Patent Publication Number: US-6707951-B1

Title: Halftone dot image discrimination method and image processing device

Description:
RELATED APPLICATIONS 
     This application is based on Application No. HEI 10-221296 filed in Japan, the content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a halftone dot image discrimination method and image processing device applicable to digital copiers and the like. 
     2. Description of the Related Art 
     In digital copiers and the like, image data obtained by reading a document are subjected to various image processing in accordance with the type of document image. Types of document images include, for example, text images, variable density images, halftone dot images and the like, and in order to discriminate these image types, the document image is divided into small block areas. For example, to discriminate whether or not an image is a halftone dot image, the halftone dot image discrimination must be performed on each block area. 
     Conventionally, the methods used to discriminate a halftone dot image determine whether or not the number of white or black isolated points within a block area exceeds a threshold value. 
     FIG. 21 illustrates the halftone dot image discrimination method. 
     In FIG. 21, the block area BE has, for example, a vertical by horizontal size of 9×41 dots, and the number of isolated points within the block area BE is detected using a 5×5 size isolated dot detection filter FD. In the detection of isolated points, the isolated point detection filter FD is applied to all pixels PX sequentially from the upper left pixel of the block area BE. The pixels matching the pixels in the center of the isolated dot detection filter FD are designated as the target pixels PXT, and these target pixels PXT are checked to determine whether or not they satisfy predetermined conditions. 
     The predetermined conditions satisfied in this instance determine if a certain target pixel PXT is a white isolated point by determining whether the target pixel PXT has a value larger than or equal to the values of the eight peripheral pixels PX, and whether the value of the center target pixel PXT is greater than or equal to the average value of two pixels PX arrayed in a row along the eight lateral, vertical, and inclined directions. These satisfied conditions also determine if target pixel is a black isolated point by determining whether the target pixel PXT has a value less than or equal to the values of the eight peripheral pixels PX, and whether the value of the center target pixel PXT is less than or equal to the average value of two pixels PX arrayed in a row along the eight lateral, vertical, and inclined directions. 
     The isolated point detection filter FD must be applied at one time for all image data of the five lines corresponding to the area size. Accordingly, the at least a 4-line memory is used to delay the image data of each line. 
     In this way the number of determined isolated points is counted. When this count value exceeds a pre-set threshold value, the block area BE is discriminated as a halftone dot area. 
     In the previously mentioned conventional halftone dot image discrimination method, a 5×5 size filter is used as the isolated dot detection filter FD. The size of the isolated dot detection filter FD is set so as to effectively detect isolated points via the isolated point detection filter FD when the image resolution is 400 dpi. 
     That is, as shown in FIG.  22 (A), when the image resolution is 400 dpi, even a large isolated point SP can be contained within the range of the isolated point detection filter FDa. Accordingly, the isolated point SP can be detected. However, when the image resolution is 600 dpi, as shown in FIG.  22 (B), the size of the isolated point detection filter FDb is relatively small. 
     Therefore, even the identical isolated point SP cannot be contained within the isolated point detection filter FD. as a result, this point cannot be detected as an isolated point SP. 
     For example, when the size of the isolated point SP is the size shown in FIG.  22 (B), the number of isolated points SP becomes [0] regardless of the presence of several isolated points SP, and since the number of isolated points SP is below the threshold value, there is concern that the area will not be discriminated as a halftone dot area. In this case, a discrimination error occurs. 
     In order to eliminate this problem, the algorithm for detecting the isolated point SP is modified to correct the detection of the isolated point SP even when the isolated point SP is large. In this instance, however, it is necessary to use a circuit or a method for detecting isolated points using a different algorithm at different resolutions. As a result, this alone complicates the algorithms and circuit construction, and is disadvantageous from a cost perspective. 
     Another methods considered for eliminating this problem increases the size of the isolated point detection filter FD when the resolution is higher. In this instance, however, the line memory must be increased in accordance with the size of the isolated point detection filter FD, circuit construction becomes more complex, and costs rise. 
     In recent years, image resolution has been generally increasing, and the types of images has tended to increase, such that the aforesaid methods of eliminating the problem have become increasingly disadvantageous from a cost perspective. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to eliminate the previously described disadvantages. 
     In view of the previously described problems, another object of the present invention is to provide a halftone dot image discrimination method and image processing device capable of accurately discriminating halftone dot images at various resolutions even when the resolution increases and when the types of resolutions increase, and which is capable of suppressing cost increases as much as possible. 
     These and other objects are attained by an image processing device having, an isolated point detection filter for detecting isolated points in image data, and a modifier for reducing the size of the isolated points included in the image data so as to be smaller than the size of the isolated point detection filter. 
