Abstract:
A half-tone image is produced by scanning a plurality of input points of an original, outputting a screened image having a plurality of black and white dots and transmitting signals representing the screened image to a marking device. The size of each plurality of dots is determined from a recursive relationship between a value of a current input point, a previous output and an error representing a difference between a value of a previous input point and the previous output.

Description:
This application is a division of application Ser. No. 07/875,210, filed Apr. 28, 1992, now U.S. Pat. No. 5,325,448 which is a continuation of application Ser. No. 07/270,809, filed Nov. 14, 1988, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image treatment method and apparatus for treating images with digital signals. More particularly, the present invention relates to an image treatment method and apparatus for expressing halftone images in a pseudo-manner by performing a quantization treatment of input data. 
     2. Related Background Art 
     Apparatuses of the type described, above such as laser beam printers (LBP) and ink jet type printers employing a binary recording method in which recording dots are treated as “printed or not” are known. When a copying treatment of images of half-tone density, such as photographs or half-tone dot original documents, is performed with a copying machine which employs the above-described binary recording method, a method is employed in which a treatment for expressing a half-tone in a pseudo-manner is used, the read-out half-tone image data being treated using an image treatment circuit. 
     As an example of the method of treatment of the above-described type of pseudo-half-tone treatment, there is, at present, a so-called “dither method” which is the method widely used. 
     This dither method has an advantage that the above described type of pseudo-half-tone processing can be performed with a low cost since the structure of the hardware is simple. However, this method raises the following problems: 
     (1) In the case where the original document is a dot image such as a print, the quality of the image deteriorates due to generation of periodical fringes (moiré patterns) in the copied image; new 
     (2) In the case where the original document contains line drawings and/or characters, sufficient reproductivity of the lines cannot be obtained, and the quality of the image deteriorates. 
     There is a method of overcoming problem (1) by performing a smoothing treatment (spatial filtering treatment) upon the read-out half-tone image data. Furthermore, there is a method of overcoming problem (2) by performing an edge exaggerating treatment. However, with the above-described methods, it is difficult to obtain images exhibiting a sufficient productivity upon all various images such as photographs, dot images, line drawings, and characters. Furthermore, the size of the circuit for performing the above-described treatment is larger. Therefore, the original advantages of the dither method can deteriorate. 
     To improve on this, as an example of a pseudo-half-tone treatment, there is a so-called “error diffusion method” which has recently attracted public attention. 
     This error diffusion method is a method in which the error in the density, which is generated when the input image data is binarized, between that of the input (not yet binarized) image data and that of the output (binarized) image data is diffused to the peripheral picture elements whereby the density can be secured. This method was published in “An Adaptive Algorithm for Spatial Grey Scale,” SID 75 Digest, literature by R. W. Floyd and L. Steinberg. 
     This error diffusion method exhibits a rather improved gradating performance and resolution with respect to the above-described dither method. However, this method raises problems that a specific fringe pattern can be generated in a portion where the density of the image is uniform, and/or granular noise can appear due to generation of dots in a diffused manner in highlight portions of the image. 
     In order to overcome these problems, a variety of methods have been disclosed in U.S. patent application Ser. Nos. 137,439, 140,029 and 192,601 and in U.S. Pat. Nos. 5,008,950, 4,876,610 and 4,878,125. 
     Furthermore, a method is disclosed in U.S. patent application Ser. No. 203,880, in which the generation of lines at the boundary portions of the images is prevented when the image is divided into a plurality of regions and the thus-divided regions are each quantization-treated in the error diffusion method. 
     On the other hand, when an original image is read and it is binarized by the error diffusion method so as to be output by a printer, there is a problem that there is a blank area in which no dot is printed as shown in FIG. 15 if the density of the original image is in a low level. Furthermore, in the region next to such blank region, dots are, as shown in FIG. 15, printed successively. As described above, the reproduced image output after treatment using the error diffusion method raises the problem that excessive deterioration in the quality of image is generated in the highlight portion in which the density of the image is in a low level. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image treatment method and apparatus capable of overcoming the problems experienced with the conventional methods and apparatuses and further capable of reproducing the image exhibiting high grade and excellent accuracy. 
     A further object of the present invention is to provide an image treatment method and apparatus capable of reproducing the highlight portion of the image well when the input data is treated using an error diffusion method. 
     Another object of the present invention is to provide an image treatment method and apparatus capable of reproducing an image exhibiting an excellent quality with a simple structure. 
     A still further object of the present invention is to provide an image treatment method and apparatus in which a reference value is determined at the time of quantizing image data and the image data is quantized using an error diffusion method depending upon the thus-determined referential value. 
