Image processing apparatus and method for erasing dirty spots in reproducing systems

An image processing apparatus which processes image data either in a character image reading mode or a character/photo mixed-image reading mode. The apparatus includes an MTF filter that emphasizes the image data, a smoothing filter that smooths the image data, a memory that stores white data, a mesh-screen image judging device that judges whether the image data is a mesh-screen image in the character image reading mode and the character/photo mixed-image reading mode, and an isolated-pixel judging device that judges whether the image data is an isolated-pixel. A first selection device selects an output of the MTF filter when the isolated-pixel judging device judges the image data to be a non-isolated-pixel and selects the white data when the isolated-pixel judging device judges the image data to be an isolated-pixel, when the mesh-screen image judging device judges the image data to be a non-mesh-screen image. A second selection device selects an output of the first selection device when the mesh-screen image judging device judges the image data to be a non-mesh-screen image and selects an output of the smoothing filter when the mesh-screen image judging device judges said image data to be a mesh-screen image.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to an improved method and apparatus
 for erasing dirty spots in the information content of image reproducing
 systems including copying systems and printing systems.
 2. Discussion of the Background
 The present invention is related to the area of analyzing the images of
 documents and determining compensation filter selections to optimize the
 reproduction fidelity of the print. FIG. 13 illustrates a general image
 processing unit (IPU) 10 which has been developed for use in an image
 forming apparatus such as, scanners, copying machines, facsimile machines,
 and so forth. The IPU 10 of FIG. 13 typically adopts a mesh-screen image
 seek operation and an isolated-pixel seek operation in order to erase
 dirty spots in the image data. The mesh-screen image seek operation is to
 distinguish ordinary images from mesh-screen images. The mesh-screen
 images are typically used with images of newspapers, for example.
 In a practical application, the IPU 10 is provided with two selectable
 image reading modes relative to the mesh-screen image seek operation. One
 is a character image mode and the other is a character/photo mixed-image
 mode. The character image mode will be used typically when the document to
 be read includes mainly characters and the character/photomixed-image mode
 will be used typically when the document to be read includes characters
 and images such as photos.
 A mesh-screen image, such as an image of newspaper, typically appears with
 images to be read in the character/photo mixed-image mode but not in the
 character image mode. Accordingly, the mesh-screen seek operation does not
 need to be operable in the character image mode in a general image
 processing apparatus including the IPU 10 of FIG. 13.
 The dirty-pixel seek operation is used to distinguish a pixel that is not
 at a white level and is isolated from ordinary pixels. Such a pixel
 (referred to as an isolated-pixel) is regarded as an undesired dirty spot
 on a document. Therefore, it may be desirable if such a dirty-spot is
 erased from the document.
 For these operations, the IPU 10 is provided with a mesh-screen image seek
 circuit 12 and an isolated-pixel seek circuit 13. In the IPU 10, a shading
 correction circuit 11 corrects the image data of a document which is
 transmitted from a CCD (charge coupled device) 9 through a video
 processing unit (VPU) 10a. The image data is then applied to a plurality
 of circuits such as the mesh-screen image seek circuit 12, the
 isolated-pixel seek circuit 13, an MTF (Modulation Transfer Function)
 correction filter 14 and a smoothing filter 15.
 The isolated-pixel seek circuit 13 judges whether each of the individual
 pixels of an image is surrounded by white pixels, either in the character
 image mode or the character/photo mixed-image mode. A pixel surrounded by
 white pixels is referred to as the above-described isolated-pixel. When an
 isolated-pixel is found, the isolated-pixel seek circuit 13 outputs an
 isolated-pixel signal to a selector 17.
 The mesh-screen image seek circuit 12 operates only in the character/photo
 mixed-image mode and judges whether the image is composed of a plurality
 of groups of spots, or a mesh-screen image such as in newspapers. The
 mesh-screen image seek circuit 12 outputs a mesh-screen image signal to a
 selector 18, indicating the pixel being transmitted is either a
 mesh-screen image or an ordinary image.
