JPH0213508B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a shading distortion correction device that can determine a shading distortion correction coefficient with high accuracy.

Conventional Structures and Their Problems Image devices such as facsimiles generally read calligraphy information using a scanner incorporating an optical system as shown in FIG. This scanner includes a lamp 12 for illuminating a white reference plate 10 or original 11, a lens 13 for forming an image of the white reference plate 10 or original 11, and an optical image formed by the lens 13. It is composed of a CCD image sensor 14 that performs photoelectric conversion, and a preamplifier 15 that amplifies the output of the CCD image sensor 14 and outputs an analog image signal a. The analog image signal a output from this scanner is subjected to distortion correction by a shading distortion correction device, and a conventional example of such a shading distortion correction device is as follows.
For example, there is one shown in FIG.

This conventional shedding distortion correction device includes a buffer amplifier 1, an A/D converter 2 that digitizes an analog image signal, and a ROM 4 that determines a correction coefficient according to the digitized image signal.
It consists of a RAM 5 that stores the correction coefficients determined in the ROM 4 for one scanning line, and a multiplier 3 that performs distortion correction on the digital image signal. The analog image signal a output from the scanner is first
The signal is input to an amplifier 1, amplified to become an analog image signal b, and input to an A/D converter 2. This A/D converter 2 converts the analog image signal b into a digital image signal c and outputs it. digital image signal c
One of them is output to the ROM 4, and the ROM 4 determines a correction coefficient d according to the digital image signal c, and outputs this correction coefficient to the RAM 5. The RAM 5 stores the correction coefficients for one line scan. The other side of the digital image signal c is input to the multiplier 3, and the correction coefficient data d' read out from the RAM 5 is input to the multiplier 3, where multiplication is performed and distortion correction is applied to the image signal. .

Such a shedding distortion correction device is
The white reference plate 10 is read in a training mode to be described later, and the ROM 4 calculates and outputs a correction coefficient d from the digital image signal c obtained by this reading operation, and stores it in the RAM 5. The correction coefficient value M at this time is expressed by the following equation.

V _io ×M/2 ⁿ = K ... (1) where, V _io : Digital image signal value when scanning with white reference plate, n: Number of quantization bits, K: Lower limit value of shading distortion correction range is a constant corresponding to .

However, in the above equation (1), since M/2 ⁿ ≦1,
If V _io is less than or equal to the value of K, the multiplication result in equation (1) cannot be K. Therefore, the digital image signal value _Vio must fall within the range between K as the lower limit and ²ⁿ as the upper limit.

Next, in the scan mode, the original is fed into the scanner and read. In this reading operation, the multiplier 3 receives the digital image signal c at this time, and also receives the training signal c.
The correction coefficient d' written in the mode is read out from the RAM 5, and multiplied by them to obtain a digital image signal f, that is, an image signal f subjected to shading distortion correction as the product. Here, the correction coefficient d' has the same content as d above. If the value of the digital image signal f at this time is _Vput , then _Vput can be obtained by the following equation.

V _put = V _p × M/2 ⁿ (2) where, V _p is the digital image signal value when the original is read and scanned.

Note that the correction coefficient can be changed in training mode.
Writing to RAM5, or reading and multiplying correction coefficients from RAM5 in scan mode is as follows:
This is performed for each pixel of the CCD image sensor 14. In order to synchronize the RAM addresses in these two modes, the address is selected by signal e.

However, in such a conventional shading distortion correction device, the correction accuracy of the value V _put of the corrected image signal f depends on the image signal value V _io when the white reference plate 10 is read and scanned. It has the characteristic of being That is, considering the quantization errors ΔV _p and ΔV _io associated with V _p and V _io , the theoretical V _put without errors and V′ _put with actual errors are as follows.
From equations (1) and (2), it is expressed as follows.

