JP6001983B2 - Image processing device - Google Patents

Description

  The present invention relates to an image processing apparatus.

2. Description of the Related Art Conventionally, in an image processing apparatus, a background removal process is performed in which an area corresponding to a document sheet, that is, a background area color is converted to white from original image data read by a scanner or the like. In the background removal processing, the original image data represented by device-dependent colors such as RGB used in a device such as a scanner has independent brightness values representing brightness and color difference component values representing saturation and color. Conversion to device-independent colors such as CIE L ^* a ^* b ^* and CIE XYZ. Then, the brightness value of the background is detected, gradation conversion is performed so that the brightness values of the pixels equal to or higher than the detected brightness value are uniformly set to the maximum brightness value, and the saturation value of the pixel having the maximum brightness value is set to 0 ( (Achromatic), the background is made to fly white.

  However, in such a background removal process, even for pixels outside the background region, the color of the pixel having a high saturation value that is a lightness value equal to or higher than the lightness value of the background is white. Therefore, in order to prevent this, after performing the gradation conversion, among the pixels having the maximum brightness value, for the pixels having a saturation value exceeding the predetermined threshold (threshold) value, the saturation value is stored, Only for pixels having a saturation value equal to or less than a predetermined threshold, the saturation value is set to 0 (see, for example, Patent Document 1).

JP 2006-67233 A

  However, in the conventional image processing apparatus, since the saturation conversion is performed in a state where the lightness values of pixels equal to or higher than the background lightness value are uniformly converted to the maximum lightness value, in different lightness values equal to or higher than the background lightness value. When pixels where saturation values are to be stored and pixels where saturation values are to be 0 are mixed, it is impossible to distinguish and process them. For example, there are color high brightness pixels that are both brighter and more saturated than the background pixels, and scanner lid (reflector) back reflector pixels, and when the color high brightness pixels have a lower saturation value than the lid back reflector pixels, Leaving the color high brightness color and erasing the color of the back cover reflector had to be in a trade-off relationship.

  The present invention solves the problems of the conventional image processing apparatus and performs a different conversion process according to the lightness value on the color difference component of the image data so as to have a lightness value equal to or higher than the lightness value of the background. Provided is an image processing apparatus capable of distinguishing and processing even when pixels that should store chroma values having different brightness values and pixels that should be 0 are mixed. For the purpose.

Therefore, in the image processing apparatus of the present invention, a mounting table on which a document is placed, a light source that irradiates light on the document placed on the placing table, and a reflection that reflects the light emitted from the light source An image acquisition unit that acquires original image data from a reading unit that includes a plate, and the original image data acquired by the image acquisition unit is represented by a lightness color difference color space in which a lightness component and a color difference component are independent. An input color space conversion unit that performs conversion processing to convert data, a background color detection unit that detects a background color from the first image data, and an extended color space that includes a lightness value that exceeds the lightness maximum value. An extended color space conversion unit that performs conversion processing to convert to the second image data represented by: a first point that indicates the lightness saturation value of the background color in the extended color space; and the lightness of the color of the reflector the indicating the reference lightness saturation value is the saturation value A saturation conversion threshold value deriving unit for deriving a saturation conversion threshold value which is determined by a straight line passing through a point and differs depending on the lightness value, and for the color difference component of the second image data, based on the saturation conversion threshold value A chrominance component conversion unit that performs conversion processing; and a luminosity component conversion unit that performs conversion processing for converting the lightness value of the second image data that has been converted by the chrominance component conversion unit into a value that is equal to or less than the maximum lightness value. An output color space conversion unit that performs conversion processing for converting the second image data that has been converted by the lightness component conversion unit into third image data in an output format color space; and outputs the third image data. An image output unit.

  According to the present invention, the image processing apparatus performs different conversion processing on the color difference component of the image data according to the lightness value. Thus, even when pixels having a lightness value equal to or higher than the lightness value of the background and having a saturation value having different lightness values and pixels to be stored in a mixture are set to 0, It can be distinguished and processed.

It is a block diagram which shows the function structure of the image processing apparatus main body in the 1st Embodiment of this invention. 1 is a perspective view illustrating an appearance of an image processing apparatus according to a first embodiment of the present invention. 1 is a block diagram illustrating a configuration of an image processing device according to a first embodiment of the present invention. It is a figure explaining the result of the background removal process of the image processing apparatus in the 1st Embodiment of this invention. It is a figure explaining the operation | movement on the color space of the background removal process of the image processing apparatus in the 1st Embodiment of this invention. It is a flowchart which shows operation | movement of the image processing apparatus in the 1st Embodiment of this invention. It is a figure which shows the example of the L value histogram in the 1st Embodiment of this invention. 4 is a flowchart illustrating a background color detection subroutine of the image processing apparatus according to the first embodiment of the present invention. It is a flowchart which shows the subroutine of the background removal of the image processing apparatus in the 1st Embodiment of this invention. It is a block diagram which shows the function structure of the image processing apparatus in the 2nd Embodiment of this invention. It is a figure explaining the result of the background removal process of the image processing apparatus in the 2nd Embodiment of this invention. It is a figure explaining the operation | movement on the color space of the background removal process of the image processing apparatus in the 2nd Embodiment of this invention. It is a flowchart which shows the subroutine of the background removal of the image processing apparatus in the 2nd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 2 is a perspective view showing the appearance of the image processing apparatus according to the first embodiment of the present invention, and FIG. 3 is a block diagram showing the configuration of the image processing apparatus according to the first embodiment of the present invention.

  In the figure, reference numeral 10 denotes an image processing apparatus, which is a color complex machine having various functions such as a scanner, a printer, a facsimile machine, and a copying machine. Here, at least a scanner 12 as a reading unit for reading a color image from a document. An image processing apparatus main body 11 that performs conversion such as processing and correction on color image data read from the original by the scanner 12, and a printer 13 as an image forming unit that prints an image on a medium such as printing paper. It explains as having.

  In the present embodiment, the image processing apparatus main body 11 includes an input I / F (interface) 14, a control unit 15, and an output I / F (interface) 22.