 The isolated-pixel seek circuit 13 uses a 5- by 5-pixel matrix, for
 example, as illustrated in FIG. 4, and picks up the values of a center
 pixel D22 and its surrounding pixels in the matrix, as illustrated in FIG.
 7, and compares these values with a predetermined threshold value X. When
 the values of the center pixel D22 and all of the surrounding pixels in
 the matrix are smaller than the predetermined threshold value X, the
 isolated-pixel seek circuit 13 judges the center pixel D22 to be an
 isolated and potentially black pixel, or a potential dirty spot. At this
 time, the isolated-pixel seek circuit 13 outputs an isolated-pixel signal
 for indicating that the pixel being transmitted is either an
 isolated-pixel or an ordinary pixel. When the pixel being transmitted is
 judged to be an isolated-pixel, this pixel needs to be changed to a white
 pixel, so that the isolated-pixel may be erased.
 The selector 17 selects the data from a white data register 16 when the
 pixel D22 is judged to be an isolated and potentially black pixel, or a
 potential dirty spot, on the basis of the isolated-pixel signal output
 from the isolated-pixel seek circuit 13. The selector 17 selects the
 correction data from the MTF correction filter 14 when the pixel D22 is
 judged not to be an isolated and potentially black pixel, or an ordinary
 image spot, on the basis of the isolated-pixel signal output from the
 isolated-pixel seek circuit 13.
 The selector 18 selects data output from either the selector 17 or the
 smoothing filter 15 on the basis of the mesh-screen image signal output
 from the mesh-screen image seek circuit 12. In the character mode,
 however, the selector 18 selects only the data output from the selector 17
 regardless of the mesh-screen image signal. In the character/photo
 mixed-image mode, the selector 18 selects the correction data output from
 the smoothing filter 15 when the target pixel is judged as a pixel in a
 mesh-screen image. Also, in the character/photo mixed-image mode, the
 selector 18 selects the data output from the selector 17 when the target
 pixel is judged as a pixel not in a mesh-screen image. The data selected
 by the selector 18 is applied to a .gamma.-correction circuit 19 and then
 to the following circuit.
 Thus, in the character mode, the image data selected is either the white
 data from the white data register 16 or the MTF correction data from the
 MTF correction filter 14, according to the isolated-pixel signal from the
 isolated-pixel seek circuit 13, and the smoothing filter data from the
 smoothing filter 15 is not selected as the image data. In the
 character/photo mixed-image mode, the smoothing filter data from the
 smoothing filter 15 is determined as the image data when the target pixel
 is judged as a pixel in a mesh-screen image. On the other hand, when the
 target pixel is judged as a pixel not in a mesh-screen image in the
 character/photo mixed-image mode, the white data from the white data
 register 16 is selected as the image data when the target pixel is judged
 as an isolated-pixel, and the MTF correction data from the MTF correction
 filter 14 is determined as the image data when the target pixel is not
 judged as the isolated-pixel.
 That is, the IPU 10 of FIG. 13 executes only the isolated-pixel seek
 operation but not the mesh-screen image seek operation when in the
 character mode. This means that in the character mode a target pixel in a
 mesh-screen image may be undesirably judged as an isolated and potentially
 black spot, or a potential black spot, through the dirty-spot seek
 operation. A countermeasure for this issue is to set the predetermined
 threshold value X to a relatively high value for a target pixel in a
 mesh-screen image. However, this countermeasure has a further drawback in
 which a real dirty spot in a mesh-screen image might not be found and
 erased with such a relatively high value X.
 On the contrary, if the threshold value X is set to such a value that the
 isolated-pixel seek operation functions on the mesh-screen image, a
 problem may occur in which many pixels of the mesh-screen and low contrast
 images may be erased. Furthermore, if the size of the matrix of FIG. 4 is
 made larger in order to avoid the above-mentioned problem, another problem
 may occur that the size of the IPU 10 may become larger as well.
 Japanese Patent Laid-open Publication 7-30751/1995 discloses an image
 processing apparatus that attempts to prevent upsizing thereof by
 combining a shift register that outputs matrix data to the isolated-pixel
 seek circuit 13 and a shift register that outputs different matrix data to
 an edge separation circuit (not shown).
