Patent Publication Number: US-2011075004-A1

Title: Method and apparatus for processing images using black character substitution

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 11/966,217, filed Dec. 28, 2007 which is a continuation of U.S. application Ser. No. 10/790,009, filed Mar. 2, 2004, now U.S. Pat. No. 7,336,846, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image input apparatus that inputs image information using a 4-line CCD sensor, and an image processing method. 
     2. Description of the Related Art 
     A conventional 3-line CCD sensor having three line sensors for RED, GREEN and BLUE has been used to read a color image. The 3-line CCD sensor is configured such that color filters of RED, GREEN and BLUE are disposed on light-receiving surfaces of the respective three line sensors, and the three line sensors are arranged in a one-dimensional fashion. Thus, one location on an original cannot be read at the same time by all the line sensors. A resultant displacement in the original scan direction is corrected by performing position correction for image signals read by the respective line sensors, with use of a memory circuit comprising, e.g. a line memory. 
     Recently, a 4-line CCD sensor having a 4-line architecture has been marketed as a product. The 4-line CCD sensor is configured such that a monochromatic line sensor for reading a monochromatic image, which has no color filter on its light-receiving surface, is added to the aforementioned 3-line CCD sensor for reading a color image. 
     Jpn. Pat. Appln. KOKAI Publication No. 2003-87556 discloses a technique wherein a monochromatic original is read with a high resolution and a color original is read with a high sensitivity. In this technique, the light receiving areas of pixels are made different between a line sensor for reading a monochromatic image and three line sensors for reading a color image. 
     However, this CCD line sensor comprises the line sensor for reading a monochromatic image and the three line sensors for reading a color image and, compared to the line sensor for reading a monochromatic image, the three line sensors for reading a color image are unable to read one location on the original at the same time, as mentioned above. Consequently, color overlapping of a black character becomes difficult when the magnification for reading is changed or non-uniformity occurs in reading speed. 
     When the color original is read with high sensitivity by making the light receiving areas of pixels different between the line sensor for reading a monochromatic image and the line sensors for reading a color image, the resolution of the color original lowers. Although the resolution can be increased by simple linear interpolation, reproduction of a fine character is degraded by linear interpolation in the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of an aspect of the present invention is to provide an image input apparatus and an image processing method, which can enhance the quality of image information that is input using a 4-line CCD sensor. 
     According to an aspect of the present invention, there is provided an image input apparatus that inputs an image of an original, comprising: photoelectric conversion means including a first line sensor and a second line sensor, the first line sensor being composed of a plurality of line sensors having different color filters on light receiving surfaces thereof, and the second line sensor having no color filter on a light receiving surface thereof; and correction means for correcting output signals from the plural line sensors of the first line sensor of the photoelectric conversion means, using an output signal from the second line sensor of the photoelectric conversion means. 
     According to another aspect of the present invention, there is provided an image processing method for an image input apparatus that inputs an image of an original, comprising: scanning the original using photoelectric conversion means including a first line sensor and a second line sensor, the first line sensor being composed of a plurality of line sensors having different color filters on light receiving surfaces thereof, and the second line sensor having no color filter on a light receiving surface thereof; and correcting output signals from the plural line sensors of the first line sensor, using an output signal that is produced from the second line sensor of the photoelectric conversion means by the scanning of the original. 
     Additional objects and advantages of an aspect of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of an aspect of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of an aspect of the invention. 
         FIG. 1  schematically shows the structure of an image input apparatus according to the present invention; 
         FIG. 2  schematically shows the structure of a control board; 
         FIG. 3  schematically shows the structure of CCD line sensors; 
         FIG. 4  illustrates processing performed successively by the CCD sensor, a various analog processing circuit and an image processing section; 
         FIG. 5  shows an example of arrangement of the CCD sensor; 
         FIG. 6  is a view for explaining the pixel size of each line sensor; 
         FIG. 7  is a block diagram schematically showing the structure of an image processing section in a first embodiment of the invention; 
         FIG. 8  shows structures of respective signals that are input to an image correction section; 
         FIG. 9  shows structures of respective signals that are subjected to a resolution-enhancing process in the image correction section and are output; 
         FIG. 10  is a block diagram schematically showing the structure of an image processing section in a modification of the first embodiment of the invention; 
         FIG. 11  shows structures of respective signals that are subjected to a resolution-enhancing process in the image correction section and are output; 
         FIG. 12  shows a high-resolution BLACK output signal; 
         FIG. 13  shows an example of a low-resolution color output signal, which is read at the same time as the high-resolution BLACK signal; 
         FIG. 14  illustrates an example of the resolution-enhancing process; 
         FIG. 15  shows an example in which the resolution of the low-resolution color signal is enhanced by simple linear interpolation; 
         FIG. 16  shows an example of arrangement of a CCD sensor according to a second embodiment of the invention; 
         FIG. 17  is a view for explaining the pixel size of each line sensor; 
         FIG. 18  is a block diagram schematically showing the structure of an image processing section in the second embodiment of the invention; 
         FIG. 19  shows structures of respective signals that are input to an image correction section; 
         FIG. 20  shows structures of respective signals that are subjected to a correction process in the image correction section and are output; 
         FIG. 21  is a block diagram schematically showing the structure of an image processing section in a modification of the second embodiment of the invention; 
         FIG. 22  shows structures of respective signals that are subjected to a correction process in the image correction section and are output; 
         FIG. 23  shows an example in which black lines with the same density and thickness are arranged; 
         FIG. 24  shows an example of color output signals in a case where luminance values are calculated; and 
         FIG. 25  shows a BLACK output signal at the same position as in the example of the color output signals. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. 