     The objects of the present invention are attained by an image processing device having, an isolated point detection filter for detecting isolated points in image data, a resolution detection unit for detecting the resolution of the image data, and a modifier for reducing the size of the isolated points included in the image data in accordance with the resolution of the image data. 
    
    
     The invention itself, together with further objects and attendant advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the overall construction of a color copier; 
     FIG. 2 is a block diagram of the construction of an image processing device in the image reading unit; 
     FIG. 3 is a block diagram of the construction of an image processing device in the image reading unit; 
     FIG. 4 is a block diagram showing the construction of the area discrimination unit; 
     FIG. 5 is a block diagram showing an example of the construction of a halftone dot pre-processing unit and a halftone dot image discrimination unit; 
     FIG. 6 shows an example of the circuit of the image culling processor; 
     FIG. 7 is a timing chart showing the operation of the image culling processor; 
     FIG. 8 shows the reduction of the isolated point by the image culling processor; 
     FIG. 9 shows the isolated point detection filter; 
     FIG. 10 shows an example of a circuit of the halftone dot discrimination unit; 
     FIG. 11 shows an example of the circuit of the color/monochrome discrimination unit; 
     FIG. 12 shows an example of the construction of the halftone dot output unit; 
     FIG. 13 is a block diagram showing an example of the construction of the image correction unit; 
     FIG. 14 shows the attenuation process performed on a monochrome halftone dot image; 
     FIG. 15 shows the smoothing process; 
     FIG. 16 is a block diagram showing an example of the construction of the halftone dot pre-processing unit; 
     FIG. 17 shows the size of a black isolated dot before and after the opening process; 
     FIG. 18 is a block diagram showing an example of the construction of the halftone dot pre-processing unit; 
     FIG. 19 shows an example of a filter; 
     FIG. 20 shows an example of the construction of the halftone dot pre-processing unit and the color/monochrome discrimination unit; 
     FIG. 21 illustrates the halftone dot image discrimination method; and 
     FIGS.  22 (A) and  22 (B) show the reduction of the isolated point by the culling process. 
     In the following description, like parts are designated by like reference numbers throughout the several drawings. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the overall construction of a digital color copier  1  utilizing the halftone dot image discrimination method of the present invention. 
     In FIG. 1, the copier  1  comprises an automatic document feeder  100 , an image reader  200 , and an image forming unit  300 . Typically, a document transported to an image reading position by the automatic document feeder  100  is scanned by the image reader  200 , and the obtained image data are transmitted to the image forming unit  300 , which forms an image on a recording sheet. This operation is the copy function. This copier  1  can be connected to peripheral devices via an interface  207 . This arrangement allows the realization of an image reading function wherein the image data obtained by the image reader  200  are output to an external device, and a printing function wherein an image is formed by the image forming unit  300  based on image data input from an external device. 
     The automatic document feeder  100  feeds a document set on a document tray  101  to the image reading position of o the image reader  200 , and after the image is scanned, ejects the document to a document discharge tray  103 . The document transport operation is accomplished in accordance with instructions from an operation panel not shown in the illustration, and the document discharge operation is accomplished based on a reading end signal from the image reader  200 . When a plurality of documents are stacked on the document tray  101 , these control signals are continuously generated to efficiently perform the various operations to transport documents, read images, and discharge documents. 
     In the image reader  200  the light reflected from the document on a document glass  208  illuminated by an exposure lamp  201  is directed to a lens  203  by a 3-mirror element  202 , and forms an image on a CCD sensor  204 . The exposure lamp  201  and a first mirror are drivably moved in the arrow direction via a scan motor  209  at a speed V corresponding to the magnification, so as to scan the entire surface of a document placed on the document glass  208 . In conjunction with the scanning by the exposure lamp  201  and the first mirror, the second mirror and the third mirror move in the same direction at a speed V/2. The position of the exposure lamp  201  is calculated and controlled via the amount of movement from the home position, i.e., via detection signals from a scan home position sensor  210  and the number of steps of the drive motor. The light reflected by the document and entering the CCD sensor  204  is converted to electrical signals within the CCD sensor  204 , and subjected to analog processing, analog-to-digital (A/D) conversion, and digital image processing by an image processing unit  205 , then transmitted via an interface  207  to the image forming unit  300 . A white shading correction panel  206  is arranged in proximity to the document reading start position of the document glass  208 . Before the document image is read, the shading correction panel  206  is read to generate correction data for a shading correction table. 
     The image forming unit  300  is described below. The exposure and imaging processes are described first. Image data transmitted from the image reader  200  or the interface  207  are converted to print data of each color cyan (C), magenta (M), yellow (Y), and black (K), and transmitted to a controller of each exposure head not shown in the drawings. In the controller of each exposure head, a laser beam is emitted in accordance with the value of the transmitted image data, and this beam performs a unidimensional scan via a polygonal mirror  301 , and optically exposes the photosensitive members within each imaging unit  302   c ,  302   m ,  302   y , and  302   k.    
