JPS6172472A - Shading correction system - Google Patents

Abstract

PURPOSE:To speed up a correction and to make it accurate by combining and correcting shading information read out at every main scan and read-out data on an original surface. CONSTITUTION:In accordance with chip select signals 3m-3m+2 read out from a ternary counter 8, only information on data R, G and B among an original read-out data Dc and a shading information Dy1 are read in ROMs 61-63. When data obtained by calculating inherent gamma correction characteristics is stored in the ROMs 61-63, processed data Dc(R), De(G) and De(B) which are subjected not only to the shading correction but also to the prescribed gamma correction can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scanner that reads image information pixel by pixel by optically scanning a document surface, and particularly relates to a method for performing shading correction on the read image information.

In general, in scanners that read image information pixel by pixel by optically scanning the document surface, non-uniformity in the light distribution characteristics in the optical system or in each element in the line image sensor consisting of COD etc. Since so-called shading occurs in which the output level of the image sensor fluctuates due to variations in sensitivity, etc., it is necessary to perform shading correction to keep the output level constant.

Conventionally, this type of shading correction.

A storage section with a memory capacity corresponding to the line image sensor is provided, and a comparison section that compares the AD conversion output of the image sensor with the data stored in the storage section and outputs the larger signal. the maximum output value for each image sensor element in the sub-scanning direction is stored in the storage unit based on the comparison result from the comparison unit;
The correction ROM uses the accumulated data in the storage section as an address.
Read the correction signal from 5I! The shading correction is performed by applying a predetermined multiplication process to the AD conversion output of the image sensor (see Japanese Patent Laid-Open No. 59-27675).

However, in order to perform such shading correction, it is necessary to increase the resolution of the AD converter, that is, the number of bits, in order to increase the accuracy, and the output accuracy is determined by the number of input bits of the multiplier. It has become. In particular, since a multiplier is required, it is difficult to perform high-speed processing.

I]η The present invention has been made in consideration of the above points, and provides a shading correction method that allows high-speed shading correction to be performed with high accuracy by using simple means. .

In order to achieve the object, the present invention includes a first memory in which shading information is stored, and an address that is accessed according to a combination of data read from the first memory and read data to be corrected. a second memory, the second memory stores in advance data corresponding to the case where the read data to be corrected is corrected in association with the read data from the first memory; The data read out is used as correction data.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Figure 2 shows the output level characteristics in the main scanning direction consisting of N CODs in a line image sensor to explain shading. There are three factors: (1) due to optical variations in the light source, (2) deterioration of the light source due to aging and temperature characteristics, and (2) variations in the sensitivity of the CCD. In relation to Figure 2,
The curve of the A or B characteristic is due to the factor (2), the difference between the A characteristic and the B characteristic is due to the factor (2), and the enlarged unevenness of the curve is due to the factor (2). Here, n-3, n -2, n-1, [1, ・
. . . indicates the arrangement order of the CCDs.

FIG. 1 shows an example of a configuration for concretely implementing the shading correction method according to the present invention.
An amplifier 2 that amplifies the level of an output signal a read out in time series from a line image sensor in which OD is arranged in a line, and an AD converter that converts the signal amplified to the required level into a digital signal C. 3, a peak hold circuit 4 that performs peak hold according to the AD conversion output, that is, the read data C of the original, and adjusts the gain of the amplifier 2 according to the peak hold signal f; and a peak hold circuit 4 that optically scans the reference white surface in advance. A RAM 5 in which shading information obtained by reading is written and predetermined shading information d is read out according to the read data C, and a RAM 5 in which shading correction data is stored in advance and the combination of the read data C and the shading information d It is comprised of a ROM 6 from which predetermined shading correction data e is read out in response, and a control section 7 that controls each section at a predetermined timing.

