JP3492460B2 - Background processing method for color image forming apparatus - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a background processing method for a color image forming apparatus such as a digital color copying machine. 2. Description of the Related Art In a color image forming apparatus which does not perform background processing, for example, when an image of an old document having a discolored paper color is formed, the background of the old document is faithfully imaged in pale yellow as it is. You. However, in many cases, it is not necessary to faithfully reproduce the background color of an old document, and the background is colored, which makes it difficult to read an imaged old document. To solve this problem, see, for example,
In 213073 discloses performs background elimination process using a histogram, based on the density distribution of the three primary color signals of the original image, detect those high frequency as the background density,
A copying apparatus that forms a clear image by changing a portion lighter than the background density to a predetermined density is disclosed. [0003] In the copying apparatus according to the above-mentioned disclosure, a high frequency component is detected as a background density based on the density distribution of three primary color signals of a document image, and even if the color is a chromatic color. Since an achromatic color is also removed, it is impossible to respond to a request to leave the background color as it is when forming an image because the background is a chromatic color. In some cases, the background is a chromatic color, and it is desired to remove only the color component and not to affect other color components. However, in the copying apparatus according to the disclosure, the background is removed by gamma processing. If so, removing the background will affect other color components. SUMMARY OF THE INVENTION The present invention has been made in view of the background of the background removal of a color image forming apparatus as described above, and an object of the invention is to remove the background under optimum conditions corresponding to a document image to obtain a high-quality background. An object of the present invention is to provide a background processing method for a color image forming apparatus capable of forming an image. SUMMARY OF THE INVENTION A color image forming method of the present invention is provided.
The background image processing method of the original image in the apparatus
The background density of the three primary colors based on the density distribution of the corresponding three primary color signals
Detecting the degree, and detecting the ground of the three primary colors.
Determining each difference value of the density;
Each difference value is compared with the achromatic level.
If it is determined to be smaller, the gamma correction unit
A step for replacing the level of the curve characteristic below a predetermined density with 0.
-Up and, the least one is said achromatic record of each difference value
If it is determined that the difference value is larger than the bell,
Each difference value is smaller than the chromatic color level compared to the color level.
If it is determined that the color correction coefficient is
Replacing the white level with the ground level.
It is characterized by doing. An embodiment of the present invention will be described below with reference to FIGS. 1 to 12 when a color image forming apparatus is a color copying machine. FIG. 1 is a block diagram showing an overall configuration of an embodiment of the present invention, FIG. 2 is a block diagram showing an example of a configuration of an image processing unit in FIG. 1, and FIG. 3 is a block showing an example of a configuration of a background detection unit in FIG. FIG . In this embodiment, as shown in FIG. 1, an image processing section 12 for performing signal processing on RGB image signals output from the scanner section 11 is connected to a scanner section 11 for reading a document. The image processing unit 12 includes
A printer unit 13 for forming an image on transfer paper based on the image recording information of each color output from the printer unit is connected.
The scanner unit 11, the image processing unit 12, and the printer unit 13 are connected to an operation unit 14 that controls the operations of the scanner unit 11, the image processing unit 12, and the printer unit 13 by a user operation. The image processing unit 12 shown in FIG. 1 has a RAM 26a and a ROM 26b as shown in FIG.
A scanner gamma correction unit 21 that converts the GB image data from linear reflectance to linear density data and a background detection unit 25 that detects background density are connected, and the scanner gamma correction unit 21 and the background detection unit 25 It is connected. Similarly, the CPU 26 transmits a signal R
The GB signal is converted to a YMC signal, a BK signal is calculated from the YMC signal as achromatic data, a BK toner amount is obtained,
A color correction unit 22 for correcting the YMC toner amount based on the BK toner amount is connected, and a scanner gamma correction unit 21 is connected to the color correction unit 22. Similarly, a CPU 26 is connected via a bus B to a printer gamma correction unit 23 for performing conversion in accordance with the density characteristics of the printer, and a gradation processing unit 24 for performing dither processing and the like. Is connected to the printer gamma correction unit 23, and the printer gamma correction unit 23
It is connected to the. The background detector 25 shown in FIG. 2 comprises a background detector 31 and a memory 32 connected to the background detector 31, as shown in FIG. In the present embodiment, the CPU 26 has a function of storing the color correction coefficient data and the gamma curve characteristic data and executing a related operation. The operation of the present embodiment having such a configuration is described below.
This will be described below in order. In the present embodiment, the scanner unit 1
The document on the document table is illuminated by the first illumination lamp, and light reflected from the document is reflected by a mirror and is incident on an image forming lens. Formed on a dichroic prism, R
The light is split into three types of light of (red) G (green) and B (blue). The three wavelengths of light thus split are:
CCD for R, CCD for G, CC for B for each wavelength
D respectively, and the incident light is the corresponding CC
The signal is photoelectrically converted by D, converted into a digital signal by an AD converter, subjected to necessary signal processing by an image processing unit 12, and recorded in black (BK), yellow (Y), magenta (M), and cyan (C). It is converted into a signal for formation. When RGB image data is input from the scanner unit 11 to the image processing unit 12, the input R
The GB image data is converted from linear reflectance data to linear density data by the scanner gamma correction unit 21 and input to the color correction unit 22 and the background detection unit 25. The background density is detected by the background detection unit 25, and the data is transmitted to the CPU 26 via the bus B.