  The scanner 12 includes a document placing table, a light source, a reflecting plate, a light receiving element, and a signal processing unit (not shown), and irradiates the document placed on the document placing table with light from the light source. The light is reflected by the reflector on the back of the document and the scanner lid, the light receiving element receives the reflected light, reads the image signal, and performs signal processing such as A / D conversion and shading correction by the signal processing unit. Get image data. Then, the acquired image data is transmitted to the control unit 15 of the image processing apparatus main body 11 via the input I / F 14.

  The input I / F 14 is an interface for connecting to an external device in order to acquire image data from the external device. The input I / F 14 is connected to the scanner 12 and has three RGB color components for each pixel read by the scanner 12. Acquires color image data represented by a value.

  The control unit 15 includes a RAM 16 that is a volatile memory, a ROM 17 that is a read-only memory, and a CPU 21 that is a processor, and performs control such as temporary storage of image data and calculation and execution of an image processing program.

  The output I / F 22 is an interface for connecting to an external device in order to output the color image data processed by the control unit 15 of the image processing apparatus main body 11 to the external device. The output I / F 22 is connected to the printer 13 and receives CMYK image data. Is output to the printer 13.

  The printer 13 is, for example, an electrophotographic printer, and forms an image using a toner color material on a medium such as printing paper in accordance with the received CMYK image data.

  Next, the image processing apparatus main body 11 will be described from the viewpoint of functions.

  FIG. 1 is a block diagram showing a functional configuration of the image processing apparatus main body according to the first embodiment of the present invention.

  As shown in the figure, from the viewpoint of function, the image processing apparatus body 11 includes an image acquisition unit 23 that acquires original image data, and the original image data acquired by the image acquisition unit 23 as a lightness component and a color difference component. An input color space conversion unit 24 that performs conversion processing into first image data represented in a lightness color difference color space, a background color detection unit 25 that detects a background color from the first image data, and background removal processing A background removal unit 26 that performs the conversion process, an output color space conversion unit 34 that performs a conversion process that converts the second image data that has been converted by the lightness component conversion unit 33 into the third image data in the color space of the output format, and an image And an output unit 35. The background removal unit 26 performs an extension color space conversion unit 27 that performs conversion processing for converting the first image data into second image data represented by an extended color space including a brightness value exceeding the maximum brightness value, A saturation conversion threshold value deriving unit 31 for deriving a saturation conversion threshold value that differs according to the lightness value, a color difference component conversion unit 32 that performs a different conversion process according to the lightness value for the color difference component of the second image data, A lightness component conversion unit 33 that performs conversion processing for converting the lightness value of the second image data that has been subjected to the conversion processing by the color difference component conversion unit 32 into a value that is equal to or less than the maximum lightness value; Note that the communication control, write control, and image processing calculations in each unit are performed by the CPU 21 in the control unit 15 using the RAM 16 as a working memory and executing a program stored in the ROM 17.

  The image acquisition unit 23 acquires RGB image data read by the scanner 12 through the input I / F 14 and stores the acquired RGB image data in the RAM 16. In the present embodiment, description will be made on the assumption that the RGB image data transmitted from the scanner 12 is acquired. However, a storage object such as a PC (personal computer) or an HDD (hard disk drive) connected to a network, Alternatively, a portable storage medium such as a USB memory stick may be connected to the input I / F 14 to acquire RGB image data. Further, the acquired image data is not limited to RGB image data, and may be image data in a color space other than RGB.

The input color space conversion unit 24 converts the RGB image data stored in the RAM 16 into a CIE L ^* a ^* b ^* (hereinafter referred to as “Lab”) space, which is a uniform color space in which the brightness component and the color difference component are independent. The image data is converted into Lab image data as first image data. In the present embodiment, a three-dimensional lookup table for deriving Lab values (L _in , a _in , b _in ) corresponding to combinations of input RGB values (R _in , G _in , B _in ) is created in advance. In addition, the RGB value is read for each pixel of the image data, and converted into the corresponding Lab value with reference to the three-dimensional lookup table. As a color space conversion method using a three-dimensional lookup table, a known conversion method, for example, a conversion method described in JP-A-9-208389 is used. The Lab image data converted by the input color space conversion unit 24 is transferred to the background color detection unit 25 and the extended color space conversion unit 27 of the background removal unit 26.

The background color detection unit 25 refers to the Lab image data converted by the input color space conversion unit 24, and determines the lightness value corresponding to the lowest value among the lightness values of the background region as a background lightness value by a detection method described later. Then, a background color Lab value (L _ws , a _ws , b _ws ) using the average value of the color difference component values of the background region as the background color difference component value is detected. The background color Lab values (L _ws , a _ws , b _ws ) detected by the background color detection unit 25 are transferred to the extended color space conversion unit 27 and the saturation conversion threshold value derivation unit 31 of the background removal unit 26. .

The background removal unit 26 includes an extended color space conversion unit 27 that converts the input color space into an extended color space, a saturation conversion threshold value derivation unit 31 that derives a saturation conversion threshold value that varies depending on the lightness value, and a saturation conversion threshold value. chrominance component values in accordance with the extended lightness value L _p based on (a _p, b _p) and chrominance component transformer 32 for converting the brightness component conversion unit that converts the extended lightness value L _p in the range of the original lightness value L 33.

The extended color space conversion unit 27 uses the input Lab value (L _in , a _in , b _in ), which is the Lab value of the input Lab space converted by the input color space conversion unit 24, as the maximum value of the lightness value L. Extended Lab values (L _p , a _p , b) that are Lab values of the second image data represented in the extended color space including a lightness value exceeding the maximum value L _max (L _max = 100 in the present embodiment). _p ). Specifically, a monotonically increasing lightness conversion function is derived such that the background color lightness value L _ws acquired from the background color detection unit 25 is converted to the lightness maximum value L _max , and input using the derived lightness conversion function The brightness value L _in is converted into an extended brightness value L _p . The extended color difference component values (a _p , b _p ) are the same values as the input color difference component values (a _in , b _in ).

