JPS5848571A - Picture signal compensating system - Google Patents

Abstract

PURPOSE:To suppress the deterioration in the sharpness at a high spatial frequency region, by expanding the amplitude of a picture signal in the ratio in response to the spatial frequency of an original picture detected from the picture signal. CONSTITUTION:A picture signal obtained from a picture information reader (not shown) is converted into a digital code (a) in 4-bit expressing intermediate tone 16-level and inputted. A clock (b) synchronized with a picture signal (a) from an input terminal 9 is inputted, a reset pulse (d) is from an input terminal 11, and a latch clock (e) is inputted from an input terminal 13. Each input signal is applied to registers 5-8, an MSB change count circuit 10, and a latch circuit 12, a spatial frequency of an original picture is detected from the picture signal, and the amplitude of the said picture signal can be expanded in the ratio in response to the value of the spatial frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image signal correction method for correcting the amplitude of an image signal obtained by an image information reading device in a facsimile machine, etc. The purpose is to improve.

MTF is generally used as a means of evaluating image sharpness.
(Modulation Tzansfer Func
tion) is used. This MTF is defined by .

Here, B((c))ma! *B$)min are the maximum value and minimum value of the sinusoidal response at the spatial frequency ω, respectively.

Spatial frequency (Hon/Sono on the horizontal axis, MTFt&:g axis on the 34
− If you draw a graph plotted for each,
Generally, the higher the spatial frequency, the smaller the MTF.

In other words, as the spatial frequency increases, the sharpness of the image decreases.

However, in conventional facsimile machines, no measures have been taken to deal with the reduction in sharpness in the high spatial frequency region that occurs in the lens system or image sensor.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image signal correction method that can suppress a decrease in sharpness in a high spatial frequency region.

The image signal correction method according to the present invention detects the spatial frequency of the original image from the image signal obtained by the image information reading device, and expands the amplitude of the image signal at a ratio according to the value of this spatial frequency. This improves sharpness in areas with high spatial frequencies.

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

FIG. 1 shows an example of a circuit for implementing the present invention. An image signal obtained by an apparatus is converted into a 4-pit digital code representing 16 levels of intermediate tone, and then inputted. 1
~4 is the 4-pit digitally encoded image signal a
This is an image signal input terminal for inputting each bit of , LSB is input to terminal 1 and MSB is input to terminal 4. Numerals 6 to 8 are 8-pit serial input and serial output shift registers, respectively, which input the image signal a from the input terminals 1 to 4.
Input each bit signal. Further, these shift registers 6 to 8 are supplied with a clock b synchronized with the image signal a from a clock input terminal 9. 10 is an MSB change counting circuit which inputs the MSB of the image signal a from the input terminal 4, and calculates that the MSB is @0'' between two consecutive pixels.
The number of changes from @1" to @1" and from @1" to @0" is counted. In addition, this M, S., 7 conversion counting circuit 10 receives a reset pulse d inputted from a reset pulse input terminal 11. It is reset every 8 pixels of the image signal a. 12 is a latch circuit, and MSB change input terminal 13
It is latched at the rising edge of the latch clock e supplied from. 14 is a ROM, which inputs the outputs of shift registers 6 to 8 to four of its address input terminals on the LSB side, and inputs the three bits of the latch circuit 12 to three of its address input terminals on the MSB side. Enter the output signal amount. This ROM14 will be explained in detail later.
Correction data for pits are written in advance, and these correction data are output to the output terminal 16 in response to address input.
~1B as a corrected image signal q.

Next, the operation of this embodiment will be explained using the signal waveform diagram shown in FIG. Incidentally, 龜 to q in FIG. 2 correspond to the respective signals B- and -IJ in FIG. 1, respectively. Each bit of the image signal d input from the image signal input terminals 1 to 4 to the shift registers 6 to 8 is transferred to the output side one bit at a time at each rising edge of the clock b. Therefore, as shown in FIG. 2C, a signal C (hereinafter referred to as image signal C) obtained by delaying the image signal a by eight cycles of the clock b appears on the output side of the shift registers 6 to 8.

