JP2001285635A - Image processing apparatus, image processing method and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide am image processing method and an image forming device that can suitably and efficiently apply proper density control in response to various originals whose density condition differs from each other. SOLUTION: A histogram generating section 22 generates s density histogram by correlating each density with frequency of occurrence of a pixel with the density on the basis of the density of each pixel of image data and a background density discrimination setting section 32 sets a background density discrimination area in the density histogram on the basis of a predetermined background density area threshold. The background density discrimination section 23 takes notice of a maximum point of the density that is the closest to the background density area threshold in maximum points and minimum points of the density histogram within the background density area extracted by a minimum point and maximum point detection section 33 and uses the density at the maximum point and the density of the minimum point having a density that is the closest to the maximum point to discriminate the background density.

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 an image processing method, and more particularly, to a method for creating a histogram of pixel densities of an object image and creating a density correction curve suitable for the object image based on the result. And an image processing method for performing an automatic exposure adjustment process based on the density correction curve.

[0002]

2. Description of the Related Art In an analog copying machine, an automatic exposure function, that is, a method of detecting the density of a document by a document density sensor and changing the brightness of an exposure lamp to obtain an optimum image quality is known as a general function.

In recent years, digital image forming apparatuses such as electronic copying machines have become widespread in addition to conventional analog image forming apparatuses. Under such circumstances, in a digital copying machine, as a method of realizing the automatic exposure function, Japanese Patent Publication No. Hei 3-30143, Japanese Patent Application Laid-Open No. H8-204963, Japanese Patent Application Laid-Open No. 9-43915, 10-164
No. 368 proposes an automatic density adjustment method using a density histogram.

In the image quality adjusting device described in Japanese Patent Publication No. Hei 3-30143, a density histogram is created by sampling the density of a document image at a constant cycle and by sampling even when the density value becomes an extreme value. are doing. By comparing the density histogram pattern obtained in this way with the data in the ROM, the type of the original (white background printing original, color background printing original, white background pencil original, etc.) is determined, and the image density control signal is output. Is output.

[0005] In the image forming apparatus described in Japanese Patent Application Laid-Open No. H08-204963, a reference value is determined by obtaining two peak positions of white and black from the created density histogram, and the type of the image is determined from the density histogram. The judgment is made and the reference value is corrected, and gradation correction is performed using the corrected reference value.

In an image forming apparatus described in Japanese Patent Application Laid-Open No. 9-43915, in an automatic density adjustment method for obtaining a reference value for density correction by referring to a numerical value of a density histogram, even if a document cover is open, the A method is described in which a histogram is created only for an area and automatic density adjustment is performed without being affected by data in an area outside the document.

In the image processing apparatus described in Japanese Patent Application Laid-Open No. H10-164368, a density histogram is
Original discrimination is performed based on two peak positions and a peak in an area considered as a background, and gradation correction for removing the background with the lowest density is performed.

[0008]

However, the above-mentioned conventional technique has the following problems.

In these prior arts, when adjusting the density using the density histogram, the background density is determined based on the position where the density frequency peaks. For this reason, in a document having a plurality of bases,
There is a problem that it is not possible to simultaneously and reliably remove the plurality of bases.

For example, in a case where a cutout of a newspaper is pasted on a white mount having a larger area and the original is copied as a manuscript, the manuscript has two bases, a base of the newspaper and a base of the mount.
There will be one base. At this time, when the original is read, a density histogram is obtained in which the frequency increases in both the background density of the newspaper and the background density of the mount.
However, at this time, if the frequency of the background density of the mount is larger, that is, if the frequency of the background density of the mount becomes a peak, the background density of the mount is determined as the background density of the document. For this reason, it is determined that the pasted area of the newspaper is not the background, and density correction for increasing the density as a whole is performed in this area, making it difficult to read characters.

Similarly, in a document having a plurality of backgrounds, if the areas of the backgrounds are substantially equal, the density frequency of each of the backgrounds may be different for each reading due to a problem of the reading accuracy of a scanner or the like. For this reason, in the method of determining the background density based on the position where the frequency of the density peaks, there is a problem that the density determined to be the background differs for each reading, and it is difficult to obtain a constant output image. .

Further, in the method disclosed in Japanese Patent Publication No. Hei 3-30143, since the data of the ROM is used for discriminating the type of the original, when the detailed discrimination is performed, the data corresponding to the discrimination is stored in advance. And the capacity of the ROM increases. In addition, it is determined that a special image that does not match the data in the ROM cannot be determined, and the range of application is limited.

JP-A-8-204963 and JP-A-9-43915 both use a method of obtaining a reference value for gradation correction from a density histogram of a target image. If the density histogram does not accurately represent the density distribution state on the document, such as indistinguishable, mixed pictures and characters, or multiple backgrounds, the optimal correction reference value can be calculated. The automatic density adjustment is impossible or a defective image is output.

In Japanese Patent Application Laid-Open No. 9-43915, different processes are performed by opening and closing the document cover, and there is a problem that the processes become complicated.

Similarly, in Japanese Patent Application Laid-Open No. 10-164368, it is difficult to remove a plurality of backgrounds because the original is discriminated and the background with the lowest density is removed. In addition, there is a case where the same processing is not performed in spite of the same original, such as a case where a density value varies due to the accuracy of a scanner or the like and a density histogram is not faithfully created.

Further, in many of the conventional digital image forming apparatuses, a manual density adjusting device is incorporated, but a user makes a trial copy and adjusts to an optimum density based on the output image. Alternatively, the adjustment is performed based on experience, but in any case, there is a problem that useless copying is performed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. An object of the present invention is to create a density histogram of the pixel density of a target image and easily determine the background density based on the density histogram. The present invention provides an image processing method and an image forming apparatus capable of forming a density correction curve based on the background density value and performing appropriate density control appropriately and efficiently according to various documents having different density conditions. Is to do.

[0018]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, an image processing apparatus according to the present invention determines the background density of a document with respect to image data input by an image input device, and determines the determined background density. In an image processing apparatus for performing an automatic exposure adjustment process for removing pixel data of a portion corresponding to a background region of the image data based on a density, the image processing apparatus includes each density and the density based on a density of each pixel of the image data. A histogram creating means for creating a density histogram in which the frequencies of the pixels are associated with each other; and a density correction for determining the background density and correcting the density of the image data based on the result of the density histogram created by the histogram creating means. Means for determining a background density based on a predetermined background density area threshold value in the density histogram. Background density determination area setting means for setting an area, and minimum point / maximum point detection means for extracting a maximum point and a minimum point of the density histogram in the background density determination area, wherein the density correction means Attention is paid to the maximum point having the density closest to the background density area threshold from the extracted maximum points, and the density of the maximum point and the density of the minimum point having the density closest to the maximum point are used to calculate the background density. It is characterized by having a background density determining means for determining.