 It is believed that there is no image processing apparatus that is capable
 of properly performing the dirty-spot erasing operation in the character
 and character/photo mixed-image modes without causing an enlargement of
 the unit size.
 SUMMARY OF THE INVENTION
 In light of the above problems, an object of the present invention is to
 provide an image processing apparatus that can properly perform the
 dirty-spot erasing operation in the character and character/photo
 mixed-image modes without causing an enlargement of the unit size.
 These and other objects are achieved by providing a novel image processing
 apparatus that includes an MTF filter that emphasizes an image data, a
 smoothing filter that smooths the image data, a memory that stores white
 data, a mesh-screen image judging device that judges whether the image
 data is a mesh-screen image in the character image reading mode and the
 character/photo mixed-image reading mode, an isolated-pixel judging device
 that judges whether the image data is an isolated-pixel, a first selection
 device that selects an output of the MTF filter when the isolated-pixel
 judging device judges the image data to be a non-isolated-pixel and that
 selects the white data when the isolated-pixel judging device judges the
 image data to be an isolated-pixel, when the mesh-screen image judging
 device judges the image data to be a non-mesh-screen image; and a second
 selection device that selects an output of the first selection device when
 the mesh-screen image judging device judges the image data to be a
 non-mesh-screen image and that selects an output of the smoothing filter
 when the mesh-screen image judging device judges the image data to be a
 mesh-screen image.
 Likewise, another image processing apparatus according to the present
 invention includes an MTF filter that emphasizes the image data, a
 smoothing filter that smooths the image data, a memory that stores white
 data, a mesh-screen image judging device that judges whether the image
 data is a mesh-screen image in the character image reading mode and the
 character/photo mixed-image reading mode, an isolated-pixel judging device
 that judges whether the image data is an isolated-pixel so as to make an
 isolated-pixel erasing ability strong when the mesh-screen image judging
 device judges the image data to be a non-mesh-screen image, and that
 judges whether the image data is an isolated-pixel so as to make the
 isolated-pixel erasing ability weak when the mesh-screen image judging
 device judges the image data to be a mesh-screen image, a first selection
 device that selects an output of the MTF filter when the isolated-pixel
 judging device judges the image data to be a non-isolated-pixel and that
 selects the white data when the isolated-pixel judging device judges the
 image data to be an isolated-pixel, and a second selection device that
 selects an output of the first selection device when the mesh-screen image
 judging device judges the image data to be a non-mesh-screen image and
 that selects an output of the smoothing filter when the mesh-screen image
 judging device judges the image data to be a mesh-screen image.
 The novel method according to the present invention includes the steps of
 emphasizing the image data with an MTF filter, smoothing the image data
 with a smoothing filter, storing white data with a memory, judging whether
 the image data is a mesh-screen image in the character image reading mode
 and the character/photo mixed-image reading mode with a mesh-screen image
 judging device, judging whether the image data is an isolated-pixel with
 an isolated-pixel judging device, selecting an output of the MTF filter
 when the isolated-pixel judging device judges the image data to be a
 non-isolated-pixel and selecting the white data when the isolated-pixel
 judging device judges the image data to be an isolated-pixel, when the
 mesh-screen image judging device judges the image data to be a
 non-mesh-screen image with a first selection device, and selecting an
 output of the first selection device when the mesh-screen image judging
 device judges the image data to be a non-mesh-screen image and selecting
 an output of the smoothing filter when the mesh-screen image judging
 device judges the image data to be a mesh-screen image with a second
 selection device.