     The other object of the present invention is to provide an image treatment method and apparatus wherein whether there is a dot printed in the region in which treatment has been performed in the periphery of the subject picture element or not is determined, and this subject picture element is quantized using an error diffusion method in accordance with the result of the determination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram for use in a first embodiment of the present invention; 
     FIG. 2 is a block diagram illustrating in detail a threshold value setting circuit  4 ; 
     FIG. 3 is a block diagram illustrating in detail a binarization circuit  5 ; 
     FIG. 4 is a view illustrating an example of a weight coefficient; 
     FIG. 5 is a block diagram illustrating in detail a judgment circuit  6 ; 
     FIG. 6 is a block diagram in a case where the judgment circuit  6  is changed; 
     FIG. 7 is a block diagram for use in a case where the present invention is applied to a multi-value treatment; 
     FIG. 8 is a block diagram according to a second embodiment of the present invention; 
     FIG. 9A is a block diagram illustrating in detail a threshold value setting circuit  104 ; 
     FIG. 9B is a view illustrating a ROM table stored in a ROM  108 ; 
     FIG. 10 is a block diagram illustrating in detail a binarization circuit  105 ; 
     FIG. 11 is a block diagram illustrating in detail a judgment circuit  106 ; 
     FIG. 12 is a block diagram illustrating a case where the threshold setting circuit  104  is changed; 
     FIG. 13 illustrates the relationship between a signal  1100  and a signal  1410 ; 
     FIG. 14 is a block diagram used in a case where the judgment circuit  106  is changed; and 
     FIG. 15 is a view illustrating the problem experienced with the conventional treatment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First, the cause for deterioration in image quality in the highlight portion of the image described in the prior art will be described prior to the description of embodiments of the present invention. 
     The causes for the printing of extraneous dots in a region in which the density of the image is low can be considered to be as follows: 
     The error diffusion method is a method of performing a successive quantization of input images by diffusing the quantization error between the density of the input image and the same of the output image to the as yet non-treated picture elements. However, when the portion in which the density of the image is in a low level is binarized in the error diffusion method, the positive error gathered in the subject picture element can be made in a low level since the position error is diffused, at the time of binarization, to the periphery portion is in the low level. As a result of this, it is difficult to make the density of the subject picture element exceed the threshold value (when the input image is 8 bits=256 levels, the threshold value is usually 127) in the binarization. As a result of this, a portion in which no dot is printed can be generated, as shown in FIG.  15 . 
     Furthermore, the cause of generation of the successive dots can be considered as follows. The errors generated in the blank areas shown in FIG.  15  and generated at the time of binarization become positive values since the density of the input image is low. Furthermore, since these positive errors are diffused to the region next to the blank area, dense generation of the dots occurs in this area. 
     Then, an embodiment of the present invention capable of preventing the dot blank phenomenon in the highlight portions of the image due to the above-described cause or the phenomenon in which dots are successively generated, will be described. 
     First Embodiment 
     A first embodiment of the present invention will now be described with reference to the accompanying drawings. 
     FIG. 1 is a block diagram of circuitry used in an image treatment apparatus according to a first embodiment of the present invention. 
     Image data read out by an input device  1  comprising a phototransducing element such as a CCD and a driving system for scanning the same are successively supplied to an A/D converter  2 . In this A/D converter  2 , data upon each of the picture elements are converted to, for example, digital data of 8 bits. As a result of this, the image data is quantized to 256-gradations data. Next, a correction such as shading correction for correcting for nonuniform sensitivity of the sensor or for nonuniform illuminance due to the power source for illumination is performed in a correction circuit  3  in a digital calculation treatment. Then, a signal  100  which has been subjected to the above-described correction is input to a binarization circuit  5  and a judgment circuit  6 . In a threshold value setting circuit  4 , the threshold for realizing the binarization is set in response to a judgment signal  400  output from the judgment circuit  6  so that a threshold signal  200  is output. In the binarization circuit  5 , the corrected signal  100  output from the correction circuit  3  is binarized by the threshold value signal  200  output from the threshold value setting circuit  4  so that a binary signal  300  is output therefrom. In the judgment circuit  6 , by using the binary signal  300  output from the binarization circuit  5  and the corrected signal  100  output from the correction circuit  3 , the binarized regions in the periphery of the subject picture element to be binarized are made reference for the purpose of performing a determination whether there is a dot which has been turned on or not. As a result of this, a judgment signal  400  is output. An output device  7  comprises a laser beam printer or ink jet printer and performs an image formation of the binary signal  300  output from the binarization circuit  5  by way of turning on/off of the dots. 
     FIG. 2 is a block diagram illustrating in detail the threshold value setting circuit  4 . 
     The judgment signal  400  output from the judgment circuit  6  is input to a selector  8 . The selector  8 , in response to the judgment signal  400 , selects a threshold value T1=300 when the judgment signal  400  is “1”, while the same selects a threshold value of T2=127 when the judgment signal  400  is “0” so that the threshold signal  200  is output. 
     When the judgment signal  400  is “1”, the threshold T1=300 is selected since it is a highlight portion of the image and furthermore a dot is present in the peripheral portion of the subject picture element so that the successive printing of the dots is prevented. 
     Although T1=300 in this case, the level of the value T1 only needs to exceed the maximum value of the corrected signal  100 . Furthermore, while T2=127 in this case, it can be set to another value. 
     FIG. 3 is a block diagram illustrating in detail the binarization circuit  5 . The corrected signal  100  (the density of the subject picture element) output from the correction circuit  3  is added to the error E ij  (the total sum of the errors distributed from the peripheral picture elements to the subject picture element) stored in an error buffer memory  10  by an adder  9 . As a result of this, an error corrected signal  210  is output. 