       FIG. 1  schematically shows the structure of an image input apparatus using a CCD line sensor. The image input apparatus, which is a general scanner, comprises a first carriage  4  including a light source  1 , a reflector  2  for correcting the light distribution of the light source  1 , and a first mirror  3 ; a second carriage  7  including a second mirror  5  and a third mirror  6 ; a converging lens  8 ; a CCD sensor board  10  on which a CCD sensor  9  is mounted; a control board  11  for controlling the CCD sensor  9  and performing various processings; a white reference plate  12  serving as a reference of white color; an original glass  13  on which an original org is placed; an original hold cover  14  for holding the original org; and a scanner casing  15  in which all the structural components are disposed. 
     The present invention relates to the CCD sensor  9  and control board  11 . 
     The operation of the image input apparatus is described in brief with reference to  FIG. 1 . 
     Light radiated from the light source  1  passes through the original glass  13  and illuminates the original org. Distribution of light radiated from the light source  1  is not uniform, and non-uniformity in illuminance occurs on the original org. To solve the problem, reflective light from the reflector  2  is also radiated on the original org, thereby uniformizing light distribution on the original org. 
     The reflective light from the original org is reflected by the first mirror  3 , second mirror  5  and third mirror  6 . Light coming from the third mirror  6  passes through the converging lens  8  and is focused on the light receiving surface of the CCD sensor  9 . The CCD sensor  9  is mounted on the CCD sensor board  10  and is controlled by a control signal input from the control board  11 . 
     The original hold cover  14  serves to bring the to-be-read surface of the original org, which is placed on the original glass  13 , into close contact with the original glass  13 . 
     An analog signal that is output from the CCD sensor  9  includes a high-frequency distortion due to a conversion efficiency variance among photoelectric converters, and a low-frequency distortion due to an aberration resulting from the use of a reduction-type optical system that includes the converging lens  8 . 
     This requires data that is to be used as a reference for normalization correction. In the present embodiment, the reference data is image data that is obtained by reading the white reference plate  12 . 
       FIG. 2  schematically shows the structure of the control board  11 . 
     The control board  11  comprises a process IC (CPU)  11 A, a various timing generating circuit  11 B, a various analog processing circuit  11 C, a line memory circuit  11 D, and an image processing section  11 E. 
     The process IC (CPU)  11  performs various processes. 
     The various timing generating circuit  11 B generates various timings. 
     The various analog processing circuit  11 C performs various processings that range from the processing of analog signals input from the CCD sensor  9  to the conversion of the analog signals to digital signals. 
     The image processing section  11 E performs image correction such as shading correction for correcting high-frequency and low-frequency distortions in digital signals produced from the various analog processing circuit  11 C, and inter-line correction for correcting an inter-line displacement between a plurality of line sensors. 
     The line memory circuit  11 D delays image data in units of a line when the inter-line correction process is carried out. 
     The process IC (CPU)  11 A controls a CCD sensor control circuit  19 A that is mounted on the CCD sensor board  10 , a light source control circuit  16  that controls light emission of the light source  1 , and a drive system control circuit  17  that controls a motor  18  for driving the first carriage  4  and second carriage  7 . 
     The CCD sensor board  10  comprises the CCD sensor  9 , the CCD sensor control circuit  10 A for driving the CCD sensor  9 , and a CCD driver  10 B that receives an output from the CCD sensor control circuit  10 A and generates proper driving conditions for the CCD sensor  9 . 
       FIG. 3  schematically shows the structure of the CCD sensor  9 . 
     The CCD sensor  9  is a 4-line CCD sensor and comprises a RED line sensor  9 R, a GREEN line sensor  9 G, a BLUE line sensor  9 B and a BLACK line sensor  9 K. 
     The RED line sensor  9 R comprises a RED photodiode array  9 R 1 , a shift gate  9 R 2 , an analog shift register  9 R 3 , and an output amplifier  9 R 4 . A RED color filter (not shown) is disposed on the light receiving surface of the RED photodiode array  9 R 1 . The RED photodiode array  9 R 1  converts (photoelectric conversion) incident light to a charge corresponding to the amount of light, and accumulates the charge in the respective photodiodes. The accumulated charge is transferred to the analog shift register  9 R 3  via the shift gate  9 R 2  by a control signal SH 1  that is applied to the shift gate  9 R 2 . The charge that has been transferred to the analog shift register  9 R 3  is successively delivered to the rear-stage output amplifier  9 R 4  by control signals φ 1  and φ 2 . The output amplifier  9 R 4  outputs the charge as an output signal OUT 1 . 