In the image processing method of the present invention, the background density of a document is determined for image data input by an image input device, and the image data corresponds to the background area of the image data based on the determined background density. In an image processing method for performing an automatic exposure adjustment process for removing pixel data of a portion, a density histogram is created in which each density is associated with a frequency of a pixel having the density based on the density of each pixel of the image data. A histogram creation step; and a density correction step of determining a background density based on a result of the density histogram created by the histogram creation unit and correcting the density of the image data. The histogram creation step further includes: A background density determination area setting step of setting a background density determination area in the density histogram based on a predetermined background density area threshold. If, minimum point-to extract the maximum and minimum points of the density histogram of the background density determination region
A maximum point detection step, wherein the density correction step comprises:
Attention is paid to the maximum point having the density closest to the background density area threshold from the extracted maximum points, and the density of the maximum point and the density of the minimum point having the density closest to the maximum point are used to calculate the background density. It is characterized by having a background density discriminating step of discriminating.

According to the above configuration, when determining the background density using the maximum point and the minimum point in the density histogram, the position of the local maximum point and the background density area threshold value is determined not by the frequency of the maximum point. It is possible to determine the background density. For this reason, when there are a plurality of backgrounds in the image data, if these background densities appearing as the maximum points on the density histogram have a density smaller than the background density area threshold, Regardless of the frequency, these backgrounds can be reliably removed in the automatic exposure adjustment processing.

Further, the frequency of the maximum point changes under the influence of noise and blur due to the accuracy of the hardware (for example, a scanner) of the image input device, and the background density to be determined differs every reading, so that a constant output image can be obtained. The problem of becoming difficult can be solved. As a result, the same processing can always be performed on the same input image, and a suitable output image can be obtained by performing optimal gradation processing on all images.

In the above-mentioned image processing apparatus, the background density discriminating means compares the density value within a density width threshold value indicating a density range centered on the density value of the maximum point with the density value of the maximum point of interest. Has, and from among the minimum points whose frequency difference with the maximum point is within the frequency difference threshold, a minimum point having the density closest to the maximum point is selected, and the density of the maximum point and the selected density are selected. It is preferable to determine the background density using the density of the minimum point.

Similarly, in the above image processing method, the background density discriminating step includes the step of determining the density value of the maximum point of interest within a density width threshold value indicating a density range centered on the density of the maximum point. The minimum point having the density closest to the local maximum point is selected from the minimum points having a frequency difference between the local maximum point and the frequency difference between the local maximum point and the density difference between the local maximum point and the density of the local maximum point. It is preferable that the background density be determined using the obtained density of the minimum point.

According to the above arrangement, the minimum value that satisfies the density width threshold value and the frequency difference threshold value is selected when determining the background density. At this time, if there is no minimum value that satisfies each of the above threshold values with respect to the maximum point of interest, the maximum value is considered to be a part of an image that is not a base having a certain density distribution near the density. It can be said that it is inappropriate as a maximum point. Therefore, by the above method,
The determination of the background density using an inappropriate maximum point is avoided, and the determination of the background can be accurately performed by a simple method quickly and without being affected even if the accuracy of the hardware (scanner) is somewhat poor. it can.

In the above image processing apparatus, the background density discriminating means has, for a maximum point of interest, a density existing within a density width threshold value indicating a density range centered on the density of the maximum point, In addition, when there is no local minimum point where the frequency difference between the local maximum points is within the frequency difference threshold, the local maximum point of interest is changed to the local maximum point having a density next to the background density area threshold to determine the background density. Is preferred.

Similarly, in the above-described image processing method, the background density discriminating step includes, for the maximum point of interest, a density existing within a density width threshold value indicating a density range centered on the density of the maximum point. If there is no local minimum point where the frequency difference between the local maximum points is within the frequency difference threshold, the local maximum point of interest is changed to the local maximum point having a density next to the background density area threshold to determine the background density. It is preferable to adopt a configuration in which:

According to the above configuration, even if there is no local minimum point satisfying the above thresholds with respect to the local maximum point, the local maximum point is changed to the local maximum point having the next closest density. This makes it possible to always determine the optimal background density.

In the above-mentioned image processing apparatus, the background density discriminating means adds or subtracts a value obtained by adding an absolute value of a difference between the density of the maximum point and the density of the minimum point to the density of the noted maximum point. It is preferable that the value be determined as the background density.

Similarly, in the above-described image processing method, the background density discriminating step includes calculating a value obtained by adding an absolute value of a difference between the density of the maximum point and the density of the minimum point to the density of the noted maximum point. Alternatively, it is preferable that the subtracted value is determined as the background density.

According to the above arrangement, the background density determined by this is a value obtained by adding or subtracting the absolute value of the difference between the density of the maximum point and the density of the minimum point with respect to the maximum point of interest. As a result, the density of the local maximum point which seems to indicate the background density in the image data is reliably removed in the automatic exposure adjustment processing. The value obtained by adding the absolute value of the difference between the density at the maximum point and the density at the minimum point is determined as the background density because, for example, CMY which is color-corrected RGB-based signals input from a scanner or the like as image data. The value obtained by subtracting the absolute value of the difference between the density of the local maximum point and the density of the local minimum point is determined as the background density, for example, when RGB values input from a scanner or the like as image data are used.
This is a case where the system signal is used as it is.

In the above image processing apparatus, the density correction means further includes a reference density correction curve for setting a correction relationship between an input density value and an output density value prepared in advance.
Density correction curve creating means for creating a density correction curve by uniformly compressing the reference density correction curve using the density determined as the background density by the background density determination means,
The density correction curve creating means sets a plurality of correction points on the reference density correction curve, and sets the background density as a reference point for providing an output density value of 0 for the density correction curve to be created. The correction points are equally compressed according to the compression ratio between the reference point and the starting point for giving an output density value of 0 in the correction curve, and a density correction curve is created. It is preferable that the density correction at the time of the automatic exposure adjustment processing is performed using the density correction curve created by the means.

Similarly, in the above-mentioned image processing method, the above-mentioned density correction step further includes the step of preparing the base density based on a reference density correction curve for setting a correction relationship between an input density value and an output density value prepared in advance. A density correction curve creating step of uniformly compressing the reference density correction curve using the density determined as the background density by the determination means to create a density correction curve; and the density correction curve creating step includes: A plurality of correction points are provided on the correction curve, the background density is used as a reference point for providing an output density value of 0 for a density correction curve to be created, and a starting point for providing an output density value of 0 in the reference density correction curve. In accordance with the compression ratio between the correction point and the reference value, the correction points are equally compressed to create a density correction curve, and the density correction step is performed by the density correction curve generation means. It is preferable to have a structure to perform density correction of the automatic exposure adjustment processing by using the density correction curve generated Te.

According to the above configuration, the density correction curve used for performing the above automatic exposure adjustment processing can be realized by a simple calculation such as compression processing of correction points on the reference density correction curve. Optimal and more accurate density correction curve creation is possible. Further, the processing speed and the circuit scale in creating the density correction curve can be reduced, and the hardware for performing the above processing is greatly simplified.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS.

FIG. 2 is a front sectional view showing a schematic configuration of a digital color copying machine as an image forming apparatus according to the present embodiment.

An original table 1 is provided on the upper surface of the image forming apparatus main body 1.
11 and an operation unit (not shown) are provided, and an image input device 110 and an image output device 210 are provided inside the image forming apparatus main body 1.