In this embodiment, in order to simplify the explanation, only the lightness value is converted and the input value is used as the color difference component value. However, in general, an extended Lab value (L _p , A _p , b _p ). For example, Lab 3 × 3 matrix operation or the like can be used as in the following equation (1).

In the present embodiment, in the equation (1), coefficient s ₁₁ = s ₂₂ = 1, coefficient s ₀₁ = s ₀₂ = s ₁₀ = s ₁₂ = s ₂₀ = s ₂₁ = 0, constant t ₀ = t ₁ = A lightness conversion function is derived by deriving a coefficient s ₀₀ with t ₂ = 0.

The converted extended Lab values (L _p , a _p , b _p ) are transferred to the color difference component conversion unit 32.

The saturation conversion threshold value deriving unit 31 includes a background color brightness saturation value (L _ws , C _ws ) obtained from the background color Lab values (L _ws , a _ws , b _ws ) detected by the background color detection unit 25, and Based on predetermined reference lightness saturation values (L _f , C _f ), a saturation conversion threshold value C _th (l) that differs depending on the lightness value is derived.

該明degree component converter 33, the color difference components converted Lab values by the color difference component conversion unit _{32 (L p, a q,} b q) of the brightness value L _p of the brightness maximum value L _max greater lightness value L Convert to a value less than _max . The Lab values (L _q , a _q , b _q ) obtained by converting the lightness values are transferred to the output color space conversion unit 34.

  The output color space conversion unit 34 converts the second image data converted by the background removal unit 26 into CMYK image data as third image data in the color space of the output format. In the present embodiment, a three-dimensional lookup table for deriving an output CMYK value corresponding to a combination of input Lab values is created in advance, and the Lab value is read for each pixel of image data. Are converted into corresponding CMYK values. The converted CMYK image data is transferred to the image output unit 35.

  The image output unit 35 outputs CMYK image data to the printer 13 through the output I / F 22. Then, in accordance with the received CMYK image data, the printer 13 forms an image with a color material such as toner on a medium such as printing paper.

  Next, the background removal process executed by the background removal unit 26 having the above configuration will be described in detail.

  FIG. 4 is a diagram for explaining the result of the background removal processing of the image processing apparatus according to the first embodiment of the present invention, and FIG. 5 is the color space of the background removal processing of the image processing apparatus according to the first embodiment of the present invention. It is a figure explaining the operation | movement above. 4A is a diagram showing image data acquired by the image acquisition unit, FIG. 4B is a diagram showing image data after performing a conventional background removal process, and FIG. The figure which shows the image data after performing a background removal process, (d) is a figure which shows the image data after performing the background removal process in the 1st Embodiment of this invention, In FIG. -1) is a diagram for explaining the operation of the extended color space conversion unit in the first embodiment of the present invention, and (a-2) is for explaining the operation of the color difference component conversion unit in the first embodiment of the present invention. (A-3) is a diagram for explaining the operation of the lightness component conversion unit in the first embodiment of the present invention, (b-1) is a diagram for explaining the operation of the conventional lightness component conversion, (b) FIG. 2B is a diagram illustrating a conventional saturation conversion operation.

  As shown in FIG. 4A, when a document having a size smaller than the reading size of the scanner 12 is read by the scanner 12, the RGB image data acquired by the image acquisition unit 23 corresponds to the color of the document. A background color 51 and a reflector color 52 on the back of the lid of the scanner 12 are included. Further, a color image portion 53 having a low lightness and a color image portion 54 having a high lightness exist in the document area.

In the background removal process executed by the background removal unit 26 in the present embodiment, as shown in FIG. 4D, the color information of the color image parts 53 and 54 that are images is not removed, but the background color 51 and the reflection are obtained. Only the color information of the plate color 52 is removed.

The points 61, 62, and 63 all have a lightness value equal to or higher than the lightness value _Lws of the background color 51. Therefore, the lightness value is the lightness maximum value _Lmax = 100. And point 66.

  Thus, in the lightness component conversion in the conventional background removal processing, the difference in the saturation value can be distinguished after the conversion, but information on the lightness value difference is lost.

  FIG. 5B-2 shows the operation of saturation conversion in the conventional background removal processing.

Here, a predetermined threshold value (for example, the background indicated by the broken line 71) among the pixel values having the maximum brightness value L _max by the brightness component conversion operation in the conventional background removal process shown in FIG. It is assumed that the saturation value below the saturation value C _ws ) of the color 51 is set to 0, that is, converted to a point 67 and the saturation value greater than the threshold value is not changed. In this case, the color high brightness in which the saturation value is to be stored between the saturation value of the background color 51 (point 64) and the saturation value (point 66) of the reflector color 52 which should have a saturation value of 0. Since there is a color saturation value (point 65), the reflector color 52 remains together with the color high brightness color. As a result, as shown in FIG. 4B, the reflector color 52 is not removed.

  On the other hand, when the threshold value is set large in order to erase the reflector color 52, all of the points 64, 65 and 66 are converted into points 67, and the color high brightness color disappears together with the reflector color 52. . As a result, as shown in FIG. 4C, the color image portion 54 with high brightness disappears.

  As described above, in the conventional background removal processing, the saturation conversion operation is performed in a state where the brightness values of the pixels equal to or higher than the brightness value of the background color 51 are uniformly converted to the maximum brightness value. It is not possible to leave 54 and erase the reflector color 52 at the same time.

  In contrast, FIG. 5A-1 shows the operation of the extended color space conversion unit 27 of the background removal unit 26 in the present embodiment.

Here, the point 72, the point 73, and the point 74 were subjected to extended color space conversion for the background color 51 (point 61), the color high brightness color (point 62), and the reflector color 52 (point 63), respectively. It is a later point. Since the extended color space includes lightness values that exceed the lightness maximum value L _max , the converted points 72, 73, and 74 can be distinguished from each other in terms of lightness values and saturation values. In addition, _let the brightness value and saturation value of the reflector color 52 (point 63) which can be acquired beforehand be a standard brightness saturation value ( _Lf , _Cf ).