On the other hand, the MSB change counting circuit 1o is reset every 8 pixels as mentioned above, so that the MSB of the image signal a changes from '0' to '11' and from '1' to '0'. is counted every 87 primes, and the counting results are output.

Here, the image signal a has 16 levels from "oooo" to "1111", and when the minimum level is [level oJ] and the maximum level is "level 16", [level 71.
! = 4 level 8'' and is centered at the center and fluctuates above and below it, then at a level higher than the middle IL% level, the MSB is 1'', while at a level lower than the center level, ldMsB is '0''. ” becomes. Therefore, in a predetermined number of consecutive pixels, the total number of times that the MSB of the image signal a changes from 0, 1 to 1, and 1 to @o is equal to the spatial frequency of the original image. It can be regarded as a numerical value corresponding to .

In this embodiment, since the counting section for the number of changes in the MSH is 8 pixels, the result of counting the number of changes in the MSB is as follows.
The minimum is 0 and the maximum is 7. Therefore, the output of the MSB change counting circuit 1o becomes a 3-pit parallel signal.

The output of the MSB change counting circuit 10 is latched by the latch circuit 12 based on the latch pulse e just before the circuit 10 is reset by the reset pulse d, and inputted from the latch circuit 12 to the address input terminal of the ROM 14. be done.

As described above, the address input terminal of ROM14 is
A 4-pit image signal C obtained by delaying the image signal a by 8 cycles of the clock b, and a rough one-chip circuit 1 that conveys spatial frequency information.
(Note that the output signal f of the latch circuit 12, which is input to the ROM 14 at the same time as the pixel with the image signal C, is input to the 8 pixels including the pixel corresponding to the pixel X. (This is the spatial frequency information detected from the fractional signal a).

Therefore, by appropriately setting the contents of the correction data stored in the ROM 14, the corrected image signal q
can be output from the ROM 14, but especially in this circuit, the larger the value of the output signal f of the latch circuit 12, that is, the higher the spatial frequency, the more the amplitude of the image signal C is expanded by a larger ratio. The correction data is set so that the image signal q is obtained.

Therefore, as is clear from equation O), the corrected image signal q suppresses the decrease in MTF in the high spatial frequency region.

In this embodiment, (g) the number of times a pixel at a level lower than the center level appears next to a pixel at a level higher than a predetermined center level, and (b) conversely, the number of times a pixel at a level lower than the center level appears Next, the spatial frequency is detected by detecting the total number of times that pixels of a level higher than the center level appear, but by detecting only the number of times of either (a) or (b) above, the spatial frequency is detected. Frequency may also be detected.

As is clear from the above description, by outputting an image signal according to the present invention and expanding the image signal at a ratio according to the value of the spatial frequency, it is possible to suppress a decrease in sharpness in a region with a high spatial frequency. Therefore, for example, even if thin image information such as text is mixed in a halftone photo original, it can be output clearly, and even when expressing only black and white binary values, the threshold level of the image signal can be adjusted. You can get clear output by fixing it.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a processing circuit to which the image signal correction method according to the present invention is applied, and FIG. 2 is a signal waveform diagram of the main part thereof.

Claims (3)

[Claims] (1) An image signal correction method characterized by detecting the spatial frequency of an original image from an image signal obtained by an image information reading device, and expanding the amplitude of the image signal at a ratio according to the value of this spatial frequency. (2) In an image signal for a predetermined number of continuous pixels, the number of times a pixel with a level lower than the center level appears next to a pixel with a level higher than the predetermined center level. and conversely, the spatial frequency of the original image is detected by detecting the total number of times a pixel at a level higher than the center level appears next to a pixel at a level lower than the center level. Image signal correction method. (3) In an image signal for a predetermined number of consecutive pixels, the number of times a pixel with a level lower than the center level appears next to a pixel with a level higher than the predetermined center level. Or conversely, the spatial frequency of the original image is detected by detecting only the number of times a pixel at a level higher than the center level appears next to a pixel at a level lower than the center level. Image signal correction method described in section.