     The GREEN line sensor  9 G comprises a GREEN photodiode array  9 G 1 , a shift gate  9 G 2 , an analog shift register  9 G 3 , and an output amplifier  9 G 4  A GREEN color filter (not shown) is disposed on the light receiving surface of the GREEN photodiode array  9 G 1 . The GREEN photodiode array  9 G 1  converts (photoelectric conversion) incident light to a charge corresponding to the amount of light, and accumulates the charge in the respective photodiodes. The accumulated charge is transferred to the analog shift register  9 G 3  via the shift gate  9 G 2  by a control signal SH 2  that is applied to the shift gate  9 G 2 . The charge that has been transferred to the analog shift register  9 G 3  is successively delivered to the rear-stage output amplifier  9 G 4  by control signals φ 1  and φ 2 . The output amplifier  9 G 4  outputs the charge as an output signal OUT 2 . 
     The BLUE line sensor  9 B comprises a BLUE photodiode array  9 B 1 , a shift gate  9 B 2 , an analog shift register  9 B 3 , and an output amplifier  9 B 4 . A BLUE color filter (not shown) is disposed on the light receiving surface of the BLUE photodiode array  9 B 1 . The BLUE photodiode array  9 B 1  converts (photoelectric conversion) incident light to a charge corresponding to the amount of light, and accumulates the charge in the respective photodiodes. The accumulated charge is transferred to the analog shift register  9 B 3  via the shift gate  9 B 2  by a control signal SH 3  that is applied to the shift gate  9 B 2 . The charge that has been transferred to the analog shift register  9 B 3  is successively delivered to the rear-stage output amplifier  9 B 4  by control signals φ 1  and φ 2 . The output amplifier  9 B 4  outputs the charge as an output signal OUT 3 . 
     The BLACK line sensor  9 K comprises a BLACK photodiode array  9 K 1 , a shift gate  9 K 2 , an analog shift register  9 K 3 , and an output amplifier  9 K 4 . No color filter is disposed on the light receiving surface of the BLACK photodiode array  9 K 1 . The BLACK photodiode array  9 K 1  converts (photoelectric conversion) incident light to a charge corresponding to the amount of light, and accumulates the charge in the respective photodiodes. The accumulated charge is transferred to the analog shift register  9 K 3  via the shift gate  9 K 2  by a control signal SH 4  that is applied to the shift gate  9 K 2 . The charge that has been transferred to the analog shift register  9 K 3  is successively delivered to the rear-stage output amplifier  9 K 4  by control signals φ 1  and φ 2 . The output amplifier  9 K 4  outputs the charge as an output signal OUT 4 . 
     In the CCD sensor  9 , the above-described photodiode arrays, shift gates, analog shift registers and output amplifiers are disposed at predetermined positions. 
       FIG. 4  illustrates processing that is performed successively by the CCD sensor  9 , the various analog processing circuit  11 C and the image processing section  11 E. In  FIG. 4 , depiction of the above-described structure of the various analog processing circuit  11 C is partly omitted. 
     The output signal OUT 1  from the RED line sensor  9 R of the CCD sensor  9  is subjected to amplitude adjustment in a gain amplifier circuit (PGA-R)  22 R. An ADC section  25 R converts the amplitude-adjusted analog signal to a digital signal, and outputs the digital signal to the image processing section  11 E. 
     Similarly, the output signal OUT 2  from the GREEN line sensor  9 G of the CCD sensor  9  is subjected to amplitude adjustment in a gain amplifier circuit (PGA-G)  22 G. An ADC section  25 G converts the amplitude-adjusted analog signal to a digital signal, and outputs the digital signal to the image processing section  11 E. 
     Similarly, the output signal OUT 3  from the BLUE line sensor  9 B of the CCD sensor  9  is subjected to amplitude adjustment in a gain amplifier circuit (PGA-B)  22 B. An ADC section  25 B converts the amplitude-adjusted analog signal to a digital signal, and outputs the digital signal to the image processing section  11 E. 
     Similarly, the output signal OUT 4  from the BLACK line sensor  9 K of the CCD sensor  9  is subjected to amplitude adjustment in a gain amplifier circuit (PGA-BLACK)  22 K. An ADC section  25 K converts the amplitude-adjusted analog signal to a digital signal, and outputs the digital signal to the image processing section  11 E. 
     The image processing section  11 E performs, for instance, shading correction, inter-line correction, and a process for correcting the RED, GREEN and BLUE output signals on the basis of the BLACK output signal. The processed signals are output to the rear-stage image processing system (engine section). 
     According to the present invention, in the 4-line CCD sensor that outputs RED, GREEN, BLUE and BLACK signals, the image processing section  11 E shown in  FIG. 4  performs an image process for correcting the RED, GREEN and BLUE output signals by using the BLACK output signal, thereby producing corrected RED, GREEN and BLUE signals. 
     Next, a first embodiment of the invention with the above-described structure will be described. 
     The first embodiment is associated with a process in a case where the resolution of each of the output signals from the RED line sensor  9 R, GREEN line sensor  9 G and BLUE line sensor  9 B differs from the resolution of the output signal from the BLACK line sensor  9 K. 