The upper surface of the document table 111 is supported so as to be openable and closable with respect to the document table 111, and has a predetermined positional relationship with respect to the surface of the document table 111, and a double-sided automatic document feeder (RA
A DF (Reversing Automatic Document Feeder) 112 is mounted.

Further, the automatic two-sided automatic document feeder 112
First, the original is conveyed so that one side of the original faces the image input device 110 at a predetermined position on the original platen 111, and after the image reading on this one side is completed, the other side is placed on the original The document is turned over so as to face the image input device 110 at the position, and is conveyed toward the document table 111. Then, after the two-sided image reading for one document is completed, the double-sided automatic document feeder 112 discharges this document and performs a double-sided conveyance operation for the next document. The above-described operation of conveying the document and reversing the front and back is controlled in relation to the operation of the entire image forming apparatus.

The image input device 110 is arranged below the document table 111 for reading an image of a document conveyed onto the document table 111 by the automatic duplex document feeder 112. The image input device 110 includes first and second scanning units 113 and 114 that reciprocate in parallel along the lower surface of the document table 111, an optical lens 115, and a CCD (Charge Coupled Device) line sensor that is a photoelectric conversion element. 116.

The first scanning unit 113 has an exposure lamp for exposing the surface of the original image and a first mirror for deflecting a reflected light image from the original in a predetermined direction.
11 reciprocates in parallel at a predetermined scanning speed while maintaining a constant distance with respect to the lower surface. The second scanning unit 114 has second and third mirrors for further deflecting the reflected light image from the document deflected by the first mirror of the first scanning unit 113 in a predetermined direction,
It reciprocates in parallel with the first scanning unit 113 while maintaining a constant speed relationship.

The optical lens 115 reduces the reflected light image from the document deflected by the third mirror of the second scanning unit 114 and converts the reduced light image into the CCD line sensor 11.
6 to form an image at a predetermined position.

The CCD line sensor 116 photoelectrically converts the formed light image sequentially and outputs it as an electric signal. The CCD line sensor 116 reads a black-and-white image or a color image, and outputs R (red), G (green), and B (blue).
This is a three-line color CCD capable of outputting line data color-separated into each color component. The document image information converted into an electric signal by the CCD line sensor 116 is further transferred to an image processing device (shown in FIGS. 3 and 4), which will be described later, and subjected to predetermined image data processing.

Next, the configuration of the image output device 210 and the configuration of each unit related to the image output device 210 will be described.

Below the image output device 210, there is provided a paper feed mechanism 211 for separating the paper (recording medium) P stored in the paper tray one by one and supplying it to the image output device 210. ing. The sheets P separated and supplied one by one are conveyed to the image output device 210 with the timing controlled by a pair of registration rollers 212 disposed in front of the image output device 210. Further, the sheet P on which an image is formed on one side is re-supplied and conveyed to the image output device 210 in synchronization with the image forming timing of the image output device 210.

Below the image output device 210, a transfer and transport belt mechanism 213 is disposed. The transfer / transport belt mechanism 213 is provided between the drive roller 214 and the driven roller 215 to extend substantially in parallel with the transfer / transport belt 2.
The sheet P is electrostatically attracted to the sheet 16 and transported. Then, in the vicinity of the lower side of the transfer conveyance belt 216,
A pattern image detection unit 232 is provided.

Further, a fixing device 217 for fixing the toner image transferred and formed on the paper P on the paper P is provided on the downstream side of the transfer and transport belt mechanism 213 in the paper transport path.
Is arranged. The sheet P that has passed through the nip between the pair of fixing rollers of the fixing device 217 passes through a conveyance direction switching gate 218 and is discharged by a discharge roller 219 to a discharge tray 22 attached to an outer wall of the image forming apparatus main body 1.
Drained above zero.

The switching gate 218 is connected to the sheet P after fixing.
Is selectively switched between a path for discharging the sheet P to the outside of the image forming apparatus main body 1 and a path for re-supplying the sheet P to the image output apparatus 210. The sheet P whose transport direction has been switched again toward the image output device 210 by the switching gate 218 is turned upside down via the switchback transport path 221 and then supplied again to the image output device 210.

The first image forming unit Pa and the second image forming unit Pb are located above the transfer belt 216 in the image output device 210 and close to the transfer belt 216.
A third image forming unit Pc and a fourth image forming unit Pd
Are arranged in order from the upstream side of the paper transport path.

The transfer conveying belt 216 is driven by a driving roller 214.
Is driven in the direction indicated by arrow Z in FIG.
As described above, the sheet P that is fed through the sheet feeding mechanism 211 is carried, and the sheet P is sequentially conveyed to the image forming units Pa to Pd.

Each of the image forming units Pa to Pd has substantially the same configuration. Each image forming unit Pa / Pb / Pc /
Pd includes photosensitive drums 222a, 222b, 222c, and 222d that are driven to rotate in the direction of arrow F shown in FIG. Each photoconductor drum 222a to 222
Around d, charging devices 223a to 223d for uniformly charging the photosensitive drums 222a to 222d, and developing devices 224a to 224d for developing the electrostatic latent images formed on the photosensitive drums 222a to 222d, respectively. And transfer members 225a to 225d for transferring the developed toner images on the photosensitive drums 222a to 222d onto the sheet P, and cleaning devices 226a to 226d for removing the toner remaining on the photosensitive drums 222a to 222d. They are sequentially arranged along the rotation direction of the body drums 222a to 222d.

Each of the photosensitive drums 222a to 222d
Above the laser beam scanner unit 227a
To 227d are provided respectively. The laser beam scanner units 227a to 227d include a semiconductor laser device (not shown) that emits light modulated according to image data, and a polygon mirror (deflecting device) for deflecting the laser beam from the semiconductor laser device in the main scanning direction. ) 2
40a to 240d, polygon mirrors 240a to 240d
The laser beam deflected by the photosensitive drum 222a
F.theta. Lens 241a for forming an image on the surface of .about.222d
To 241d, mirrors 242a to 242d and 243a to 2
43d and the like.

Laser beam scanner unit 227a
The pixel signal corresponding to the black component image of the color original image, and the laser beam scanner unit 227b the pixel signal corresponding to the cyan component image of the color original image.
A pixel signal corresponding to the image of the magenta color component of the color original image is input to the laser beam scanner unit 227c, and a pixel signal corresponding to the image of the yellow color component of the color original image is input to the laser beam scanner unit 227d. You.

As a result, an electrostatic latent image corresponding to the color-converted document image information is formed on each of the photosensitive drums 222a to 222d. The developing device 224a receives the black toner, the developing device 224b receives the cyan toner, the developing device 224c receives the magenta toner, and the developing device 224c.
24d contains yellow toner respectively, and the electrostatic latent images on the photosensitive drums 222a to 222d are:
The image is developed with the toner of each color. As a result, the image output device 210 reproduces the document image information as a toner image of each color.

