JPH0690356A - Picture reader - Google Patents

Abstract

PURPOSE:To obtain an output picture with fidelity to density gradation of an original picture by providing a new A/D converter means that takes shading correction for correcting output distortion in an optical system into account in the reader. CONSTITUTION:A photoelectric conversion means 2 forms an image of a light reflected from an original 1 receiving a light source 7 through a lens 8 onto a CCD 6. Then an output voltage (x) from the CCD 6 is inputted to an A/D converter means 3 in proportion to a reflectivity of the original 1, in which the voltage is converted by a function y=f (ax)=b.f(x) (where a, b are constants) and an output voltage y is outputted, Then the output voltage y is inputted to a shading correction means 4, in which output distortion with respect to picture elements of one line is corrected and the result is inputted to a gamma correction means 5. The gradation density unevenness is remarkably reduced from the picture obtained in this way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing technique of an image reading device for reading a still image by photoelectric conversion.

[0002]

2. Description of the Related Art An image reading apparatus reads each pixel of a still image as an analog image signal by a photoelectric conversion means and digitizes the analog pixel signal by an A / D conversion means to store numerical data in a storage means such as a memory. The image is stored as.

Conventionally, the A / D converting means for converting the analog image signal into numerical data outputs a numerical value proportional to an input value. The original image can be restored by outputting the image data stored as the numerical data to an output device such as a CRT or a printer.

By the way, CRTs and printers, which are output devices for restoring original images, each have characteristics of density reproducibility, and the digitized image data obtained by the A / D conversion means is directly input to the output device. In many cases, the density is large and the image is often restored in a form different from the original image. Therefore, the image data is corrected by a gamma correction unit that converts the image data so as to match the characteristics of each output device, and is input to each output device. This allows each output device to faithfully restore the original image.

Further, in order to correct the output distortion caused by the nonuniformity of the optical system for forming the original image on the photoelectric conversion means,
It is also common to provide a shading correction means.

By these techniques, uneven density gradation and output distortion of the output image are removed, and as a result, the output image read by the image reading apparatus becomes closer to the original image.

[0007]

However, in the A / D conversion by the conventional method, the CCD output voltage which is a photoelectric conversion element is equally spaced (for example, 0 to 1V, 1 to 2V, 2 to 3V, ...
.), And the values are assigned to integers at equal intervals, and when gamma correction (FIG. 6A) in which the CCD output voltage changes abruptly is performed, CC after correction is corrected.
Regarding the density gradation output of the image with respect to the D output voltage, the density gradation output value that can be taken in the vicinity of the small CCD output voltage is greatly skipped (FIG. 6B), and as a result, when the entire image is viewed, the density gradation There is considerable unevenness in key expression.

In other words, although the original document density in the conventional method and the image data after A / D conversion are in a proportional relationship, the density level is changed by the gamma correction performed when the image data is output to the output device. There is a problem that uneven density gradation occurs in a part of the tones.

It is possible to use a high resolution A / D converter as one of the methods for solving such a problem.
It was unavoidable that the device was expensive.

The present invention has been conceived in order to solve such a problem, and an object thereof is to solve the drawbacks of the conventional gamma correction in consideration of shading correction. A / D conversion means or gamma correction pre-processing means corresponding to the A / D conversion means is created to more finely correspond to the density gradation output of the image with respect to the CCD output voltage, eliminate the density gradation unevenness, and more faithfully match the density gradation of the original image. Another object is to provide a read image device that outputs various density gradations.

[0011]

In order to solve such a problem, an image reading apparatus of the present invention has a photoelectric conversion unit for reading a still image and an analog image signal read by the photoelectric conversion unit as a digital signal. A / D conversion means for converting the output signal to the shading correction means, shading correction means for correcting the output distortion of the digital signal obtained by the A / D conversion means, and the output obtained by the shading correction means. And a gamma correction means for satisfying the following relational expression.

Y = f (a.x) = b.f (x) where y: output value of A / D conversion means x: input value of A / D conversion means a, b: constant

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram for explaining the flow of image processing in the present invention, in which 1 is an original, 2 is a photoelectric conversion means,
3 is an A / D conversion means, 4 is a shading correction means, 5
Denote gamma correction means, respectively. The photoelectric conversion means 2 forms an image of the reflected light from the light source 7 for illuminating the original 1 and the original 1 on the CCD 6 through the lens 8. In the figure, x is a value indicating the voltage output from the CCD 6 in proportion to the reflectance of the read document 1. After that, each pixel data obtained by the photoelectric conversion means 2 is subjected to A / D conversion and gamma conversion. Is performed.