With such a configuration, if, for example, a reference white surface in an indentation is optically scanned prior to reading a document, shading information with characteristics as shown in A in Fig. 2 will be obtained. Write to RAM5. Then, the sub-scanning progresses and reading of the original is started, and when the read data C at that time is corrected using the previous shading information, it is read out from RΔM5 as a signal d and used. Further, at this time, the peak hold circuit 4 detects the peak level when the reference white surface was optically scanned before the operation of writing the shading information into the RAM 5, and holds that level. The gain of the amplifier 2 is adjusted by the output signal f corresponding to the peak level at that time, and during shading, in order to correct in advance the variation caused by the optical system described in (2) with rough accuracy, for example, during peak detection, the gain of the amplifier 2 is adjusted. Even if it is at the level shown in the B characteristic, after it is amplified, the gain is adjusted so that it becomes the level shown in the A characteristic. As a result, the input level of the AD converter 3 is optimized, and conversion accuracy is improved. It should be noted that precise shading correction, including those due to variations in the optical system, is performed by the original shading correction according to the present invention. In this way A
When the input level of the D/TL converter 3 is optimized, the gain of the amplifier 2 is fixed to maintain that state, and from then on, writing of shading information and reading of image information of the original are performed in that state. It is done. Next, when reading the original, the ROM 6 is accessed using the read data C and the shading information d read from the RAM 5, and the shading correction data e corresponding to the combination of the read data C and the shading information d is accessed according to the address. It is read out as appropriate. Sequence and timing control of each part, sub-scanning control, etc. are output from the control unit 7 (appropriately performed according to the a group t).The peak level is detected not only by the AD conversion output but also by the line image sensor. The peak level of the output signal a of the amplifier 1 or the output signal of the amplifier 2 may be detected.

The main points of the shading method according to the present invention are enumerated in sequence as follows.

(1) After setting the gain of the amplifier that amplifies the image sensor output to an initial value, the reference white surface is optically scanned, and a peak value corresponding to the image sensor output signal obtained at that time is detected.

(2) Perform main scanning of the reference white surface at least once, and adjust the gain of the amplifier according to the obtained peak value.

(3) After that, the reference white surface is optically scanned at least once in the main scanning direction, and the shading information obtained at that time is written into the RAM.

(4) RA is synchronized with each main scan when reading the original.
The shading information is read from M, and the ROM is accessed in accordance with the combination of the read shading information and the read data of the document surface to obtain shading correction data.

FIG. 3 shows a specific configuration example of the amplifier 2 and the peak hold circuit 4. Here, a method is used to detect and hold the peak by digital logic groove formation, and as mentioned above, the gain adjustment of the amplifier 2 is rough. Since accuracy is sufficient, the read data of the upper 4 bits of the 6 bits of the AD conversion output is supplied to the peak hold circuit 4.

With this configuration, the comparator 41 first compares the new input signal C with the peak value cp that had been held in the latch 43 up to that point, and according to the result, selects 1-1 to the S terminal of the selector 42. give a signal. The selector 42 selects either the new input signal C or the previous peak hold value Cp in response to the latch 43.
Send to. The selection condition at this time is c>c
If p, select fa number C; if C≦cp, select signal c
p will be selected. Thereby, the latch 43 updates and holds the new input signal C when it is larger than the previous peak value cp. The above operations are sequentially repeated for each main scan, and the peak values detected for each main scan are successively updated and held in the latch 43.

Next, the peak value detected for each main scan is transferred to the latch 44, and the latch output is sent to the amplifier 2 as a gain adjustment signal "
given as. Note that the clock tit3 and clear signal t2. t4 is given from the control section 7. Also, the gain adjustment signal f is the input resistance selection switch (analog switch) of amplifier 2.
The switching signals SWI to SW4 are used to switch the input resistance of the operational amplifier 21 based on the combination of on and off states of the switches SWI to SW4. At this time, since the gain adjustment signal f is 4 bits, the input resistance value can be changed in 16 ways, and therefore the gain of the operational amplifier 21 is adjusted in 16 steps. Here, the 16 stages of gain are K/16, K/15,
K/14,...K/3. ni/2. /I. At this time, the gain has a relationship of /f (f is 16 levels).

Further, FIG. 4 shows an example of a one-channel configuration of the AD converter 3, in which the output signal S of the amplifier 2 is converted into a 6-bit digital signal C. Let's now functionally express the reference voltages as Vr+ and Vr-, ignoring their analog and digital properties.

c= (b (Vr-))/ ((Vr+)
(Vr)) A,, X64
(level). However, when C≦Vr-, c=1. When c≧Vr+, c=64 (or an overflow signal is generated).