 Likewise another novel method according to the present invention includes
 the steps of emphasizing image data with an MTF, smoothing the image data
 with a smoothing filter, storing white data with a memory, judging whether
 the image data is a mesh-screen image in the character image reading mode
 and the character/photo mixed-image reading mode with a mesh-screen image
 judging device, judging whether the image data is an isolated-pixel so as
 to make an isolated-pixel erasing ability strong when the mesh-screen
 image judging device judges the image data to be a non-mesh-screen image,
 and that judges whether the image data is an isolated-pixel so as to make
 the isolated-pixel erasing ability weak when the mesh-screen image judging
 device judges the image data to be a mesh-screen image with an
 isolated-pixel judging device, selecting an output of the MTF filter when
 the isolated-pixel judging device judges the image data to be a
 non-isolated-pixel and selecting the white data when the isolated-pixel
 judging device judges the image data to be an isolated-pixel with a first
 selection device, and selecting an output of the first selection device
 when the mesh-screen image judging device judges the image data to be a
 non-mesh-screen image and selecting an output of the smoothing filter when
 the mesh-screen image judging device judges the image data to be a
 mesh-screen image with a second selection device.
 Other objects, features and advantages of the present invention will become
 apparent upon consideration of the following description of the preferred
 embodiments of the present invention taken in conjunction with the
 accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An embodiment of the present invention is described in detail in the
 drawings, wherein like reference numerals indicate identical or
 corresponding parts throughout the several views.
 In the reading apparatus 1 of FIG. 1, an original document (not shown)
 placed on a contact-glass 2 is illuminated by a light source 4. A
 reflection light from the original document is successively reflected by a
 first mirror 3, a second mirror 6, and a third mirror 7. Then, the
 reflected light is focused onto a surface of a CCD (charge coupled device)
 line image sensor 9 by a lens 8. Thereby, an image of the original
 document is scanned in a main-scanning direction with the CCD line image
 sensor 9. The scanned image is then converted into image data through a
 photo-to-electronic conversion. The first mirror 3 and the light source 4
 are mounted on a first moving member 5a. The second mirror 6 and the third
 mirror 7 are mounted on a second moving member 5b. The original document
 on the contact-glass 2 is scanned in a sub-scanning direction by moving
 the first and second moving members 5a and 5b in the sub-scanning
 direction. The moving members 5a and 5b move at a speed ratio of 2:1.
 In FIG. 2, an image signal, which is read by the CCD line image sensor 9,
 is applied with an appropriate gain and then converted into a digital
 signal, through a VPU (video processing unit) 10a. Thereby, eight-bit
 digital image data GDT0-GDT7, which are synchronized with a 16-MHZ pixel
 clock signal CLK, are generated and applied to an IPU (image processing
 unit) 10. In addition, the IPU 10 applies the pixel clock signal CLK and a
 signal CCDSTN (not shown) that determines read-out timing for the image
 data of the CCD line image sensor 9 to the VPU 10a.
 In the IPU 10, a shading correction circuit 11 corrects the amount of light
 of the image data GDT0-GDT7 due to the uneven light-production of the
 light source 4 in a main-scanning direction and for the uneven
 light-receiving sensitivity of each pixel of the CCD line image sensor 9.
 This operation of the shading correction circuit 11 is called a shading
 correction. In the shading correction, the CCD line image sensor 9 reads a
 reference white plate that has a uniform density in a main-scanning
 direction before reading the image of the original document. Then, the
 white data of the reference white plate read by the CCD line image sensor
 9 are stored pixel by pixel into a memory (not shown).
 Thereafter, the CCD line image sensor 9 reads the original document and the
 image data GDT0-GDT7 are generated. The image data GDT0-GDT7 for each
 pixel are divided by the white data of the corresponding pixel stored in
 the memory (not shown). Then, the division result is used as the image
 data.
 The image data GDT0-GDT7 that are corrected through the shading correction
 circuit 11 are applied to a mesh-screen image seek circuit 12, an
 isolated-pixel seek circuit 13, an MTF correction filter 14, and a
 smoothing filter 15. The MTF correction filter 14 includes, for each bit,
 four FIFO (first-in first-out) memories for four data lines and 4.times.5
 D-FFs (D-type flip-flop circuits), as shown in FIG. 3, and forms a 5- by
 5-pixel matrix, as shown in FIG. 4. Accordingly, the MTF correction filter
 14 has 5 lines in the main scanning direction, 5 lines in the sub-scanning
 direction, and 8 bits in depth. The MTF correction filter 14 includes a
 plurality of predetermined coefficients, as shown in FIG. 5. The MTF
 correction filter 14 multiplies each pixel of the image data by the
 corresponding coefficient, and adds each multiplication result to the
 pixel being examined. This operation emphasizes especially high-frequency
 image data in order to minimize optically-made frequency characteristics.
