Image processing apparatus and method for reproducing high quality image in low-density portion

When an image is adjusted to compensate for individual differences among image forming units, there are cases where a phenomenon occurs in which a contour that is originally non-existent appears in a low-density portion, namely an area other than that where the output density characteristic is linear. In such instances image quality deteriorates markedly when importance is placed upon tonality. To solve this problem, a printer is caused to print an image diagnostic pattern, the image diagnostic pattern that has been printed is read by a reader and the linear portion of the output density characteristic of a design center value and the linear portion of the read output density characteristic are approximated by straight lines. Both of the output density characteristics are compared and image adjustment coefficients A, B are compared. The coefficients A, B thus obtained are set in a primary converter and gamma tables in a gamma converter are changed over in conformity with the values of the coefficients A, B.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to an image processing apparatus and method. More
 particularly, the invention relates to an image processing apparatus and
 method for processing input image data in such a manner that a
 predetermined output characteristic is obtained.
 2. Description of the Related Art
 The gamma table in an image forming apparatus is engineered in such a
 manner that the design center value of an output density characteristic
 under conditions in which a conversion is not being applied will be
 converted to a desired output density characteristic. However, owing to
 error attributable to the variety of component parts that constitute an
 image forming apparatus, the finished products exhibit output density
 characteristics that differ from one apparatus to the another.
 FIG. 1 is a diagram showing an example of the output density characteristic
 of such an apparatus. Here an image signal value is plotted along the
 horizontal axis and output density along the vertical axis. The curve
 indicated at 2-a represents the design center value of the output density
 characteristic, and the curve indicated at 2-b represents an output
 density characteristic that has deviated from the design central value.
 Recent technological advances have led to the appearance of many image
 forming apparatus equipped with photographic and other modes for
 reproducing high-definition images exhibiting a high degree of tonality.
 Individual differences in output density characteristics in such apparatus
 greatly detract from image quality. Accordingly, methods of printing an
 image based upon a prescribed image signal, measuring the output density
 characteristic of the apparatus by reading the image and then correcting
 the output density that has been measured have been disclosed in U.S. Pat.
 No. 5,258,783 and U.S. Pat. No. 4,888,636.
 Though a variety of these correction methods have been developed, it is
 generally necessary to implement them upon balancing cost against required
 image quality in the case of a monochromatic image forming apparatus. More
 specifically, since the output density characteristic in the vicinity of
 intermediate density is linear, as shown in FIG. 1, a primary conversion
 which makes this linear portion conform to the design center value is
 applied to the image signal of an apparatus whose output density
 characteristic has deviated. This shall be referred to as a "linear image
 adjustment" below. This adjustment makes it possible to obtain the desired
 output density characteristic irrespective of individual differences in
 the image forming apparatus.
 However, a problem which arises is that when a linear image adjustment is
 carried out, there are instances where the output density characteristic
 exhibits a characteristic different from the design center value in a
 density area other than that where the output density characteristic is
 linear. In such instances there is no problem in terms of density
 reproducibility of image halftones but there are cases where a phenomenon
 occurs in which a contour that is originally non-existent appears in
 portions where density is low. This shall be referred to as a "false
 contour phenomenon" below. In particular, when importance is placed upon
 gradation, as in the case of the photograph mode, image quality
 deteriorates markedly if the false contour phenomenon occurs.
 SUMMARY OF THE INVENTION
 An object of the present invention is to solve the aforementioned problems
 and provide an image processing apparatus and method in which individual
 differences in apparatus output characteristics are compensated for and in
 which it is possible to prevent a deterioration in image quality due to
 the false contour phenomenon.
 According to the present invention, the foregoing object is attained by
 providing an image processing apparatus comprising converting means for
 converting input image data in such a manner that a predetermined output
 characteristic is obtained, and correcting means for correcting the image
 data, which have been converted by the converting means, in such a manner
 that a standard output characteristic is obtained, wherein the converting
 means alters a conversion characteristic in dependence upon a correction
 characteristic of the correcting means.
