JPS58173974A - Processing method of picture sharpening - Google Patents

Abstract

PURPOSE:To improve the processing method of multi-value quantizing picture and to attain sharpening of picture, by applying the MTF compensation to a noted picture element when there exist one or more signals in which the noted picture element has a larger density level than a prescribed value. CONSTITUTION:Outputs of registers 102B, 102D, 102E, 102F, and 102H, that is the noted picture element E and density data of picture elements B, D, F, H positioned up/down/left/right to the E, are inputted to a discrimination circuit 106. The circuit 106 consists of four comparators 106B, 106D, 106F, 106H and an OR gate 109 taking the OR of the outputs of each comparator. The comparator 106B compares the density level difference (absolute value) ¦B-E¦ between the peripheral picture element B and the noted picture element E with a prescribed discrimination threshold value K, outputs ''1'' signal when ¦B-E¦>=K, and outputs ''0'' signal when not. The OR signal of the output signals of the comparators 106B, 106D, 106F and 106H is given to a selector 108 as the output signal of the discrimination circuit 106 from the OR gate 109.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to image processing for sharpening multilevel quantized images.

In facsimile machines, digital copying machines, etc., the original is scanned to sample the image density.
A multivalued quantized image is obtained by quantization, which is then subjected to binarization and other necessary processing. MTF (Modulat 1 on Transfer) of the image reading section of such equipment
When a scanner such as a CCD image sensor is used, the r function is generally 30 to 40 inches.
It is a degree of chi. Therefore, MTF correction is often performed to improve image quality.

This MTF correction is normally performed on surrounding pixels B, D, and F, assuming that the pixel of interest is E in FIG.

This is done according to the following formula with reference to H.

E'-αE+β(B+D+F-1-H)...
- Equation (1) where E is the density level of the pixel of interest before correction, E' is the density level of the pixel of interest after correction, B, D, F
.

H is the density level of surrounding pixels B, D, F, IT. α and β are coefficients that determine the degree of correction.

If the MTF is on the order of 309b to 40ch, the coefficients α and β in equation (1) are selected to be on the order of α-3, β--0, and 5.

Applying such M'l''P correction has the effect of sharpening line images such as thin lines and small characters.However, when the density level of the background is close to the intermediate level between white and black,
In addition, if there is a small level difference, the level difference is emphasized, causing granular noise (black dots), resulting in a problem that the quality of the reproduced image deteriorates.

For example, when sending and receiving thin slips with carbon on the back using Fuji SIJ, black spots may appear on the carbon portion of the received image, but this is a side effect of MTF correction.

An object of the present invention is to provide a method for sharpening a multi-level quantized image, which improves the above-mentioned drawbacks.

According to the present invention, the difference in density level between a pixel of interest in a multi-level quantized image and each of its surrounding pixels is detected. If even one of the surrounding pixels has this level difference greater than a predetermined value, MTF correction is performed for the pixel of interest. If all of the level differences are less than a predetermined value, MTF correction is applied to the pixel of interest by weakening the degree of correction.

In the latter case, it also includes weakening the degree of correction to zero. In other words, the MTIi'' correction is not performed.

The present invention will be specifically described below with reference to the drawings.

FIG. 2 shows a functional block diagram of an example of a device that performs image sharpening processing according to the present invention.

A pixel density signal read by a known Sukiyana 100 is digitized by an A/D converter 101. In this example, the number of output bits of the A/D converter 101 is 4, and the density of each pixel is set from level O (white) to level 15 (black).
It is assumed that 16-value quantization is performed within the range of .

The pixel density data output from the A/D converter 101 is input to the register 102 of the register matrix 102, and at the same time, it is input to the 1-line buffer fi+ 103. (2) The output of the line buffer fil 103 is input to the register 102F of the register matrix 102, and is also input to the 1-line buffer +21104 at the next stage. (2) The output of the line buffer +21104 is input to the register 102C of the register matrix 102.