       FIG. 5  shows an example of arrangement of the CCD sensor  9 . In this example of arrangement, the RED photodiode array  9 R 1  is disposed as a first array in the original scan direction. Next, the GREEN photodiode array  9 G 1  is disposed. Then, the BLUE photodiode array  9 B 1  is disposed. At last, the BLACK photodiode array  9 K 1  is disposed. 
     The size of each photodiode (hereinafter referred to as “pixel size”) in each of the RED photodiode array  9 R 1 , GREEN photodiode array  9 G 1  and BLUE photodiode array  9 B 1  is larger than the pixel size of the BLACK photodiode array  9 K 1 . Accordingly, the number of pixels in each of the RED photodiode array  9 R 1 , GREEN photodiode array  9 G 1  and BLUE photodiode array  9 B 1  is less than the number of pixels in the BLACK photodiode array  9 K 1 . 
     In the first embodiment, as shown in  FIG. 6 , the pixel size of the BLACK photodiode array is set at a×a, and the pixel size of each of the RED photodiode array  9 R 1 , GREEN photodiode array  9 G 1  and BLUE photodiode array  9 B 1  is set at (a× 2 ) ×(a×2). 
     Since each photodiode array has the same longitudinal length, the number of pixels of each of the RED photodiode array  9 R 1 , GREEN photodiode array  9 G 1  and BLUE photodiode array  9 B 1  is half the number of pixels of the BLACK photodiode array  9 K 1 . 
     When an A4-size paper sheet with a longitudinal length of 297 mm is read with a resolution of 600 dpi, the number of pixels that is required is 600 dpi/25.4 mm×297 mm=7015.7. Thus, the BLACK photodiode array  9 K 1  needs to have at least 7016 pixels. 
     If an error in attachment of the CCD sensor  9  or an error in placing the original org is taken into account, the BLACK photodiode array  9 K 1  needs to have (7016+a) pixels. Assume that the number of pixels of the BLACK photodiode array  9 K 1  is 7500. 
     In this case, the number of pixels in each of the RED photodiode array  9 R 1 , GREEN photodiode array  9 G 1  and BLUE photodiode array  9 B 1  is 3750 (½ of 7500). 
     When an image is read by using the CCD sensor  9  of the first embodiment, the resolution of each of the output signals from the RED photodiode array  9 R 1 , GREEN photodiode array  9 G 1  and BLUE photodiode array  9 B 1  is lower than the resolution of the output signal from the BLACK photodiode array  9 K 1 . 
     In this case, in the first embodiment, the output signals from the RED photodiode array  9 R 1 , GREEN photodiode array  9 G 1  and BLUE photodiode array  9 B 1  are corrected on the basis of the output signal from the BLACK photodiode array  9 K 1  (hereinafter referred to as “resolution-enhancing process”). The resolution-enhancing process is carried out to produce RED, GREEN and BLUE signals each having a resolution substantially equal to the resolution of the output signal from the BLACK photodiode array  9 K 1 . 
     Next, image processes including the resolution-enhancing process in the image processing section  11 E of the first embodiment are described. 
       FIG. 7  schematically shows the structure of the image processing section  11 E. The image processing section  11 E comprises a shading correction section  31 , an inter-line correction section  32 , an image correction section  33 , a filter process section  34 , an image region discrimination section  35  and a color conversion section  36 . 
     Assume that an output signal from the BLACK photodiode array  9 K 1  is a K signal, an output signal from the RED photodiode array  9 R 1  is an R signal, an output signal from the GREEN photodiode array  9 G 1  is a G signal, and an output signal from the BLUE photodiode array  9 B 1  is a B signal. 
     The K signal, R signal, G signal and B signal are subjected to shading correction in the shading correction section  31 . The shading-corrected signals are subjected to inter-line correction for positioning in the inter-line correction section  32 . The K signal, R signal, G signal and B signal, which have been subjected to inter-line correction, are input to the image correction section  33 . 
     Using the input K signal, the image correction section  33  subjects the R signal, G signal and B signal to the resolution-enhancing process. The image correction section  33  outputs four signals: an Rc 1  signal, a Gc 1  signal and a Bc 1  signal, which are produced by the resolution-enhancing process, and the K signal that is used for the resolution-enhancing process. 
     The resolution-enhancing process in the image correction section  33  is described in greater detail. 
     The pixel size of RED, GREEN and BLUE is double the pixel size of BLACK. Thus, if the K signal that is input to the image correction section  33  comprises components K 01 , K 02 , K 03  and K 04 , the R signal comprises components R 01  and R 02 , the G signal comprises components G 01  and G 02 , and the B signal comprises components B 01  and B 02 . 
       FIG. 8  shows the configuration of the K signal (K 01 , K 02 , K 03 , K 04 ), the R signal (R 01 , R 02 ), the G signal (G 01 , G 02 ) and the B signal (B 01 , B 02 ). 