 Further, according to the present invention, the foregoing object is
 attained by providing an image processing method using a reading device
 for reading an original and generating image data indicative of the
 original read, and an output device for forming an image on a medium in
 dependence upon the image data, the method comprising the steps of forming
 a reference image by the output device, deciding a correction
 characteristic, which is for correcting an output characteristic of the
 output device, using image data obtained by reading the reference image by
 the reading device. The output device has a plurality of output modes
 corresponding to a plurality of output characteristics that differ from
 one another and decides the correction characteristic in conformity with
 the output mode.
 Another object of the invention is to speed up the correction of an image
 forming apparatus based upon an output image.
 A further object of the invention is to correct an image forming apparatus
 based upon an output image while holding down cost.
 Yet another object of the invention is to provide a novel method of
 correcting density.
 Other features and advantages of the present invention will be apparent
 from the following description taken in conjunction with the accompanying
 drawings, in which like reference characters designate the same or similar
 parts throughout the figures thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An image processing apparatus according to embodiments of the invention
 will now be described in detail with reference to the drawings.
 First Embodiment
 [Configuration]
 FIG. 2 is a schematic view illustrating the construction of a digital
 copier to which the present invention is applied, and FIG. 3 is a block
 diagram illustrating image processing blocks of the digital copier shown
 in FIG. 2.
 As shown in FIG. 2, an original placed upon a glass platen 2 is illuminated
 by light emitted by a lamp 3. Light reflected from the original 1 is
 condensed on a CCD line sensor 9 via mirrors 4.about.6, a lens 7 and a
 mirror 8. An analog signal outputted by the CCD line sensor 9 is quantized
 and converted to an eight-bit (256-level) digital signal by an AD
 converter 101 shown in FIG. 3. After being subjected to image processing
 by an edge emphasizer 102 and zoom processor 103, the digital image signal
 undergoes a luminance-to-density data conversion in a LOG converter 104.
 The digital signal that has undergone the conversion to density data is
 density-converted (gamma-converted) by a .gamma.-converter 106 to an image
 signal that matches the image reproducing characteristic of a printer. A
 resolution converter 108 effects a conversion to a resolution conforming
 to the recording density of the printer. A primary converter 107 will be
 described later.
 In a case where the printer outputs two values per pixel or when it is
 desired to output a bi-level image, the digital signal that was subjected
 to the resolution conversion is binarized by a binarizer 109. In a case
 where it is desired to output a multivalued image by a printer capable of
 outputting multiple values per pixel, the digital signal is not binarized
 but is converted to an analog signal by a D/A converter 110, and the
 analog signal is sent to a laser driver 111. The latter drives a laser
 element (not shown), whereby a laser beam is emitted.
 The laser beam outputted by the laser element is scanned by a polygon
 mirror 10. An electrostatic latent image is formed on a photosensitive
 drum 15, the surface of which has been uniformly charged by a primary
 corona discharge device 14, via an f-.theta. lens 11 and mirrors 13, 12.
 The latent image that has been formed on the photosensitive drum 15 is
 developed by a developing device 19 using toner. The toner image formed is
 transferred to recording paper 21 supplied by a paper supply roll 20. The
 recording paper 21 to which the toner image has been transferred is
 conveyed by a conveyor belt 22 and the toner image is fixed on the
 recording paper 21 semipermanently by a fixing device 23. The recording
 paper is then discharged from the apparatus.
 Meanwhile, the photosensitive drum 15 from which the toner image has been
 transferred has remaining toner removed by a cleaner 16, after which the
 drum is charged by the primary corona discharge device 14 to prepare for
 formation of the next latent image.
 The gamma table in the gamma converter 106 has been engineered in such a
 manner that the output density characteristic of the apparatus will be the
 design center value. However, there are many cases where the output
 density characteristic of the design center value is not obtained owing to
 individual differences among the component parts of the apparatus. In
 order to compensate for these individual differences, the apparatus is
 provided with the primary converter 107, which serves as image adjusting
 means for subjecting the gamma-corrected image signal to processing that
 corrects the deviation in the output density characteristic.
 A CPU 121 shown in FIG. 3 controls the aforementioned blocks in accordance
 with a control program stored in a ROM 123. In accordance with the program
 stored in the ROM 123, the CPU 121 executes various image processing as
 well as image adjustment processing, described later, using a RAM 122 as a
 working memory.