  FIG. 5A-2 shows the operation of the color difference component conversion unit 32 of the background removal unit 26 in the present embodiment.

Here, the broken line 75 indicates the saturation conversion threshold value C _th (l) derived by the saturation conversion threshold value deriving unit 31. The saturation conversion threshold value C _th (l) passes through a point 74 where the background color 51 (point 61) is subjected to the extended color space conversion and the reflector color 52 (point 63) is subjected to the extended color space conversion. It is derived as a half line.

Then, the chrominance component conversion unit 32 sets the point 72 and the point 74 in which the input extended saturation value C _p is equal to or less than the saturation conversion threshold value C _th (L _p ) in the input lightness value, respectively. Convert to points 76 and 77 which are 0 (a _q = b _q = 0). Further, the saturation value (color difference component value) is not converted for the point 73 where the extended saturation value C _p is larger than the saturation conversion threshold C _th (L _p ) (a _q = a _p , b _q = B _p ).

  Further, FIG. 5A-3 shows the operation of the lightness component conversion unit 33 of the background removal unit 26 in the present embodiment.

Here, the background color 51 (point 61), the color high brightness color (point 62), and the reflector color 52 (point 63) are converted into points 76, 73, and 77 by the color difference component converter 32, respectively. ing. Since the point 73 and the point 77 are lightness values larger than the lightness maximum value L _max , the lightness component conversion unit 33 converts these lightness values into predetermined lightness values within the reproduction range of the lightness value L ( In this embodiment, it is converted to L _max ). Thereby, the point 73 is converted to a point 78, and the point 77 is converted to a point 76 (white) which is a conversion value of the background color 51 because it is on the achromatic color axis (vertical axis).

  As a result, the background color 51 (point 61) and the reflector color 52 (point 63) are converted to white (point 76), and the color high brightness color (point 62) is a color (point) with a maximum brightness value and a saturation value. 78). Therefore, the target output can be obtained as shown in FIG. That is, the background color 51 and the reflector color 52 can be erased while the color image portion 53 having a low brightness and the color image portion 54 having a high brightness remain.

  As described above, in the background removal process executed by the background removal unit 26 in the present embodiment, pixels that have brightness values that are equal to or higher than the background brightness value and that should store saturation values that have different brightness values. Even when pixels that should be set to 0 are mixed, it is possible to distinguish and appropriately process them.

  Next, the operation of the image processing apparatus 10 having the above configuration will be described.

  FIG. 6 is a flowchart showing the operation of the image processing apparatus according to the first embodiment of the present invention.

  First, the image acquisition unit 23 acquires RGB image data. Specifically, RGB image data read by the scanner 12 is acquired through the input I / F 14, recorded in the RAM 16 and stored.

  Subsequently, the input color space conversion unit 24 performs color space conversion into Lab image data. Specifically, the RGB image data stored in the RAM 16 is converted into Lab image data as image data in the Lab space, which is a uniform color space in which the brightness component and the color difference component are independent.

  Subsequently, the background color detection unit 25 executes background color detection processing. Specifically, the background color is detected with reference to the Lab image data converted by the input color space conversion unit 24.

  Subsequently, the background removal unit 26 performs background removal processing. Specifically, the operations shown in FIGS. 5A-1 to 5A-3 are performed.

  Subsequently, the output color space conversion unit 34 changes to output CMYK image data. Specifically, the Lab image data converted by the background removal unit 26 is converted into CMYK image data with reference to a three-dimensional lookup table.

  Finally, the image output unit 35 outputs CMYK image data. Specifically, the CMYK image data is output to the printer 13 via the output I / F 22. Then, the process ends.

Next, a flowchart will be described.
Step S1: The image acquisition unit 23 acquires RGB image data.
Step S2: The input color space conversion unit 24 performs color space conversion to Lab image data.
Step S3 The background color detection unit 25 executes background color detection processing.
Step S4 The background removal unit 26 performs background removal processing.
Step S5: The output color space conversion unit 34 changes the output color space to output CMYK image data.
Step S6: The image output unit 35 outputs CMYK image data, and the process ends.

  Next, a background color detection subroutine will be described.

  FIG. 7 is a diagram showing an example of an L value histogram in the first embodiment of the present invention, and FIG. 8 is a flowchart showing a background color detection subroutine of the image processing apparatus in the first embodiment of the present invention.

  First, the background color detection unit 25 refers to the Lab value of each pixel of the Lab image data converted by the input color space conversion unit 24 and creates an L value histogram as a lightness histogram of the Lab image data. The L value histogram is, for example, as shown in FIG. 7. The vertical axis indicates the appearance frequency histgram [L], and the horizontal axis indicates the lightness value L. In the example shown in FIG. 7, three peaks are seen, and in order from the highest brightness value, the brightness value of the reflector on the back of the scanner lid, the brightness value including the document surface to be detected as the background, and This corresponds to the brightness value of an image such as a character or halftone dot image.

  Subsequently, the background color detection unit 25 refers to the Lab value of each pixel of the Lab image data, and sums sum_a [L] of a values for each L value and sum_b [L] of sums of b values for each L value. To get.

Subsequently, the background color detection unit 25, the L value histogram [L], and searches the lightness threshold value L _th to the low brightness side, for detecting a first peak start lightness value Start_L. The peak start brightness value is the brightness value 42 corresponding to the skirt (hem) portion on the high brightness side of the peak of the background color in the example shown in FIG. Further, L _th is a known predetermined value that is excluded in advance from the background peak detection when the value is equal to or higher than the brightness value of the document, such as the minimum brightness value of the reflector on the back of the scanner lid.

In this embodiment, L = L _th will lower the brightness value from the histogram is the first time histgram [L]> 0 and since L value peak start lightness value start_L a.

  Subsequently, the background color detection unit 25 searches further from the start_L to the lower lightness side, and detects the peak vertex lightness value top_L. In the example shown in FIG. 7, the peak vertex lightness value is the lightness value 43 when the peak frequency on the right side is the highest. In this embodiment, the histogram is first displayed as histgram [L] <histgram [ It is assumed that the L value is L + 1].