     Elements required for electrophotographic processing are provided around the periphery of the photosensitive member within each imaging unit  302   c ,  302   m ,  302   y , and  302   k . A continuous image forming process is accomplished by rotating the C, M, Y, K photosensitive members in a clockwise direction. The imaging units necessary for image formation are integrated for each process, and are detachable from the body. The electrostatic latent image formed on the surface of the photosensitive member within each image forming unit  302   c ,  302   m ,  302   y ,  302   k  is developed by their respective developing devices. The toner image formed on the surface of the photosensitive member is transferred onto a recording sheet on a sheet transport belt  304  via transfer chargers  302   c ,  302   m ,  303   y ,  303   k  disposed opposite the photosensitive member within the sheet transport belt  304 . 
     The paper feeding, transport, and fixing operations are described below. The transported sheet is fed to the transfer positions in the following sequence, and the image is formed thereon. Sheets of various sizes are loaded beforehand in the paper cassettes  310   a ˜ 310   c , and a sheet of a desired size is fed to the transport path by take-up rollers  312  mounted on each paper cassette  310   a ˜ 310   c.    
     The sheet fed to the transport path is transported onto the transport belt  304  by a pair of transport rollers  313 . At this time, the transport timing alignment of the transported sheet is accomplished by detecting a standard mark on the transport belt  304  via a timing sensor  306 . Three individual registration correction sensors  312  are arranged along the scanning direction on the upstream side of the imaging units  302   c ,  302   m ,  302   y ,  302   k . When forming a registration pattern on the transport belt  304 , the amount of color dislocation is detected in the main direction and the sub direction of the C, M, Y, K images. Print image correction and image distortion correction are performed by a print image controller (PIC unit) based on the detection results. As a result, color dislocation is prevented on the recording sheet. The toner image transferred onto the recording sheet is fused and fixed thereon via heating by a pair of fixing rollers  307 , then the recording sheet is ejected to a discharge tray  311 . In the case of duplex copies, in order to form an image on the back side of the recording sheet, the sheet bearing the toner image fixed by the pair of fixing rollers  307  is inverted by a sheet inverting unit  309 , and is guided to a duplex unit  308 , which re-feeds the recording sheet. The transport belt  304  can be retracted from he C, M, and Y imaging units  302   c ,  302   m ,  302   y  via the operation of a belt retracting roller  305 , so that the sheet transport belt  304  does not come into contact with the photosensitive member. When forming a monochrome image, the operation of the imaging units  302   c ,  302   m ,  302   y  can be stopped to eliminate wear on the photosensitive member and the peripheral processing. 
     Signal processing in the image reader  200  is described below. 
     FIGS. 2 and 3 are block diagrams showing the construction of an image processor  20 - 5  in the image reader  200 . FIG. 2 shows the anterior half of the image processor, and FIG. 3 shows the posterior half of the image processor. 
     In these drawings, the document image is converted to R, G, B color-separated electric signals in accordance with the intensity of the light reflected from the surface of the document via the CCD sensor  204 . The reading resolution of the CCD sensor  204  can be switched among 400 dpi, 600 dpi, and 1200 dpi. The AD converter  401  converts the analog signal output from the CCD sensor  204  to 8-bit digital data of 256 gradients for the R, G, B information based on the timing signal output from a standard drive pulse generator  411 . 
     In the shading correction unit  402 , the data obtained by reading the shading correction panel  206  for each color independently are stored in an internal shading memory as standard data to eliminate uneven light in the main scan direction of the R, G, B image data. When scanning a document, correction is accomplished by reciprocal conversion of the standard data and multiplying by the image data. 
     In the line interval correction unit  403 , the image data of each color are delayed in line units using the internal field memory in accordance with the scanning speed to align the reading position in the scan direction of the R, G, B sensor chip. 
     The R, G, B reading phase difference increases near the document edge on the main scan side due to a color aberration phenomenon generated by the optical lens. There is concern that, besides simple color aberration, this influence may produce discrimination errors in black text discrimination and ACS determination described later. This R, G, B phase difference is corrected in the color aberration correction unit  404  based on chroma information. 
     In the variable magnification/moving processor  405 , the a main scan direction variable magnification/movement process is executed by independently controlling the write/read timing to alternate the input/output of each one line using two separate variable magnification line memories for each R, G, B image data. That is, reduction is accomplished by culling data when writing to memory, and enlargement is accomplished by diluting data when reading from memory. In these controls, a complementation process is executed before writing to memory on the reduction side in accordance with variable magnification, and after reading from memory on the enlargement side in accordance with variable magnification, so as to prevent image loss and superscripting. The combination of block control and scan control is performed not only for reduction and enlargement, but also centering, image repeat, enlargement series, bound reduction and the like. 
