JPS6260874B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses the amplitude of an image signal obtained by an image information reading device in a facsimile machine or the like to detect changes in the density of the background of a read document, the fluorescent light of the reading device, etc. The present invention relates to an automatic contrast adjustment circuit that automatically adjusts the contrast of a reproduced image by correcting it according to changes in the light amount of a light source.

FIG. 1 shows a conventional automatic contrast adjustment circuit of this type. In the figure, reference numeral 1 denotes an image sensor that reads a document through an optical system (not shown), and its output passes through a buffer 2 to become an original image signal a, and this original image signal a is further amplified by a variable amplifier 3. As a result, it is converted into a corrected image signal b. 4 is a peak hold circuit which holds the peak value of the corrected image signal b;
This hold value c is fed back to the gain control section of the variable amplifier 3. This variable amplifier 3 is
As described above, the original image signal a is amplified with a gain corresponding to the hold value c to produce a corrected image signal b.

Here, the hold value c of the peak hold circuit 4 is designed to attenuate with a predetermined time constant, so that the hold value c is sequentially changed even when a lower peak value appears than before. Updated. Therefore, as mentioned above, the variable amplifier 3
By changing the gain of , in accordance with the hold value c, the corrected image signal b obtained as the output of the variable amplifier 3 has its amplitude corrected for changes in the light amount of the light source and the density of the background of the original. Become something.

However, such a conventional circuit has the disadvantage that it cannot follow sudden changes in the background density of the document due to the time constant of the hold value c of the peak hold circuit 4. In addition, as the peak value held in the peak hold circuit 4 is updated sequentially, the gain of the variable amplifier 3 changes even within the same line, so that the corrected image signal for image information of the same density even within the same line. There was also a drawback that the amplitude of b changes and this appears as density unevenness in the reproduced image.

The present invention eliminates the above-mentioned conventional drawbacks, can quickly follow sudden changes in the density of the background of a document, etc., can stably correct the amplitude of an image signal with high precision, and can An object of the present invention is to provide an automatic contrast adjustment circuit in which the degree of correction of an image signal does not change during a line.

The automatic contrast adjustment circuit according to the present invention stores the peak value of the original image signal when reading a white reference plane in a memory as a reference peak value, and outputs the reference peak value from the memory and the peak value detection circuit. The peak value of each line of the original picture signal is compared with the peak value of each line, and a correction coefficient is determined to correct the original picture signal having the same value as the peak value of each line to an amplitude equal to the reference peak value. The amplitude of the original image signal of the line next to the line whose peak values were compared is corrected.

The present invention will be described below based on embodiments shown in the drawings.

In FIG. 2, 1 is an image sensor and 2 is a buffer, which correspond to the same reference numerals in FIG. 5 is an A/D converter that receives the original image signal a, and 6 is a peak value detection circuit that receives the output of the A/D converter 5 as input. Reference numeral 7 denotes a memory for holding reference peak values, in which the peak value d of the output of the A/D converter 5 when the image sensor 1 reads the white reference plane is inputted from the peak value detection circuit 6 and stored therein. 8 is a control circuit which compares the reference peak value e outputted from the reference peak value holding memory 7 and the peak value d for each line of the original image signal a outputted from the peak value detection circuit 6, and detects both peaks. A difference signal f between values d and e is output. Reference numeral 9 denotes a correction coefficient memory, in which correction coefficient values corresponding to various values that the difference signal f can take are written, and by inputting the difference signal f as an address, it corresponds to the value of the difference signal f. The correction coefficient g of the value is read out. The way to set the value of this correction coefficient g is as follows:
I will explain this in more detail later. 10 is a multiplier that takes the original image signal a as a multiplicand input and the correction coefficient g as a multiplier input; 11 is an amplifier that amplifies the output of the multiplier 10; and from the output of this amplifier 11, a corrected image signal h
is obtained.

FIG. 3 shows the signal waveforms of the main parts of the above embodiment, where A is the line section signal, B is the original image signal a obtained from the buffer 2, and C is the output signal h of the amplifier 11.

Next, the operation of the above embodiment will be explained. First, before reading a document, the image sensor 1 is caused to read a white reference plane provided on the document reading surface. Then, the original image signal a in this case is converted to A/D by the A/D converter 5.
After being D-converted, it is input to the peak value detection circuit 6, the peak value d is digitally detected by the peak value detection circuit 6, and the same peak value d is stored in the reference peak value holding memory 7.