First, the CCD output x is input to the A / D conversion means 3, is converted in the form of passing through the function y = f (x), and y is output. Next, the value of y is input to the shading correction means 4, where the output distortion of one line of pixels is corrected, converted into Y, and input to the gamma correction means 5. z is an output from the gamma correction means 5 having a function of z = h (Y). Finally, based on this value, for example, the toner amount of YMC is determined in the case of a printer or RGB of CRT. The color amount is determined.

FIG. 2 is a diagram specifically showing the A / D conversion means 3 of the present invention. Reference numeral 9 is a resistor network for generating a CCD output voltage divided into 32 steps by 32 resistors having different resistance values from R1 to R32 with respect to a reference voltage between −Vr and + Vr, and 10 is R33. It is a resistor network for producing a CCD output voltage that is evenly divided in 32 steps by 32 identical resistors from R64 to R64. 11 is a resistance network 9 or 1
This is a switch circuit for switching the CCD output voltage x from 0 by the SEL signal and inputting it to the comparator 12. SE
Switch resistor network between 9 and 10 by L signal,
A / D conversion that selects different y = f (x) functions can be employed. Reference numeral 13 is an encoder that converts the output of the comparator 12 into a binary number, and the result is output in 5 bits from D0 to D4.

Now, assuming that the ideal voltage obtained by the actual density of the original image is X, the shading correction means performs the correction according to the following relational expression.

G (X) = (f (X) −f (Xb)) / (f (Xw) −f (Xb)) (1) Xb: ideal output voltage based on black Xw: ideal based on white Output voltage As a result, the output voltage X is linearly normalized using the white reference data and the black reference data.

By the way, the actual CCD output voltage x has a relationship of X = a.x due to the influence of the distortion of the optical system for forming the original image on the CCD as described above. Therefore, the output f (x) satisfying the following equation is provided to the A / D conversion means.

F (a _x · x) = b _x · f (x) (2) a _x , b _x : output distortion coefficient generated by the optical system of the CCD Therefore, the output distortion is added to the equation (1). By _{transforming, g (a x · x)} = (f (a x · x) -f (a x · xb)) / (f (a x · xw) -f (a x · xb)) = b x · (f (x) -f ( xb)) / b x · (f (xw) -f (xb)) = (f (x) -f (xb)) / (f (xw) -f (xb) ) = G (x) (3) xb and xw are the black reference and white reference densities of the original image itself. This means that if the CCD output x is A / D-converted through a function that satisfies the equation (2), it is not necessary to consider the influence of the output distortion of each pixel in the gamma correction means 5. is doing. That is, by giving the A / D conversion means the characteristic of the equation (2), it is possible to produce the effect that the output distortion term is not included in the data after shading correction.

In the following, the resolution is set to 32 steps from 0 to 31 by setting the data bit length to 5 bits, and the A / D
An explanation will be given by comparing cases where two function expressions (4) and (5) satisfying the expression (2) are used as the function f (x) of the conversion means 3.

Y = x (4) y = x ^{1 / 2.2} (5) Here, the output z with respect to the input x to be finally obtained is both Expression (6).

Z = 1 + (1 / 2.42) logx (6) FIG. 3 shows a value obtained by converting the CCD output voltage x into an integer by an A / D converter having the functions of the equations (4) and (5). There is. Since the input is set so that the value of f (x) takes a continuous integer such as 0, 1, 2, 3, ..., For example, in the equation (4), when x is 0 to 1, f (X) is 0,
When x is 1-2, f (x) is 1, and when x is 2-3, f (x) is f.
(X) has a degree of 2.

On the other hand, in the formula (5), x is 0 to 0.01.
6, f (x) is 0, x is 0.016 to 0.074, f (x) is 1, x is 0.074 to 0.182, f (x) is 2, x Is 0.182 to 0.342 f
(X) has become like 3.

A / D having the characteristic of equation (4) or (5)
With respect to the output Y obtained by shading-correcting the image data y obtained by the conversion, the function h (Y) of the gamma correction performed to obtain the output result z of the expression (6) becomes the expressions (7) and (8).