 [Image adjusting produce]
 Image adjustment processing for measuring the output density characteristic
 of the apparatus and then correcting the measured output density
 characteristic will now be described.
 FIG. 4 is a flowchart illustrating image adjustment processing. Step S1 of
 FIG. 4 calls for the printer to print an image diagnostic pattern based
 upon an image signal representing a reference density pattern. The image
 diagnostic pattern that has been printed is read at step S2. This image
 diagnostic pattern is a pattern composed of a patch having a plurality of
 gray levels.
 FIG. 5 is a diagram showing an example of an output density characteristic.
 The curve indicated at 5-a represents the design center value of the
 output density characteristic, and the curve indicated at 5-b represents
 an output density characteristic that has deviated from the design central
 value, namely the output density characteristic that has been measured.
 Since the output density characteristic in the vicinity of intermediate
 density is linear, as shown in FIG. 5, it will suffice for a primary
 conversion which makes this linear portion conform to the design center
 value to be applied to the image signal of an apparatus whose output
 density characteristic has deviated.
 More specifically, the linear portion of the output density characteristic
 of the design center value is linearly approximated to the following
 equation at step S3:
EQU N=A1(X-B1) (1)
 where X represents the image signal value, N the output density, A1 the
 slope of the straight line and B1 an offset.
 On the other hand, the linear portion of the output density characteristic
 of the apparatus whose output density characteristic has deviated is
 linearly approximated to the following equation at step S3:
EQU N'=A2(X'-B2) (2)
 where X' represents the image signal value, N' the output density, A2 the
 slope of the straight line and B2 an offset.
 Next, the two output density characteristics are compared at step S4. More
 specifically, with regard to an image signal Y calculated by applying a
 primary conversion Y=A(X-B) to an image signal X, it will suffice to
 define image adjustment coefficients A, B in accordance with Equations
 (1), (2) such that N=A2 (Y-B2) will hold at all times. The image
 adjustment coefficients A, B are calculated in accordance with the
 following equations at step S5:
EQU A=A1/A2
EQU B=B1-A2.multidot.B2/A1 (3)
 The coefficients A, B thus obtained are set in the primary converter 107 at
 step S6 to establish a state in which the gamma-corrected image signal X
 can be converted and the corrected image signal Y obtained.
EQU Y=A(X-B)=A1(X-B1+A2.multidot.B2/A1)/A2 (4)
 By performing a first image adjustment in accordance with this method, the
 desired output density characteristic can be obtained irrespective of
 individual differences in the apparatus. This will be described later with
 regard to step S7 shown in FIG. 4.
 [Problem encountered with first image adjustment]
 The first image adjustment mentioned above is very effective with regard to
 an intermediate-density portion where the characteristic is linear.
 However, the characteristic of a low-density portion where the slope of
 the output density characteristic differs from that of the
 intermediate-density portion is not corrected satisfactorily. Accordingly,
 if merely the above-described image adjustment is applied to an apparatus
 for which the output density characteristic has deviated from the design
 center value, an individual difference will appear in the output density
 of the delivered image in the low-density portion thereof.
 In a case where three image output modes having image reproducing
 characteristics that differ from one another, namely a "character mode"
 suited to the reproduction of character images, a "character/photograph"
 mode suited to the reproduction of images that are a mixture of characters
 and photographs, and a "photograph mode" suited to the reproduction of
 photographic images, are provided, the density reproducibility of
 low-density portions is not much of a problem with regard to the character
 mode. However, in a case where fine grayscale reproducibility of a
 low-density portion is desired, as in the photograph mode, the fact that
 the output density characteristic of the low-density portion differs is a
 major problem.
 FIG. 6 shows an example of the output density characteristic in the
 photograph mode. The design is such that the relationship between input
 density and output density will be a straight line having a slope of
 approximately 45.degree.. FIG. 7 shows an example of the output density
 characteristic (referred to as the "straight-thru output density
 characteristic" below) in a case where the primary conversion or gamma
 conversion is not carried out in an apparatus operating at the design
 center value. FIG. 8 shows a gamma table for obtaining an output density
 characteristic of the kind shown in FIG. 6 as opposed to the straight-thru
 output density characteristic of FIG. 7.