Next, when an actual original is read by the image sensor 1, the original image signal a in this case is also A/D converted by the A/D converter 5 and then input to the peak value detection circuit 6. Then, the peak value detection circuit 6 digitally detects the peak value d for each line in this case. The reference peak value e and the peak value d are stored in the memory 7 and the peak value detection circuit 6.
is input to the control circuit 8 at the same time, and the control circuit 8
outputs a difference signal f between both peak values d and e. As a result, the difference signal f is stored in the correction coefficient memory 9.
A correction coefficient g corresponding to the value of is output. and,
This correction coefficient g is multiplied by the multiplier 10 by the original image signal a of the line next to the line from which the peak value e was obtained.

Here, if the specific value of the reference peak value e is V _A as shown in FIG. 3B, and the specific value of the line peak value d is V _B , then the difference signal f is The specific value is (V _A - V _B ). However, the correction coefficient g at this time is set to a value such that the corrected image signal h becomes _VA when the original image signal a is V _B. Therefore, the original image signal a of the line next to the line where the specific value of the peak value d is V _B is:
The multiplier 10 and the amplifier 11 amplify the signal by a factor of V _A /V _B and convert it into a corrected image signal h. For this reason,
The specific value of the peak value d of the next line is V _C
If so, the peak value of the corrected image signal h of the line will be corrected to V _C ×V _A /V _B =V ₀ . As a result, the corrected image signal h becomes a signal that has been corrected for changes in the background density of the document and the light amount of the light source.

In this embodiment, since the amplitude of the image signal is corrected by comparing the peak value d of each line of the original image signal a with the reference peak value e, it is possible to correct sudden changes in the density of the background of the original. can be quickly tracked, and stable and highly accurate correction can be performed. Also,
Since correction is performed using the same correction coefficient g during the same line, the amplitude of the corrected image signal h for image information of the same density changes during the same line, which may appear as uneven density in the reproduced image. do not have.

In addition, in the conventional circuit shown in Figure 1, all black lines and lines scanned from documents with extremely high background density are also subject to correction, and the peak values of these lines are also raised to a predetermined level, improving the image quality. was causing deterioration. For example, in an all-black line,
A low-level peak value such as V _E in Figure 3 can be obtained, but if such an all-black line is corrected in exactly the same way as other lines, there is a risk that the all-black line will be regarded as an all-white line. There is.

However, in this embodiment, the correction target range is set for the peak value d of each line of the original image signal a based on the reference peak value e, and when the peak value d is outside the correction target range, By not performing correction on that line, the all-black line etc. can be excluded from the correction target, and deterioration of image quality can be prevented.

Here, the correction target range can be set through the contents of the correction coefficient memory 9, or can be set using the contents of the correction coefficient memory 9.
It can also be set by moving the memory area to be referenced.

As is clear from the above description, the automatic contrast adjustment circuit according to the present invention compares the peak value of each line of the original picture signal with a reference peak value, and determines whether the original picture signal has the same value as the peak value of each line. By correcting the amplitude of the next line's original image signal using a correction coefficient that corrects the amplitude to be equal to the reference peak value, it is possible to quickly follow sudden changes in the density of the background of the original, etc. An excellent effect is obtained in that stable and highly accurate correction can be performed, and the degree of correction of the image signal does not change during the same line, causing density unevenness in the reproduced image.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional automatic contrast adjustment circuit, FIG. 2 is a block diagram of an automatic contrast adjustment circuit according to an embodiment of the present invention, and FIG. 3 is a signal waveform diagram of the main parts thereof. DESCRIPTION OF SYMBOLS 1... Image sensor, 7... Memory for holding reference peak value, 8... Control circuit, 9... Correction coefficient memory, 10... Multiplier, 11... Amplifier.

Claims (1)

[Claims] 1. A peak value detection circuit that detects a peak value of an original image signal, and a peak value of the original image signal when a white reference plane is read, which is input from the peak value detection circuit and stored as a reference peak value. The memory compares the reference peak value output from the memory with the peak value for each line of the original image signal when reading the original output from the peak value detection circuit, and calculates the peak value for each line. and a circuit for determining a correction coefficient for correcting the value to an amplitude equal to the reference peak value, and using the correction coefficient to correct the amplitude of the original image signal of the line next to the line in which the peak value was compared. An automatic contrast adjustment circuit characterized by: 2. A correction target range is set for the peak value of the original image signal for each line, and correction is performed only for lines whose peak values are within the correction target range. Automatic contrast adjustment circuit.