     The image correction section  33  subjects the R signal to the resolution-enhancing process so that it may have a pixel size that is equal to the pixel size of the K signal (BLACK). Thus, the image correction section  33  outputs a RED correction signal comprising Rc 01 , Rc 02 , Rc 03  and Rc 04 . Similarly, the image correction section  33  subjects the G signal to the resolution-enhancing process and outputs a GREEN correction signal comprising Gc 01 , Gc 02 , Gc 03  and Gc 04 . The image correction section  33  subjects the B signal to the resolution-enhancing process and outputs a BLUE correction signal comprising Bc 01 , Bc 02 , Bc 03  and Bc 04 . In addition, the image correction section  33  outputs K 01 , K 02 , K 03  and K 04  of the K signal, which have been used for the resolution-enhancing process. 
       FIG. 9  shows the configuration of the K signal (K 01 , K 02 , K 03 , K 04 ), the R signal (Rc 01 , Rc 02 , Rc 03 , Rc 04 ), the G signal (Gc 01 , Gc 02 , Gc 03 , Gc 04 ) and the B signal (Bc 01 , Bc 02 , Bc 03 , Bc 04 ), which are produced by the resolution-enhancing process in the image correction section  33 . 
     In  FIG. 7 , these four signals are output from the image correction section  33  as the K signal, Rc 1  signal, Gc 1  signal and Bc 1  signal. 
     The K signal, Rc 1  signal Gc 1  signal and Bc 1  signal are successively subjected to a filter process in the filter process section  34 , an image region discrimination process in the image region discrimination section  35 , and a color conversion process in the color conversion section  36 . 
     In the image process section  11 E, a Kc 1  signal, a Cc 1  signal (CYAN), an Mc 1  signal (MAGENTA) and a Yc 1  signal (YELLOW), which are output from the color conversion section  36 , are delivered to the rear-stage image processing system. 
       FIG. 10  schematically shows the structure of an image processing section  11 E according to a modification of the first embodiment shown in  FIG. 7 . The image processing section  11 E comprises a shading correction section  31 , an inter-line correction section  32 , an image correction section  43 , a filter process section  34 , an image region discrimination section  35  and a color conversion section  36 . The parts common to those in the first embodiment are denoted by like reference numerals, and a description thereof is omitted. 
     The K signal, R signal, G signal and B signal are subjected to shading correction in the shading correction section  31 . The shading-corrected signals are subjected to inter-line correction for positioning in the inter-line correction section  32 . The K signal, R signal, G signal and B signal, which have been subjected to inter-line correction, are input to the image correction section  43 . 
     Using the input K signal, the image correction section  43  subjects the R signal, G signal and B signal to the resolution-enhancing process. The image correction section  43  outputs an Rc 2  signal, a Gc 2  signal and a Bc 2  signal, which are produced by the resolution-enhancing process. 
     The resolution-enhancing process in the image correction section  43  is described in greater detail. 
     The pixel size of RED, GREEN and BLUE is double the pixel size of BLACK. Thus, if the K signal that is input to the image correction section  43  comprises components K 01 , K 02 , K 03  and K 04 , the R signal comprises components R 01  and R 02 , the G signal comprises components G 01  and G 02 , and the B signal comprises components B 01  and B 02 . This is the same as in the structure shown in  FIG. 8 . 
     The image correction section  43  subjects the R signal to the resolution-enhancing process so that it may have a pixel size that is equal to the pixel size of the K signal (BLACK). Thus, the image correction section  43  outputs a RED correction signal comprising Rc 11 , Rc 12 , Rc 13  and Rc 14 . Similarly, the image correction section  43  subjects the G signal to the resolution-enhancing process and outputs a GREEN correction signal comprising Gc 11 , Gc 12 , Gc 13  and Gc 14 . The image correction section  43  subjects the B signal to the resolution-enhancing process and outputs a RED correction signal comprising Bc 11 , Bc 12 , Bc 13  and Bc 14 . 
       FIG. 11  shows the configuration of the R signal (Rc 11 , Rc 12 , Rc 13 , Rc 14 ), the G signal (Gc 11 , Gc 12 , Gc 13 , Gc 14 ) and the B signal (Bc 11 , Bc 12 , Bc 13 , Bc 14 ), which are produced by the resolution-enhancing process in the image correction section  43 . 
     In  FIG. 10 , these three signals are output from the image correction section  43  as the Rc 2  signal, Gc 2  signal and Bc 2  signal. 
     The Rc 2  signal Gc 2  signal and Bc 2  signal are successively subjected to a filter process in the filter process section  34 , an image region discrimination process in the image region discrimination section  35 , and a color conversion process in the color conversion section  36 . 
     In the image process section  11 E, a Cc 2  signal (CYAN), an Mc 2  signal (MAGENTA) and a Yc 2  signal (YELLOW), which are output from the color conversion section  36 , are delivered to the rear-stage image processing system. 
     The details of the resolution-enhancing process are described below. 
     In an example of the resolution-enhancing process, RED, GREEN and BLUE output signals are corrected with reference to a high-resolution BLACK output signal. 
       FIG. 12  shows a high-resolution BLACK output signal. 
       FIG. 13  shows an example of a low-resolution color output signal, which is read at the same time as the high-resolution BLACK signal shown in  FIG. 12 . 
     The ordinate indicates an output signal luminance value, and the abscissa indicates the number of pixels. Broken lines indicate division lines in units of 300 dpi. If the reading by a high-resolution sensor and the reading by a low-resolution sensor are compared, it is understood that high-luminance portions and low-luminance portions of the output signal are rounded in the case of the low-resolution reading. In the resolution-enhancing process, the above two output signals are used to obtain a high-resolution color signal. 