Further, the first image forming unit Pa and the paper feed mechanism 2
A paper-charging charger 228 is provided between the transfer-belt 11 and the sheet-charging charger 11. As a result, the sheet P supplied from the sheet feeding mechanism 211 is transported without shifting between the first image forming unit Pa and the fourth image forming unit Pd in a state where the sheet P is securely attracted onto the transfer and transport belt 216. Is done.

On the other hand, the fourth image forming portion Pd and the fixing device 2
A static eliminator 229 is provided almost directly above the drive roller 214 between the roller 17 and the roller 17. This static eliminator has a conveyor belt 2
The transfer conveyance belt 21 transfers the paper P electrostatically attracted to the
6 is applied.

In the digital color image forming apparatus having the above-described configuration, cut sheet-shaped paper is used as the paper P. When the paper P is sent out of the paper feed cassette and supplied into the guide of the paper feed path of the paper feed mechanism 211,
The leading end of the sheet P is detected by a sensor (not shown), and is temporarily stopped by a pair of registration rollers 212 based on a detection signal output from the sensor.

The paper P is stored in each of the image forming portions Pa to Pd.
At this time, the sheet is fed onto the transfer / conveying belt 216 rotating in the direction of arrow Z in FIG. At this time, since the transfer / conveyance belt 216 is charged with a predetermined charge by the attraction charger 228 as described above, the paper P is stably conveyed and supplied while passing through the image forming units Pa to Pd. You.

In each of the image forming units Pa to Pd, a toner image of each color is formed,
The sheet P is superposed on the supporting surface of the sheet P which is electrostatically attracted and conveyed by 6. When the transfer of the image by the fourth image forming unit Pd is completed, the sheet P is sequentially separated from the transfer conveyance belt 216 by the discharging unit 229 from the leading end thereof, and is guided to the fixing device 217. Finally, the paper P on which the toner image has been fixed is discharged onto a paper discharge tray 220 from a paper discharge port (not shown).

In the above description, the laser beam scanner units 227a to 227d scan and expose a laser beam to perform light writing on a photosensitive member. However, instead of the laser beam scanner unit, A writing optical system including a light emitting diode array and an imaging lens array may be used. LED (LED:
Light Emitting Diode heads are smaller in size than laser beam scanner units, and have no moving parts and are silent. Therefore, it can be suitably used in a tandem-type digital color image forming apparatus or the like that requires a plurality of optical writing units.

Next, FIG. 3 shows the configuration of the image processing apparatus according to the present embodiment. An image processing apparatus (image processing means) 1 shown in FIG. 3 performs image processing on image data input from an image input apparatus (image input means) 2, and outputs the image data to an image output apparatus (image output means). 3 is modified to a form suitable for output. The image data that has been subjected to the image processing by the image processing device 1 is output to the image output device 3, and output processing is performed by the image output device 3.

The image input device 2 is, for example, an image input device 1 provided as a scanner unit in FIG.
10 and the reflected light image from the original is represented by CC.
D, and outputs the obtained RGB analog signals to the image processing apparatus 1 as image data. The image output device 3 is, for example, the image output device 2 in FIG.
Equivalent to 10. The image processing apparatus 1 is configured as shown in FIG.
Although not shown in the figure, it is provided as an image processing unit.

As shown in FIG. 3, the image processing apparatus 1 includes an A / D (analog / digital) conversion unit 11, a shading correction unit 12, an input gradation correction unit 13, and a color correction unit 1.
4. Black generation / under color removal unit 15, spatial filter processing unit 1
6, an output tone correction section 17, a tone reproduction processing section 18, and an area separation processing section 19.

The image data input from the image input device 2 to the image processing device 1 is first converted by the A / D converter 11 into R
The analog signal of GB is converted into a digital signal, and the shading correction unit 12 controls the illumination system of the image input device 2.
Shading correction is performed to remove various types of distortion generated in the imaging system / imaging system. After that, the image data is subjected to an input tone correction process in which the input tone correction unit 13 adjusts the color balance of the RGB reflectance signal and simultaneously converts the RGB reflectance signal into a signal such as a density signal which can be easily handled by the image processing system. Is done.

Next, the color correction section 14 performs a color correction process for removing color turbidity based on the spectral characteristics of the CMY color materials containing unnecessary absorption components in order to realize the fidelity of color reproduction.
The black generation / under color removal unit 15 generates a black (K) signal from the CMY three-color signals after color correction, and generates the original C signal.
By subtracting the K signal obtained by black generation from the MY signal, a new C
The under color removal processing for generating the MY signal is performed, and the CMY 3
The color signals are converted into CMYK four-color signals.

Next, the obtained image signal is subjected to a spatial filter processing by a digital filter in the spatial filter processing section 16 to correct the spatial frequency characteristic, thereby preventing the output image from being blurred or grainy deterioration. It is processed as follows. Then, the output tone correction unit 17 performs an output tone correction process for converting a signal such as a density signal into a dot area ratio which is a characteristic value of the image forming apparatus. A tone reproduction process (halftone generation process) is performed to divide an image into pixels and reproduce the respective tones.

After the above-described color correction processing, in order to enhance the reproducibility of the character portion especially in the text and photograph mixed original, the region separation processing section 19 extracts the black character (including the color character in some cases). The image area is filtered by the sharpness enhancement processing in the spatial filter processing unit 16 so that the enhancement amount of the high frequency band is increased. At the same time, in the image area, in the halftone generation processing in the gradation reproduction processing section 18, binarization or multi-value processing on a high-resolution screen suitable for high-frequency reproduction is selected.

On the other hand, for the area determined to be a photograph by the area separation processing section 19, the spatial filter processing section 16 performs low-pass filter processing for removing an input halftone dot component. At the same time, in the halftone generation process in the tone reproduction processing unit 18, binarization or multi-value processing is performed on a screen that emphasizes tone reproducibility. The image data subjected to each of the above-described processes is temporarily stored in the storage unit, read out at a predetermined timing, and input to the image output device 3.

The automatic exposure adjustment processing performed by the image processing apparatus 1 according to the present embodiment
It is performed in. That is, if the automatic exposure adjustment mode is selected in the image forming apparatus, as shown in FIG.
RGB signals as input image data (or the RGB
An automatic exposure adjustment process is performed by the input tone correction unit 13 using a single color signal selected from the B signal converted CMY signal), and the processed single color signal is converted into a K signal. , Area separation processing, spatial filter processing, output gradation correction processing, and gradation reproduction processing (halftone generation processing) are performed and output. However, the automatic exposure adjustment processing shown in FIG. 4 is an example in which the processing is performed at the time of pre-scanning a document, and the above-described color correction and black generation / under color removal processing are omitted.

As shown in FIG. 1, the input tone correction section 13 for performing the above automatic exposure adjustment processing includes a monochromatic signal conversion section 21, a histogram creation section (histogram creation means) 22, a background density determination section (background density determination section). ) 23, a density correction curve creation unit (density correction curve creation means) 24, a density correction point creation unit 25, and a signal conversion unit 26. still,
The density correction means described in the claims comprises the above-described background density determination section 23, density correction curve creation section 24, density correction point creation section 25, and signal conversion section 26.