 When the above-described image adjustment is carried out to correct for the
 individual difference in output density characteristic, the output density
 characteristic in low-density portions will deviate from the ideal line
 depending upon the values of the coefficients A, B. FIG. 9 is a diagram
 showing the correlation between values of the image correction
 coefficients A, B and the output density characteristic of a low-density
 portion. The larger the value of the coefficient A and the smaller the
 value of the coefficient B, the more the output density characteristic of
 the low-density portion deviates from the ideal line and the darker the
 image becomes. Conversely, the smaller the coefficient A and the larger
 the coefficient B, the lighter the image becomes.
 This is ascribable to the shape of the low-density portion of the gamma
 table. FIG. 10 is a diagram showing the low-density region of the gamma
 table of FIG. 8 in enlarged form. Here the conversion slope of the gamma
 table is steepest where the input signal value is 10. Owing to application
 of the primary conversion, the region of the low-density portion where the
 slope is steep indicates a change larger than that of the
 intermediate-to-high density portion. Accordingly, in a case where the
 primary conversion is applied in a direction that will increase density,
 the low-density portion becomes more dense. Conversely, if the primary
 conversion is applied in a direction that will decrease density, the
 low-density portion becomes less dense.
 Thus, in a case where the output density characteristic in the low-density
 portion deviates from the ideal line by a wide margin, the false-contour
 phenomenon often occurs in an image where low- and intermediate-density
 portions are continuous. When this phenomenon occurs, there is a
 pronounced decline in image quality, such as when reproducing the skin of
 a human being, and a reproduction that looks natural cannot be obtained.
 In order to prevent the occurrence of this false-contour phenomenon, this
 embodiment is so adapted that the gamma table is changed over at step S7
 in FIG. 4 depending upon the values of the coefficients A, B obtained by
 image adjustment. This embodiment has at least two types of gamma tables,
 of the kind shown in FIG. 11, in addition to the gamma table illustrated
 in FIG. 8. It should be noted that these two types of gamma tables are
 exactly the same except for the low-density portions thereof.
 FIG. 12 is a diagram useful in describing control for switching between two
 types of gamma tables. The values of the coefficients A and B are plotted
 along the horizontal and vertical axes, respectively. The gamma table is
 selected depending upon in which of two areas, which are demarcated by the
 straight line B=.alpha.A -.beta., the values of the coefficients A and B
 reside as the result of the first image adjustment. It should be noted
 that .alpha.=24, .beta.=30 in this embodiment.
 In a case where the values of the coefficients A and B reside in area 1
 shown in FIG. 12 as the result of the first image adjustment, control is
 performed in such a manner that the gamma table shown in FIG. 11 (namely
 the gamma table having the characteristic for which density decreases in
 the low-density portion) is selected. In a case where the values of the
 coefficients A and B reside in area 2 as the result of the first image
 adjustment, control is performed in such a manner that the ordinary gamma
 table shown in FIG. 8 is selected. This control shall be referred to as
 "automatic gamma selection" below.
 The plurality of gamma tables mentioned above can be implemented using a
 RAM or ROM, and the selection of the gamma table is executed by the CPU
 121 based upon the procedure described above.
 In accordance with this embodiment, as described above, the target output
 density characteristic can be obtained by making the output density
 characteristic follow an ideal characteristic at densities above
 intermediate density through primary conversion control (first image
 adjustment) and automatic gamma selection control (second image
 adjustment). Such control also prevents the occurrence of the
 false-contour phenomenon in the low-density portion.
 To deal with output modes such as the character mode and photograph mode
 mentioned above, the output density characteristic is changed in the
 resolution converter 108 or the developing bias in the developing device
 or discharge voltage in the corona discharge device is changed, thereby
 making it possible to change the density reproducibility. Further, in
 accordance with this embodiment, while simplifying the first image
 adjustment to make possible high-speed processing and lower cost, the
 correction of the first image adjustment can be carried out by the second
 image adjustment to make possible the formation of an excellent image.