       FIG. 14  illustrates an example of the resolution-enhancing process. The ordinate indicates an output signal luminance value, and the abscissa indicates the number of pixels. Broken lines indicate division lines in units of 300 dpi. A thick solid line indicates an output signal after the resolution-enhancing process, and a thick broken line indicates an input low-resolution color signal. Arrows indicate directions of correction. 
     The high-resolution BLACK signal and low-resolution color signal are superimposed on each other. Referring to the BLACK output signal, the low-resolution signal is corrected so as to conform to the BLACK signal. By carrying out this resolution-enhancing process, a high-resolution color image can be obtained. 
       FIG. 15  shows an example in which the resolution of the low-resolution color signal shown in  FIG. 13  is enhanced by simple linear interpolation. If  FIG. 14  and  FIG. 15  are compared, it is understood that in  FIG. 14  high-luminance portions and low-luminance portions of the output signal are clearly reproduced. 
     As illustrated in the first embodiment and the modification thereof, each of the low-resolution RED, GREEN and BLUE output signals can be made to conform to the resolution of the BLACK output signal by the above-described resolution-enhancing process. Thus, high-resolution RED, GREEN and BLUE correction signals are obtained. 
     A second embodiment of the present-invention will now be described. 
     The second embodiment is associated with a process in a case where the resolution of each of the output signals from the RED, GREEN and BLUE sensors is equal to the resolution of the output signal from the BLACK sensor. 
       FIG. 16  shows an example of arrangement of the CCD sensor  9  according to the second embodiment. In this example of arrangement, a RED photodiode array  9 R 1 - 2  is disposed as a first array in the original scan direction. A GREEN photodiode array  9 G 1 - 2  is then disposed. Subsequently, a BLUE photodiode array  9 B 1 - 2  is disposed. At last, a BLACK photodiode array  9 K 1 - 2  is disposed. 
     The size of each photodiode (hereinafter referred to as “pixel size”) in each of the RED photodiode array  9 R 1 - 2 , GREEN photodiode array  9 G 1 - 2  and BLUE photodiode array  9 B 1 - 2  is equal to the pixel size of the BLACK photodiode array  9 K 1 - 2 . 
     In the second embodiment, as shown in  FIG. 17 , the pixel size of the BLACK photodiode array  9 K 1 - 2  is set at a×a, and the pixel size of each of the RED photodiode array  9 R 1 - 2 , GREEN photodiode array  9 G 1 - 2  and BLUE photodiode array  9 B 1 - 2  is set at a×a. Since each photodiode array has the same longitudinal length, the number of pixels of each of the RED photodiode array  9 R 1 - 2 , GREEN photodiode array  9 G 1 - 2  and BLUE photodiode array  9 B 1 - 2  is equal to the number of pixels of the BLACK photodiode array  9 K 1 - 2 . 
     When an A4-size paper sheet with a longitudinal length of 297 mm is read with a resolution, of 600 dpi, the number of pixels that is required is 600 dpi/25.4 mm×297 mm=7015.7. Thus, the BLACK photodiode array  9 K 1 - 2  needs to have at least 7016 pixels. 
     If an error in attachment of the CCD sensor  9  or an error in placing the original org is taken into account, the BLACK photodiode array  9 K 1 - 2  needs to have (7016+a) pixels. Assume that the number of pixels of the BLACK photodiode array  9 K 1 - 2  is 7500. 
     In this case, the number of pixels in each of the RED photodiode array  9 R 1 - 2 , GREEN photodiode array  9 G 1 - 2  and BLUE photodiode array  9 B 1 - 2  is 7500, which is equal to the number of pixels in the BLACK photodiode array  9 K 1 - 2 . 
     When an image is read by using the CCD sensor  9  of the second embodiment, the output signals from the RED photodiode array  9 R 1 - 2 , GREEN photodiode array  9 G 1 - 2 , BLUE photodiode array  9 B 1 - 2  and the BLACK photodiode array  9 K 1 - 2  have the same resolution. 
     In this case, in the second embodiment, a black character substitution process is performed for the output signals from the RED photodiode array  9 R 1 - 2 , GREEN photodiode array  9 G 1 - 2  and BLUE photodiode array  9 B 1 - 2  on the basis of the output signal from the BLACK photodiode array  9 K 1 - 2 . The details of the black character substitution process will be described later. 
     Next, image processes including the correction process in the image processing section  11 E of the second embodiment are described. 
       FIG. 18  schematically shows the structure of the image processing section  11 E according to the second embodiment. The image processing section  11 E comprises a shading correction section  51 , an inter-line correction section  52 , an image correction section  53 , a filter process section  54 , an image region discrimination section  55 , a color conversion section  56 , and an image region discrimination section  57 . 
     Assume that an output signal from the BLACK photodiode array  9 K 1 - 2  is a K signal, an output signal from the RED photodiode array  9 R 1 - 2  is an R signal, an output signal from the GREEN photodiode array  9 G 1 - 2  is a G signal, and an output signal from the BLUE photodiode array  9 B 1 - 2  is a B signal. 