 The MTF correction filter 14 then outputs the resultant image data to the
 selector 17.
 The smoothing filter 15 forms, for each bit, a matrix of 5.times.5 pixels,
 as also shown in FIG. 4, including four FIFO (first-in first-out) memories
 for four lines and 4.times.5 D-FFs (D-type flip-flop circuits) as shown in
 FIG. 3. The smoothing filter 15 also includes a plurality of predetermined
 coefficients, as shown in FIG. 6. The smoothing filter 15 multiplies each
 pixel of the image data by the corresponding coefficient in a similar
 manner as mentioned above. Then, the smoothing filter 15 adds each
 multiplication result to the image data being examined, and outputs the
 resultant image data to a selector 18. This operation cuts high-frequency
 data.
 The isolated-pixel seek circuit 13 forms a matrix of 5.times.5 pixels, as
 shown in FIG. 4. The isolated-pixel seek circuit 13 includes, for each
 bit, four FIFO (first-in first-out) memories for four lines and 4.times.5
 D-FFs (D-type flip-flop circuits), as shown in FIG. 3, in a manner similar
 to that mentioned above. The isolated-pixel seek circuit 13 picks up a
 target pixel D22 and its surrounding pixels in the matrix, as shown in
 FIG. 7. Then, the isolated-pixel seek circuit 13 compares the values of
 the target pixel D22 and the surrounding pixels in the matrix with a
 predetermined threshold value X which is stored in a threshold register
 23. Then, the isolated-pixel seek circuit 13 outputs an isolated-pixel
 signal to an AND gate 22 when the values of the target pixel D22 and all
 of the surrounding pixels in the matrix are smaller than the value of the
 threshold value X. The isolated-pixel signal indicates that the target
 pixel is an isolated-pixel, and is used to change the pixel to a white
 pixel, as described later on. Hereupon, the isolated-pixel seek circuit 13
 judges the target pixel D22 and the surrounding pixels in the matrix as
 follows:
 D44&lt;X
 and D43&lt;X
 and D42&lt;X
 and D41&lt;X
 and D40&lt;X
 and D34&lt;X
 and D30&lt;X
 and D24&lt;X
 and D22&lt;X
 and D20&lt;X
 and D14&lt;X
 and D10&lt;X
 and D04&lt;X
 and D03&lt;X
 and D02&lt;X
 and D01&lt;X
 and D00&lt;X
 Furthermore, the isolation-point removing circuit 13 outputs a high signal
 (H) when an isolated-pixel is detected.
 FIG. 8 illustrates a construction of the mesh-screen image seek circuit 12
 in detail. A peak value detecting section 201 of the mesh-screen image
 seek circuit 12 analyzes density variations of the input image data and
 detects a peak of a crest or a trough (hereinafter called crest peak and
 trough peak, respectively) of the density variations. This peak value
 detecting operation compares the values of a target pixel and its
 surrounding pixels. FIG. 9 shows an exemplary 3- by 3-pixel matrix used
 for this operation, having a target pixel p and eight surrounding pixels
 d00-d22.
 Crest Peak Value Detection
 In the crest peak value detection, the value of the target pixel p is
 judged as a crest peak pixel when the following two conditions are met.