And stored in the RAM 26a. On the other hand, the color correction unit 22 converts the RGB signals into YMC signals,
A BK signal is calculated as achromatic data from the MC signal,
The BK toner amount is determined. Further, the YMC toner amount is corrected according to the obtained BK toner amount (UC
R / UCA). The YMCBK signal obtained by the color correction unit 22 is input to a printer gamma correction unit 23, which performs conversion according to the density characteristics of the printer, The image data is input to the processing unit 24 and subjected to area gradation processing such as dither processing in the gradation processing unit 24 to reduce the bit width of one pixel and stabilize the density. In the background detection operation of the present embodiment, the background detector 31 detects the background at the first scan of the document, and the detected RGB data is written into the memory 32 to perform one scan of the document. Is completed, the CPU 26 reads the RGB data from the memory 32, the background removal level is obtained in accordance with the RGB data, and the coefficient is loaded from the CPU 26 to the color correction unit 22 or the printer gamma correction unit 23. After the second scan, the color correction unit 22 or the printer gamma correction unit 23 performs background removal to form an image. Here, the first scan may be a pre-scan, but in the case of a photographic mode in which the start point of starting BK addition is late (low UCR), background detection is performed at the time of the BK version and at the time of the CMY version. It is also possible to remove the background. The background detection operation of this embodiment will be described with reference to FIGS. FIG. 4 is an explanatory view of a first method of background level detection in the embodiment, FIG. 5 is an explanatory view of color background level detection by the method of FIG. 4, and FIG. 6 is a second view of background level detection in the present embodiment. FIG. 4 is an explanatory diagram of the method. In this embodiment, two methods are possible for the background level detection. The first method is a method of detecting the background using a density histogram, and as shown in FIG.
Naturally, in the density distribution of an original with a background, since a high-frequency area exists in a low-density portion, the area up to the maximum density (point C in FIG. 4) of the distribution is replaced with 0, that is, white. In this case, the hardware only needs to have a memory for storing all data, and a histogram is created by counting the density of the data, so that it can handle thinned data and compressed data. In the first method, as shown in FIG.
The background density detection is performed for each of RGB, and the background density (Rb, Gb, Bb) having the highest frequency is detected and the amount to be actually removed is (Rc, Gc, Bc). Concentrations (Rb, Gb, Bb) are used. The second method is a background detection method using the threshold value TH. FIG. 6A shows the density distribution in the X direction (main scanning direction), and FIG. 6B shows the density distribution in the Y direction (sub scanning direction). First, a threshold value TH is set in the X direction, and when the document density exceeds TH by scanning and then falls below the threshold value TH, the density is tracked and the minimum density is stored in the memory as a background candidate point. Generally, background candidate points are detected in this way for a plurality of lines. In this case, the end of the document is set as an invalid area because the density is uneven due to the floating of the document. In this manner, when a background candidate point in one line is detected, the background candidate point is developed in the Y direction, and a line that does not intersect the threshold value TH is invalidated without the threshold value TH crossing. In other regions, the lowest density point (point B in FIG. 6) among the background candidate points is selected as the background density in the document. The background processing operation of this embodiment will be described with reference to FIGS. FIG. 7 is an explanatory diagram of a γ curve after background removal according to the present embodiment, FIG. 8 is an explanatory diagram of hue division primary masking of the present embodiment, and FIG. 9 is an explanatory diagram of a color space according to the present embodiment. FIG. 10 is a diagram illustrating a comparison with a conventional method of removing a background in an intermediate chromatic color according to the present embodiment, FIG. 11 is a flowchart illustrating the operation of the present embodiment, and FIG. 12 is a diagram illustrating a calculation of a color correction coefficient according to the present embodiment. FIG. In the flowchart of FIG. 11, when the start key is operated to start the operation, the detection of the background density is started by the above-described first or second method, and the end of one scan is confirmed in step S1. And step S2
The background densities Rb, Gb and Bb are detected in step S3.
And the background density difference signal △ rg = | Rb−Gb
|, Δgb = | Gb−Bb |, Δbr = | Bb−Rb |
Is calculated. Next, in step S4, the difference signals △ rg, △ gb, △ br are compared with the achromatic color level W1, and in step S4, the difference signals △ rg, △ gb, △ br
Are determined to be smaller than the achromatic color level W1, it is determined to be achromatic, and the process proceeds to step S6.