     The K signal, R signal, G signal and B signal are subjected to shading correction in the shading correction section  51 . The shading-corrected signals are subjected to inter-line correction for positioning in the inter-line correction section  52 . The K signal, R signal, G signal and B signal, which have been subjected to inter-line correction, are input to the image correction section  53 . At the same time, the R signal, G signal and B signal are input to the image region discrimination section  57 . 
     The image region discrimination section  57  determines (discriminates) the position of a black part, such as a black character or a black line, on the basis of the input R signal, G signal and B signal, and delivers the discrimination information to the image correction section  53 . 
     Based on the information from the image region discrimination section  57 , the image correction section  53  performs the black character substitution process for the R signal, G signal and B signal by using the K signal. The image correction section  53  outputs four signals: an Rc 3  signal, a Gc 3  signal and a Bc 3  signal, which have been subjected to the black character substitution process, and the K signal that has been used for the black character substitution process. 
     The pixel size of RED, GREEN and BLUE is equal. Thus, if the K signal that is input to the image correction section  53  comprises components K 21 , K 22 , K 23  and K 24 , the R signal comprises components R 21 , R 22 , R 23  and R 24 , the G signal comprises components G 21 , G 22 , G 23  and G 24 , and the B signal comprises components B 21 , B 22 , B 23  and B 24 . 
       FIG. 19  shows the configuration of the K signal (K 21 , K 22 , K 23  and K 24 ), the R signal (R 21 , R 22 , R 23  and R 24 ), the G signal (G 21 , G 22 , G 23  and G 24 ) and the B signal (B 21 , B 22 , B 23  and B 24 ). 
     The image correction section  53  subjects the R signal, G signal and B signal to the black character substitution process by using K 21 , K 22 , K 23  and K 24  of the K signal, on the basis of the information from the image region discrimination section  57 . Thus, the image correction section  53  outputs a RED correction signal comprising Rc 21 , Rc 22 , Rc 23  and Rc 24 , a GREEN correction signal comprising Gc 21 , Gc 22 , Gc 23  and Gc 24 , a BLUE correction signal comprising Bc 21 , Bc 22 , Bc 23  and Bc 24 . Each of the RED correction signal, GREEN correction signal and BLUE correction signal has the same pixel size as the K signal. In addition, the image correction section  53  outputs K 21 , K 22 , K 23  and K 24  of the K signal, which have been used for the correction process. 
       FIG. 20  shows the configuration of the K signal (K 21 , K 22 , K 23 , K 24 ), the R signal (Rc 21 , Rc 22 , Rc 23 , Rc 24 ), the G signal (Gc 21 , Gc 22 , Gc 23 , Gc 24 ) and the B signal (Bc 21 , Bc 22 , Bc 23 , Bc 24 ), which are produced by the correction process in the image correction section  53 . 
     In  FIG. 18 , these four signals are output as the K signal, Rc 3  signal, Gc 3  signal and Bc 3  signal. 
     The K signal, Rc 3  signal Gc 3  signal and Bc 3  signal are successively subjected to a filter process in the filter process section  54 , an image region discrimination process in the image region discrimination section  55 , and a color conversion process in the color conversion section  56 . 
     In the image process section  11 E, a Kc 3  signal, a Cc 3  signal (CYAN), an Mc 3  signal (MAGENTA) and a Yc 3  signal (YELLOW), which are output from the color conversion section  56 , are delivered to the rear-stage image processing system. 
       FIG. 21  schematically shows the structure of an image processing section  11 E according to a modification of the second embodiment shown in  FIG. 18 . The image processing section  11 E according to the modification of the second embodiment comprises a shading correction section  51 , an inter-line correction section  52 , an image correction section  63 , a filter process section  54 , an image region discrimination section  55 , a color conversion section  56  and an image region discrimination section  57 . The parts common to those in the second embodiment are denoted by like reference numerals, and a description thereof is omitted. 
     The K signal, R signal, G signal and B signal are subjected to shading correction in the shading correction section  51 . The shading-corrected signals are subjected to inter-line correction for positioning in the inter-line correction section  52 . In addition, the R signal, G signal and B signal are input to the image region discrimination section  57 . 
     The image region discrimination section  57  discriminates the position of a black part, such as a black character or a black line, on the basis of the input R signal, G signal and B signal, and delivers the discrimination information to the image correction section  63 . 
     Based on the information from the image region discrimination section  57 , the image correction section  63  performs the black character substitution process for the R signal, G signal and B signal by using the K signal. The image correction section  63  outputs an Rc 4  signal, a Gc 4  signal and a Bc 4  signal, which have been subjected to the black character substitution process. 
     The correction process in the image correction section  63  is described in greater detail. 
     The pixel size of RED, GREEN and BLUE is equal. Thus, if the K signal that is input to the image correction section  63  comprises components K 21 , K 22 , K 23  and K 24 , the R signal comprises components R 21 , R 22 , R 23  and R 24 , the G signal comprises components G 21 , G 22 , G 23  and G 24 , and the B signal comprises components B 21 , B 22 , B 23  and B 24 . This is the same as in the structure shown in  FIG. 19 . 