     The error corrected signal  210  is input to a comparator  11  wherein it is compared with the threshold signal  200 . If the error corrects signal  210  is greater than the threshold signal  200 , “1” is output, while, the same is smaller than the threshold signal  200 , “0” is output as the binary signal  300 . on the other hand, in a converter  12 , if the thus-input binary signal  300  is “0”, the value as it is, is output, while if the same is “1”, the value converted to “Dmax” is output as a signal  220 . The value  210  and the values  220  are input to a calculator  13 . In this calculator  13 , the difference between these two signals is calculated and its result is output as a signal  230  (ΔE ij ). This signal  230  is input to an weighing circuit  14 , wherein an weighing (a k1 ) is performed. Next, it is added to an error upon the picture element at a predetermined position in the error buffer. FIG. 4 illustrates an example of weighing coefficients (a k1 ), wherein the symbol * corresponds to the position of the subject picture element (I, J). By repeating the above-described operation, the binarization with the error diffusion method is performed. In this embodiment, since the corrected signal  100  is treated in an 8 bits manner: 
     
       
         Dmax−255 
       
     
     However, if the corrected signal  100  is treated in an m bits manner: 
     
       
         Dmax−2 m−1 +2 m−2 +2 2 . . . +2 0   
       
     
     FIG. 5 illustrates in detail a block diagram for use as the judgment circuit  6 . 
     The binary signal  300  output from the binarization circuit  5  is latched immediately when the same is input to a line buffer  17 . The signal read out from the line buffer  17  is also latched immediately when the same is input to a line buffer  16 . That is, assuming that the position of the subject picture element to be now treated is (I, J), the binarized data picture elements at 12 peripheral positions are latched, the 12 positions being (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I−2, J−1), (I−1, J−1), (I, J−1), (I+1, J−1), (I+2, J−1), (I−2, J), and (I−1, J). The thus-latched data for the 12 picture elements are input to the OR circuit  18 , wherein the ‘OR’ of the data for the 12 picture elements are calculated, and the results are output as a signal  520 . 
     The corrected signal  100  is input to the comparator  15  wherein it is compared with the threshold D=20, wherein if the signal  100  is greater than the threshold D, “1” is, while the same is smaller than the threshold D, “0”, is output as a signal  510 . 
     As a result of this, the density of the image can be determined. 
     A selector  19 , in accordance with the value of the signal  510 , outputs a signal  520  if the signal  510  is “0”, while the same outputs a signal  530  if the signal  510  is “1” as a signal  400 . However, the value of the signal  530  is “0”. 
     That is, in a case where the picture element whose density of the image is in a low level, the binarized data in the periphery portion of the subject picture element is examined, and if there is a signal for making the dot turn on, the signal  520  becomes “1”. Furthermore, since the threshold T1=300 is selected in the threshold setting circuit, the dot of the subject picture element necessarily is turned off. In this state, if there is no signal which makes the dot turn in the periphery portion of the subject picture element, the signal  520  becomes “0”. Therefore, the threshold T2=127 is selected in the threshold value setting circuit  4  so that the binarization treatment is performed. 
     In a case where the picture element whose density of image is high, since the signal  530  is selected by the selector  19 , the threshold T2=127 is selected in the threshold setting circuit  4  so that the binarization treatment is performed. 
     As a of the examination of the binarized data in the peripheral portion of the subject element with the structure described above, if there is a dot in the periphery, in a portion whose density of image is low, the dot is forcedly turned off. Consequently, a phenomenon that dots are printed in closely positioned manner can be prevented. 
     FIG. 6 is a block diagram illustrating a case where the judgment circuit  5  described in the aforesaid embodiment is changed. 
     The binarization signal  300  is latched immediately when the same is input to a line buffer  20 . The signal read out from the line buffer  20  is also immediately latched when the same is input to a line buffer  21 . That is, assuming that the position of the subject picture element to be now treated is (I, J), the binarized data picture elements at 12 peripheral positions are latched, the 12 positions being (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I−2, J−1), (I−1, J−1), (I, J−1), (I+1, J−1), (I+2, J−1), (I−2, J), and (I−1, J). 
     In an OR circuit  22 , the ‘OR’ of the binarized data for 4 picture elements at the positions of the picture elements (I−1, J−1), (I, J−1), (I+1, J−1), and (I−1, J) are calculated, and as a result of this, a signal  630  is output. 
     In an OR circuit  23 , the ‘OR’ of the binarized data for 8 picture elements at the positions of the picture elements (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I−2, J−1), (I+2, J−1), and (I−2, J) are calculated, and as a result of this, a signal  620  is output. 
     In an LUT  24 , a switch signal  610  set in three levels corresponding to the input corrected signal  100  is output. The switch signal  610  is set to “1” when the corrected signal  100  is 20 or less, is set to “2” when the same is 21 or more and 50 or less, and is set to “0” when the same is 51 or more. 
     A selective OR circuit  25  outputs, in response to a selection signal  610  output from the LUT  24 , “0” as a judgment signal  400  when the selection signal  610  is “0”, outputs the ‘OR’ of the signal  620  and the signal  630  when the same is “1”, and outputs the signal  630  when the same is “2”. For example, the corrected signal  100  is  18 , the switch signal  610  become “1”, and in the state if the signal  620  is “1” and the signal  630  is “0”, the judgement signal  400  becomes “1”. 