Levels below c, Gc, and Bc are replaced with 0. In step S4, the difference signals △ rg, △ g
If it is determined that at least one of b and △ br is larger than the achromatic color level W1, the process proceeds to step S5, where the printer gamma of the normal copy is loaded into the printer gamma correction unit 23, and the present embodiment is executed. Is completed. Next, in step S7, it is determined whether at least one of the difference signals △ rg, △ gb, △ br is larger than the chromatic color level W2, and the difference signal 、 rg
If it is determined that at least one of g, △ gb, and △ br is greater than the chromatic color level W2, it is determined that the color is a high chromatic color, and the process proceeds to step S8 , where the background removal itself is not performed. On the other hand, in step S7, the difference signal {rg,
If it is determined that none of the Δgb and Δbr is greater than the chromatic color level W2, it is determined that the color is an intermediate chromatic color.
In step S9 , the white level of the color correction coefficient is set to the background color (Rc, Gc, Bc), and the color correction coefficient is recalculated and loaded. Thus, all the steps of the present embodiment are completed. In the present embodiment, the primary masking method of the hue division type is used for the color correction. As a method of hue division, a method of cutting a hue space into a dice pattern or a triangular prism shape is used. In the present embodiment, a hue space is divided into two adjacent colors, RY, 6 in each space of YG, GC, CB, BM, MR
The method of dividing is taken. For example, in FIG. 8, a hatched portion indicates an RY space, and this space includes R, Y, W, B
It is composed of a triangular pyramid with K as the vertex. In the present embodiment, the densities at the vertices of the respective triangular pyramids are obtained for the scanner and the printer, and the masking coefficients are obtained by the matrix operation. RY
To explain the space, the density on the scanner side is expressed as W =
(0, 0, 0, 0), BK = (255, 255, 25)
5, 255), R = (30, 200, 220, 25)
5), Y = (5, 15, 200, 255), the density on the printer side is W = (0, 0, 0, 0), BK = (255,
255, 255, 255), R = (0, 255, 22)
0, 0) and Y = (0, 0, 255, 0), R
The masking coefficient in the Y space is represented by an equation shown in FIG. As described above, the background density is determined to be an intermediate chromatic color, and as shown in FIG.
(0, 15, 50, 0), the white vector is replaced with (0, 0, 0, 0), recalculated and loaded as shown in FIG. This conversion is described using a color space in FIG.
Originally, the white W is located at the origin, but since the background density is set to 0, the entire space is distorted. However, the fact that the background is colored means that the background color is also added to other colors, which is essentially unnecessary, and this method is effective. Incidentally, FIG.
(A) is a background removal space according to the conventional method, in which a rectangular parallelepiped having a diagonal line between the origin and white is a space to be removed. In the case of an achromatic color, this method may be used. As described above, according to the present embodiment, based on the difference signals △ rg, △ gb, △ br of the three primary colors of the background density, the background density can be any of achromatic, intermediate chromatic, and high chromatic. If it is determined that the color is an achromatic color or an intermediate chromatic color, the background is removed.If it is determined that the color is a high chromatic color, the background processing is not performed. High-quality images can be reproduced as they are, achromatic and intermediate chromatic backgrounds can be removed, and high-quality images can be formed from original images with accurate and clear color reproducibility. According to the present invention, high chromatic color background can be obtained with high quality.
As it is, and the density conversion
Performs skin removal and removes the background of the intermediate chromatic background by color correction.
This allows accurate and clear color reproducibility from the original image
High quality images can be formed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing an overall configuration of an embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of an image processing unit in FIG. 1; FIG. 3 is an explanatory diagram illustrating a configuration of a background detection unit in FIG. 2; FIG. 4 is an explanatory diagram of a first method of background level detection in the embodiment. FIG. 5 is an explanatory diagram of color background level detection by the method of FIG. 4; FIG. 6 is an explanatory diagram of a second method of background level detection in the embodiment. FIG. 7 is an explanatory diagram of a γ curve after background removal according to the embodiment. FIG. 8 is an explanatory diagram of hue division primary masking of the embodiment. FIG. 9 is an explanatory diagram of a color space according to the embodiment. FIG. 10 is a diagram illustrating a comparison with a conventional method of background removal in an intermediate chromatic color according to the embodiment. FIG. 11 is a flowchart showing the operation of the embodiment. FIG. 12 is an explanatory diagram of a color correction coefficient calculation according to the embodiment; [Description of Signs] 11 Scanner unit 12 Image processing unit 13 Printer unit 14 Operation unit 21 Scanner gamma correction unit 22 Color correction unit 23 Printer gamma correction unit 24 Gradation processing unit 25 Background detection unit 26 CPU

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 1/407 G06T 5/00 100 H04N 1/46

Claims (1)

(57) [Claims 1] An original image in a color image forming apparatus
A background processing method, comprising the steps of:
Detecting the background density of the primary color; and determining each difference value of the detected background density of the three primary colors.
Step and each of the difference values are compared with the achromatic color level.
If it is determined to be smaller than the achromatic color level,
The correction unit determines the level of the gamma curve
Replacing with 0, at least one of the difference values is smaller than the achromatic color level.
Is determined to be greater than the chromatic color level.
And it is determined that all the difference values are smaller than the chromatic color level.
If it is set, the color correction unit
Background processing of the color image forming apparatus, comprising: the step of replacing a level to background level, the
Method.