Further, in FIG. 4, the analog switch SW5 is switched in accordance with the control signal t5 from the control unit 7, and the reference voltage Vr- is set to v2 by the control signal t5 during the writing period of shading information to the RAM 5, and during other periods. The reference voltage Vr+ is connected to GND. This is done for the purpose of increasing the accuracy when writing shading information, as will be described later.

Further, FIG. 5 shows a specific configuration example of the RAM 5 and ROM 6 portions.

First, the shading information is stored in RAM 5.
When writing data to C, data C corresponds to the output signal for each pixel of the line image sensor 1 when the reference white surface is optically scanned.
is input to the Din terminal of the RAM main body 51. that time,
Addressing of the RAM main body 51 is performed by an address control signal t6 given from the control unit 7 for each data input in pixel units.6 At the same time, a write signal t7 is given to the V lower terminal of the RAM main body 5I in synchronization with it. As a result, shading information for each pixel in the main scanning direction is written into the RAM main body 51. Next, each read data C in pixel units in the main scanning direction when reading the original is given to the ROM 6, and the RAM main body 51 is accessed by the address control signal t6 in synchronization with the pixel clock to read the previously written shading data. Information d is read out and temporarily held in latch 52. The latch 52 is provided with a clock [8] and is provided as necessary so that it can be used even if the access time is long for the RAM main body 51 and the ROM 6. Further, when the latch 52 is provided, the timing of access in the RAM main body 51 is controlled in order to synchronize the read data C and the shading information d.

Next, the read data C of the original and the shading information d read out from the RAM 5 are used to store shading correction values corresponding to various combinations of these data in advance.
M6 was dressed 7 times.

In response, a predetermined shading correction signal e is read out from the ROM 6. Here, each data C1d is 6
It consists of a bit configuration, and addresses AO to ALL of ROM6.
Among them, read data C is made to correspond to addresses AO to A5, and shading information d is made to correspond to addresses 86 to All. In this example, 200
A line image sensor 1 with 0 CCDs arranged in the main scanning direction is used, RAM 5 has a capacity of 2 x 6 bits, and ROM 6 has a capacity of 4K x 6 bits. I use each of them.

Next, the accuracy of the shading information d will be explained below with reference to FIG.

FIG. 6 shows density level characteristics for each pixel 1 to N in the main scanning direction.

When the gain of amplifier 2 is set to the initial value, that is, all switches SWI to SW4 are turned off, the gain of amplifier 2 is /16, and the reference white surface is scanned with light in that state. Let the data be B. Assume that the peak value of the read data B is Bp and the minimum value is Bm, and that, for example, Bp=27 and B m =18 are obtained. That is, in this case, the read data B is 18 to 17.
This means that it is in the range of 10 levels. In Figure 7, 10
A comparison table is shown in which levels expressed in base numbers are expressed as digital values in 6-bit and 4-bit binary. If Bp=27 is expressed as a 6-bit digital value as shown in FIG.
=0110 (level 7), and therefore the gain of amplifier 2 at that time is adjusted to /7.

Next, when optical scanning for writing shading information is performed on the reference white surface, the level of input signal a of amplifier 2 is as shown in characteristic B in FIG. 6, the same as when optical scanning is performed for gain adjustment. Become. Strictly speaking, some difference will occur due to unevenness in the light reflectance of the reference white surface, noise in the electrical system, etc.

However, since the gain of amplifier 2 that amplifies the read data B of the reference white surface at that time is set to /7, the output signal of amplifier 2 at that time is BXK.
/7. Therefore, the amplifier output Bp' for Bp=27 is r3p'=BρXK/7=27/7, and the amplified output Bm' for Bm=15 is.

Bm'=BmXK/7=18/7. (The actual value of
This is achieved by appropriately selecting each of R4 and the feedback resistor Rf. Also, setting it to about 16 is because when the B characteristic in Figure 6 is corrected (or it can be considered as normalization) as the A characteristic by the gain adjustment, Bp' is the highest. For example, Bp' and Bm' mentioned above are intended to be at level 64 or below 64 and close to it.