     In the histogram generator  412  and auto color selector (ACS) unit  413 , brightness data are generated from the R, G, B image data obtained in a preliminary scan prior to operation for copying the document to create a histogram therefrom in memory, and a determination is made as to whether or not each dot is a color dot via the chroma data, and the number of color dots in each dot mesh in all directions of document  512  is generated in memory. Based on these results, a copy background level auto control (AE process) and a color copy operation or monochrome copy operation auto color selection (ACS process) are executed. 
     The line buffer  414  is provided with a memory capable of storing one line of the R, G, B image data read by the image reader  200 , and performs monitoring of image data used for image analysis for auto sensitivity correction of the CCD sensor and auto clamp correction by the AD converter  401 . 
     In the paper currency recognition unit  415 , when paper currency such as negotiable securities are stacked for copying on the document glass  208 , a determination is made as to whether or not the documents are securities via pattern matching whenever R, G, B data are extracted so as to prevent making a normal copy image. When documents are determined to be paper currency, the CPU controlling the reading operation of the image reader  200  and the image processor  205  immediately outputs a solid print signal (-PNT=“L”) to the print image controller, and the print image controller switches the k data to solid so as to prohibit normal copying. 
     In the HVC converter  421 , R, G, B data input via the data selector  422  are once converted to brightness (V data) and color difference signals (Cr, Cb data) via 3×3 matrix calculation. 
     Then, in the AE processor  423 , the V data are corrected based on the aforesaid background level control values, and the Cr, Cb data are corrected in accordance with the chroma level and hue level set on the operation panel. Thereafter, in the inverse HVC converter  421 , the data are reconverted to the R, G, B data by a 3×3 inverse matrix calculation. 
     In the color correction unit  430 , after the R, G, B data are converted to density data (DR, DG, DB data) by the LOG correction unit  431 , the minimum color levels of the DR, DG, DB data are detected as document background components in the black extractor unit  432 , and the gradient level difference of the maximum color and minimum color of the DR., DG, DB data are detected as document chroma data. 
     The DR, DG, DB data are subjected to a 3×6 nonlinear matrix calculation process by the masking calculator  433 , and converted to color data (C, M, Y, K data) matching the color toner of the printer. 
     In the background color removal/black print processor (UCR/BP processor)  434 , the UCR/BP coefficients corresponding to the document chroma data are calculated relative to the aforesaid document undercolor component (Min(R,G,B)), and the amount of UCR/BP is determined by a multiplicative process. The amount of undercolor removal (UCR) is differentiated from the C, M, Y data after the masking calculation, and the C, M, Y data and K data (=amount of BP) are calculated. In the monochrome data generator  435 , the brightness component is generated from the R, G, B data, and subjected to LOG correction, then the black data (DV data) are output. Finally, in the color data selector  436 , the C, M, Y, K data of the color copy image, and the DV data (C, M, Y are white) of the monochrome copy image are selected. 
     In the area discrimination unit  440 , discrimination is performed on each block to determine whether or not the area is a halftone dot image, and if the area is a halftone dot image, to determined whether or not the image is a monochrome image or a color image based on the R, G, B image data input via the data selector  422 . The discrimination result is output as a color halftone signal S 15  or a monochrome halftone signal S 16 . 
     In the discrimination of a halftone dot image, the isolated points SP are detected by applying the isolated point detection filter FD shown in FIG. 1 to the image data. The number of isolated points SP detected within the block area are counted. In this instance, processing is executed to reduce the size of the isolated point SP included in the image data so as to be smaller than the size of the isolated point detection filter FD as shown in FIG.  22 (A) and  22 (B), in accordance with the image data resolution, and the processed image data are subjected to filtering by the isolated point detection filter FD. 
     In the determination as to whether or not the image is a monochrome image or a color image, the difference [Max(R,G,B)−Min(R,G,B)] between the minimum color ([Min(R,G,B)] and the maximum color [Max(R,G,B)] is detected. Text shading correction data are generated when discriminating black text, and the transmitted together with the discrimination result to the image correction unit  451 . At the same time, the attribute signals are generated for switching the gradient reproduction method transmitted to the print image controller and the print head controller. 
     In the image correction unit  451 , the C, M, Y, K data output from the color correction unit  430  are subjected to edge correction, smoothing text edge removal and the like performed on each discrimination area based on the area discrimination result output from the area discrimination unit  440 . Then, the C, M, Y, K data are subjected to mage correction in accordance with the sharpness, color balance, gamma level and the like specified on the operation panel, and the gradient reproduction attribute signals-LOMOS are transmitted to the print image controller interface  453 . The C, M, Y, and K data are transmitted to the image interface unit  462  via the data selector  461 . 