The pixel density data in the l line buffers fi+103 and +21104 are sequentially shifted to the right in synchronization with the pixel transfer timing of the scanner 100. Therefore, in l line buffer fi+103, +21104 there are 2
Pixel density data for a line is temporarily accumulated. Also within the master raster matrix 102,
In synchronization with the pixel transfer timing, register 102 I
, 102F, and 102c are shifted to the right registers, respectively.

In the register matrix 102, the register 102
The contents of E are the density data of the pixel of interest as the processing target. In other words, when compared to FIG. 1, pixel E
The density data of pixels before and after that on the same line -1- is stored in the register 102E, and the density data of F is stored in the register 102E.
D, 102 F, the picture of the previous line, IB,
The density data of C is stored in registers 102B and 102C, and the density data of pixels H and I of the next line are stored in register 10.
2H and 102I, respectively.

Registers 102B, 102D, 102B, 10
The output of 2F and 102H, that is, the pixel of interest E and above,
The density data of the lower, left, and right pixels B, I), F, and H are manually input to the determination circuit 106. This judgment circuit 106 includes four comparators 106B, 106D, 106F,
106H, and an OR gate 109 for calculating the logical sum of the outputs of each comparator. Comparator 106
B is the density level difference (absolute value) between peripheral pixel B and target pixel E
Compare IB-El with a certain determination threshold K, and find that IB-El>
If it is K, it outputs l" signal, otherwise it outputs "0'"
Output a signal. Another comparator 106D.

Similarly, for 106 F and 106 H, surrounding pixels, F
, )) and the pixel of interest E, the density level difference ID-El, IP
-El, IH-El and the judgment threshold are compared respectively, and if it is II)-El)K, the "1" signal is sent from the comparator 106D, and if it is IF'-El'2, the comparator 1 is sent.
If the "'1" signal from 06F is IH-El)K, a "1" signal is output from the comparator 106H.

Otherwise, comparators 106D, 106F.

106H outputs a "0" signal. Comparator 106
The OR signal of the output signals of B, 106D, 106F, and 106H is sent from the OFL gate 109 to the selector 108 as the output signal of the determination circuit 106. Note that, for example, =2 is selected as the determination threshold value.

The adder 105 includes registers 102B and 102D.
, 102F, 102H to four pixels B above, below, left and right of the pixel of interest E.

1), F, I-(, and the sum of the density levels of these four pixels S = B 1D -1-F +
Calculate H.

Density data of the pixel of interest in the register 102E and adder 1
The output data (S) of 05 is converted R, OM (read/
only memory) 107. This conversion RO
M107 stores the algorithm of equation (1) above in advance, and calculates the density level E' of the pixel of interest after MTP correction.
Output. Then, the data of the corrected density level E' and the data of the density level E before correction are input to the selector 108.

When the selector 108 receives the “1” signal from the determination circuit 106, the selector 108 selects the MT output from the conversion ROM 107.
The density data E' after F correction is selected and output. vice versa,
When the output of the determination circuit 106 is 0'', the density data before MTF correction is selected by the selector 108 and output.

Note that the function of the selector 108 is the conversion R, OM 107
It is also easy to realize this. In other words, the determination circuit 106
If the output of
F−l−H), and if the output of the determination circuit 106 is 0”, the conversion R, OM 10
It is also possible to create 7 internal patterns. Of course, it is also possible to implement the conversion ROM 1070 function by hardware.

Furthermore, if binarization processing is performed after the selector 108, this processing may also be performed simultaneously in the conversion ROM 107.

Furthermore, the determination circuit 106 may be implemented as a ROM.

Here, the effects of the present invention (two methods) will be specifically explained in comparison with the conventional method.

FIG. 3 shows the processing results according to the conventional method. The density level of the pixel of interest after MTF correction when the pixel pattern shown in the left column is E'-αE1β(B+D-)-F-1-
H) is shown in the middle column, and its binarized output is shown in the right column. However, α=3, β=−0,5, the pixel density level is 16 values from level O (white) to level 15 (black), and the binarization threshold is 7.5, which is 50 levels.

In pixel patterns (a) and (b), the density level of the pixel of interest is lower than the binarization threshold before MTF correction, but M T
After F correction, exceed the binarization threshold. Therefore, the binary output will be ``1'' in both cases.Such pixel patterns often occur in the white area of thin slips where the carbon on the back side is transparent, and the binary output is originally ``0''. What happens if a pixel that should be ``' becomes ``1''?