Subsequently, the background color detection unit 25 determines the background lightness value L _ws using the detected peak start lightness value start_L and peak vertex lightness value top_L. Since it is considered that the brightness value of the pixel corresponding to the background is not a single point but is normally distributed, in the present embodiment, the minimum brightness value that the background pixel can take is determined as the background brightness value L _ws . As a result, all background pixels having a brightness value higher than the background brightness value L _ws can be targeted for removal.

Further, the lower lightness side skirt of the background lightness value L _ws is a position advanced from the peak vertex lightness value top_L to the low lightness side by a width from the peak start lightness value start_L to the peak vertex lightness value top_L (lightness value 44 in FIG. 7). ), The value calculated by the following equation (2) is set as the background lightness value L _ws .
L _ws = top_L− (start_L−top_L) (2)
However, the peak of the peak apex lightness value Top_L (brightness value 43), since not always normally distributed, the background brightness value L _ws or lightness values, may be included the brightness value of the non-image background.

Finally, the background color detection unit 25 determines background color difference component values (a _ws , b _ws ). In this embodiment, the back views difference component values (a _ws, b _ws) value of each of a and b values of the pixels with a brightness value between the background lightness value L _ws to peak start lightness value start_L The average value of Therefore, using the L value [L] that has already been created, the sum sum_a [L] of the a values for each L value that has already been acquired, and the sum smu_b [L] of the b values for each L value, And the value calculated by (4) is used as the background color difference component value (a _ws , b _ws ).

As described above, the background color Lab values (L _ws , a _ws , b _ws ) are detected, and thus the background color detection process ends.

Next, a flowchart will be described.
Step S3-1: An L value histogram of Lab image data is created.
Step S3-2: A sum sum_a [L] of a values for each L value and a sum sum_b [L] of b values for each L value are acquired.
Step S3-3 L value histogram [L], and searches the lightness threshold value L _th to the low brightness side, for detecting a first peak start lightness value Start_L.
Step S3-4 The search is further performed from the start_L to the lower lightness side, and the peak vertex lightness value top_L is detected.
Step S3-5: Determine the background brightness value L _ws .
Step S3-6: Determine the background color difference component values (a _ws , b _ws ) and end the background color detection process.

  Next, a background removal subroutine will be described.

  FIG. 9 is a flowchart showing a background removal subroutine of the image processing apparatus according to the first embodiment of the present invention.

First, the background removal unit 26 acquires the background color Lab values (L _ws , a _ws , b _ws ) detected by the background color detection unit 25.

Subsequently, the extended color space conversion unit 27 of the background removal unit 26 refers to the background color brightness value L _ws of the acquired background color Lab values (L _ws , a _ws , b _ws ), and determines the input brightness value L _in as An extended color space conversion function T1 (l) for converting to an extended brightness value L _p including a brightness value exceeding the maximum brightness value L _max is derived.

  The following equation (5) is an example of the extended color space conversion function T1 (l).

Note that the extended color space conversion function T1 (l) is not limited to that expressed by the above formula (5), and the background color lightness value L _ws is converted to the maximum lightness value L _max and the background color lightness value L _{ws is obtained.} Any function can be applied as long as it is a continuous function in which the above brightness value is converted into a brightness value equal to or greater than the brightness maximum value L _max with a simple increase.

Subsequently, the extended color space conversion unit 27 uses the already derived extended color space conversion function T1 (l) to convert the input lightness value Lin into the lightness value of the extended color space as _in the following equation (6). To do. Thus, the input Lab values (L _in , a _in , b _in ) that are Lab values of the input Lab space converted by the input color space conversion unit 24 are extended Lab values (L _p , a _p , b _p ).

In the present embodiment, the color difference component values (a _p , b _p ) after the extended color space conversion are the same as the input color difference component values (a _in , b _in ).

Subsequently, the saturation conversion threshold value deriving unit 31 uses the background color lightness saturation value (L _ws , C _ws ) and the predetermined reference lightness saturation value (L _f , C _f ) to obtain the extended color that has already been obtained. Different saturation conversion threshold values C _th (l) are derived according to the lightness values of the space.

The following equation (7) is an example of the saturation conversion threshold C _th (l).

Subsequently, the color difference component conversion unit 32 determines whether or not the extended saturation value C _p is equal to or less than the saturation conversion threshold C _th (L _p ) derived by the saturation conversion threshold deriving unit 31.

When the extended saturation value C _p is equal to or less than the saturation conversion threshold C _th (L _p ), the color difference component conversion unit 32 performs color difference component conversion that sets the color difference component value to 0. Lab values subjected to color difference component conversion are Lab values (L _p , a _q , b _q ), and a _q = 0 and b _q = 0.

When the extended saturation value C _p is not equal to or less than the saturation conversion threshold C _th (L _p ), the color difference component conversion unit 32 does not perform color difference component conversion. In this case, a _q = a _p and b _q = b _p .

Finally, the lightness component conversion unit 33 of the background removal unit 26 receives the Lab values (L _p , a _q , b _q ) that have been subjected to the color difference component conversion by the color difference component conversion unit 32, and the lightness value L _p is the maximum lightness. the brightness value of the value L _max greater than the pixel, and converts the lightness maximum value L _max. The Lab values (L _q , a _q , b _q ) obtained by converting the lightness values are transferred to the output color space conversion unit 34, and the background removal process ends.