     The image interface unit  462  outputs image data to external devices. Simultaneous Input/output of the R, G, B data and sequential input/output of the R, G, B data are possible via the image interface  462 . An external device may use the scanning function and printing function of the copier  1 . 
     The area discrimination unit  440  is described below. 
     FIG. 4 is a block diagram showing the construction of the area discrimination unit  440 . 
     In FIG. 4, the area discrimination unit  440  comprises a brightness/chroma detection unit  441 , a halftone preprocessor  442 , a halftone dot image discrimination unit  443 , a color/monochrome discrimination unit  444 , and a halftone dot image output unit  445 . 
     The brightness/chroma detection unit  441  detects the brightness V and the chroma W from the input R, G, B data, and outputs the image data S 10  representing the brightness V and image data S 11  representing the chroma W. For example, the average value of the R, G, B data may be determined to obtain the image data S 10 . The difference DF between the maximum color and minimum color of the R, G, B data may be determined to obtain the image data S 11  (DF=[Max(R,G,B)−Min(R,G,B)]). In the case of monochrome images, the difference DF generally approaches zero [0]. 
     The halftone preprocessing unit  442  executes processing to reduce the size of the isolated points SP included in the image data S 10  so as to be smaller than the size of the isolated point detection filter FD in accordance with the resolution of the image data S 10 . 
     The halftone image discrimination unit  443  detects isolated points SP in the image data S 12  output from the halftone preprocessing unit  442  using the isolated point detection filter FD, and counts the number of isolated points SP detected within the block area BE. The image data S 10  are discriminated to determined whether or not the image is a halftone dot image by determining whether or not the number of isolated points SP within the block area BE exceeds a previously set threshold value, and the discrimination result is output as discrimination signal S 13 . 
     The color/monochrome discrimination unit  444  discriminates whether the image is a color image or a monochrome image for each block area BE based on the image data S 10  and S 11 . The discrimination result is output as discrimination signal S 14 . 
     The halftone dot image output unit  445  outputs a color halftone dot image signal S 15  representing the color halftone image, or outputs a monochrome halftone dot image signal S 16  representing a monochrome image based on the discrimination signal S 13  of the halftone dot image discrimination unit and the discrimination signal S 14  of the color/monochrome discrimination unit  444 . In the image correction unit  451 , image processing is performed on the respective discrimination results for each block area BE based on the color halftone signal S 15  and the monochrome halftone dot image signal S 16 . 
     The construction of each part of the area discrimination unit  440  is described below. The first embodiment is described first. 
     First Embodiment 
     FIG. 5 is a block diagram showing the construction of the halftone preprocessing unit  442 A and the halftone image discrimination unit  443 A. 
     In FIG. 5, the halftone preprocessing unit  442 A reduces the isolated points SP via a culling process performed on the image data S 10 . The halftone preprocessing unit  442 A comprises an image culling processor  4421 , a resolution detector  4423 , and a selector  4422 . 
     The image culling processor  4421  performs a culling process on the image data S 10  in accordance with the resolution RS of the image data S 10 , and outputs image data S 10 r. In the culling process, the number of pixels is reduced by culling specific predetermined pixels from the input image data S 10 . For example, when the resolution RS is 600 dpi, ⅓ of the pixels are culled from the image data S 10 , and the image data S 10 r ⅔ the number of pixels are output. The culling process reduces the number of isolated points SP included in the image data S 10  to a size detectable by the isolated point detection filter FD identical to a resolution of 400 dpi. 
     FIG. 6 is a circuit diagram of the image culling processor  4421 , FIG. 7 is a timing chart of the operation performed by the image culling processor  4421 , FIG. 8 shows the reduced condition of the isolated points SP accomplished via the image culling processor  4421 , FIG. 9 shows the isolated point detection filter FD, and FIG. 10 is a circuit diagram of the halftone dot image discrimination circuit  4435 . 
     In FIG. 6, the culling process circuit  44211  sequentially writes the image data S 10  synchronously with a clock signal WCLK input to the write clock pin WCK. The written image data S 10  are output synchronously with a clock signal CLK input to the read clock pin RCK. The clock signal CLK synchronizes the output timing of image data S 10  of each pixel. The clock signal WCLK omits one of three signals, as shown in FIG.  7 . In this way, the only ⅔ of the serially input image data S 10  are written to the culling process circuit  44211 . Accordingly, the image data S 10 r read synchronously with the clock signal CLK have been culled ⅔ from the image data S 10 . The construction and operation of the image culling process circuit  4421  itself are well known. 
     As shown in FIG. 8, when the isolated points SP are larger than the isolated point detection filter FD, the number of isolated points is reduced via the culling process so that the isolated points SP can be detected by the isolated point detection filter FD. 