     In this embodiment, the region to be made a reference with respect to the value of the corrected signal  100  is set to three stages (that is, the three stages that the periphery of the subject picture element is not examined, that the peripheral four picture elements are examined, and that the peripheral 12 picture elements are examined). As a result of this, since the range of peripheral regions to be examined is enlarged as the density of the image becomes lower, the dots can be diffused in accordance with the density. As a result of this, closely positioned printing of dots performed in the portion whose density is low can be prevented. 
     By increase in the number of the line buffers, the latches and the OR circuits at need, the regions to be made the reference can be made a multistage. An example in which the region is set to four stages will now be described. 
     It is assumed that the position of the subject picture element to be now treated is (I, J) and there are line buffers and latches needed for maintaining the binarized data for picture elements at 24 positions, the 24 positions being: (I−3, J−3), (I−2, J−3), (I−1, J−3), (I, J−3), (I+1, J−3), (I+2, J−3), (I+3, J−3), (I−3, J−2), (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I+3, J−2), (I−3, J−1), (I−2, J−1), (I−1, J−1), (I, J−1), (I+1, J−1), (I+2, J−1), (I+3, J−1), (I−3, J), (I−2, J), and (I−1, J). 
     Furthermore, it is assumed that three OR circuits (a, b, c) and one selective OR circuit (d) are included. In the OR circuit a, the ‘OR’ of the binarized data upon four picture elements at the positions (I−1, J−1), (I, J−1), (I+1, J−1), and (I−1, J) are calculated. As a result of this, a signal e is output. In the OR circuit b, the ‘OR’ of the binarized data upon the eight picture elements at the positions (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I−2, J−1), (I+2, J−1), and (I−2, J) are calculated. As a result of this, a signal f is output. In the OR circuit c, the ‘OR’ of the binarized data upon 12 picture elements at the positions (I−3, J−3), (I−2, J−3), (I−1, J−3), (I, J−3), (I+1, J−3), (I+2, J−3), (I+3, J−3), (I−3, J−2), (I+3, J−2), (I−3, J−1), (I+3, J−1), and (I−3, J). As a result of this, a signal g is output. In the selective OR circuit d: if the corrected signal  100  is 10 or less, the result of the ‘OR’ of the signal e, the signal f, and the signal g; if the corrected signal  100  is 11 or more and simultaneously 20 or less, the result of the ‘OR’ of the signal e and the signal f; if the corrected signal  100  is 21 or more and 50 or less, the result of the ‘OR’ of the signal e; and if the corrected signal  100  is 51 or more, “0”, may be output as the judgment signal. In this embodiment the level of the corrected signal  100  is set to four stage; 10 or less, 11 or more and 20 or less, 21 or more and 50 or less, and 51 or more. However, this is described only as an example. 
     Furthermore, in a case where the color image, it can be realized in such a manner that a predetermined number of the circuits described in this embodiment may be provided corresponding to the number of the colors. 
     FIG. 7 is a block diagram in a case where the embodiment shown in FIG. 1 is applied to a ternarization treatment. 
     The input sensor  1 , the A/D converter  2 , and the correction circuit  3  are the same as those shown in FIG.  1 . The corrected signal  100  is input to a ternarization circuit  27  and a judgment circuit  28 . 
     In a threshold value setting circuit  26 , a threshold for ternarization is set by judgment signal  750   760  output from the judgment circuit  28 . For example, if the judgment signal  750  is “0”,  80  is output, while if the same is “1”,  160  is output as a threshold signal  710 . If the judgment signal  760  is “0”,  160  is output, while if the same is “1”,  300  is output as a threshold signal  720 . This threshold value setting circuit  26  can be realized by two circuits such as that shown in FIG.  2 . 
     In the ternarization circuit  27 , the corrected signal  100  output from the correction circuit  3  is ternarized by the threshold signals  710  and  720  output from the threshold value setting circuit  26  so that signals  730  and  740  are output. For example, if the signal  100  is smaller than the signal  710 , both signals  730  and  740  are set to “0”, and if the signal  100  is greater than the signal  710  and smaller than the signal  720 , the signal  730  is set to “1”. Furthermore, if the signal  100  is greater than the signal  720 , the signal  710  is set to “0”, and the signal  740  is set to “1”. The ternarization treatment in this state is performed with an error diffusion method in which the difference between the density of the output image and that of the input image is diffused to the peripheral picture elements. 
     The judgment circuit  28  can be realized by two of the circuits shown in FIG. 5 or FIG.  6 . In this state, by the signal output from the ternarization circuit  27  and the corrected signal  100  output from the correction circuit  3 , the ternarized region in the periphery of the subject picture element to be ternarized is made reference, and whether there is a printed dot in this region or not is judged. As a result of this, judgment signal  750  and  760  are output. For example, if the signal  100  is 20 or less, the presence of the dot in the treated region in the periphery of the subject picture element is examined in the circuit to which the signal  730  is input. The result of this is output as the signal  750 . In this state, the signal  760  becomes “0”. If the signal  100  is 128 or more and 150 or less, the presence of the dot in the treated region in the periphery of the subject picture element is examined in the circuit to which the signal  740  is input. The result of this is output as the signal  760 . In this state, the signal  750  becomes “0”. If the signal  100  is 21 or more and less than 128, or the same is 150 or more, both signals  750  and  760  become “0”. 