Next, a flowchart will be described.
Step S4-1: The background removal unit 26 acquires background color Lab values (L _ws , a _ws , b _ws ).
Step S4-2: The extended color space conversion unit 27 derives an extended color space conversion function T1 (l).
Step S4-3: The input Lab values (L _in , a _in , b _in ) are converted into extended Lab values (L _p , a _p , b _p ).
Step S4-4 The saturation conversion threshold value deriving unit 31 derives the background color saturation value _Cws .
Step S4-5 The saturation conversion threshold value deriving unit 31 derives a different saturation conversion threshold value C _th (l) according to the brightness value of the already obtained extended color space.
Step S4-6 chrominance component transformer 32 obtains the extended chroma value C _p.
Step S4-7: The color difference component converter 32 determines whether or not the extended saturation value C _p is equal to or less than the saturation conversion threshold C _th (L _p ). If the extended saturation value C _p is less than or equal to the saturation conversion threshold C _th (L _p ), the process proceeds to step S4-8, and if the extended saturation value C _p is not less than or equal to the saturation conversion threshold C _th (L _p ). Proceed to step S4-9.
Step S4-8 The color difference component conversion unit 32 performs color difference component conversion for setting the color difference component value to zero.
Step S4-9: The color difference component conversion unit 32 does not perform color difference component conversion.
Step S4-10 brightness component converter 33, the lightness value L _p lightness value of the lightness maximum value L _max greater than the pixel, and converts the lightness maximum value L _max, the background removal process is completed.

  As described above, in the present embodiment, a pixel having a lightness value equal to or higher than the background lightness value and a pixel to store a saturation value having a different lightness value and a pixel to be set to 0 are mixed. However, it is possible to distinguish and process the pixel whose chroma value is to be stored and the pixel whose chroma value is to be 0.

  Next, a second embodiment of the present invention will be described. In addition, about the thing which has the same structure as 1st Embodiment, the description is abbreviate | omitted by providing the same code | symbol. The description of the same operation and the same effect as those of the first embodiment is also omitted.

  FIG. 10 is a block diagram showing a functional configuration of the image processing apparatus according to the second embodiment of the present invention.

  In the present embodiment, the background removal unit 26 includes a continuous saturation in addition to the extended color space conversion unit 27, the saturation conversion threshold value derivation unit 31, the color difference component conversion unit 32, and the lightness component conversion unit 33. A saturation conversion function deriving unit 81 for deriving a saturation conversion function for converting different color difference components according to the lightness value while maintaining the characteristics.

The saturation conversion threshold value deriving unit 31 includes a background color brightness saturation value (L _ws , C _ws ) obtained from the background color Lab values (L _ws , a _ws , b _ws ) detected by the background color detection unit 25, and Based on the predetermined reference lightness saturation value (L _f , C _f ), a saturation conversion threshold value C _th (l) that differs depending on the lightness value is derived. At this time, in order to prevent a saturation gradation gap due to a difference in saturation conversion between the lightness maximum value L _max and the lightness value L _max −1, a threshold value is also derived for lightness values equal to or lower than the lightness maximum value L _max .

Then, the saturation conversion function deriving unit 81 does not generate a gradation gap in the saturation direction with the saturation conversion threshold C _th (l) that differs depending on the lightness value derived by the saturation conversion threshold deriving unit 31 as a boundary. Such a saturation conversion function S (l, c) is derived.

In addition, the color difference component conversion unit 32 uses the saturation conversion function S (l, c) derived by the saturation conversion function deriving unit 81 to use the extended Lab value (L _p , a _p, the chrominance component values of b _p) (a _p, the b _p), while maintaining the continuity of chroma (a _q, converted Lab values _{b q) (L p, a} q, b q) Is derived.

  Since the configuration of other points is the same as that of the first embodiment, description thereof is omitted.

  Next, the background removal process executed by the background removal unit 26 in the present embodiment will be described in detail.

  FIG. 11 is a diagram for explaining the result of the background removal processing of the image processing apparatus according to the second embodiment of the present invention, and FIG. 12 is the color space of the background removal processing of the image processing apparatus according to the second embodiment of the present invention. It is a figure explaining the operation | movement above. In FIG. 11, (a) is a diagram showing image data acquired by the image acquisition unit, (b) is a diagram showing image data after performing background removal processing in the first embodiment of the present invention, (C) is a figure which shows the image data after performing the background removal process in the 2nd Embodiment of this invention, In FIG. 12, (a-1) is in the 2nd Embodiment of this invention. The figure explaining operation | movement of an extended color space conversion part, (a-2) is a figure explaining operation | movement of the color difference component conversion part in the 2nd Embodiment of this invention, (a-3) is 2nd figure of this invention. It is a figure explaining the operation | movement of the lightness component conversion part in the embodiment.

  As shown in FIG. 11A, when a document having a size smaller than the reading size of the scanner 12 is read by the scanner 12, the RGB image data acquired by the image acquisition unit 23 is included in the first embodiment. Similar to the form, a background color 51 corresponding to the color of the document and a reflector color 52 on the back of the lid of the scanner 12 are included. Also, in the document area, there are a color image portion 54 having a high lightness and a gradation image from a color image portion 55 having a low lightness and a high saturation value to a color image portion 56 having a high lightness value and a low saturation value. To do.

In the first embodiment, it is possible to process a pixel having a lightness value equal to or higher than the background lightness value while distinguishing between a pixel that should store a saturation value and a pixel that should be set to 0. When an image such as an image with continuously changing lightness and saturation is included, as indicated by 82 in FIG. 11B, the floor in the saturation direction with the saturation conversion threshold C _th (l) as a boundary. There is a case where a gap occurs in the tone or the gradation in the saturation direction between the lightness maximum value L _max and the lightness value L _max −1.

  In the background removal process executed by the background removal unit 26 in the present embodiment, as shown in FIG. 11C, the saturation of the color image portion 54 with high brightness is slightly lowered (part 93), and the gradation image Although there is a portion where the saturation is slightly low (portion 92), a saturation gradation gap in the high brightness and low saturation portion of the gradation does not occur and a natural gradation is maintained (portion 94).

  In FIG. 12, the vertical axis and the horizontal axis indicate the lightness value L and the saturation value C, as in FIG. 5 described in the first embodiment.

  FIG. 12A-1 shows the operation of the extended color space conversion unit 27 of the background removal unit 26 in the present embodiment.

