^* is less than or equal to the threshold value TH set based on the background density detected by the background density detection unit 36,
It is a table of characteristics that makes L ^* 'of the output' 0 '. As a result, the so-called background fogging phenomenon can be suppressed. The lightness correction unit 38B includes a lightness correction lookup table 39B having characteristics suitable for the binary mode, like the lightness correction unit 38A.

The brightness correction section 38A uses the brightness correction lookup table 39A having the characteristics shown in FIG.
The output L ^* 'corresponding to the L ^* input to T44A is obtained. Then, the YMC corresponding to the obtained L ^* ', a ^* , b ^*
The value of K is calculated from DLUT44A, and this calculated YMCK
The DLUT 44A is rewritten with the value of.

For example, if the input value is (L^*, A^*, B^*) =
If it is (48, 16, 16), it is L from FIG.^*’
Is '26'. And (L^*’、 A^*, B^*) =
The value of YMCK corresponding to (26, 16, 16) is DLU
Obtained from T44A, DLUT with the obtained YMCK value
Rewrite 44A. As a result, the input value becomes (L^*,
a^*, B ^*) = (48,16,16),
(L^*’、 A^*, B^*) = (26,16,16)
The value of YMCK is output to the gradation correction unit 34B. That
Then, the gradation correction unit 34B can perform gradation correction suitable for the binary mode.
After being applied, the image is output to the image output unit 16.

As described above, the IN described in the first embodiment is used.
The DLUT 44A can perform the same correction as the background correction by the LUT. The same applies to the brightness correction unit 38B.

[0053]

As described above, according to the present invention,
Even if the background correction lookup table is set for a predetermined output format, even if the output format is changed because the three-dimensional lookup table corresponding to an output format different from the predetermined output format is changed. This has the effect of suppressing the deviation in gradation reproducibility.

[Brief description of drawings]

FIG. 1 is a schematic block diagram showing a schematic configuration of an image processing apparatus according to a first embodiment.

FIG. 2 is a schematic block diagram illustrating a schematic configuration of a color space conversion unit according to the first embodiment.

FIG. 3 is a diagram showing input / output characteristics of a look-up table of a brightness correction unit.

FIG. 4 is a flowchart showing a processing flow of an image processing unit.

FIG. 5 is a schematic block diagram showing a schematic configuration of an image processing apparatus according to a second embodiment.

FIG. 6 is a schematic block diagram illustrating a schematic configuration of a color space conversion unit according to a second embodiment.

FIG. 7 is a diagram showing input / output characteristics of a look-up table of a brightness correction unit.

[Explanation of symbols]

10 image processing device 12 image input unit 14 image processing unit 16 image output unit 20 A / D converter 22 correction unit 24 color space conversion unit 26 scaling unit 28 filter 30 INLUT (look-up table for background correction) 32 image processing unit 32 gradation correction section 32 color space conversion section 34 gradation correction section 36 background density detection section (detection means) 38 brightness correction section (change means) 39 brightness correction lookup table 40 output format data 42 selector (three-dimensional lookup table) ) 44A, 44B DLUT (three-dimensional lookup table)

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Claims (3)

[Claims] 1. An input image for converting a first color signal corresponding to a first color system into a second color signal corresponding to a second color system and outputting the input image. A plurality of three-dimensional lookup tables of conversion relations corresponding to each of a plurality of output formats of the image output unit, detection means for detecting the background density of the input image from the first color signal of the input image, Set based on the background density and a predetermined output format of the plurality of output formats of the image output unit,
A background correction lookup table for correcting the background level of the input image, and a different output of the background level-corrected input image corresponding to an output format different from the predetermined output format. If the output format of the image output unit is set to the different output format, it is based on the detected background density. The brightness correction lookup table is set, at least a part of the first color signal is corrected by the brightness correction lookup table, and the second color corresponding to the corrected first color signal is set. A signal is obtained from the three-dimensional look-up table corresponding to the different output format, and the obtained second color signal is the first color signal before correction. Image processing apparatus, the changing means changing the three-dimensional lookup table corresponding to the different output format so as to correspond to the output format. 2. An input image for converting a first color signal corresponding to a first color system into a second color signal corresponding to a second color system and outputting the input image. A plurality of three-dimensional look-up tables having a conversion relationship corresponding to each of a plurality of output formats of the image output section, and the input image set based on a predetermined output format of the plurality of output formats of the image output section. And a background correction lookup table for correcting the background level of the image processing apparatus, the background level correction input corresponding to an output format different from the predetermined output format, and the background level correction input according to the predetermined output format. At least a part of the first color signal is corrected by a brightness correction lookup table for correcting the image into the background-level-corrected input image in the different output format, and the correction is performed. The second color signal corresponding to the generated first color signal is obtained from the three-dimensional lookup table corresponding to the different output format, and the obtained second color signal is the first color before correction. Image processing characterized in that the three-dimensional lookup table corresponding to the different output format is changed in advance by the changing means for changing the three-dimensional lookup table corresponding to the different output format so as to correspond to the signal. apparatus. 3. The image processing apparatus according to claim 1, wherein the output format includes at least one of a screen pattern of the image output unit and a halftone processing method.