     An outputting device  29  comprises a laser beam printer or an ink jet printer, wherein an image formation is performed by the signal  730  and  740  output from the ternarization circuit  27 . 
     Although the ternarization circuit is described in this embodiment, if (N−1) of the threshold value setting circuit shown in FIG.  2  and the same number of the judgment circuit shown in FIG. 6 are used, the present invention can be applied to an N-value treatment. 
     A color image can be realized by providing the circuits shown in the above-described embodiments in numbers corresponding to the predetermined number of colors. 
     As described above, according to this embodiment, the presence of the dot in the periphery of the subject picture element is determined at the time of performing N-value treatment so that the binarization (N-value) treatment is performed. As a result of this, the problem experienced with the error diffusion method, the close positioned printing of dots, can be prevented. 
     Furthermore, by changing the binarized regions to be made the reference in accordance with the density of the images, dots can be printed in accordance with the density of the image so that the quality of the image can be improved. 
     Furthermore, according to the present invention, the white noise generated due to the fact that no dot is printed in the region in which the density of the image is high, can be prevented. In this case, the structure may be constituted in such manner that a dot is arranged to be printed in the subject picture element if a dot is not printed in the referential region, while if dots are completely printed, it is binarized at the usual threshold. 
     As described above, according to the present invention, he presence of the dot printed in the treated region in the periphery of the subject picture element is determined and the subject picture element is quantized in accordance with the result of the judgment. Therefore, the phenomenon of closely positioned dots printed in the region in which the image density is low can be prevented. Consequently the quality of the image can be improved. 
     Next, as a second embodiment, an embodiment capable of, in addition to the effect obtained in the first embodiment, preventing blanking in which no dot is printed in the portions in which the image density is low will be described. 
     Second Embodiment 
     According to the accompanying drawings, a second embodiment of the present invention will now be described. 
     FIG. 8 is a block diagram for use in an image treatment device according to this embodiment. 
     Image data read out by an input device  101  comprising a photo-transducing element such as a CCD and a driving system for scanning the same are successively supplied to an A/D converter  102 . In this A/D converter  102 , data upon each of the picture elements are converted to, for example, digital data of 8 bits. As a result of this, the image data is quantized to 256-gradations data. Next, a correction such as shading correction for correcting for nonuniform sensitivity of the sensor of for nonuniform illuminance due to the power source for illumination is performed in a correction circuit  3  in a digital calculation treatment. Then, a signal  1100  which has been subjected to the above-described correction is input to a threshold value setting circuit  104 , a binarization circuit  105  and a judgment circuit  106 . In a threshold value setting circuit  104 , the threshold for realizing the binarization is set in response to a judgment signal  1400  output from the judgment circuit  106  and the corrected signal  1100  output from the correction circuit  103  so that a threshold signal  1200  is output. In the binarization circuit  105 , the corrected signal  1100  output from the correction circuit  103  is binarized by a threshold value signal  1200  output from the threshold value setting circuit  104  so that a binary signal  1300  is output therefrom. In the judgment circuit  106 , by using the binary signal  1300  output from the binarization circuit  105  and the corrected signal  1100  output from the correction circuit  103 , the binarized regions in the periphery of the subject picture element to be binarized are made a reference for the purpose of performing a determination whether there is a dot which has been turned on or not is made. As a result of this, a judgment signal  1400  is output. An output device  107  comprises a laser beam printer or ink jet printer and performing an image formation of the binary signal  1300  output from the binarization circuit  105  by way of turning on/off of the dots. 
     FIGS. 9A is a block diagram illustrating in detail the threshold value setting circuit  104 . 
     The judgment signal  1400  output from the judgment circuit  106  and the corrected signal  1100  output from the correction circuit  103  are input to ROM  108 . The ROM  108  outputs as a signal  1100 : “0” when the judgment signal  1400  is “0” and the signal  110  is 1 more and less than 5; “1” when the judgment signal  1400  is “0” and the signal  1100  is 5 or more and less than 15; “2” when the judgment signal  1400  is “0” and the signal  1100  is 15 or more and less than 30; “3” when the judgment signal  1400  is “0” and the signal  1100  is 30 or more; and “4” regardless of the value of the signal  1100  when the judgment signal  1400  is “1”. 
     FIG. 9B illustrates a ROM table stored in the ROM  108 , wherein the above-described signal  1100  is accessed in accordance with the signal  1110  and the signal  1400 . 