Here, similarly to the first embodiment, the points 72, 73, and 74 are the background color 51 (point 61), the color high brightness color (point 62), and the reflector color 52 (point 63), respectively. ) For the extended color space conversion. A point 84 is a point after the extended color space conversion is performed on the point 83 having the same saturation value as that of the background color 51 and a lightness value. In addition, _let the brightness value and saturation value of the reflector color 52 (point 63) which can be acquired beforehand be a standard brightness saturation value ( _Lf , _Cf ).

  FIG. 12 (a-2) shows the operation of the color difference component conversion unit 32 of the background removal unit 26 in the present embodiment.

Here, a broken line 85 indicates the saturation conversion threshold C _th (l) derived by the saturation conversion threshold deriving unit 31. The saturation conversion threshold C _th (l) is derived as a straight line passing through points 72 and 74 where the background color 51 (point 61) and the reflector color 52 (point 63) have been subjected to extended color space conversion, respectively. Yes.

Then, the color difference component conversion unit 32 sets the color difference component value a _q = b _q = 0 as a point where the input extended saturation value C _p is equal to or less than the saturation conversion threshold C _th (L _p ) in the input brightness value. Achromatic color. For example, the points 72 and 74 are converted into points 76 and 77 having a saturation value of 0, respectively.

Further, at the point 73 where the extended saturation value C _p is larger than the saturation conversion threshold C _th (L _p ), the gradation gap in the saturation direction derived by the saturation conversion function deriving unit 81 does not occur. The saturation value of the point at which the extended saturation value C _p is equal to or less than the saturation conversion threshold value C _th (L _p ) is set to 0 using the saturation conversion function S (l, c). While reducing the value, conversion is performed so that the saturation information remains without using an achromatic color. For example, a point 73 and a point 84 are converted into a point 86 and a point 87, respectively.

  Further, FIG. 12 (a-3) shows the operation of the lightness component conversion unit 33 of the background removal unit 26 in the present embodiment.

Here, the background color 51 (point 61), the color high brightness color (point 62), the reflector color 52 (point 63), and the point 83 having the same saturation value as the background color 51 and a slightly small brightness value are color differences. The component conversion unit 32 converts the points into points 76, 86, 77, and 87, respectively. Since the point 86 and the point 77 are lightness values larger than the lightness maximum value L _max , the lightness component conversion unit 33 converts these lightness values into predetermined lightness values within the reproduction range of the lightness value L ( In this embodiment, it is converted to L _max ). Thereby, the point 86 is converted to the point 91, and the point 77 is converted to the point 76 (white) which is the conversion value of the background color 51 because it is on the achromatic color axis (vertical axis).

As a result, as in the first embodiment, the background color 51 (point 61) and the reflector color 52 (point 63) are converted to white (point 76). The color high brightness color (point 62) is converted to a color (point 91) having a maximum brightness value L _max and a saturation value slightly lower than the original saturation value, and the same saturation as the background color 51 (point 61). A point 83 having a value and a slightly low brightness value is converted to a color (point 87) having a brightness less than the maximum brightness value L _{max and} a saturation value slightly lower than the original saturation value. Therefore, the target output can be obtained as shown in FIG. That is, the saturation of the color image portion 54 with high lightness is slightly low (part 93) and there is a portion (part 92) where the saturation of the gradation image is slightly low (part 92), but the saturation of the high lightness and low saturation portion of the gradation is present. A gradation gap does not occur and a natural gradation is maintained (portion 94), and the background color 51 and the reflector color 52 can be erased as in the first embodiment.

  As described above, in the background removal process executed by the background removal unit 26 in the present embodiment, pixels that have brightness values that are equal to or higher than the background brightness value and that should store saturation values that have different brightness values. Even if pixels that should be 0 are mixed, they are distinguished and processed appropriately, and saturation gradation gap is also applied to images with continuous gradation such as natural images. Can be prevented.

  Next, the operation of the image processing apparatus 10 in the present embodiment will be described. Here, only the background removal subroutine will be described, and the other operations are the same as those in the first embodiment, and the description thereof will be omitted.

  FIG. 13 is a flowchart showing a background removal subroutine of the image processing apparatus according to the second embodiment of the present invention.

The operation from when the background removal unit 26 acquires the background color Lab values (L _ws , a _ws , b _ws ) until the color difference component conversion unit 32 of the background removal unit 26 obtains the extended saturation value C _p , That is, the operations in steps S4-11 to S4-16 in FIG. 13 are the same as the operations in steps S4-1 to S4-6 in the first embodiment, and a description thereof will be omitted.

When the color difference component conversion unit 32 obtains the extended saturation value C _p , the saturation conversion function derivation unit 81 of the background removal unit 26 subsequently acquires the acquired background color Lab value (L _ws , a _ws , b _ws ). and brightness value L _ws of, with reference to the background color saturation value C _ws the chroma conversion threshold derivation unit 31 derives derives different saturation transform function S (l, c) according to the brightness value.

  The following equations (8) and (9) are examples of the saturation conversion function S (l, c).

C _max is, for example, C _max = 128.

Subsequently, the color difference component conversion unit 32 of the background removal unit 26 performs saturation conversion and color difference component conversion. Specifically, using the saturation conversion function S (l, c) derived by the saturation conversion function deriving unit 81, the saturation value C _q after the saturation conversion is obtained by the following equation (10).
C _q = S (L _p , C _p ) (10)
Further, the color difference component values (a _p , b _p ) are converted into (a _q , b _q ) by the following equations (11) and (12) using the saturation value C _q .

Finally, the lightness component conversion unit 33 of the background removal unit 26 receives the Lab values (L _p , a _q , b _q ) that have been subjected to the color difference component conversion by the color difference component conversion unit 32, and the lightness value L _p is the maximum lightness. the brightness value of the value L _max greater than the pixel, and converts the lightness maximum value L _max. The Lab values (L _q , a _q , b _q ) obtained by converting the lightness values are transferred to the output color space conversion unit 34, and the background removal process ends.