Z = 1 + (1 / 2.42) logY (7) z = 1 + (2.2 / 2.42) logY (8) FIG. 4 shows the possible values 0 to 31 of f (x) shown in FIG.
It is the figure which plotted and showed the integer value which expression (7) and (8) can take with respect to Y which carried out the shading correction of integer value y of. From this figure, the gamma correction according to the conventional equation (7) can take only 16 levels such as 0, 11, 15, 18 ,. On the other hand, in the gamma correction by the formula (8), 0, 2, 6, 8, 1
It is possible to take 23 levels, such as 0, ...

The final output z with respect to the CCD output voltage x to be finally obtained is obtained by combining FIGS. 3 and 4, and is expressed by the formula: the conventional method (combinations of formulas (4) and (7)). ) And the method according to the present invention (combination of equations (5) and (8)), the final output z for input x can be expressed by equation (6). However, in the present invention, the number of possible levels of the final output z is greater than that of the conventional method.

FIG. 5 shows this conventional method (FIG. 5 (a)).
FIG. 6 is a diagram for visually explaining that the levels that the output value can take by the method (FIG. 5B) according to the present invention are different. In (a), the point of the final output z that is rounded to an integer value with respect to the value of the CCD output x that can actually be taken is x =
There are 16 locations such as 0, 1, 2, 3, 4, 5, ..., And in (b), x = 0, 0.18, 0.34, 0.5.
There are 23 locations such as 6, 0.83, ... As is apparent from this graph, in the portion where the value of the CCD output voltage x is small, the value of the conventional example suddenly changes from 0 to 11, then to 0, 2, 6, 8, 10, 12 in the present invention. The resolution can be increased. In addition, there are 23 places for the phrase segmentation of x in comparison with the 16 places in the conventional example, and the portion where the value of x is large is almost the same as the resolution of the conventional example.

These can be realized by configuring the resistance network in the A / D conversion means by the detection resistances having different resistance values (FIG. 2). That is, assuming a function for obtaining the final output z desired to be obtained with respect to the CCD output voltage x, and determining the resistor network for detection of the A / D converter to be used in accordance with the function, the unevenness in density gradation can be obtained. It is possible to realize a suppressed high-quality image reading device.

Further, if a plurality of types of resistance networks shown in FIG. 2 are prepared and a plurality of types of resistance networks of the A / D converter used according to the function to be finally obtained are provided, variations in correction can be increased.

In the embodiment of the present invention, the method of providing the A / D conversion means with the correction function is shown.
It is obvious that the same effect can be obtained by using the amplifier having the function represented by the equation (2) before the D conversion means to perform the same operation.

[0031]

As described above, according to the present invention, y = f (a.x) = b.f is taken into consideration in consideration of the shading correction for correcting the output distortion of the optical system of the image reading apparatus.
By giving the function (x) to the A / D converter, the term of output distortion is removed from the output after shading correction. Furthermore, by configuring the detection resistor network of the A / D converter with resistors having different resistance values, expensive high resolution A
It is possible to increase the resolution of the digital output after gamma correction with respect to the CCD output voltage without using the / D converter. As a result, the density gradation unevenness of the obtained image can be significantly reduced as compared with the conventional method.

[Brief description of drawings]

FIG. 1 is a block diagram of image processing according to the present invention.

FIG. 2 is a diagram showing a circuit configuration of an A / D converter according to an embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between inputs and outputs of an A / D converter.

FIG. 4 is a diagram showing an input / output relationship of a gamma conversion means.

FIG. 5 is a diagram showing a relationship of a final output z with respect to a CCD voltage x.

FIG. 6 is a diagram illustrating the principle of conventional gamma correction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 original 2 photoelectric conversion unit 3 A / D conversion means 4 shading correction means 5 gamma correction means 6 CCD 7 light source 8 lens 9 resistance network 10 resistance network 11 switch 12 comparator 13 encoder

Claims (2)

[Claims] 1. A photoelectric conversion unit for reading a still image, an A / D conversion unit for converting an analog image signal read by the photoelectric conversion unit into a digital signal, and the A / D conversion unit. In an image reading apparatus having shading correction means for correcting output distortion of a digital signal, and gamma correction means for correcting output obtained by the shading correction means, the A / D conversion means is: An image reading apparatus characterized by satisfying the relational expression of. y = f (ax) = b · f (x) where: y: output value of A / D conversion means x: input value of A / D conversion means a, b: constant 2. The A / D conversion means according to claim 1,
2. A plurality of functions satisfying the above described relational expressions are provided, and one is selected from a plurality of relational expressions of the A / D conversion according to the gamma correction means used. Image reading device.