Bp' = 27/7Φ61.7 B m' = 18 K/7''-41,1.

Therefore, by adjusting the gain of the amplifier 2 by the peak hold value Bp of the input signal B shown by the B characteristic in FIG. 6 when the gain of the amplifier 2 is at its initial value, the amplified output for the input signal shown by the B characteristic in the figure will be It will be corrected like the A characteristic.

Note that this is not limited to the case of FIG. 6, but for example, in the case of Bp=40, Bp:100111. f:1001 (level 10)
Therefore, in this case BP' = BpxK/
f=64, and the gain of amplifier 2 is /f=
16/lo.

In this way, the gain of the amplifier 2 is adjusted according to the peak hold value Bp, and the amplified output Bp' is corrected to a level close to 64 at or below 64.

As shown in FIG. 6, the input signal having the B characteristic is corrected to have the A characteristic, thereby improving the level resolution. That is, in the case of B characteristic, B p -B m + 1 = 1 O level, and in the case of A characteristic, Bp' - Bm' +1 = 22 levels, and the resolution is improved from 10 levels to 22 levels. However, the resolution in this case is considered to be the level digitized by the AD converter 3 to 6 bits, that is, 64 levels.

Next, improvement in resolution when writing shading information will be described below.

As mentioned above, the reference voltages Vr+ and Vr- in the AD converter 3 are (Vr+)=V1, (Vr-)=V
It is set at 2. vl has the relationship shown in FIG. 6, and V 2 <
It is an appropriate value that satisfies Bm', and here it is the minimum possible value of Bm' in characteristic A in Figure 6 when shading due to deterioration of the light source, variations in the optical system or CCD, etc. is within the specified range. The smaller value is selected, and the relationship is V2=V1/2. That is, from the relationship of the y-axis in Fig. 6, V1 = 64
level, V2=32 level.

The output level of the AD converter 3 with respect to vl and v2 is illustrated on the y1 axis shown in FIG. “In other words, 32 levels between 32 and 64 on the y-axis are
This means that it is expressed at 64 levels. here.

yl=2y-64 (32<y≦64) yl=1. (y≦32) yl=64 or overflow (y>64).

In this way, (V r ) = V 2 = V 1 /
By making the relationship 2, 32 levels on yH become 64 levels on yl axis, which means that the resolution is 2.
This shows that it has doubled. From another perspective,
Even if the same 6 bits are expressed in binary, it will be 100 on the y-axis.
32 levels from 000 to 111111, oo on the yI thin axis
The A characteristic is expressed in 64 levels from oooo to 111111, and while on the y-axis, only 5 bits out of 6 bits contribute to the resolution, on the yt fine axis, 6 bits are contributed to the resolution. You will end up using all of them.

For example, in Figure 6, if B p' (y=61.7) is expressed as the y1 coordinate, Bp' (yl)=2x61.7-64=59.4, and Bm' (yl)=2X41 .1-64=18.2, therefore BP' (y 1) Bm' (y 1)+1
It becomes Φ42.

That is, in the B characteristic shown in FIG. 6, Bp-8m+1=1
What was 0 level has been corrected to the A characteristic, so that the resolution has increased to B p' - Bm' +1:2 levels, and further, the resolution has been increased to 42 levels by AD conversion using the yt axis.

The yl axis shown in FIG.
This shows that when AD conversion is performed with - set to GND, the resolution of the AD converter 3 needs to be 128 levels, that is, 7 bits, in order to obtain a resolution equivalent to that of the yt axis.

Furthermore, if an amplifier 2 that corrects the B characteristic to the A characteristic
In order to obtain the same resolution as the yt axis when writing shading information using the B characteristic without adjusting the gain of the Is required. In addition, when the number of bits is increased, a problem arises in that the SN ratio per LLSB deteriorates. According to the present invention, however, shading information can be taken in with high resolution without increasing the number of bits of the AD converter 3 as described above.

In this way, when writing shading information, the AD converter 3
By controlling the reference voltage Vr- at , the resolution can be improved. In that case, the relationship (Vr-) = V2 = V1/2 is only when writing shading information, and when reading the original, Vr-
will be connected to GND.