V _put = V _p × K / V _io ... (3) V' _put = (V _p + ΔV _p ) × K / (V _io + ΔV _io ) V _p × K / V _io + ΔV _p × K / V _io = V _put +ΔV _p ×K/V _io ……(4) In other words, the error in the value V′ _put of the corrected image signal f is:
Value V _io of image signal c during reading scan of white reference plate 10
The larger the value, the smaller the value. Therefore, in addition to satisfying the condition K≦V _io ≦2 ⁿ , it is necessary for V _io to be brought as close to the level of 2 ⁿ as possible in order _to improve the correction accuracy.

However, the light intensity of the lamp 12 etc. built into the scanner changes as the lighting time elapses, and for this reason, the value of the image signal when reading the white reference plate 10 changes.
V _io changes with time, and this value is 2 ⁿ
It is not easy to maintain this level. Therefore, in addition to the drawback that the gray level of the image signal obtained as the output of the shading distortion correction device changes over time, there is also the problem that correction accuracy cannot be maintained at a high level.

This problem can be solved by providing an AGC circuit before the buffer amplifier 1 and controlling the amplitude of the analog image signal to an optimum value.

However, the amplitude of the analog image signal is not a constant value, but has shedding distortion as described above, and this distortion also differs depending on the scanner, so AGC In the circuit, control over the optimum value becomes unstable.

OBJECTS OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and provides an apparatus capable of correcting shading distortion of an image signal while maintaining correction accuracy at a high level. The purpose is to solve problems.

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an attenuator at the front stage of a buffer amplifier that amplifies an analog image signal, and when determining the attenuation ratio, the analog image signal is The output signal of the comparator that compares the level of the signal is used, while the overflow signal of the shedding distortion correction device is used in the scan mode. The gist of this is that it is controlled by voltage.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 3 is a diagram showing a shading distortion correction device according to an embodiment of the present invention. In this figure, 7 is an attenuator that attenuates the analog image signal a input from the scanner, 1 is a buffer amplifier that amplifies the output signal b' of the attenuator 7 as a subsequent stage,
2 is an A/D converter that converts analog image signal b into digital image signal c, 4 is ROM that outputs correction coefficient d for digital image signal c, and 5 is correction coefficient d.
3 is a multiplier for multiplying the digital image signal c by the correction coefficient d'. Also,
9 is a comparator that compares the analog image signal b' with a certain fixed level at which the image signal is to be controlled; 8 is the output signal h of the comparator 9 in the training mode;
is a selector that selects and outputs the overflow signal g of the multiplier 3 in the scan mode, and a selector 6 integrates the output j of the selector 8 into an analog voltage signal k, and a gain determined by this signal k. This is an integrator circuit that outputs to the attenuator 7.

The scanner has the same configuration as the general example of the scanner explained in connection with FIG. An image is formed on the light receiving surface of the CCD image sensor 14, and photoelectrically converted into an analog image signal by the CCD image sensor 14. Among the reading scans for the white reference plate 10 or original 11, main scanning is performed by a CCD image sensor, and sub-scanning is performed by moving the original (however, sub-scanning for the white reference plate 10 is not performed). . The analog image signal output from the CCD image sensor 14 is amplified by the preamplifier 15, and then input into the shading distortion correction device as an analog image signal a.

The operation of the shading distortion correction device according to this embodiment will be explained below.

