JP2602199B2 - Image processing method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method, and more particularly, to an image processing method which is suitably used for adjusting the density of a digital copying machine which reads the density of an original with an image sensor.

<Prior Art> Conventionally, when a read image is binarized, white and black are distinguished by comparing each read image signal with a binarization threshold.

As the binarization threshold, there are a threshold that uses only one predetermined threshold, and a threshold that is used by appropriately selecting from a plurality of predetermined thresholds. Of the above, the former cannot obtain high-quality processed images for various types of originals such as blue-printed originals, newspapers, and copy originals, while the latter can obtain good-quality processed images for a wide range of originals. Can be obtained, which is preferable.

As the method of setting the plurality of thresholds, there are a method of presetting a threshold value which is considered to be optimal in accordance with the type of the original such as a blueprint original, a newspaper, a copy original, and the like. There are two methods for setting the threshold value in a proper manner. The former method of setting the threshold that seems to be optimal is
Since only one threshold is set for each document type,
High quality processed image signals cannot be obtained for all originals. More specifically, even if a threshold value that seems to be optimal is set according to the type of the original, there is considerable variation in density even for the same type of original. Is not guaranteed, and in the worst case, there is an inconvenience that only a processed image which cannot be determined at all is obtained. When the thresholds are set differentially, the number of set thresholds must be increased in order to obtain high-quality processed images for various documents, but the number of set thresholds is actually increased. That is, since there is a restriction in terms of usability of the operation keys and the like, there is also a disadvantage that a high-quality processed image cannot be obtained for all originals.

In particular, for a document in which images having different density are mixed, if the selected binarization threshold is inappropriate, solid black,
Alternatively, there is a problem that white dropout or the like is easily generated.

The present invention has been made in view of the above-described problems, and has as its object to provide an image processing method that can obtain a high-quality processed image for various read images only by setting a small number of threshold values. .

<Structure> In order to achieve the above object, the image processing method of the present invention is based on a maximum level and a minimum level of a read image signal of a document, based on a reference threshold value and a maximum threshold value for a read image,
And the minimum threshold is calculated, the difference between the maximum threshold and the reference threshold is set to be larger than the difference between the reference threshold and the minimum threshold, and the maximum threshold, and the intermediate threshold set between the minimum threshold and the reference threshold Is set so that the difference between the threshold values becomes smaller as the threshold value becomes closer to the reference threshold value, one of the set threshold values is selected, and the read image signal is binarized. By changing the close threshold value little by little, a fine threshold value change can be achieved in the vicinity of the reference threshold value, and the threshold value can be changed over a wide range. , The threshold value can be selected for a read image in which white dropout is likely to occur, and a high-quality processed image for various read images can be obtained.

<Embodiment> An embodiment applied to a digital copying machine will be described in detail below.

The digital copying machine has a conventionally known configuration, and has a prescan function (a function of performing an exposure operation for reading a document density prior to performing an original exposure operation for a copy operation).

Therefore, by performing the pre-scan operation, it is possible to obtain the maximum level Max corresponding to the low density portion of the document and the minimum level Min corresponding to the high density portion.

And the maximum level Max obtained as above,
Based on the minimum level Min, first, the minimum threshold T
H (0), reference threshold TH (4), and maximum threshold TH (8) can be calculated.

TH (0) = Min + C1, TH (4) = C2 / (Max-Min) −C3, TH (8) = TH (0) + C4 (Max-Min) ² + C5 where C1 to C5 are constants.

Next, based on the above TH (0), TH (4) and TH (8),
Α and α ′ are calculated by the following equations.

α = (TH (4) −TH (0) + C6) / C7 α ′ = (TH (8) −TH (4) + C6) / C7 where C6 and C7 are constants.

Then, based on TH (4), α and α ′, the intermediate threshold values TH (1), TH (2), TH (3), TH (5), TH
(6), TH (7) is calculated.