Next, a process in which the ROM 6 is accessed using the read data C and shading information d when reading a document and predetermined shading correction data e is obtained will be described below.

In FIG. 5, the original reading data C consists of 6 bits, and this data is designated as Dc. Also, the shading information d is 6 bits and corresponds to the yt axis in Fig. 6,
Let this data be Dyl.

Further, the level on the y2 axis corresponding to the data Dyl is assumed to be DY2. Furthermore, Dc= (dc5.dc 4, -
1dcO) (dc5=MsB), Dy1= (dy5.
dy4. ..., dyo) (dy5=MSB) and the addresses AO to All in the ROM 6 are as follows.

dco=AO dc 1 = A 1 d c 5- A 5 dyO=A7 dyl=A7 dy5=A11 This correspondence is hereinafter expressed as an address (Dy 1.DC). Further, the shading correction data stored in the ROM 6 is assumed to be De.

In the above, ROM6 at address (Dy l, Dc)
The data De in is.

The result calculated using the formula De=(DcX128/D)'2) (rank) is stored in the ROM6. Here, the data De has a 6-bit configuration. Also, the data DC given as an address is 6 bits, but
The DC used in the above calculation does not need to be 6 bits. Furthermore, there is no restriction as to how many bits the numbers DY2 and 128 in the calculation formula should be expressed. Namely.

Each element in the calculation formula and its calculation result are calculated with the required precision or number of bits, for example, 8 bits + 8 bits = 16 bits, and the top 6 bits of the results are used as data De. as ROM6
I am trying to store it in . In this case, it is in the lower bits below the 7th bit, and in A, numbers above a certain number are rounded up, and numbers below a certain number are rounded down.

For example, if Bp' is expressed on the y2 axis in Figure 6, then y2
=27.

Bp' (y2)=2X61.7=123.4.

Let Np be the position in the main scanning direction that is now BP', and the read data at the position Np when reading the original is, for example, Dc=
Consider the address of the ROM 6 and the read data De at that time when the level is 38.

First, address of ROM6 (D y 1 + D c )
as.

Dy1=59.4 (level) now 111010Dc=38
(Level)=100101, and therefore address 111010100101=(EC5)H, where (EC5)H is a hexadecimal representation.

That is, when Np and Dc=38, address (EC5)H in ROM6 is accessed.

Contents D of ROM6 corresponding to that address (EC5)
e is.

Since De = Dc
=lOO110 (binary 6 bits).

In the reverse case, data 100110 may be written in advance to the 7th address (EC5) H of the ROM6.

As described above, according to the shading correction method according to the present invention, by setting the gain of the amplifier 2 so that the B characteristic becomes the A characteristic in the relationship shown in FIG. The resolution of AD conversion 3 (
Even if the number of bits is small, shading correction can be performed with high precision.

Further, according to the present invention, it is possible to obtain shading correction data Da having arbitrary density characteristics when implementing the above-described shading correction method.

This will be explained below.

Figures 8(a), (b), and (c) show various density conversion characteristics (
The graphs respectively show examples of gamma characteristics), and show the correspondence between the output density level and the input density level at which the original was read.

Now let the function of the read data DC of the manuscript be F (DC),
Address in ROM6 (D y 1 p D c
), let us consider that De==F (Dc) X 128/Dy 2.

Here, if F (Dc) = Dc, the contents of De at that time are the ROM6 at the time of shading correction described above.
The data content is the same as that of , and the density characteristic in that case becomes linear as shown in FIG. 8(a). At that time, F
If (Dc)=kl・DC (kl is a constant), the same figure (
In the characteristic a), 1 gives the slope of the straight line. Also, for example, if F (DC)=k 2 ・Dc", then (
(k2 is a constant), it becomes possible to obtain the square density characteristic shown in FIG. 8(c). That is, F (Dc)
Calculate De using the above formula for the function and use the result as RO
By storing the data in M6, shading correction data De having arbitrary density characteristics can be obtained.

Now, the case where a color image of a document is read will be explained below.6 Figure 9 shows the arrangement of each CCD sensor section in the line image sensor-', and each sensor section is arranged in R, G, G in the main scanning direction. , B are arranged in this order. Here, R is red, G
is a sensor section that is sensitive to green and B is sensitive to blue, respectively.
Each of these sensor sections is obtained, for example, by disposing each color filter on the light-receiving surface of a CCD. These R
, G, and B sensor sections including color filters are shown in FIG.

