Image forming apparatus correcting the density of image information according to the type of manuscript

Claim 2

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the reference value computing means contains means for calculating the reference value using the following equation: the reference value K=(the peak density value P×(the density width of the density histogram W)+W/2+ ##EQU1## where the frequency H P+1! is the frequency adjacent to the frequency H P! on the higher density side in the density histogram and the frequency H P-1! is the frequency adjacent to the frequency H P! on the lower density side in the density histogram.

Claim 3

An image forming apparatus according to claim 1, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area.

Claim 4

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has no peak density value, determines the highest density value of the pixel area to be the peak density value.

Claim 5

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has no peak density value, determines the highest density value of the pixel area to be the peak density value.

Claim 6

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has no peak density value, determines the lowest density value of the pixel area to be the peak density value.

Claim 7

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has no peak density value, determines the lowest density value of the pixel area to be the peak density value.

Claim 8

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the Peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the Peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has no peak density value, determines the medium density value of the pixel area to be the peak density value.

Claim 9

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the Peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the Peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has no peak density value, determines the medium density value of the pixel area to be the peak density value.

Claim 10

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks, determines the highest density value of the pixel area to be the peak density value.

Claim 11

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks, determines the highest density value of the pixel area to be the peak density value.

Claim 12

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the Peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks, determines the lowest density value of the pixel area to be the peak density value.

Claim 13

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the Peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks, determines the lowest density value of the pixel area to be the peak density value.

Claim 14

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks, determines the one of the highest frequency to be the peak density value.

Claim 15

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks, determines the one of the highest frequency to be the peak density value.

Claim 16

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has three density values with peaks, determines the middle density to be the peak density value.

Claim 17

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has three density values with peaks, determines the middle density to be the peak density value.

Claim 18

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the Peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks of the same frequency, determines the highest density value to be the peak density value.

Claim 19

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the Peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks of the same frequency, determines the highest density value to be the peak density value.

Claim 20

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks of the same frequency, determines the lowest density value to be the peak density value.

Claim 21

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks of the same frequency, determines the lowest density value to be the peak density value.

Claim 22

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks of the same frequency, determines the medium one of them to be the peak density value.

Claim 23

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the peak density determining means contains means that, when either the white pixel area or the black pixel area in the density histogram has a plurality of density values with peaks of the same frequency, determines the medium one of them to be the peak density value.

Claim 24

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the reference value computing means contains means for computing a second reference value for density correction of the digital image at regular intervals on the basis of the frequency of the peak density value and means for comparing the computed reference value with a third reference value computed a specific period of time ago, correcting the second reference value so that the difference may be less than a specific value, and outputting the corrected second reference value.

Claim 25

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the reference value computing means contains means for computing a second reference value for density correction of the digital image at regular intervals on the basis of the frequency of the peak density value and means for comparing the computed reference value with a third reference value computed a specific period of time ago, correcting the second reference value so that the difference may be less than a specific value, and outputting the corrected second reference value.

Claim 26

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the reference value computing means contains means for computing a second reference value for density correction of the digital image at regular intervals on the basis of the frequency of the peak density value and means for comparing the computed reference value with a third reference value computed a specific period of time ago and outputting the corrected second reference value if the difference is less than a specific value and a specific reference value instead of the second reference value if the difference is larger than the specific value.

Claim 27

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the reference value computing means contains means for computing a second reference value for density correction of the digital image at regular intervals on the basis of the frequency of the peak density value and means for comparing the computed reference value with a third reference value computed a specific period of time ago and outputting the corrected second reference value if the difference is less than a specific value and a specific reference value instead of the second reference value if the difference is larger than the specific value.

Claim 28

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for Pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the reference value computing means contains: first judging means for judging whether or not the image has continued to be judged by the judging means to be a photograph area for more than a specific period of time; second judging means for judging whether or not the image has continued to be judged by the judging means to be a character area for more than the specific period of time; and means for correcting the reference value on the basis of the judging results of the first and second judging means.

Claim 29

An image forming apparatus which reads an image in lines containing density information on each pixel, said image forming apparatus comprising: means for forming a density histogram having a frequency of each density on the basis of the density information about the digital image in a particular area; means for determining a peak density value at which the frequency of the density histogram peaks; means for computing a reference value for density correction of the digital image on the basis of the frequency of the peak density value; means for detecting a type of the image on the basis of the density histogram; means for correcting the reference value on the basis of the detected type of the image; means for correcting the density information about the digital image on the basis of the reference value corrected at the correcting means; and means for forming an image on the basis of the corrected density information, wherein the peak density determining means contains means for Pinpointing a white pixel area and a black pixel area from the density histogram and determining the peak density values for the individual areas, one peak density for each area, wherein the reference value computing means contains: first judging means for judging whether or not the image has continued to be judged by the judging means to be a photograph area for more than a specific period of time; second judging means for judging whether or not the image has continued to be judged by the judging means to be a character area for more than the specific period of time; and means for correcting the reference value on the basis of the judging results of the first and second judging means.

Claim 30

An image forming apparatus according to claim 1, wherein the reference value correcting means contains: means for, when the image has continued to be detected by the detecting means to be a photograph area during a period shorter than a specific period of time, correcting the reference value so that the reference value may be set at the reference value computed at the time when the image was detected to be a photograph area and then be set at the reference value computed at the time when the image was detected to be a character area; means for, when the image has continued to be detected by the detecting means to be a photograph area during a period longer than the specific period of time, correcting the reference value so that the reference value may be kept at the reference value computed at the time when the image was detected to be a photograph area; means for, when the image has continued to be detected by the detecting means to be a character area during a period shorter than the specific period of time, correcting the reference value so that the reference value may be kept at the reference value computed at the time when the image was detected to be a character area; and means for, when the image has continued to be detected by the detecting means to be a character area during a period longer than the specific period of time, correcting the reference value so that the reference value may be kept at the reference value computed at the time when the image was detected to be a character area.