Next, a flowchart will be described.
Step S4-11: The background removal unit 26 acquires a background color Lab value (L _ws , a _ws , b _ws ).
Step S4-12: The extended color space conversion unit 27 derives an extended color space conversion function T1 (l).
Step S4-13: The input Lab values (L _in , a _in , b _in ) are converted into extended Lab values (L _p , a _p , b _p ).
Step S4-14 chroma conversion threshold derivation unit 31 derives the background color saturation value C _ws.
Step S4-15 The saturation conversion threshold value deriving unit 31 derives a different saturation conversion threshold value C _th (l) according to the already obtained brightness value of the extended color space.
Step S4-16 chrominance component transformer 32 obtains the extended chroma value C _p.
Step S4-17 The saturation conversion function deriving unit 81 derives the saturation conversion function S (l, c).
Step S4-18 The color difference component conversion unit 32 performs saturation conversion and color difference component conversion.
Step S4-19 brightness component converter 33, the lightness value L _p lightness value of the lightness maximum value L _max greater than the pixel, and converts the lightness maximum value L _max, the background removal process is completed.

  As described above, in the present embodiment, a pixel having a lightness value equal to or higher than the background lightness value and a pixel to store a saturation value having a different lightness value and a pixel to be set to 0 are mixed. However, it is possible to distinguish and process the pixels for which the saturation value should be stored and the pixels for which the saturation value should be set to 0, and the saturation gradation for an image having a continuous gradation such as a natural image. A gap can be prevented.

  In the first and second embodiments, the case where color image data (CMYK image data) is output to the printer 13 via the output I / F 22 has been described. However, the output destination of the color image data is this. The output device may be another output device such as an HDD or a monitor, and may be output to an output device built in the image processing apparatus 10 without going through the output I / F 22. Good.

Furthermore, in the first and second embodiments, only the example in which CIE L ^* a ^* b ^* is applied as the lightness color difference color space in which the lightness component and the color difference component are independent has been described. However, the YCbCr color space, etc. As described above, any color space can be applied as long as the brightness component and the color component are independent.

  The present invention is not limited to the above-described embodiment, and various modifications can be made based on the spirit of the present invention, and they are not excluded from the scope of the present invention.

  The present invention can be used in an image processing apparatus.

DESCRIPTION OF SYMBOLS 10 Image processing apparatus 12 Scanner 13 Printer 23 Image acquisition part 24 Input color space conversion part 25 Background color detection part 27 Extended color space conversion part 31 Saturation conversion threshold value derivation part 32 Color difference component conversion part 33 Lightness component conversion part 34 Output color space Conversion unit 35 Image output unit 51 Background color 81 Saturation conversion function derivation unit

Claims (13)

Original image data is obtained from a reading unit including a placing table for placing a document, a light source for irradiating light on the document placed on the placing table, and a reflecting plate for reflecting light emitted from the light source. An image acquisition unit to
An input color space conversion unit that performs a conversion process for converting the original image data acquired by the image acquisition unit into first image data represented by a lightness color difference color space in which a lightness component and a color difference component are independent;
A background color detection unit for detecting a background color from the first image data;
An extended color space conversion unit that performs conversion processing for converting the first image data into second image data represented by an extended color space including a lightness value that exceeds a lightness maximum value;
In the extended color space, it is defined by a straight line passing through a first point indicating the lightness saturation value of the background color and a second point indicating a reference lightness saturation value which is the lightness saturation value of the color of the reflector. A saturation conversion threshold value derivation unit for deriving a saturation conversion threshold value that differs according to the lightness value;
A color difference component conversion unit that performs a conversion process on the color difference component of the second image data based on the saturation conversion threshold;
A lightness component conversion unit that performs conversion processing for converting the lightness value of the second image data that has been converted by the color difference component conversion unit into a value that is equal to or less than the lightness maximum value;
An output color space conversion unit that performs conversion processing for converting the second image data that has been converted by the lightness component conversion unit into third image data in a color space of an output format;
An image output unit for outputting the third image data;
An image processing apparatus comprising: The image processing apparatus according to claim 1, wherein the color difference component conversion unit sets a saturation value of the second image data to 0 when a saturation value of the second image data is equal to or less than the saturation conversion threshold. Saturation that lowers the saturation value of the second image data to a predetermined value other than 0 when the saturation conversion threshold is a boundary and the saturation value of the second image data is larger than the saturation conversion threshold A saturation conversion function deriving unit for deriving the conversion function;
The image processing apparatus according to claim 2, wherein the color difference component conversion unit performs a conversion process according to the saturation conversion function on the color difference component of the second image data. The image processing apparatus according to claim 1, wherein the input color space conversion unit refers to a three-dimensional lookup table. The image processing apparatus according to claim 1, wherein the background color detection unit detects a background color from a brightness histogram of the first image data. The extended color space conversion unit converts the lightness value of the background color detected by the background color detection unit to the lightness maximum value, and converts the lightness value equal to or higher than the lightness value of the background color to a lightness value equal to or higher than the lightness maximum value. The image processing apparatus according to claim 1, wherein the image processing apparatus converts the image processing apparatus. The image processing apparatus according to claim 1, wherein the lightness component conversion unit converts a lightness value exceeding the lightness maximum value into the lightness maximum value. The output color space conversion unit converts the second image data converted by the lightness component conversion unit into image data in a CMYK color space, which is a color space of an output device as a color space of the output format. The image processing apparatus according to claim 1. The output color space conversion unit converts the second image data converted by the lightness component conversion unit into image data in an RGB color space that is a color space of an output device as a color space of the output format. The image processing apparatus according to claim 1. The output color space conversion unit converts the second image data converted by the lightness component conversion unit into image data in a CIE L ^* a ^* b ^* space as a color space of the output format. The image processing apparatus according to any one of 1 to 7. The image processing apparatus according to claim 1, wherein the output color space conversion unit refers to a three-dimensional lookup table. The reading unit is read-color image data from the document, the image acquiring unit is an image processing apparatus according to any one of claims 1 to 11 for acquiring color image data in which the reading unit has read. The image processing apparatus according to claim 1, further comprising an image forming unit that forms an image on a medium using a color material, wherein the image output unit outputs the third image data to the image forming unit. .