 One condition is that the density of the target pixel p is greater than
 those of the surrounding pixels d00-d22. The other condition is that a
 difference between the density levels of the target pixel p and a mean
 value of the density levels of the pixels that are symmetrically located
 at both sides of the target pixel D22 in diagonal, top and bottom, and
 right and left directions (in FIG. 9) is greater than a predetermined
 threshold value Y. The predetermined threshold value Y is stored in a
 threshold value register 205. The above-mentioned conditions are expressed
 by a following inequality:
 P&gt;d00 and p&gt;d01 and p&gt;d02
 and p&gt;d10 and p&gt;d12
 and p&gt;d20 and p&gt;d21 and p&gt;d22 and
 {2.times.p-(d00+d22)}&gt;Y
 and {2.times.p-(d01+d21)}&gt;Y
 and {2.times.p-(d02+d20)}&gt;Y
 and {2.times.p-(d10+d12)}&gt;Y.
 TROUGH PEAK VALUE DETECTION
 In a trough peak value detection, the value of the target pixel is judged
 as a trough peak when the following two conditions are met. One condition
 is that a density of the target pixel p is smaller than the surrounding
 pixels d00-d22. The other condition is that a difference of the density
 level between the mean value of the surrounding pixels of the target pixel
 D22 that are symmetrically located at both sides of the target pixel D22
 in diagonal, top and bottom, and right and left directions (in FIG. 9) and
 the value of the target pixel is greater than the threshold value Y. These
 conditions are expressed in the following equation:
 P&lt;d00 and p&lt;d01 and p&lt;d02
 and p&lt;d10 and p&lt;d12
 and p&lt;d20 and p&lt;d21 and p&lt;d22 and
 {(d00+d22)-2.times.p}&gt;Y
 and {(d01+d21)-2.times.p}&gt;Y
 and {(d02+d20)-2.times.p}&gt;Y
 and {(d10+d12)-2.times.p}&gt;Y
 The result of this judgement is transmitted to a potential mesh-screen
 image detect circuit 202. Then, the potential mesh-screen image detect
 circuit 202 detects a potential mesh-screen image area with a pattern
 matching of the peak pixels. Then, a correcting section 203 corrects the
 erroneous detection of the potential mesh-screen image area made through
 the pattern matching operation. Thereafter, a mesh-screen image
 determining circuit 204 extends the potential mesh-screen image area and
 ultimately determines the mesh-screen image area. Then, the mesh-screen
 image determining circuit 204 outputs the mesh-screen image signal to a
 NOT gate 21 and the selector 18. Hereupon, the mesh-screen image seek
 circuit 12 operates in both the character mode and the character/photo
 mixed-image mode. The image separation signal becomes H (high) at the
 state of mesh-screen image, and L (low) at the state of non-mesh-screen
 image.
 Then, an output signal of the NOT gate 21 and an output signal of the
 isolated-pixel seek circuit 13 are logically operated on by the AND gate
 22. The selector 17 selects the output signal either from the MTF
 correction filter 14 or from the white data register 16 in accordance with
 the output signal from the AND gate 22. On the other hand, the selector 18
 selects the output signal either from the selector 17 or from the
 smoothing filter 15 in accordance with the mesh-screen image signal.
 FIG. 2 shows the image processing operation of the IPU 10 in the character
 mode. When the mesh-screen image signal is H (high), the isolated-pixel
 signal is never made effective due to the AND gate 22 which does not open
 with the inversed signal of the isolated-pixel signal that is H (high).
 However, when the mesh-screen image signal from the mesh-screen image seek
 circuit 12 is L (low), the isolated-pixel signal becomes effective at the
 AND gate 22 because the mesh-screen image signal is applied to the AND
 gate 22 through the NOT gate 21. Then, the selector 17 selects the white
 data and, subsequently, the selector 18 also selects the white data. As a
 result, the IPU 10 erases the isolated-pixel, or the dirty spot, in the
 non-mesh-screen image area in the character mode.
 FIG. 10 shows the image processing process of the IPU 10 in the
 character/photo mixed-image mode. When the mesh-screen image signal is H
 (high), the output of the AND gate 22 is L (low), meaning that the pixel
 being examined is not the isolated-pixel, and the MTF correction data is
 selected by the selector 17. However, in this case, the selector 18 does
 not select the MTF correction data. When the mesh-screen image signal from
 the mesh-screen image seek circuit 12 is L (low), the isolated-pixel
 signal becomes effective at the AND gate 22. Then, either the MTF
 correction data or the white data is selected by the selector 17 on the
 basis of the isolated-pixel signal. The selector 18 accordingly selects
 the same data that the selector 17 selected.