The monochromatic signal converter 21 extracts a CMY monochromatic signal from the RGB signals input from the shading corrector 12. The histogram creation unit 22 creates a density histogram from the density and frequency of each pixel. The background density determination unit 23 determines the background density from the density histogram created by the histogram creation unit 22. The density correction curve creation unit 24 creates a reference density correction curve based on the result of the background density determination unit 23. The density correction point creation unit 25 creates a density correction point based on a reference density correction point prepared in advance when creating a density correction curve. The signal converter 26 converts the monochrome signal into a K signal.

The histogram creating section 22 includes a background density area threshold value setting section 31 and a background density determination area setting section 3
2 and a minimum point / maximum point detection unit 33. Background density area threshold setting unit (background density area threshold setting means) 3
2 sets a threshold value for creating a background density determination area to be a background density determination target. Background density determination area creation unit 3
3 creates a background density determination area based on the threshold value. Minimum point / maximum point detection unit (minimum point / maximum point detection means) 31
Detects the minimum value and the maximum value of the density histogram in the background density determination area.

The flow of the automatic exposure adjustment processing will be described with reference to the flowchart of FIG. In the following description,
A case where processing is performed using a single color signal selected from CMY will be described.

First, in step 1 (S1: hereinafter, step is described as S), an automatic exposure adjustment mode is set. The setting of the automatic exposure adjustment mode can be performed by, for example, providing an automatic exposure adjustment mode setting button on the operation unit of the digital color image forming apparatus. When the automatic exposure adjustment mode is set, a pre-scan is started (S2), and a monochromatic signal (for example, an M signal) is obtained from a CMY signal obtained by performing color correction on the RGB signals of all pixels of the read document. Selected (S3).

Here, the reason why a monochromatic signal is used in the above automatic exposure adjustment processing is as follows. That is, in the automatic exposure adjustment process, a process of determining the background density of the document and removing density data lower than the background density is performed. At this time, the background density is a sufficiently small density,
In the density distribution of each monochromatic signal, it is easily predicted that the frequency distribution near the background density shows almost the same tendency. For this reason, even if the background density is determined for one single-color signal, a sufficiently appropriate result is obtained, and the processing can be simplified by performing the automatic exposure adjustment processing using one single-color signal.

Next, in extracting the background density of the document, the background density to be extracted is regarded as a value equal to or lower than a certain predetermined density, and the predetermined value is set as a background density area threshold to set a background density determination area. The background density determination area is an area having a density equal to or lower than the background density area threshold. The background density area threshold is a preset value, but the threshold can be manually reset at this time. That is, at this time, it is determined whether or not to manually set the background density determination area in S4. If the background density area threshold is manually set, the setting is performed in S5. A method of setting the background density area threshold will be described later.

Next, a density histogram is created using the single color signal selected in S3 (S6). Based on the created density histogram, the minimum value and the maximum value in the above-described background density determination area are determined in S7, and the background density (reference value) is determined in S8. Next, in S9, S
A density correction curve is created based on the background density extracted in step S8.

Next, an image is output using the density correction curve created in S9 (S10). In S11, it is determined whether or not further density adjustment is to be performed on the output image. That is, when the user does not obtain a satisfactory result with respect to the image output in S10,
A desired result can be obtained by resetting the threshold value of the background density area and the like. When the density is further adjusted, the background density area threshold value is set in the manual mode in S12, and the processing from S7 to S10 is performed again.

If the image output in S10 is satisfactory, and it is not necessary to perform density adjustment in S11, a series of processing ends. If the automatic exposure adjustment mode is not set in S1, the normal copy operation (S1
3) is performed.

In the above description, when performing the density adjustment in S11, the background density area threshold is set in the manual mode, and the detected background density is changed. However, as another control example, as shown in FIG. 6, the user can directly set the reference value (that is, the background density) in the manual mode. A method for setting the reference value will be described later. In the processing procedure shown in FIG. 6, when the density adjustment is performed in S11, the reference value is set in S12 ′.
Thereafter, the processing of S9 to S11 is repeated. Otherwise, the processing procedure is the same as that shown in FIG.

Note that S12 and S1 in FIGS.
In 2 ′, the threshold value of the background density area is set after S6.
This is because appropriate processing can be performed on the output image.

Next, a method of determining the background density from the density histogram and a method of creating a density correction curve based on the result will be described below with reference to FIGS.

In the density histogram created in the automatic exposure adjustment processing according to the present embodiment, as shown in FIG. 7, each frequency is represented for each of 256 levels of density. Of course, if hardware simplification is considered, a method of dividing the density in 256 levels of the density histogram can be considered. However, more accurate processing can be performed by obtaining the frequency for each density without dividing the 256 levels of density.

In order to determine the background density, up to which density value is determined to be the background, that is, the maximum value of the density value determined to be the background is set in advance as a background density area threshold value as shown in FIG. . Further, all the minimum points and the maximum points are detected using the frequency of the density histogram in the density determination area that is a density area equal to or less than the threshold value of the density density area.

Using the minimum point and the maximum point obtained in the background density determination area of the density histogram, the background density is obtained as follows.

First, a density width threshold value and a frequency difference threshold value for background density determination are set. As shown in FIG. 9, the density width threshold value is a threshold value that limits the difference between the density value of the local maximum point of interest and the density value determined as the background density. If the density width threshold is H, the background density is set at a density value between X−H / 2 and X + H / 2. Further, the frequency difference threshold is a threshold that limits the difference between the frequency value of the minimum point and the frequency value of the maximum point of interest within the density width threshold, and the frequency of the maximum point of interest is m, and the density value of the minimum point is m. Is set to n and the frequency difference threshold is set to v, a minimum point satisfying mn> v is selected.

In the above description, as a method of selecting the maximum point of interest, selection can be made sequentially from the low density side or the high density side of the background density determination area. It is preferable to compare from the side. By comparing from the high concentration side, it is possible to easily and quickly select the maximum point of the highest concentration satisfying the above conditions. In addition, by selecting the maximum point having the highest density, a plurality of backgrounds can be removed.

When the local maximum point to be determined is determined, the local minimum point existing in the hatched area shown in FIG. 9 is used as a target when determining the background density based on the density width threshold value and the frequency difference threshold value. If there are multiple such minimum points,
The point whose density difference is closest to the density difference at the maximum point is selected. That is, two minimum points of density values α and β exist in the hatched area, and | x−β |> | x−α |
If so, a point with the density value α is selected.

The minimum point thus selected is used for setting a reference point (background density). Specifically, if the density value of the minimum point used for setting the reference point is α, X + | X−α |
Is the reference point (background density). The density value at this reference point is input as a reference value of a corrected density curve described below. If no minimum point exists in the hatched area, the maximum point of interest is changed to the maximum point having the next highest density value, and the minimum point used for setting the reference point is determined in the same procedure. .

Next, a density correction curve is created with respect to the reference density correction curve based on the reference values obtained as described above. An example of a method for creating a density correction curve will be described with reference to FIG.
Note that the reference density correction curve in FIG. 10 is an example of a density correction curve serving as a reference curve for automatic exposure adjustment (a gamma for not performing automatic exposure adjustment may be prepared when automatic exposure adjustment is not performed).