In other words, this is an example in which unnecessary black spots are generated due to MTF correction.

Conversely, the binarized output of the pixel of interest in pixel patterns (e) and (f) should be "1", but since the level after MTF correction is lower than the binarized threshold of 1, the binarized output is becomes 0″″, and so-called white spots occur. In general, even in image areas with the same density, variations in density levels (density unevenness) on the order of ± lebel, including quantization noise, are unavoidable, so white spots like the one above occur relatively often. .

This is also a drawback of conventional processing methods.

Figure 4 shows the results of the second method of the present invention.
Concentration level E'-αE+βS of the target pixel after MTF correction for the pixel pattern shown in the left column, level difference IH-El
, IF'-El, ID-El, IB-El and the judgment threshold, and the level output as a processing result are shown in the middle column, and the binarized output for that output level is shown in the right column. be. However, S = B 1D 4-
F + H, α-3, β = -, 0, 5, is chosen as = 2,
The pixel density level is level 15 (white) to level 15.
(black) is quantized into 16 values. Further, the binarization threshold value is selected to be 7.5, which is the 50% level. Note that the pixel patterns (a) to (f) are the same as those in FIG. 3.

Now, in the pixel patterns (a) and (b), the density level (before correction) E of the pixel of interest and the a level H of each surrounding pixel.
, F, D, B level difference IH-El, IP-El, I
Both D-El and IB-El are smaller than the determination threshold. Therefore, since the density level E before MTF correction is selected as the output level, its binarized output becomes "0". In the conventional method, for these pixel patterns, 2
The digitized output became “1#” and black dots appeared as noise, but
According to the present invention, generation of such noise can be prevented.

The same can be said about pixel pattern 00.

On the other hand, pixel pattern (e) had a problem with white spots.

For (f), the level difference IH-E l, l F
-El.

Since both ID-El and IB-El are smaller than the determination threshold, the level E before MTF correction is selected as the output level, so the binary output is "1". Therefore, white spots can be prevented.

As described in detail above, according to the present invention (-), it is possible to prevent the generation of unnecessary black point noise and white spots as in the conventional method, and to obtain a high-quality image in which the original line portions are made clear. .

Note that for a pixel of interest for which the dark IW level difference between all surrounding pixels and the pixel of interest is lower than the determination threshold, only the case where no MTF correction is applied (zero correction acid) has been described as an example; however, the degree of correction may be weakened. As mentioned above, the M T'F correction may be applied based on □.

It is. The latter is based on, for example, the condition that the output of the determination circuit 106 is "I" in the conversion ROM 107 in FIG.
It is clear that this can be easily realized by a configuration in which the output of 06 generates a correction level for each of the conditions of °゛0''', and one of the correction levels is selected by the selector 108 according to the output of the determination circuit 106. be.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of pixel positions, FIG. 2 is a functional block diagram showing an example of a device that executes image sharpening processing according to the present invention, FIG. 3 is a diagram for explaining processing results according to the prior art, and FIG. FIG. 4 is a diagram for explaining the processing results according to the present invention. 100...Scanner, 101...A/D converter, 10
2...Register matrix, 102 A-102I・
...Register, 103, 104...l line buffer, 105...adder, 106...judgment circuit, 1
06B, 106D, 106F, 1061]... Comparator, 107... Conversion RoM, 1os...
・Selector, 109...OR gate. Agent Patent Attorney Seihi Suzuki, Ugly＼11 Figure 3 JP-A-58-173974 (5) Figure 4

Claims (1)

[Claims] (1) Among a specific number of pixels surrounding a pixel of interest (pixel of interest) in a multilevel quantized image, there is one or more pixels whose density level difference with the pixel of interest is equal to or larger than a predetermined value. -]2 exists, the pixel of interest has M T F (Modulation
n Transfer Function ) correction, and if not, the pixel of interest is subjected to MTP correction with a weaker degree of correction, or IVI TF' correction is not applied.