 In this way, the IPU 10 can properly perform the dirty-spot erasing
 operation in both the character mode and character/photo mixed-image mode
 without causing an enlargement of the unit size.
 Next, a modified embodiment of the IPU 10 according to the present
 invention is explained referring to FIGS. 11 and 12. The modified
 embodiment is provided with threshold value registers 23a and 23b, and a
 selector 24 is provided instead of the aforementioned NOT circuit 21 and
 the AND circuit 22 in FIG. 10. The selector 24 is constructed to select a
 threshold value X1 (&gt;X2) of the threshold register 23a on the basis of the
 mesh-screen image signal at a non-mesh-screen image area. The selector 24
 also selects a threshold value X2 of the threshold value register 23b at a
 mesh-screen image area. The selected X1 or X2 is transmitted to the
 isolated-pixel seek circuit 13. This operation is to set an ordinary
 threshold value X1 in the isolated-pixel seek circuit 13 when the pixel is
 in the mesh-screen image area and an efficiently lower threshold level X2
 when the pixel is in the non-mesh-screen image area. That is, the
 threshold value X2 needs to be such a value that a pixel in the
 mesh-screen image area is not erroneously judged as an isolation-pixel.
 FIG. 11 shows the image processing operation of the modified embodiment of
 the IPU 10 in the character image mode. When the mesh-screen image signal
 is L (low), the threshold value X1 that strongly judges the isolated-pixel
 is set in the isolated-pixel seek circuit 13. When the mesh-screen image
 signal is H (high), the threshold value X2 that weakly judges the
 isolated-pixel is set in the isolated-pixel circuit 13. The selector 17
 selects either the MTF correction data or the white data on the basis of
 the isolated-pixel signal.
 FIG. 12 shows the image processing operation of the modified embodiment of
 the IPU 10 in the character/photo mixed-image mode. The selector 18
 selects the output signal either from the selector 17 or smoothing filter
 15 on the basis of the mesh-screen image signal. When the pixel is the
 non-mesh-screen image, the selector 18 selects the output of the selector
 17. When the pixel is the mesh-screen image, the selector 18 selects the
 output of the smoothing filter 15. Further, the threshold value X1 that
 strongly judges the isolated-pixel or the threshold value X2 that weakly
 judges the isolated-pixel on the basis of the mesh-screen image signal is
 set in the isolated-pixel seek circuit 13. Then, the selector 17 selects
 either the MTF correction data or the white data on the basis of the
 isolated-pixel signal. Thereafter, the selector 18 selects the output of
 the selector 17, which is one of the outputs from the MTF correction data
 and the white data. In this way, the modified embodiment of the IPU 10 can
 properly perform the dirty-spot erasing operation in the character image
 and character/photo mixed-image modes without causing an enlargement of
 the unit size.
 The controller of this invention may be conveniently implemented using a
 conventional general purpose digital computer or microprocessor programmed
 according to the teachings of the present specification, as is apparent to
 those skilled in the computer technology. Appropriate software coding can
 readily be prepared by skilled programmers based on the teachings of the
 present disclosure, as will be apparent to those skilled in the software
 art. The invention may also be implemented by the preparation of
 application specific integrated circuits or by interconnecting an
 appropriate network of conventional component circuits, as will be readily
 apparent to those skilled in the art.
 Obviously, numerous modifications and variations of the present invention
 are possible in light of the above teachings. It is therefore to be
 understood that within the scope of the appended claims, the invention may
 be practiced otherwise than as specifically described herein.
 Having now fully described the invention, it will be apparent to one of
 ordinary skill in the art that many changes and modifications can be made
 thereto without departing from the spirit and scope of the invention as
 set forth herein. This application is based on Japanese patent
 applications JPAP09-262208 filed on Sep. 26, 1997, the entire contents of
 which are hereby incorporated by reference.