The density value of the reference value determined as the background density is
This is the starting point of the density correction curve, that is, the point at which a correction value 0 is given to the input density value. That is, in the density correction curve, the correction value of the density is 0 for a pixel whose input density value is smaller than the reference value, and the background is removed from the output image using the correction value. The end point of the density correction curve coincides with the end point of the reference density correction curve prepared in advance.

The density correction curve is created based on a reference density correction curve prepared in advance. At this time, a correction point is set on the reference density correction curve. It is desirable that the correction points are set for all the density values. However, as shown in FIG. 10, points to be corrected are defined in advance, and these points are corrected so that all the density values correspond. Is also good.

For example, in FIG.
The reference correction point (a₁, B ₁), (A_Two,
b_Two), (A_Three, B_Three), (A_Four, B_Four), ...
Set it. When the density value p serving as the reference value is determined,
Input density value a of each reference correction point₁, A_Two, A_Three,
a_Four, ..., c_i= P + (a_i× ((255-
Set the correction points to be p) / 255))
Density value b₁, B_Two, B_Three, B_FourTie ...
Thereby, a density correction curve is created. Like this
To create a density correction curve for the input image.
With this, more optimal density correction can be performed and hardware
Is greatly simplified.

Next, when the main scan is started, for example, the density below the reference value shown in FIG. 10 is removed based on the density correction curve created as described above, and the density value corresponding to each input value is removed. Is output.
However, in some cases, the user may want to output a desired image. At this time, the reference value in the created density correction curve is manually set to, for example, an exposure adjustment button (density correction amount adjusting means) provided on an operation unit of the image forming apparatus for each of the low density and the high density. By adjusting, it is possible to output a desired image. The method of adjusting the density correction curve manually by the user is as follows.

In other words, two thresholds, ie, a background density area threshold and a frequency difference threshold, which are used in preparing the density correction curve, are set as reference values in advance.
By arbitrarily adjusting these thresholds, the starting point of the density correction curve, that is, the reference value (base density) can be changed. As a result, the created density correction curve changes and the background density to be removed in the output image can be changed, so that a wider range of processing can be performed.

Regarding the background density area threshold and the frequency difference threshold, a plurality of expected values are stored in a ROM (Read Onl
y memory) or the like, and the value stored in the storage means such as the ROM can be selectively set by a switch or the like as necessary. Alternatively, as described below, the user may be able to set the above-described background density region threshold value and frequency difference threshold value to arbitrary values.

For example, as shown in FIG. 11, a band of 256 levels of density is displayed on a display unit 51 such as a liquid crystal display panel of an operation unit of a digital color image forming apparatus, and is displayed on the density band. The pointing unit 52 such as a cursor is moved right and left by the position setting means 53 of the pointing unit composed of two buttons or the like, and a predetermined density (a density to be determined as a background density) is selected. As a result, the selected density value is stored in the storage means as the background density area threshold value, and it is easy to adjust which density value is determined to be the background. As a result, it is possible to select whether to remove the background or to leave the background and output the background regardless of the density of the target.

Numerical value display section 54 in display section 51
Displays the density at the position pointed to by the instruction section 52 in a numerical value, and the enter button 55 is a button pressed when adopting the selected density. In the above description, an example of displaying a band having a density of 256 gradations is shown. However, the present invention is not limited to this, and a density of each predetermined step may be displayed. A pointing device such as a mouse may be used instead of two buttons that can move the pointing unit left and right. In the above description, the background density area threshold is adjusted. However, if the density selected by the instruction unit 52 is not the background density area threshold but the reference value of the density correction curve, It is also possible to adjust the reference value (background density) itself in exactly the same way.

Similarly, when adjusting the frequency difference threshold, FIG.
As shown in FIG. 2, the size of the background to be removed (the size of the background area occupying the entire document, for example, is displayed on the display unit 51 of the operation unit of the digital color image forming apparatus.
In this case, the minimum size is postcard and the maximum size is A3
Is displayed, and the size of the background is selected by moving the indicator 52 with two setting buttons or a mouse in the same manner as when setting the background density threshold area. . The size of the background to be set can be set not by the size as shown in FIG. 12 but by the ratio of the background area to the whole original image.

When the size of the background is selected, the number of pixels corresponding to the size of the selected background is stored in the storage means as a frequency difference threshold. In this way, it is possible to easily adjust how much frequency difference is determined to be the background, thereby enabling the removal of the background or the output of the background for any size of the background of interest.

When the above two threshold values are input from the operation unit of the digital color image forming apparatus shown in FIGS. 11 and 12, a value having a high density is input so that it is impossible to judge the background. Sometimes. In such a case, for example, a message such as "Please select a light color" is displayed on a message display section (not shown), and data is sequentially input to corresponding thresholds. You only have to input according to the message.

Further, in order to prevent a malfunction caused by inputting an erroneous numerical value to each of the above-mentioned background density area threshold value and frequency difference threshold value, an upper limit value and a lower limit value are determined for each threshold value. Prohibition processing is performed on a high numerical value that cannot be considered as the density of, and an error message is displayed. Alternatively, in the density band and the band indicating the size of the background displayed on the display unit 51 of FIGS. 11 and 12, only the range corresponding to the upper limit value / lower limit value of the set threshold is displayed. Is also good.

In the above description, the case where the single-color signal used for creating the density histogram is a CMY-based signal has been described. However, when the RGB-based single-color signal is used, the density histogram is not less than the background density area threshold. The density area becomes a background density determination area. Then, the maximum point of interest is selected sequentially from the lower density side in the background density determination area. The minimum point used for the determination of the background density is one that satisfies the density width threshold value and the frequency difference threshold value, and has the density closest to the maximum point of interest, as in the case of using the RGB single color signal. A point is selected. If the density value of the selected minimum point is α with respect to the density value X of the local maximum point of interest,
(X− | X−α |). Other processing may be performed in the same manner as in the case of using CMY-based single-color signals.

In the above description, the case where the automatic exposure adjustment processing is performed using the data obtained by the pre-scan has been described. In addition, the input data obtained by the main scan is separated into two parts, Once stored in a storage means such as an image memory, the density correction curve is obtained by performing the automatic density adjustment of the present invention using the other data, and then input data is read out from the storage means to obtain the density correction curve. Alternatively, a method of outputting an image may be used.

In the above description, a digital copying machine using an electrophotographic process has been described as an example. However, the present invention is not limited to the above example, and information is input from image input means. Of course, the present invention can be applied to an image forming apparatus that performs a predetermined image processing and outputs the result, for example, an image forming apparatus using an inkjet recording method or a sublimation type recording method.

[0105]

As described above, the image processing apparatus of the present invention creates a density histogram in which each density is associated with the frequency of a pixel having the density based on the density of each pixel of the image data. A histogram correction unit configured to determine a background density based on a result of the density histogram generated by the histogram generation unit and correct a density of the image data; the histogram generation unit further includes: In the density histogram, a background density determination area setting means for setting a background density determination area based on a predetermined background density area threshold, and a maximum point and a minimum point of the density histogram in the background density determination area A minimum point / maximum point detecting unit to be extracted, wherein the density correction unit includes a local maximum point having a density closest to the background density area threshold value among the extracted maximum points. Noted, the concentration of polar large point is a configuration that includes a background density determining means for determining a background density by using the density of minimum point having a density closest to the ultra large point.