     The signal  1110  output from the ROM  108  is input to a selector  112  wherein, in accordance with the value of the signal  1110 : when the signal  1110  is “0” a signal  1120  from RAM  109 ; when the signal  1110  is “1” a signal  1130  from a RAM  110 ; when the signals  1110  is “2” a signal  1140  from a RAM  111 ; when the signal  1110  is “3” a signal  1150 ; when the signal  1110  is “4” a signal  1160 , is selected, and is output as a threshold signal  1200 . In the RAM  109 , a uniform random number column (integer) of 20 or more and 230 or less is stored, while in the RAM  110 , a uniform random number column (integer) of 50 or more and 200 or less is stored, and while in the RAM  111 , a uniform random number column (integer) of 100 or more and 150 or less is stored. In this state, the signal  1150  is arranged to be 127, while the signal  1160  is arranged to be 255. 
     The threshold stored in the RAM  109  is selected when the density of the signal  1100  is low. That is, since the threshold stored in the RAM  109  includes small valued ones, a dot can be output if the density of the signal  1100  is low. 
     The judgment signal  1400  is a signal representing whether the dot is present in the region in the peripheral portion of the binarized subject picture element. If this judgment signal  1400  is 1, a dot is present in the peripheral portion of the subject picture element. Therefore, the signal  1160  (“255”) is selected as the threshold so as not to let any dot appear in the binarization of the subject picture element. The signal  1160  is, to be described later, selected only in the highlight portion of the image. Therefore, in the highlight portion of the image, successive printing of dots can be prevented. 
     In this embodiment shown in FIG. 9A, a three staged uniform random number column is used as the threshold by using three RAMs. However, the number of the RAM may be increased so as to use the multistage uniform random column. In this case, it is preferable to widen the region in which the random number is generated in the portions in which the density of the image is low, and the higher then density becomes, the narrower the region in which the random number is generated. The signal  1160  only needs to be a value above 255. 
     In the portion whose density is 0, in order to prevent generation of the dots, the stationary threshold (for example “127”) is made the threshold signal  1200  if the density is 0. 
     As a result of this, the dot generated, for example, in the background of the characters can be prevented. 
     By lowering the threshold for binarization in a certain probability in the portion whose density is low level, the blanking in which no dot is printed and generated in the portion whose image density is low can be prevented. Furthermore, by controlling the size of the threshold in accordance with the image density, the deterioration in the character portion can be prevented and the smoothness of the image can be secured. 
     Furthermore, since a random number is used as the threshold, the uniforming of the portion whose image density is in an allowable level after the binarization an be improved. 
     FIG. 10 is a block diagram illustrating in detail the binarization circuit  5 . The corrected signal  1100  (the density of the subject picture element) output from the correction circuit  3  is added to an error E ij  (the total sum of the errors distributed to the subject picture elements) stored in an error buffer memory  114  by an adder  113 . As a result, an error corrected signal  1210  is output. 
     Next, the error corrected signal  1210  is input to a comparator  115  wherein it is compared with a threshold signal  1200  output from the threshold value setting circuit  104 . If the error corrected signal  1210  is greater than the threshold signal  1200 , “1” is output, while if it is smaller than the same “0” is output as the binary signal  1300 . 
     On the other hand, a converter  116  outputs the “0”as it is when the input binary signal  1300  is the same, while if it is “1”, the value converted to “Dmax” is output as the signal  1220 . The signal  1210  an the signal  1220  are input to a calculator  117 , wherein the difference between the two signals is calculated and is output as a signal  1230  (ΔE ij ). This signal  1230  is input to a weighting circuit  118  wherein weighting is performed. Next, it is added to the error at the picture element at a predetermined position in the error buffer. This weighting coefficient (a k1 ) is the same as that shown in FIG.  4 . By repeating the above-described operation, the binarization with the error diffusion method is performed. In this embodiment, since the corrected signal  1100  is treated in an 8-bits manner: 
     
       
         Dmax=255 
       
     
     However, if the corrected signal  1100  is treated in an m bits manner: 
     
       
         Dmax=2 m−1 +2 m−2 +2+ . . . +2 0   
       
     
     FIG. 11 illustrates in detail a block diagram for use as a judgment circuit  106 . 
     The binary signal  1300  output from the binarization circuit  5  is latched immediately when the same is input to a line buffer  1019 . That is, assuming that the position of the subject picture element to be now treated is (I,J), the binarized data picture elements at 12 peripheral positions are latched, the 12 positions being (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+1, J−2), (I+2, J−2), (I−2,  5 −1), (I−1, J−1), (I, J−1), (I+1, J−1), (I+2, J−1), (I−2, J), and (I−1, J). The thus-latched data for the 12 picture elements are input to the OR circuit  20 , wherein the ‘OR’ of the data for the  112  picture elements are calculated, and the results are output as a signal  1320 . 
     The corrected signal  1100  output from the correction circuit  103  is input to a comparator  117  wherein it is compared with the threshold D=20, wherein if the signal  1100  is greater than the threshold D, “1” is output, while if the same is smaller than the threshold D, “0”, is output as a signal  1310 . 
     As a result of this, the density of the image can be determined. 
     A selector  121 , in accordance with the value of the signal  1310 , outputs a signal  1320  if the signal  1310  is “0”, while the same outputs a signal  1330  if the signal  1310  is “1” as a signal  1400 . However, the value of the signal  1330  is “0”. 
