JP3182974B2 - Background removal equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a background removing apparatus for detecting and removing background density from a digital image signal input by reading a document.

[0002]

2. Description of the Related Art In an image recording apparatus such as a copying machine or a facsimile,
In addition to originals using normal white paper, originals using various papers such as newspapers, straw half-paper, recycled paper, and colored paper are read. Documents that do not use ordinary paper have a high background density, so if they are read by a document reading means such as a CCD sensor and the image data is output as it is, the reproduced document will have a background with a dirty background. Will be.

Conventionally, for a document having a constant background density, a predetermined area such as the center of the document is read, the average light amount is detected, and the density is adjusted by adjusting the developing bias. Is performed so that the highlight portion of the background is saturated to reduce the density of the background.

However, in the above-described conventional background removal method, only the average amount of light in a certain area is detected, so that the detection level varies depending on the image density of the document, and whether the document has a dark background or not. There is a problem that it is not possible to correctly determine whether a document is bright. In addition, if the processing for shifting the conversion curve between the input and output to saturate the highlight side is performed as background removal, the density is reduced as a whole.

Therefore, original information is read by pre-scan and image data is recorded by main scan. Density detecting means for detecting the density of each pixel of the original, and a histogram by counting the number of pixels for each density And a threshold determining means for detecting a peak from the histogram to determine a threshold value for background removal, and generating a histogram of density distribution from read information on the entire surface of a document by pre-scanning to generate a peak. A background removal apparatus that detects and determines a background removal threshold has been proposed (for example, Japanese Patent Application No. Hei.
No. 145100, Japanese Patent Application No. 2-145101). In this background removing device, the density detecting means and the histogram creating means divide the density into a plurality of areas only in the low density area where the density of the background can be detected, and sample the density at each predetermined interval for density detection and pixel Counting numbers.

[0006] Another background processing method is a CCD processing method.
In some cases, the background is removed by returning a signal before reading and shading correction by a sensor to a reference voltage of an A / D converter.

[0007]

However, in the conventional method of creating a density histogram and detecting a peak to determine a threshold value for background removal, a pre-scan is always required, so the original must be read twice. In addition, there is a problem that processing takes time.

Further, in the method of returning to the reference voltage of the A / D converter, the maximum value is searched for in data before shading correction. However, since this data includes unevenness of the optical system, it is practically required. The maximum value can only be found around the center of the original. For this reason, there is a problem that if the image quality is high at the center of the document, the image is skipped too much.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a background removal apparatus capable of detecting and removing background density in consideration of the entire area of a document without performing prescan. Is what you do. Another object of the present invention is to enable stable background removal without fluctuation of control. Still another object of the present invention is to improve the accuracy of background density determination.

[0010]

According to the present invention, there is provided a background removing apparatus for reading a document and detecting and removing a background density from an input digital image signal, wherein a sampling interval is determined based on the size of the document to be read. Value sampling means for sampling a pixel value for each line of a digital image signal input as input, and a background removal means for calculating a threshold value for background removal from the value sampled for each line and removing the background for each line It is characterized by having.

Further, the pixel value sampling means samples a minimum pixel value between sampling intervals, and replaces a value whose sampled value is equal to or more than a predetermined threshold value with a predetermined fixed value. It is.

The background removing means calculates an average value of the values sampled for each line, compares the average value with the background density value of the previous line to determine whether or not the average value has exceeded a certain limit, and If the limit value is exceeded, a threshold value for background removal is calculated as a background density value obtained by adding or subtracting a predetermined limit amount to the background density value of the previous line, and the sampled value is averaged over a predetermined threshold value. It is characterized in that a predetermined number of sampled values from the larger and / or the smaller of the sampled values is excluded from the average value calculation.

[0013]

According to the background removing apparatus of the present invention, pixel value sampling means for sampling a pixel value for each line of an input digital image signal, and average value calculating means for obtaining an average value of the values sampled for each line And a background removal unit that removes the background for each line using a threshold obtained by adding a predetermined offset to the background density value using the obtained average value as the background density value of the line. High-speed (real-time) skin removal is possible.

The apparatus further comprises a document size detecting means for detecting a document size to be read, and a sampling interval determining means for determining a sampling interval based on the detected document size. Since the pixel value is sampled for each line of the input digital image signal, the sampling number is always constant at any document size, the processing circuit can be simplified, and the average value obtained by the average value calculation means is obtained. Checks whether a certain limit has been exceeded by comparing it with the background density value of the previous line.If the limit is exceeded, the value obtained by adding a certain limit to the background density value of the previous line is used as the background density Since the background density determining means having the value is provided, it is possible to suppress the fluctuation of the control and perform the stable background removal.