Further, according to the image processing method of the present invention, as described above, a density histogram in which each density is associated with the frequency of a pixel having the density is created based on the density of each pixel of the image data. A histogram creation step; and a density correction step of determining a background density based on a result of the density histogram created by the histogram creation unit and correcting the density of the image data. The histogram creation step further includes: In the density histogram, a background density determination area setting step of setting a background density determination area based on a predetermined background density area threshold, and a maximum point and a minimum point of the density histogram in the background density determination area A minimum point / maximum point detection step to be extracted, and the density correction step includes the step of detecting the density closest to the background density area threshold from the extracted maximum points. Focus on large point, and the concentration of the ultra-large point,
The image processing apparatus further includes a background density determination step of determining the background density using the density of the minimum point having the density closest to the maximum point.

According to the above configuration, when determining the background density using the maximum point and the minimum point in the density histogram, the position relationship between the maximum point and the background density area threshold is used instead of the frequency of the maximum point. Since the background density is determined, when a plurality of backgrounds are present in the image data, these background densities appearing as local maxima on the density histogram are determined in the automatic exposure adjustment process regardless of the frequency. This has the effect that the underlayer can be reliably removed.

Further, it is possible to solve the problem that the background density determined by the accuracy of the hardware (for example, a scanner) of the image input device differs for each reading, and it is difficult to obtain a constant output image. As a result, the same processing can always be performed on the same input image, and an advantageous effect can be obtained in which a suitable output image can be obtained by performing optimal gradation processing on all images.

In the above-mentioned image processing apparatus, the background density discriminating means compares the density value within the density width threshold value indicating the density range centered on the density of the maximum point with the density value of the maximum point of interest. Has, and from among the minimum points whose frequency difference with the maximum point is within the frequency difference threshold, a minimum point having the density closest to the maximum point is selected, and the density of the maximum point and the selected density are selected. It is possible to adopt a configuration in which the background density is determined using the density of the minimum point.

Similarly, in the above-described image processing method, the background density discriminating step includes the step of determining the density value of the maximum point of interest within a density width threshold value indicating a density range centered on the density of the maximum point. The minimum point having the density closest to the local maximum point is selected from the minimum points having a frequency difference between the local maximum point and the frequency difference between the local maximum point and the density difference between the local maximum point and the density of the local maximum point. The background density can be determined by using the obtained density of the minimum point.

Therefore, if there is no minimum value that satisfies each of the above-mentioned thresholds with respect to the maximum point of interest, the maximum value is a part of the image which is not a base having a certain density distribution near the density. Since it is considered that there is a point and it is inappropriate as a local maximum point, the above method has an advantage that it is possible to avoid performing background density determination using an inappropriate local maximum point.

In the above-described image processing apparatus, the background density discriminating means has a density which exists within a density width threshold value indicating a density range centered on the density of the maximum point with respect to the maximum point of interest. In addition, when there is no local minimum point where the frequency difference between the local maximum points is within the frequency difference threshold, the local maximum point of interest is changed to the local maximum point having a density next to the background density area threshold to determine the background density. can do.

Similarly, in the above-described image processing method, the background density discriminating step includes, for the maximum point of interest, a density existing within a density width threshold value indicating a density range centered on the density of the maximum point. If there is no local minimum point where the frequency difference between the local maximum points is within the frequency difference threshold, the local maximum point of interest is changed to the local maximum point having a density next to the background density area threshold to determine the background density. Configuration.

Therefore, even if there is no minimum point satisfying each of the above threshold values with respect to the maximum point of interest, the optimum background density can always be determined by changing the maximum point of interest. This has the effect of becoming

In the image processing apparatus, the background density determining means subtracts or subtracts a value obtained by adding an absolute value of a difference between the density of the maximum point and the density of the minimum point to the density of the noted maximum point. A configuration may be adopted in which the value is determined as the background density.

Similarly, in the above-described image processing method, the background density discriminating step includes calculating a value obtained by adding an absolute value of a difference between the density of the maximum point and the density of the minimum point to the density of the noted maximum point. Alternatively, the subtracted value may be determined as the background density.

Therefore, there is an effect that the density of the local maximum point which seems to indicate the background density in the image data is reliably removed in the automatic exposure adjustment processing.

In the above image processing apparatus, the density correction means further includes a reference density correction curve for setting a correction relationship between an input density value and an output density value prepared in advance.
Density correction curve creating means for creating a density correction curve by uniformly compressing the reference density correction curve using the density determined as the background density by the background density determination means,
The density correction curve creating means sets a plurality of correction points on the reference density correction curve, and sets the background density as a reference point for providing an output density value of 0 for the density correction curve to be created. The correction points are equally compressed according to the compression ratio between the reference point and the starting point for giving an output density value of 0 in the correction curve, and a density correction curve is created. The density correction at the time of the automatic exposure adjustment processing can be performed using the density correction curve created by the means.

Similarly, in the above-mentioned image processing method, the above-mentioned density correction step further includes the above-mentioned base density correction curve based on a reference density correction curve for setting a correction relationship between an input density value and an output density value prepared in advance. A density correction curve creating step of uniformly compressing the reference density correction curve using the density determined as the background density by the determination means to create a density correction curve; and the density correction curve creating step includes: A plurality of correction points are provided on the correction curve, the background density is used as a reference point for providing an output density value of 0 for a density correction curve to be created, and a starting point for providing an output density value of 0 in the reference density correction curve. In accordance with the compression ratio between the correction point and the reference value, the correction points are equally compressed to create a density correction curve, and the density correction step is performed by the density correction curve generation means. It can be configured to perform density correction of the automatic exposure adjustment processing by using the density correction curve generated Te.

Therefore, the density correction curve used for performing the above-mentioned automatic exposure adjustment processing can be realized by a simple calculation such as compression processing of correction points on the reference density correction curve. The effect is that the scale can be reduced and the hardware for performing the above processing is greatly simplified.

[Brief description of the drawings]

FIG. 1 illustrates one embodiment of the present invention, and is a block diagram illustrating a configuration of an input tone correction unit of an image processing apparatus.

FIG. 2 is a cross-sectional view illustrating a schematic configuration of a digital color image forming apparatus to which the image processing apparatus is applied.

FIG. 3 is a block diagram illustrating a configuration of the image processing apparatus.

FIG. 4 is a block diagram illustrating image processing at the time of automatic exposure adjustment in the image processing apparatus.

FIG. 5 is a flowchart illustrating a flow of an input gradation correction process (automatic exposure adjustment) by the image processing apparatus.

FIG. 6 is a flowchart showing a flow different from that of FIG. 5 in the input gradation correction processing (automatic exposure adjustment) by the image processing apparatus.

FIG. 7 is an explanatory diagram illustrating an example of a density histogram created when the density is divided into 256 levels in the image processing apparatus.