     The resolution RS is detected by the resolution detector  4423 . The construction and operation of the resolution detector  4423  itself is well known. The selector  4422  selects either the input image data S 10 , or the image data S 10 r culled by the image culling process unit  4421  in accordance with the resolution RS. For example, when the resolution RS is 400 dpi, the image data S 10  input to pin B are selected, and when the resolution RS is 600 dpi, the image data S 10 r input to pin A are selected. 
     The halftone image discrimination unit  443 A comprises a white isolated point detector  4431 , a black isolated point detector  4432 , a white isolated point counter  4433 , a black isolated point counter  4434 , and a halftone dot image discriminator  4435 . 
     The white isolate point detector  4431  detects white isolated points SP using a white isolated point filter FDW. The black isolated point detector  4432  detects black isolated points SP using a black isolated point detection filter FDK. 
     As shown in FIG. 9, the isolated point detection filter FD comprises a 5×5 dot matrix. When this isolated point detection filter FD is used as the white isolated point detection filter FDW, it must be determined whether or not the following conditions are satisfied when the center window V 33  is aligned with the target pixel PXT within the block area BE. 
     V 33 ≧Max(V 22 , V 23 , V 24 , V 32 , V 34 , V 42 , V 43 , V 44 ) 
     V 33 ≧(V 11 +V 22 )/2 
     V 33 ≧(V 13 +V 23 )/2 
     V 33 ≧(V 15 +V 24 )/2 
     V 33 ≧(V 35 +V 34 )/2 
     V 33 ≧(V 55 +V 44 )/2 
     V 33 ≧(V 53 +V 43 )/2 
     V 33 ≧(V 51 +V 42 )/2 
     V 33 ≧(V 31 +V 32 )/2 
     When all the aforesaid conditions are satisfied, the target pixel PXT is determined to be a white isolated point SP. 
     When the isolated point detection filter FD is used a the black isolated point detection filter FDK, the maximum condition is changed to minimum condition, and a determination is made as to whether or not the conditions are satisfied when the direction of the inequality sign is reversed in all instances. 
     The white isolated point counter  4433  counts the number of white isolated points SP. The black isolated point counter  4434  counts the number of black isolated points SP. The halftone dot image discriminator  4435  compares the number of counted isolated points SP with a previously set threshold value, and determines the area is a halftone dot area when the count value exceeds the threshold value, and outputs this result. 
     As shown in FIG. 10, in the halftone dot image discriminator  4435 , the number of white or black isolated points SP is compared to a threshold value Th by comparators  44351  and  44352 . The threshold value Th is set at a threshold value Th 4  for 400 dpi, and a threshold value Th 6  at 600 dpi. The selector  44354  selects one of these threshold values in accordance with the resolution RS. When the number of isolated points SP exceeds the threshold value Th, a signal representing a halftone dot image is output from the comparators  44311  and  44312 , and a discrimination signal S 134  is output from the NOR element  44353 . 
     When the size of the block area BE is 9×4 dots, the threshold value Th used for discrimination is, for example, a value of about 5˜30. For example, [22] is used as the threshold value Th 4 , and a value [9] is used as the threshold value Th 6 . 
     In the example shown in FIG. 10, the number of white isolated points SP and black isolated points SP are separately compared to the threshold value Th. However, the white and black isolated points SP may be added and that total number compared to another threshold value ThA, and a signal representing a halftone image may be output based on the logical sum of the aforesaid comparison result and the separate comparison results. In this instance, the threshold value ThA is a value somewhat larger than the separate white and black threshold value Th. 
     FIG. 11 shows the circuit of the color/monochrome discrimination unit  444 A. 
     In FIG. 11, the comparator  4441  compares the brightness threshold value ThV and the image data S 10 . The comparator  4442  compares the chroma threshold value ThW and the image data S 11 , and a discrimination signal S 14  which is the logical sum of these comparisons is output from the NAND element  4443 . That is, when the image data S 10  is less than the brightness threshold value ThV and the image data S 11  is greater than the chroma threshold value ThW (i.e., when the image data have a predetermined brightness and predetermined chroma), the image data S 10  are discriminated as a color image. 
     In this way, color images having the dim brightness of yellow color or red color are not designated color images. That is, images having a high degree of brightness are not designated color images. Only dark, high density images are designated color images. This designation is because blue and green colors have high density and minimal brightness, whereas red and yellow colors have high density but relatively higher brightness. 
     FIG. 12 shows the circuit of the halftone output unit  445 . 
     In FIG. 12, a color halftone signal S 15  representing a color halftone image is output from the AND element  4451 . A monochrome halftone signal S 16  representing a monochrome halftone image is output from the AND element  4452 . 
     The construction and processes of the image correction unit  451  are described below. 
     FIG. 13 is a block diagram showing the construction of the image correction unit  451 A, FIG. 14 shows the condition when the monochrome halftone dot image is subjected to attenuation processing, and FIG. 15 shows the condition when a smoothing process is performed on data subjected to attenuation processing and data which has not been attenuated. 