The pixel value sampling means samples the minimum pixel value during the sampling interval, or replaces the sampled value with a predetermined fixed value or more, and replaces the sampled value with a predetermined fixed value. Since a predetermined number of values greater than or equal to the threshold value and larger and / or smaller of the sampled values are excluded from the average value calculation,
Even if the image portion is sampled, the accuracy of the background density determination can be improved.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a view for explaining an embodiment of a background removal apparatus of the present invention, FIG. 2 is a view showing a configuration of a copying machine provided with the background removal apparatus of the present invention, and FIG. FIG. 4 is a diagram for explaining the background removal process, FIG. 5 is a diagram showing an example of a threshold value and an image signal after the background removal, and FIG. 6 is a diagram for explaining an example of the background removal process near the threshold value. FIG.

First, an outline of a copying machine provided with the background removing device of the present invention will be described with reference to FIG. The image input device 11
The original is read by a CCD sensor, the image is input as an electric signal, converted into a digital image signal by an A / D converter, and shading correction processing and the like are performed. The background processing device 12 detects the background density of the image signal sent from the image input unit 11 and performs the background removal process. The image processing device 13 performs an editing process, a sharpening process, a smoothing process, and the like. The image output device 14 receives the processed image signal, and forms and outputs an image using an output device such as an electrophotographic system.

Next, the background treatment apparatus of the present invention is shown in FIG.
This will be described in detail with reference to FIG. Pixel value sampling means 1
The pixel value is sampled at predetermined intervals from the image signal for one line as shown in FIG. 3, and the average value calculating means 2 calculates the average value of the sampled pixel values for one line. It is. Offset adding means 3
A predetermined offset value is added to the calculated average value of the pixel values for one line to determine a threshold value used for background removal. The background removal unit 4 performs background removal by, for example, setting all signals smaller than the threshold value to 0 in the input image signal and outputting the signals as shown in FIG. 4. By performing this background removal processing, In the image signal as shown in FIG. 3, the density signal of the background portion does not become 0 as shown in FIG. 5, and the density of the signal of the image portion is preserved and remains. The above processing is performed for each line, and the background removal processing is performed while the threshold value is updated continuously.

Further, if the density signal of the background portion is simply set to 0 by comparing the threshold value with the threshold value in the background removal processing of the background removing means 4, there is a problem that a sense of incongruity appears in the density region around the threshold value. Therefore, if the image data below the threshold is not simply removed as the background, but the threshold is gradually skipped, discomfort can be reduced. FIG. 6 shows the relationship between the input image data and the output image data when this processing is performed for the threshold density region. This processing does not directly output the input image data exceeding the threshold value Th, as is apparent from the figure, but performs processing for smoothing the input image data 1.5 times the threshold value Th. When the input pixel value is equal to or smaller than the threshold value Th, the output pixel value is set to 0. When the input pixel value is equal to or larger than 1.5 times the threshold value Th, the output pixel value is used as it is. The output pixel value is multiplied by three. That is, assuming that the input pixel value is D _in and the output pixel value is D _out , D _out = (D _in -Th) × 3. By performing the processing as described above, it is possible to suppress the roughness in the image portion in the density region around the threshold value.

FIG. 7 is a diagram showing another embodiment of the background removing apparatus according to the present invention, in which a document size detecting means 15 and a sampling interval determining means 16 are added. The document size detecting means 15 detects the size of the document using, for example, a reflection type photo sensor. The sampling interval determination means 16 determines the sampling interval based on the document size detected by the document size detection means 15, and outputs, for example, a value of 1/32 of the document size as the sampling interval. Of course, this value of 1/32 may be another value, but in that case, if it is set to a power of two,
The calculation for obtaining the sampling interval may be a bit shift operation, and the subsequent calculation of the average value in the background processing unit can be performed only by the bit shift. For the standard size, a sampling interval may be determined in advance and stored, and read and used based on the document size information.

FIG. 8 is a diagram showing another embodiment of the background removing apparatus according to the present invention. The background density determining means 5 and the background density storing means 6 are provided between the average value calculating means 2 and the offset adding means 3.
Is inserted. The background density determination unit 5 compares the average value obtained by the average value calculation unit 2 with the background density value of the previous line stored in the background density storage unit 6, and if the change is large, limits the amount of change. Is to give. For example, if the background density of the previous line is determined to be 30, the average value of this line is 40, and the limit value of the variation is 5, the background density of this line is determined to be 35. By adding this processing, even if there is a singular point whose average value temporarily rises due to a horizontal line or the like, it is possible to prevent the control from fluctuating due to the singular point. In addition,
The limit value of the background density change amount may be changed depending on the sub-scanning position being processed. In the early part of the sub-scanning, the restriction may be loosened in order to quickly converge the detection value to the background density of the document, and the restriction may be gradually narrowed to prevent fluctuations in control.