FIG. 8 is an explanatory diagram showing an example of a configuration when a background density area threshold value is arbitrarily set and an example of detection of a maximum point and a minimum point in creation of a density histogram by the image processing apparatus.

FIG. 9 is an explanatory diagram illustrating an example of a background discriminating method by the image processing apparatus.

FIG. 10 is an explanatory diagram showing an example of creating a density correction curve by the image processing apparatus.

FIG. 11 is an explanatory diagram showing an example of a configuration when a background density area threshold value is arbitrarily set in creation of a density histogram by the image processing apparatus.

FIG. 12 is an explanatory diagram showing an example of a configuration when a frequency difference threshold is arbitrarily set in creating a density histogram by the image processing apparatus.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 image processing device (image processing means) 2 image input device (image input means) 3 image output device (image output means) 22 histogram creation section (histogram creation means) 23 background density determination section (density correction means, background density determination means) 24) Density correction curve creation unit (density correction means, density correction curve creation means) 25 Density correction point creation means (density correction means) 26 signal conversion unit (density correction means) 32 background density determination area setting unit (base density determination area) Setting means) 33 minimum point / maximum point detection unit (minimum point / maximum point detection means)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C262 AA24 AA26 AB13 BB36 BC05 BC09 EA04 EA11 GA02 GA19 5B057 AA11 BA12 BA24 BA30 CA16 CB16 CE06 CE11 CH09 DA08 DC23 5C077 MM20 MP08 PP15 PP25 PP32 PP33 PP43 PP47 PP51 PQ19P20

Claims (11)

[Claims] 1. An automatic image processing apparatus, comprising: determining a background density of an original from image data input by an image input device; and removing pixel data corresponding to a background area of the image data based on the determined background density. An image processing apparatus for performing an exposure adjustment process; a histogram creating means for creating a density histogram in which each density is associated with a frequency of a pixel having the density based on the density of each pixel of the image data; Density correction means for determining the background density based on the result of the density histogram created by the means, and correcting the density of the image data. The histogram creation means further comprises: A background density determination area setting means for setting a background density determination area based on the determined background density area threshold; Minimum point / maximum point detection means for extracting the maximum point and the minimum point of the density histogram of the density histogram, wherein the density correction means comprises a maximum density of the density closest to the background density area threshold value among the extracted maximum points. An image processing apparatus comprising: a background density determining unit that focuses on a point and determines a background density using the density of the local maximum point and the density of a local minimum point having a density closest to the local maximum point. . 2. The image forming apparatus according to claim 1, wherein the background density determining means has a density which is within a density width threshold value indicating a density range centered on the density of the maximum point with respect to the density value of the maximum point of interest; From the minimum points whose frequency difference with the maximum point is within the frequency difference threshold, select the minimum point having the density closest to the maximum point, the density of the maximum point and the density of the selected minimum point. 2. The image processing apparatus according to claim 1, wherein the background density is determined by using. 3. The background density discriminating means has a density within a density width threshold value indicating a density range centered on the density of the maximum point with respect to the maximum point of interest, and If there is no minimum point whose frequency difference is within the frequency difference threshold,
3. The image processing apparatus according to claim 2, wherein the target maximum value is changed to a local maximum point having a density next to the background density area threshold value and the background density is determined. 4. The background density discriminating means sets a value obtained by adding an absolute value of a difference between the density of the maximum point and the density of the minimum point to the density of the noted maximum point or subtracting the obtained value as the background density. The image processing apparatus according to claim 1, wherein the determination is performed. 5. The density correcting means further determines the background density by the density determining means based on a reference density correction curve for setting a correction relationship between an input density value and an output density value prepared in advance. Density correction curve creating means for evenly compressing the reference density correction curve using the obtained density to create a density correction curve, wherein the density correction curve creation means includes a plurality of A correction point is provided, for the density correction curve to be created, the background density is used as a reference point for providing an output density value of 0, and a compression ratio between a starting point for providing an output density value of 0 in the reference density correction curve and the reference value According to the above, the correction points are uniformly compressed to create a density correction curve, and the density correction means uses the density correction curve created by the density correction curve creation means. Claims 1 and performs density correction during dynamic exposure adjustment processing image processing apparatus according to any one of 4. 6. An automatic apparatus for determining the background density of a document from image data input by an image input device and removing pixel data of a portion corresponding to a background area of the image data based on the determined background density. An image processing method for performing exposure adjustment processing, comprising: a histogram creation step of creating a density histogram in which each density is associated with a frequency of a pixel having the density based on the density of each pixel of the image data; A density correction step of determining the background density based on the result of the density histogram created by the means and correcting the density of the image data. The histogram creation step further includes: A background density determination area setting step of setting a background density determination area based on the determined background density area threshold; A minimum point / maximum point detection step of extracting a maximum point and a minimum point of the density histogram of the density histogram. The density correction step includes the step of detecting the local maximum of the density closest to the background density area threshold from the extracted maximum points. An image processing method characterized by comprising a background density determining step of determining a background density using the density of the local maximum point and the density of the local minimum point having the density closest to the local maximum point. . 7. The background density discriminating step includes, for a density value of a local maximum point of interest, a density existing within a density width threshold value indicating a density range centered on the density of the local maximum point, and From the minimum points whose frequency difference with the maximum point is within the frequency difference threshold, select the minimum point having the density closest to the maximum point, the density of the maximum point and the density of the selected minimum point. 7. The image processing method according to claim 6, wherein the background density is determined using the following. 8. The method according to claim 1, wherein the background density determining step has a density existing within a density width threshold value indicating a density range centered on the density of the local maximum point with respect to the maximum point of interest. If there is no minimum point whose frequency difference is within the frequency difference threshold,
8. The image processing method according to claim 7, wherein the target maximum value is changed to a local maximum point having a density next to the background density area threshold value to determine the background density. 9. The background density discriminating step includes, as a background density, a value obtained by adding or subtracting the absolute value of the difference between the density of the maximum point and the density of the minimum point to the density of the noted maximum point. 9. The image processing method according to claim 6, wherein the determination is performed. 10. The density correction step further includes determining a base density by the base density determining means based on a reference density correction curve for setting a correction relationship between an input density value and an output density value prepared in advance. A density correction curve creating step of creating a density correction curve by uniformly compressing the reference density correction curve using the obtained density. The density correction curve creation step includes a plurality of steps for the reference density correction curve. A correction point is provided, for the density correction curve to be created, the background density is used as a reference point for providing an output density value of 0, and a compression ratio between a starting point for providing an output density value of 0 in the reference density correction curve and the reference value In accordance with the above, the correction points are uniformly compressed to create a density correction curve, and the density correction step uses the density correction curve created by the density correction curve creation means. The image processing method according to any one of claims 6 to 9, characterized in that to perform density correction of the automatic exposure adjustment processing. 11. An image processing means as an image processing apparatus according to claim 1, wherein: an image input means for reading an original to obtain image data; and performing an automatic exposure adjustment process on said image data. An image forming apparatus comprising: an image output unit that prints out image data that has been subjected to automatic exposure adjustment processing by the image processing unit.