     That is, in a case where the picture element whose density of the image is low, the binarized data in the periphery portion of the subject picture element is examined, and if there is a signal for making the dot turn on, the signal  1320  becomes “1”. As a result of this, the signal  1400  supplied to the threshold setting circuit  104  becomes “1”. The threshold value setting circuit  4  selects as the threshold the signal  1160  (see FIG.  9 A). As a result of this, the result of the binarization of the to the OR circuit  20 , wherein the ‘OR’ of the data for the  112  picture elements are calculated, and the results are output as a signal  1320 . 
     The corrected signal  1100  output from the correction circuit  103  is input to a comparator  117  wherein it is compared with the threshold D=20, wherein if the signal  1100  is greater than the threshold D, “1” is output, while if the same is smaller than the threshold D, “0” is output as a signal  1310 . 
     As a result of this, the density of the image can be determined. 
     A selector  121 , in accordance with the value of the signal  1310 , outputs a signal  1320  if the signal  1310  is “0”, while the same outputs a signal  1330  if the signal  1310  is “1” as a signal  1400 . However, the value of the signal  1330  is “0”. 
     That is, in a case where the picture element whose density of the image is a low, the binarized data in the periphery portion of the subject picture element is examined, and if there is a signal for making the dot turn on, the signal  1320  becomes “1”. As a result of this, the signal  1400  supplied to the threshold setting circuit  104  becomes “1”. The threshold value setting circuit  4  selects as the threshold the signal  1160  (See FIG.  9 A). As a result of this, the result of the binarization of the subject picture element become 0, so that the appearance of extraneous dots can be prevented. 
     Furthermore, in this state, if there is no signal which can turn on the dot in the peripheral portion of the subject picture element, the signal  1320  become “0”. Therefore, the threshold value setting circuit  104  selects any of the threshold signals  1120 ,  1130 ,  1140  and  1150  in accordance with the density of the subject picture element. 
     In a case of a picture element whose image density is in a high level, the signal  1330  is selected by the selector  121 . Therefore, the threshold value setting circuit  104  selects any of the threshold signal  1120 ,  1130 ,  1140  and  1150  in accordance with the density of the subject picture element so that the binarization treatment is performed. 
     As a result of the above-described structure, no dot is printed in the peripheral portion of the area in which a dot is printed in the portion whose density is low. 
     Therefore, the dot blanking phenomenon in the portion whose picture density is low can be prevented. Furthermore, close positioned dot printing in the portion whose image density is in a low level can be prevented by examining the binarized data in the peripheral portion of the subject picture element. 
     FIG. 12 is a block diagram in which the threshold value setting circuit  104  described in the above-described embodiment is changed. 
     The signal  1100  output from the correction circuit  103  is input to a ROM  122 . This ROM  122  outputs a signal  1410  in accordance with the following equation; 
     
       
         (signal  1410 )=[(L1−L2)*(signal  1100 )/255], 
       
     
     wherein [ ] represents the Gaussian integer function. In this case, it is employed that L1=“185” and L2=“20”. FIG. 13 illustrates the relationship between the signal  1100  and the signal  1410  only as an example. It is not limited to this, but need only satisfy the condition that when the signal  1100  is small, the signal  1410  is also small. 
     As for the L1 and L2, they are not limited to this description but need only satisfy the condition that L1&gt;L2. 
     The RAM  123  stores a uniform random column which is 0 or more and L3 or less, but satisfying the relationship L3+L1&lt;“255”. 
     In the adder  24 , the signal  1410  an the signal  1420  are added and the result is output as the signal  1430 . 
     The selector  125  outputs, in accordance with the judgment signal  1400 , a signal  1430  if the signal  1400  is “0”, while if the signal  1400  is “1”, it outputs a signal  1440 , as a threshold signal  1200 . 
     In this case, the signal  1400  is set to “255”, however, it may be other values, and need only exceed “255”. 
     As a result of the above-described structure, the function of setting the threshold is realized similarly to the above-described embodiments, and in addition, the size of the hardware can be reduced. 
     FIG. 14 is a block diagram for use in a case where the judgment circuit  106  described in the above-described embodiment is changed. 
     The binarization signal  1300  is latched immediately when the same is input to a line buffer  126 . The signal read out from the line buffer  126  is also immediately latched when the same is input to a line buffer  127 . That is, assuming that the position of the subject picture element to be now treated is (I, J), the binarized data picture elements at 12 peripheral positions are latched, the 12 positions being (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I−2, J−1), (I−1, J−1), (I, J−1), (I+1, J−1), (I+2, J−1), (I−2, J), and (I−1, J). 
     In an OR circuit  128 , the ‘OR’ of the binarized data for 4 picture elements at the positions of the picture elements (I−1, J−1), (I, J−1), (I+1, J−1), and (I−1, J) are calculated, and as a result of this, a signal  1520  is output. 
     In an OR circuit  129 , the ‘OR’ of the binarized data for 8 picture elements at the positions of the picture elements (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, I−2), (I−2, J−1), (I−2, J−1) and (I−2, J) are calculated, and as a result of this, signal  1530  is output. 
     In a LUT  130 , a switch signal  1510  set in three levels corresponding to the input corrected signal  1100  is output. The switch signal  1510  is set to “1” when the corrected signal  1100  is 20 or less, is set to “2” when the same is 21 or more and 50 or less, and is set to “0” when the same is 51 or more. 