FIG. 9 is a view showing another embodiment of the background removing apparatus of the present invention, and FIG. 10 is a view for explaining sampling of pixel values. The sampling value judging means 7 is inserted between them. The pixel value sampling means 1 samples the minimum value within a predetermined interval from the image signal for one line at a predetermined interval as shown in FIG. 10, and the sampling value determination means 7 Is determined to be larger than a predetermined value, and if it is larger, the sampling value is replaced with a predetermined fixed value.
Further, a value larger than a predetermined value may be excluded from the average value calculation. As a result, the sampling value can be limited to only the background density, and if the image portion is accidentally sampled by the sampling means, it is possible to suppress an increase in the average value obtained thereafter. Further, the sampling value determination means 7 may exclude a predetermined number of sampling values from the larger / smaller one of the sampled values from the average calculation.
By doing so, it is possible to remove a singular point and to improve the determination accuracy of the background density.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. For example, the background removal unit may perform background removal by applying a threshold to an image signal of a line for which the threshold is determined, that is, an image signal that is delayed by one line by FIFO or the like. Also, if the threshold to be applied is the next line where the threshold is determined, a FIFO that delays the image signal by one line
And the circuit configuration can be made inexpensive. In this case, 1
As the threshold value for the line, a preset value may be applied.

[0024]

As is clear from the above description, according to the present invention, a pre-scan is not required, so that a high-speed (real-time) background removal process can be performed. Moreover,
Since the background density is detected by sampling at predetermined intervals in the main scanning direction and averaging the signal after shading correction to detect the background density, it is possible to detect the background density in consideration of the entire original area. Further, since the background removal processing is performed digitally, it is possible to remove only the density of the background portion while maintaining the density of the image portion.

Further, since the sampling interval is determined based on the document size information, the sampling number is always constant at any document size, and the processing circuit of the background detection unit can be simplified. By feeding back the background density determination result of the previous line, the fluctuation of the control can be suppressed, and the stable background removal processing can be performed. In addition, utilizing the characteristic that the density of the background is low,
If it is considered that the minimum value is selected and sampled, and further the sampling value judging means is inserted to sample the image portion, the accuracy of the background density judgment can be improved by removing the sampled value.

[Brief description of the drawings]

FIG. 1 is a view for explaining one embodiment of a background removing device of the present invention.

FIG. 2 is a diagram showing a configuration of a copying machine provided with the background removing device of the present invention.

FIG. 3 is a diagram for explaining pixel value sampling processing.

FIG. 4 is a diagram for explaining a background removal process.

FIG. 5 is a diagram illustrating an example of a threshold value and an image signal after background removal.

FIG. 6 is a diagram illustrating an example of a background removal process near a threshold.

FIG. 7 is a view showing another embodiment of the background removing apparatus of the present invention.

FIG. 8 is a view showing another embodiment of the background removing apparatus of the present invention.

FIG. 9 is a view showing another embodiment of the background removing device of the present invention.

FIG. 10 is a diagram for explaining sampling of pixel values.

[Explanation of symbols]

1 ... pixel value sampling means, 2 ... average value calculation means, 3
... Offset addition means, 4 ... Background removal means

Continuation of front page (56) References JP-A-3-68270 (JP, A) JP-A-1-27368 (JP, A) JP-A-58-104570 (JP, A) JP-A-3-289872 (JP) JP-A-63-40469 (JP, A) JP-A-6-46255 (JP, A) JP-A-4-313744 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB (Name) H04N 1/407

Claims (6)

(57) [Claims] 1. A background removal apparatus for detecting and removing background density from a digital image signal input by reading an original, wherein each line of the input digital image signal is determined by determining a sampling interval based on the size of the original to be read. A pixel value sampling unit for sampling a pixel value for each line; and a background removal unit for calculating a threshold value for background removal from a value sampled for each line and removing the background for each line. Removal device. 2. The apparatus according to claim 1, wherein the pixel value sampling means samples a minimum pixel value between sampling intervals. 3. The background removing apparatus according to claim 1, wherein the pixel value sampling means replaces a value whose sampled value is equal to or more than a predetermined threshold value with a predetermined fixed value. 4. The background removing means obtains an average value of values sampled for each line, compares the average value with a background density value of a previous line, and checks whether or not the average value exceeds a certain limit.
2. The background removal apparatus according to claim 1, wherein a threshold value for background removal is calculated as a background density value obtained by adding or subtracting a predetermined limit amount to the background density value of the previous line when the predetermined limit amount is exceeded. . 5. The background removing apparatus according to claim 4, wherein the background removing unit excludes a value whose sampled value is equal to or more than a predetermined threshold from the average value calculation. 6. The background removing apparatus according to claim 4, wherein the background removing unit excludes a predetermined number from the larger and / or the smaller of the sampled values from the average value calculation.