Therefore, the spectral sensitivity of each color is different on the scanner side that reads the color image of the document, and the ink on the printer side that records the color image based on the read data. Each color has its own unique spectral characteristics. Therefore, from the viewpoint of improving color reproducibility during recording, it is necessary to perform different gamma corrections on each color data separated into R, G, and B. This means, for example, that the slope of the straight line in FIG. 8(a) is made different for each of RG and B.

FIG. 11 shows a specific example of the configuration for processing each color data of R, (, B, and the ROM for R data is
61, ROM62 for G data, RO for B data
It consists of M63 and ternary counter 8. Further, the ternary counter 8 counts the clock t5 from the control section, and sends a chip select signal 3 to each ROM 61 to 63.
m, 3m+1.3m+2 (m=O-1,2,...
) occurs.

Further, it is configured to be reset by a clear signal tlo from the control section. Here, the ROMs 61 to 63 and Dc and Dyl correspond to those in FIG.

With this configuration, if the arrangement of each sensor section in the main scanning direction of the line image sensor l' is as shown in FIG. The original reading data Dc
and shading information Dyl, only the R data is read into the ROM 61, and the original read data D is read in response to the chip select signal 3m+1.
Only the G data from among the C and shading information Dy1 are read into the ROM 62, and only the B data from among the document read data Dc and the shading information Dy1 are read into the ROM 62 in response to the chip select signal 3m+2.
Each will be read into M63.

Therefore, if the ROMs 61 to 63 are pre-stored with data based on the calculation results according to the formula below, each having its own unique gamma correction characteristics, not only the shading correction but also the predetermined gamma correction can be applied to the processed data De.
(R) and De (G)De (B) are obtained, respectively.

De=F (Dc)
In addition, for example, it is also possible to take a method of performing color separation by scanning three times through a red filter, a green filter, and a blue filter for each main scanning line.
In such a case, instead of the ternary counter 8 in FIG. 11, each ROM 61 to 63 is used for each scan with each color filter.
For example, a decoder or the like may be provided that can sequentially select the following.

The shading correction method according to the present invention allows high-speed shading correction to be performed with high accuracy using simple means, and gamma correction can also be performed at the same time without any separate processing. It has this excellent advantage.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a configuration example for concretely implementing the shading correction method according to the present invention, Fig. 2 is a characteristic diagram showing the shading state in data read from a document by a line image sensor, and Fig. 3 is the same. FIG. 4 is a circuit diagram showing a specific configuration example of the amplifier and peak hold circuit in the embodiment. FIG. 4 is a circuit diagram showing a specific configuration example of the AD converter in the embodiment. FIG. 5 is a circuit diagram showing a specific configuration example of the AD converter in the embodiment. and a circuit diagram showing a specific configuration example of the ROM,
Fig. 6 is a characteristic diagram showing the output level state of the AD conversion output in the same embodiment, and Fig. 7 shows the level expressed in lO base system.
Figures 8 (,), (b), and (c) show examples of various density conversion characteristics (gamma characteristics), respectively. Figure 9 is a diagram showing the arrangement of each sensor section 4 in a line image sensor, Figure 10 is RG,
FIG. 11 is a block diagram showing a specific configuration example for performing shading correction and gamma correction for each R, G, and B color data. It is.

Claims (1)

[Claims] 1. Shading of a signal obtained by AD converting an image sensor output signal when a reference surface is optically scanned prior to reading a document by a scanner using a first reference voltage value set in an AD converter. A means for writing information into a first memory, and a signal obtained by AD converting an image sensor output signal when optically scanning a document surface using a second reference voltage value set in an AD converter, and a signal from the first memory. A shading correction method that uses the read shading information to access a second memory in which shading correction data is stored in advance to obtain predetermined shading correction data. 2. Among the upper reference voltage and lower reference voltage in the AD converter, the reference voltage corresponding to the darker one based on the brightness level of the original is set as the second reference voltage value. Shading correction method as described in section.