     The image correction section  63  subjects the R signal, G signal and B signal to the correction process by using K 21 , K 22 , K 23  and K 24  of the K signal, on the basis of the information from the image region discrimination section  57 . Thus, the image correction section  63  outputs a RED correction signal comprising Rc 31 , Rc 32 , Rc 33  and Rc 34 , a GREEN correction signal comprising Gc 31 , Gc 32 , Gc 33  and Gc 34 , a BLUE correction signal comprising Bc 31 , Bc 32 , Bc 33  and Bc 34 . Each of the RED correction signal, GREEN correction signal and BLUE correction signal has the same pixel size as the K signal. 
       FIG. 22  shows the configuration of the R signal (Rc 31 , Rc 32 , Rc 33 , Rc 34 ), the G signal (Gc 31 , Gc 32 , Gc 33 , Gc 34 ) and the B signal (Bc 31 , Bc 32 , Bc 33 , Bc 34 ), which are produced by the correction process in the image correction section  63 . 
     In  FIG. 21 , these three signals are output from the image correction section  63  as the Rc 4  signal, Gc 4  signal and Bc 4  signal. 
     The Rc 4  signal Gc 4  signal and Bc 4  signal are successively subjected to a filter process in the filter process section  54 , an image region discrimination process in the image region discrimination section  55 , and a color conversion process in the color conversion section  56 . 
     In the image process section  11 E, a Cc 4  signal (CYAN), an Mc 4  signal (MAGENTA) and a Yc 4  signal (YELLOW), which are output from the color conversion section  56 , are delivered to the rear-stage image processing system. 
     The details of the black character substitution are described below. 
     In the prior-art 3-line sensor that outputs RED, GREEN and BLUE signals, when a black character is read, the black character is expressed by overlapping three colors. In this case, color misregistration may occur due to aberration of the lens or non-uniformity in rotation of the motor. Because of a failure in color overlapping, a black character with a uniform density may not be read with a uniform density, resulting in a non-uniform output signal. Further, color overlapping becomes more difficult, for example, when the magnification for reading is changed. 
     In the image processing according to the second embodiment, an image region discrimination is performed based on the RED, GREEN and BLUE output signals. If a black character is determined, the BLACK output signal is substituted for the RED, GREEN and BLUE output signals. 
     When a black character is read by the BLACK sensor, only one output is obtained. Hence, no color misregistration occurs due to aberration of the lens or non-uniformity in rotation of the motor. It is less possible that the output signal value varies when the black character with the same density is read. By executing this process, it becomes possible to decrease the non-uniformity in density of the black character or in line width, which results from aberration of the lens or non-uniform rotation of the motor. 
       FIG. 24  shows an example of color output signals that are obtained when luminance values of a region including black lines with the same density and thickness, which are arranged as shown in  FIG. 23 , are calculated in a direction indicated by a broken line in  FIG. 23 . 
       FIG. 25  shows a BLACK output signal at the same position as in the example of the color output signals shown in  FIG. 23 . 
     In  FIG. 24 , the ordinate indicates an output signal luminance value, and the abscissa indicates the number of pixels. A solid line indicates a RED output signal, a dot-and-dash line indicates a GREEN output signal, and a broken line indicates a BLUE output signal. As is understood from  FIG. 24 , high-luminance portions and low-luminance portions appear with variations due to slight differences in sensitivity of sensors. If a black signal is produced from these color signals, it is possible that the density varies from line to line. Moreover, the line width that is represented by each output signal may vary due to non-uniform rotation of the motor or aberration of the lens. If a black signal is produced from these color signals, it is also possible that the line thickness varies from line to line. 
     By contrast, in the second embodiment, the black character substitution process is executed using a single-color line (BLACK output signal) as shown in  FIG. 25 . It is thus possible to reduce the non-uniformity in density or line width. 
     As has been described above, according to the embodiments of the present invention, there is provided an image input apparatus that has a plurality of lines sensors, which have different numbers of, or the same number of, pixels, and synthesizes output signals of the line sensors, thereby to form image information. Using the output signal of the line sensor having no color filter on its light receiving surface, the output of the line sensor having a color filter on its light receiving surface is corrected. Thereby, the following advantages are obtained. 
     In a case where the number of pixels of the line sensor having the color filter on its light receiving surface is less than the number of pixels of the line sensor having no color filter on its light receiving surface, a BLACK output signal with a larger number of pixels is used to perform a resolution conversion process for RED, GREEN and BLUE output signals with less numbers of pixels. Thereby, a high image quality is obtained. 
     In a case where the number of pixels of the line sensor having the color filter on its light receiving surface is equal to the number of pixels of the line sensor having no color filter on its light receiving surface, image information that is obtained by synthesizing RED, GREEN and BLUE output signals is subjected to an image region discrimination process. If the image information is determined to be a black character or a black part, a BLACK output signal is substituted. Thereby, it becomes possible to reduce non-uniformity in density of a black character or non-uniformity in line width associated with the RED, GREEN and BLUE output signals, which results from aberration in the black character or black part or non-uniform rotation of the motor. Thus, a high image quality can be obtained. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.