 Second Embodiment
 An image processing apparatus according to second embodiment of the
 invention will now be described. Elements in the second embodiment similar
 to those of the first embodiment are designated by like reference
 characters and need not be described in detail again.
 According to this embodiment, three gamma tables of different
 characteristics are prepared, two partitioning straight lines
 B=.alpha.A-.beta. for selecting the gamma table are adopted and the gamma
 table is selected depending upon the values of the coefficients A and B
 after image adjustment.
 More specifically, the area is divided into three areas 1, 2 and 3 by
 partitioning straight lines B=.alpha.1A-.beta.1 and B=.alpha.2A-.beta.2,
 as shown in FIG. 13. If the values of A and B reside in area 1, then the
 gamma table selected is one for which the output density characteristic of
 the low-density portion is somewhat suppressed (i.e., for which density is
 somewhat low) with respect to the design center value. If the values of A
 and B reside in area 2, then the gamma table selected is one for which the
 output density characteristic of the low-density portion is in line with
 the design center value. If the values of A and B reside in area 3, then
 the gamma table selected is one for which the output density
 characteristic of the low-density portion is somewhat pronounced (i.e.,
 for which density is somewhat high). In this embodiment, it is so arranged
 that .alpha.1=24, .alpha.2=24, .beta.1=18, .beta.2=30.
 By adopting this expedient, the number of gamma tables increases. However,
 an output density characteristic in line with that desired can be obtained
 by adjustment correction and automatic gamma selection even in an
 apparatus equipped with a printer having any output density characteristic
 prior to the adjustment stage.
 Furthermore, the partitioning lines which partition the gamma tables are
 not limited to straight lines. Depending upon how the output density
 characteristic of the printer deviates from the design center value or
 depending upon the shape of the straight-thru output density
 characteristic of the design center value of the printer, the partitioning
 lines may be expressed by combinations of straight lines (polygonal lines)
 or by curves of any order.
 The present invention can be applied to a system constituted by a plurality
 of devices (e.g., a host computer, interface, reader, printer, etc.) or to
 an apparatus comprising a single device (e.g., a copier or facsimile
 machine, etc.), as in the foregoing embodiments.
 Further, it goes without saying that the object of the present invention
 can also be achieved by providing a storage medium storing the program
 codes of the software for performing the aforesaid functions of the
 embodiments to a system or an apparatus, reading the program codes with a
 computer (e.g., a CPU or MPU) of the system or apparatus from the storage
 medium, and then executing the program. In this case, the program codes
 read from the storage medium realize the functions of the embodiments, and
 the storage medium storing the program codes constitutes the invention.
 Further, the storage medium, such as a floppy disk, hard disk, optical
 disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, non-volatile type
 memory card or ROM can be used to provide the program codes.
 Furthermore, besides the case where the aforesaid functions according to
 the embodiments are implemented by executing the program codes read by a
 computer, the present invention covers a case where an operating system
 (OS) or the like working on the computer performs a part of or the entire
 process in accordance with the designation of program codes and implements
 the functions according to the embodiments.
 Furthermore, the present invention further covers a case where, after the
 program codes read from the storage medium are written in a function
 extension card inserted into the computer or in a memory provided in a
 function extension unit connected to the computer, a CPU or the like
 contained in the function extension card or function extension unit
 performs a part of or the entire process in accordance with the
 designation of program codes and implements the function of the above
 embodiments.
 Further, FIG. 3 illustrates an arrangement in which the primary converter
 107 is in back of the gamma converter 106. However, the primary converter
 107 may be provided in front of the gamma converter 106. Furthermore, the
 characteristic of the reader of the apparatus can be dealt with by
 providing the primary converter 107 in front of and in back of the gamma
 converter 106.
 Thus, in accordance with the present invention as described above, there
 are provided an image processing apparatus and method through which
 individual differences in apparatus output characteristic are compensated
 for and a decline in image quality due to the false-contour phenomenon is
 prevented.
 As many apparently widely different embodiments of the present invention
 can be made without departing from the spirit and scope thereof, it is to
 be understood that the invention is not limited to the specific
 embodiments thereof except as defined in the appended claims.