     In FIG. 13, C, M, Y, K color data output from the color correction unit  430 , color halftone dot image signal S 15  output from the area discrimination unit  440 , and monochrome halftone dot image signal S 16  are input to the image correction unit  451 A. These color data are subjected to attenuation processing via attenuation processors  4511   c, m, y, k , and the selectors  4512   c, m, y, k  select the attenuation processed data or unprocessed data. In this selection, the attenuation processed data are selected for black (K) when the color halftone dot image signal; S 15  is active, and the and the attenuation processed data are selected for C, M, Y when the monochrome halftone signal S 16  is active. 
     In this way, only the K component is reduced for color halftone images, and only the C, M, Y components are reduced for monochrome halftone dot images. 
     As shown in FIG. 14A, the K component is large and the C, M, Y components are about half that of K as the black halftone dot Mk in monochrome halftone dot images. Since attenuation processing is performed on the other C, M, Y components of the black halftone dot Mk, the K component is unchanged, but the C, M, Y components are smaller as shown in FIG.  14 B. 
     Thereafter, the smoothing process is performed on the C, M, Y, K components to eliminate moire via the smoothing processors  4513   c, m, y, k . When either the color halftone signal S 15  or the monochrome halftone signal S 16  is active, the output of the NOR element  4515  is active. As a result, the smoothed data are selected by the selectors  4514   c, m, y, k , and output to the print image control interface  453 . In this way the smoothed image data are output for halftone dot images. 
     As shown in FIG. 15A, when the smoothing process is performed on data not subjected to the attenuation process, the C, M, Y components extends markedly beyond the edge of the pixel of the black halftone dot Mk. As a result, color bleed occurs in the edge region GG. Conversely, when the attenuated data are subjected to the smoothing process, color bleed does not occur due to the very slight extension from the edge. 
     In this way, the halftone dot image is discriminated by distinguishing between the color halftone dots and monochrome halftone dots, and subjecting the respective halftone dot images to suitable image processing so as to suppress color bleed and muddiness, thereby improving the reproducibility of the halftone dot image. 
     Second Embodiment 
     A second embodiment of the area discrimination unit  440  is described below. 
     FIG. 16 is a block diagram showing the construction of the halftone dot preprocessing unit  442 B, and FIG. 17 shows the size of the black isolated points SP before and after the opening process. 
     In FIG. 16, the halftone dot preprocessing unit  442 B reduces the isolated points SP by subjecting the image data S 10  to an opening process. The halftone preprocessing unit  442 B comprises a 3×3 opening process minimum filter  4424  and maximum filter  4425 , and selector  4426 , and resolution detector  4423 . The resolution detector  4423  is identical to the resolution detector of the first embodiment. 
     The minimum filter  4424  processes the white isolated points SP, and the maximum filter  4425  processes the black isolated points SP. The respective isolated points SP are reduced by the application of the minimum filter  4424  and the maximum filter  4425 . Since the isolated points SP are refined by the opening process, the opening process is referred to as a “thinning process.” 
     For example, the maximum filter  4425  executes a process to set the maximum value of the nine pixels subjected to the maximum filter  4425  as the target pixel PXT. That is, a process is executed pursuant to the following equation. 
     
       
           b   22 =Max( b   11 ,  b   12 ,  b l 3 ,  b   21 ,  b   22 ,  b   23 ,  b   31 ,  b   32 ,  b   33 ) 
       
     
     FIG. 17A shows image data before the application of the maximum filter  4425 . The large value area is white, and the small value area is black. When areas having a value less than  100  are designated black, the black isolated points SP the black isolated points SP become the size indicated in the drawing, and is larger than the isolated point detection filter FD. When the image data are subjected to the opening process using the maximum filter  4425 , the image data value become larger overall as shown in FIG.  17 B. As a result, the black isolated points SP are reduced, and become detectable by the isolated point detection filter FD. 
     When the resolution RS is 400 dpi, the selector  4426  directly outputs the image data S 10 . When the resolution RS is 600 dpi, the selector  4426  outputs the image data treated by the opening process. Accordingly, even at a resolution RS of 600 dpi, the isolated points SP can be detected by the isolated point detection filter FD, and accurate discrimination is accomplished in the halftone image discrimination unit  443 . 
     Moreover, it is unnecessary to modify the process content and algorithm in the halftone image discrimination unit  443  even at high resolution RS, thereby providing for the general usability of the halftone image discrimination unit  443 . 
     Third Embodiment 
     A third embodiment of the area discrimination unit  440  is described below. 
     FIG. 18 is a block diagram showing the construction of the halftone dot preprocessor  442 C, and FIG. 19 shows the filter  4429   a.    