Prior to reading and scanning the original 11, the white reference plate 10
The reading scan is performed by a scanner, and an analog image signal a having a level proportional to the reflectance of the white reference plate 10 is input to the shading distortion correction device. At this time, the shading distortion correction device operates in training mode. The analog image signal a attenuated by the attenuator 7 is sent to the buffer amplifier 1.
After being amplified by the analog image signal b, the A/D
It is input to converter 2. A/D converter 2 converts analog image signal b into digital image signal c,
Output to ROM4. The ROM4 operates during the training mode period, and starts from the address specified by the input digital image signal c.
Output the correction coefficient d determined by equation (1), and output the correction coefficient d
is written to RAM5. On the other hand, in this training mode, the selector 8 selects the judgment signal h outputted by the comparator 9 by comparing the analog image signal b' with a certain fixed level, and outputs it to the integrating circuit 6 as a signal j. The integrating circuit 6 converts the frequency of the "H" state of the signal j into the magnitude of an analog voltage by integrating the signal j, which is a pulse signal.
It is output to the attenuator 7 as a signal k. The attenuator 7 is
When the signal k is large, the attenuation ratio is increased and the signal k
When is small, the attenuation ratio is made small to attenuate the image signal a, and as a result, a signal b' is output. That is, the negative feedback loop formed by the attenuator 7, the comparator 9, the selector 8, and the integrating circuit 6 increases the attenuation ratio of the attenuator 7 when the image signal b' is at a high level, so that the image signal b' When the level is small, the attenuation ratio of the attenuator 7 is made small to control the image signal b' to a constant level, forming a so-called ABC circuit. Therefore,
By setting the fixed level, which is the comparison standard of the comparator 9, high in advance, the image signal b' can be maintained at a high level. In this state, the correction coefficient d mentioned earlier is written to the RAM 5, and the main scanning 1
When the writing for the line is completed, the training
The mode ends.

Next, when the original 11 is fed into the scanner and reading scanning of the original 11 is started, the shading distortion correction device operates in a scan mode. The digital image signal c obtained by the same operation as in the training mode described above and the correction coefficient written in the RAM 5 in the training mode are read out as a signal d', and the digital image signal c and the correction coefficient are read out as the signal d'.
d' by a multiplier 3, and outputs a corrected digital image signal f. In this scan mode, the selector 8 selects the overflow signal g output from the multiplier 3 and sends the signal j to the integrating circuit 6.
Output.

The above-mentioned overflow signal g is expressed as
When V _put becomes larger than the value of K shown in equation (1), that is, the value V _p of the image signal when reading the document (scan mode) becomes the value of the image signal when reading the white reference plate (training mode). This is a signal that becomes "H" level when the signal value becomes larger than the signal value _Vio .

A negative feedback loop formed by the attenuator 7, the buffer amplifier 1, the A/D converter 2, the multiplier 3, the selector 8, and the integrating circuit 6 is configured such that when the value V _p of the image signal c exceeds V _io , the attenuator 7 By increasing the attenuation ratio and decreasing the attenuation ratio of the attenuator 7 when V _p falls below V _io , the image signal c is controlled to a constant level without being affected by external influences, which is a so-called ABC circuit. It performs its function.

Effects of the Invention As explained above, according to the present invention, the image signal is maintained at a high level by the ABC circuit that operates during training mode operation, so that the shading distortion correction coefficient can be determined with high precision.
There is also a new method that works in scan mode.
The ABC circuit controls the image signal level according to the shading distortion of the image signal.
This has the effect of obtaining a stable and highly accurate shading distortion corrected image signal without being affected by external influences.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a scanner used in an imaging device, and FIG. 2 is a block diagram of a shading distortion correction device installed in the imaging device.
FIG. 3 is a block diagram of a shading distortion correction apparatus according to an embodiment of the present invention. 2... A/D converter, 3... Multiplier, 4...
ROM, 5...RAM, 6...Integrator circuit, 7...
Attenuator, 8... Selector, 9... Comparator, 10...
...Integrator circuit, 11...Manuscript, 12...Lamp, 1
4...CCD image sensor.

Claims (1)

[Claims] 1. An attenuator that attenuates the analog image signal input from the scanner, A/D conversion means that converts the analog image signal output from the attenuator into a digital image signal and outputs it, and storage means that stores correction coefficient data. The digital image signal outputted from the A/D conversion means and the correction coefficient data read out from the storage means are calculated, and a digital image signal subjected to shading distortion correction is outputted, and the digital image signal of this digital image signal is digital calculation means for outputting an overflow signal for determining the magnitude of the level, comparison means for outputting a determination signal for determining the magnitude of the analog image signal output from the attenuator and a certain fixed level; Depending on the magnitude of the determination signal,
and control means for controlling the attenuation ratio of the attenuator according to the magnitude state of the overflow signal in the scan mode.