TH (1) = TH (4) −C8α ′ TH (2) = TH (4) −C9α ′ TH (3) = TH (4) −C10α ′ TH (5) = TH (4) + C10α TH (6) = TH (4) + C9α TH (7) = TH (4) + C8α where C8, C9 and C10 are constants.

The threshold values TH (0) to TH (8) obtained as described above
Corresponds to the indication scale of the density setting key attached to the operation panel of the digital copying machine, and has the correspondence shown in FIG. Here, the horizontal axis in FIG. 1 corresponds to the scale indicated by the density setting key set by the user, the right side corresponds to light copying, and the left side corresponds to dark copying. The vertical axis corresponds to the threshold value, the upper side corresponds to a light original, and the lower side corresponds to a dark original. FIG. 2 is a diagram more clearly showing the correspondence relationship of FIG. 1, and shows the absolute value of the difference between the threshold value corresponding to each designated scale and the reference threshold value TH (4).

As is clear from FIGS. 1 and 2, the maximum threshold
The difference between TH (8) and the reference threshold TH (4) is the reference threshold TH
The threshold difference is smaller as the threshold value is larger than the difference between (4) and the minimum threshold value TH (0) and is closer to the reference threshold value TH (4).

If the threshold value is set as described above, the standard threshold value TH (4) is used as a binarization threshold value for a standard density original among various originals such as a blueprint original, a newspaper, and a copy original. By selection, a high-quality processed image can be obtained.

Also, even if the original has a density slightly deviated from the standard density or a standard density, depending on the user's preference,
If a slightly darker or slightly lighter copy is required, the intermediate threshold TH (3) or TH closest to the reference threshold is used.
By selecting (5) as the binarization threshold, a high-quality or tailored processed image can be obtained.

In addition, if the original has a density greatly deviated from the standard density, the threshold value does not need to be changed very finely, and the number of threshold values can be reduced as a whole. Also, the intermediate threshold TH (1), TH (7) or the minimum threshold TH
(0) By selecting the maximum threshold value TH (8) as the binarization threshold value, it is possible to obtain a processed image of considerably high quality.

Further, since the difference between the thresholds above the reference threshold TH (4) is set to be larger than the difference between the thresholds below the reference threshold TH (4), it is possible to obtain a high-quality processed image for a document in which white dropout is likely to occur. .

It should be noted that the present invention is not limited to the above embodiment, and that each threshold value can be calculated based on a different mathematical expression, the number of set threshold values can be increased or decreased, and the scope of the present invention is not changed. Various design changes can be made.

<Effect> As described above, the present invention calculates a reference threshold, a maximum threshold, and a minimum threshold for a read image based on the maximum level and the minimum level of the read image signal, and calculates a difference between the maximum threshold and the reference threshold. The difference between the reference threshold and the minimum threshold is set to be larger than the difference between the reference threshold and the maximum threshold, and the intermediate threshold set between the minimum threshold and the reference threshold is set such that the closer the reference threshold is, the smaller the difference between the thresholds becomes. Therefore, a high-quality binarized image can be obtained for a wide range of originals, fine density adjustment near the reference threshold can be performed, and a wide range can be obtained without increasing the number of thresholds. Solid black covering the threshold width of
A unique effect is obtained in that a high-quality binarized image can be obtained even for a mixed image that is apt to be white.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a binarization threshold and a density setting scale. FIG. 2 is a diagram showing an interval between a binarization threshold and a reference threshold in correspondence with a density setting scale.

Claims (1)

(57) [Claims] A reference threshold, a maximum threshold, and a minimum threshold for a read image are calculated based on a maximum level of a read image signal of a document and a minimum level of the read image signal, and a difference between the maximum threshold and the reference threshold is calculated. The intermediate threshold set between the reference threshold and the minimum threshold is set to be larger than the difference between the reference threshold and the minimum threshold, and the intermediate threshold set between the maximum threshold and the minimum threshold is set to be smaller as the threshold is closer to the reference threshold. An image processing method comprising: setting a plurality of threshold values; selecting one of the plurality of set threshold values; and binarizing the read image signal.