     In FIG. 18, the halftone dot preprocessor  442 C reduces the isolated points SP by subjecting the image data S 10  to a filtering process. The halftone dot preprocessor  442 C comprises 5×5 filter  4429   a  and  4429   b , a CPU  4427 , a register  4428 , and a resolution detector  4423 . The resolution detector  4423  is identical to the resolution detector of the first embodiment. 
     The coefficients and process contents of the filters  4429   a  and  4429   b  are set in the register  4428  via a process executed by the CPU  4427 , and processing is executed in accordance with the set contents. The filters  4429   a  and  4429   b  operate as Laplacian filters or opening filters in accordance with the contents set in the register  4428 . The size of the filters is variable. 
     For example, the filter  4429   a  may be a minimum filter used for the opening process, and have a size of 5×5, 3×3 or the like. 
     When the filter  4429   a  is a 5×5 minimum filter as shown in FIG. 19, the value of the target pixel PXT satisfies the following equation. 
     
       
           d   33 =Min( d   11 ,  d   12 ,  d   13 ,  d   14 ,  d   15 ,  d   21 ,  d   22 ,  d   23 ,  d   24 ,  d   25 ,  d   31 ,  d   32 ,  d   33 ,  d   34 ,  d   35 ,  d   41 ,  d   42 ,  d   43 ,  d   44 ,  d   45 ,  d   51 ,  d   52 ,  d   53 ,  d   54 ,  d   55 ) 
       
     
     That is, the value of the target pixel PXT is the minimum value among the 25 (5×5 ) pixels. When the filter  4429   a  is a 3×3 minimum filter as shown in FIG. 19, the value of the target pixel PXT satisfies the following equation. 
     
       
           d   33 =Min( d   22 ,  d   23 ,  d   24 ,  d   32 ,  d   33 ,  d   34 ,  d   42 ,  d   43 ,  d   44 ) 
       
     
     That is the value of the target pixel PXT is the minimum value among the 9 (3×3 ) pixels. 
     In this way, the CPU  4427  sets the various coefficients in the register  4428  by executing a program, so as to variously modify the type, size, and process content of the filters  4429   a  and  4429   b . Accordingly, the memory capacity required for processing is reduced by reducing the size as much as possible, thereby realizing lower costs. 
     The halftone dot preprocessor  442 C of the third embodiment executes pa process to reduce the isolated points SP when the resolution RS is 600 dpi. As a result, the isolated points SP can be detected by the isolated point detection filter FD, and the halftone dot image discrimination unit  443  performs accurate discrimination. 
     Fourth Embodiment 
     A fourth embodiment of the area discrimination unit  440  is described below. 
     FIG. 20 shows the construction of the halftone dot preprocessor  442 D and the color/monochrome discrimination unit  444 B. 
     In FIG. 20 the halftone dot preprocessor  442 D uses the same image culling processor  4421  and selector  4422  shown in FIG.  5 . In the halftone image discrimination unit  443 A, four line memories  4436  are used, and the isolated point detection filter FD is applied to five lines simultaneously. A signal WAMI representing a white isolated point, or a signal KAMI representing a black isolated point, is output from the white isolated point detector  4431  and the black isolated point detector  4432 , respectively. This signal is input to the previously mentioned white isolated point counter  4433  or the black isolated point counter  4434 . 
     In the color/monochrome discrimination unit  444 B, a discrimination signal S 14  representing a color area is output via the comparators  4441  and  4442 , and the NAND element  4443 . The black threshold table  4444  is applied to the image data S 11  representing brightness, and this output is compared to the image data S 11  representing chroma via the comparator  4445 , and a discrimination signal S 21  representing a black area is output. 
     In the edge detector  446 , various processes are performed to accomplish edge detection using the negative-positive inverter  4461  for inverting negative-to-positive as necessary, a line memory  4462 , a primary micro filter  4463 , a secondary micro filter  4464 , a Laplacian filter  4465 , and an internal edge detection filter  4466 . 
     In the previously described embodiments, the halftone dot preprocessor  442 , halftone dot image discrimination unit  443 , color/monochrome discrimination unit  444 , halftone dot output unit  445 , area discrimination unit  440 , and image processing unit  205  may be modified entirely or in part, including construction, process content, processing sequence and the like insofar as such modification does not depart from the scope of the present invention. The present invention may be applied to various apparatuses other than a copier. 
     According to the previously described embodiments, halftone dot image discrimination can be accurately accomplished at various resolutions when the resolution increases and when the type of resolution increases. In this way cost increases can be suppressed as much as possible. 
     According to the aforesaid embodiments, when subjecting a halftone dot image to image processing, monochrome and color halftone dot images can be processed by the execution of respectively suitable processes, thereby preventing color bleed and muddiness. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modification will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.