     A selective OR circuit  131  outputs, in response to a selection signal  1510  output from the LUT  130 , “0” as a judgment signal  1400  when the selection signal  1510  is “0”, and outputs the “OR” of the signal  1520  and the signal  1530  when the same is “1”, and outputs the signal  1520  when the same is “2”. For example, if the corrected signal  1100  is 18, the switch signal  1510  becomes “1”, and in this state if the signal  1520  is “1” and the signal  1530  is “0”, the judgment signal  1400  becomes “1”. 
     In this embodiment, the region to the made reference with respect to the value of the corrected signal  1100  is set to three stages (that is, the three stages that the periphery portion of the subject picture element is not examined, that the peripheral four picture elements are examined, and that the peripheral 12 picture elements are examined). As a result of this, since the range of periphery regions to be examined is enlarged as the lower the density of the image, the dots can be diffused in accordance with the density. As a result of this, the quality of the image can be improved. 
     By increase in the number of the line buffer, the latches and the OR circuits at need, the regions to be made reference can be made a multistage. An example in which the region is set for four stages will not be described. 
     It is assumed that the position of the subject picture element to be now treated is (I, J) and there are line buffers and latches needed for maintaining the binarized data for picture elements at 24 positions, the 24 positions being: (I−3, J−3), (I−2, J−3), (I−1, J−3), (I, J−3), (I+1, J−3), (I+2, J−3), (I+3, J−3), (I−3, J−2), (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I+3, J−2), (I−3, J−1), (I−2, J−1), (I−1, J−1), (I, J−1), (I+1, J−1), (I+2, J−1), (I+3, J−1), (I−3, J), (I−2, J), and (I−1, J). 
     Furthermore, it is assumed that three OR circuits a, b, c and one selective OR circuit d are included. In the OR circuit a, the ‘OR’ of the binarized data upon four picture elements at the positions (I−1, J−1), (I, J−1), (I+1, J−1), and (I−1, J) are calculated. As a result of this, a signal e is output. In the OR circuit b, the ‘OR’ of the binarized data upon the eight picture elements at the positions (I−2, J−2), (I−1, J−2), (I, J−2), (I+1, J−2), (I+2, J−2), (I−2, J−1), I+2, J−1), and (I−2, J) are calculated. As a result of this, a signal f is output. In the OR circuit c, the ‘OR’ of the binarized data upon 12 picture elements at the positions (I−3, J−3), (I−2, J−3), (I−1, J−3), I, J−3), (I+1, J−3), (I+2, J−3), (I+3, J−3), (I−3, J−2), (I+3, J−2), (I−3, J−1), (I+3, J−1), and (I−3, J). As a result of this, signal g is output. In the selective OR circuit d: if the corrected signal  1100  is 10 or less, the result of the ‘OR’ of the signal e, the signal f, and the signal g; if the corrected signal  1100  is 11 or more and simultaneously 20 or less, the result of the ‘OR’ of the signal e and the signal f; if the corrected signal  1100  is 21 or more and 50 or less, the result of the ‘OR’ of the signal e; and if the corrected signal  1100  if 51 or more, “0”, may be output as the judgment signal. In this embodiment the level of the corrected signal  1100  is set to four stages; 10 or less, 11 or more and 20 or less, 21 or more and 50 or less, and 51 or more. However, this is described only as an example. 
     Furthermore, in a case of a color image, it can be realized in such a manner that a predetermined number of the circuits described in this embodiment may be provided corresponding to the number of the colors. 
     According to this embodiment, as described above, by lowering the threshold for b binarization in a certain probability in the portion whose density is in a low level, the blanking in which any dot is not printed and generated in the portion whose image density is in a low level can be prevented from generation. 
     Furthermore, by controlling the size of the threshold in accordance with the image density, the deterioration in the character portion can be prevented and the smoothness of the image can be also secured. 
     Furthermore, by not only lowering the threshold for binarization, but also quantizing the subject picture element in accordance with the result of judgment made upon whether there is a dot printed in the treated region in the periphery of the subject picture element or not. As a result of this, close printing the dots generated in a portion whose image density is in a low level can be prevented. 
     Furthermore, according to this embodiment, by enlarging the treated region to be made the reference as the density becomes lower, the dot can be printed in accordance with the image density. As a result of this, the quality of images can be improved. 
     Although in this embodiment, the structure is employed in which the blanking and a phenomenon that dots are closely printed in the portion whose density is low can be prevented, white noise generated due to non-printing of dots in the portion whose density is high can be also prevented. 
     In this case, by changing the threshold (random number) in accordance with density and employing an arrangement that a dot is inevitably printed if any dot is not printed in the region to be made reference in the periphery portion of a subject picture element, the dot may be printed or not in accordance with the threshold if all of the dots are printed. 
     In the above-described first and second embodiments, the examples are described in which input images are binarized and multivalued in the error diffusion method. However, the method of quantization according to the present invention is not limited to the error diffusion method. It can be used in a method of performing quantization by correcting the error between the input image and the output image, such as the average error minimum method.