JP2738818B2 - Image signal processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal processing apparatus for performing MTF correction on an image signal. 2. Description of the Related Art In reproducing an image read by a scanner in a digital copying machine, a facsimile apparatus, or the like, sharpening an edge portion of an image signal to correct "blurring" and aiming at sharpening of a reproduced image. MTF correction is performed on the image signal. This MTF correction is performed using an MTF filter as shown in FIG. 2 in the main scanning direction, and is performed using an MTF filter as shown in FIG. 3 in the sub-scanning direction. Further, by using an MTF filter as shown in FIG. 4, MTF correction in both main and sub scanning directions is performed. FIG. 5 is a waveform chart for explaining the operation of the MTF filter of FIG. When an MTF filter is applied to an input image signal having poor rising and falling characteristics (sharp waveform) as shown in the upper part of the figure, a correction signal component as shown in the middle part of the figure is added to the input image signal. Thus, a corrected output image signal having a sharp rise and fall as shown in the lower part of FIG. The content of the MTF correction is expressed by the following equation. B ′ = (1 + 2α) × B−α × (A + C) = α × {2B− (A + C)} + B (1) where B is a target pixel, and A and C are adjacent pixels before and after the target pixel. The pixel, B ′ is a correction output, and α is an MTF correction coefficient. FIG. 6 is a block diagram of a conventional image signal processing apparatus for performing MTF correction in the main scanning direction by using an MTF filter as shown in FIG. This image signal processing device (the portion within the broken line) processes a multi-tone digital image signal input from the A / D converter 1. In the image signal processing device, the latch circuits 2 and
Reference numerals 3 and 4 temporarily hold image signal information of three pixels arranged in the main scanning direction. The shift register 5 doubles the information of the pixel of interest B held in the latch circuit 3 by left-shifting it by one bit to the left, and the adder 6 generates adjacent pixels A and C held in the latch circuits 2 and 4. Is added. The subtracter 7 calculates the difference F = 2B between the output information D = 2B of the shift register 5 and the output information E = A + C of the adder 6.
2B− (A + C). The multiplier 8 calculates a product H of the MTF correction coefficient α set in the coefficient register 9 and the output information F of the subtractor 7. Adder 10
Keeps the sum of B and H, I = B + H. This I
Corresponds to the correction output B ′ represented by the above equation (1). The clamp circuit 11 is provided to cope with a case where an overflow or underflow occurs in the adder 10.
When the value exceeds the value FFH (= 255), the final corrected output image signal J is clamped to FFH, and when the value of I becomes negative, it is clamped to J = 0. However, according to such a configuration, if an attempt is made to obtain a sufficient MTF correction effect, black or white noise (notch) occurs in the binarized image of the correction output. There was a problem that it was easy. The reason will be described below. FIG. 7 shows the input image signal and its M in FIG.
FIG. 4 is a waveform diagram of a TF corrected output image signal, in which a solid line indicates a waveform of an input image signal, and a broken line indicates a waveform of an MTF corrected output image signal. However, when the MTF correction coefficient α = 1, ie, (−
1, 3, -1). As shown in the figure, even in the black solid or white solid portion of the image, the value of the input image signal is changed due to the quantization error of the scanner, the quantization error of the A / D converter, and the density unevenness of the original. It fluctuates in the positive or negative direction. Both positive and negative fluctuations are emphasized by the MTF correction as indicated by broken lines. The MTF correction output image signal is converted to a slice level L
In the case of binarization by b (in the case of binarization to a normal density), even if such fluctuation is emphasized, there is almost no problem on the binarized image. However, when binarization is performed at a high slice level La (when binarization is deep), the emphasized fluctuation in the negative direction (pixels P1, P2, etc.) appears as black noise on a white background. Further, when binarization is performed at a low slice level Lc (when binarization is lightly performed), the emphasized positive-direction fluctuation (pixel P3 or the like) appears as white noise on a black background. Such noise significantly impairs the image quality of the binarized image. The present invention has been made in view of the above-described problems, and provides an image signal processing apparatus capable of suppressing the generation of noise due to minute fluctuations of an image signal and achieving a good MTF correction effect. The purpose is to: In order to solve the above-mentioned problems, the present invention provides a detecting means for detecting an edge direction of an input image signal centering on a pixel to be MTF-corrected, Setting means for setting two types of MTF correction coefficients; and selecting means for selecting an MTF correction coefficient corresponding to the edge direction detected by the detection means from the MTF correction coefficients for each edge direction set by the setting means. The MTF correction of the input image signal is performed using the MTF correction coefficient selected by the selection means. According to the present invention, the strength of the MTF correction can be adjusted independently for each edge direction by the above-described configuration. Therefore, for example, when the slice level for binarization of the MTF-corrected output image signal is high and the black noise in the white solid portion is conspicuous, the MTF correction coefficient corresponding to the negative direction is set to a small value to thereby reduce the negative of the image signal. By weakening or not emphasizing the effect of directional fluctuation and setting the MTF correction coefficient corresponding to the positive-direction edge to a required size, noise in white solid portions can be suppressed and sufficient MTF can be obtained.
A correction effect can be achieved. Similarly, when the slice level is low and the white noise in the black solid portion is conspicuous, the MTF correction coefficients in the respective edge directions are set in an inverse relationship to suppress the noise in the black solid portion and achieve a sufficient MTF correction effect. I can give it. An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an image signal processing device according to one embodiment of the present invention, and the broken line portion is the image signal processing device. This image signal processing apparatus performs the MTF correction in the main scanning direction in the same manner as the apparatus shown in FIG. 6, but the parts corresponding to those in FIG. Hereinafter, a configuration different from that of FIG. 6 will be described. Reference numeral 12 denotes a decoder as means for setting an MTF correction coefficient for each edge direction. This decoder 12 has an MTF
According to the slice level SL for binarization of the corrected output image signal J, a positive MTF correction coefficient α1 as shown in the following table;
The negative MTF correction coefficient α2 is output. [Table 1] Reference numeral 13 denotes a selector as means for selecting an MTF correction coefficient according to the edge direction. This selector 1
3 indicates that when the sign output S of the subtractor 7 indicates positive,
The MTF correction coefficient α1 for the positive direction is selected as the MTF correction coefficient α and input to the multiplier 8. When the sign output S indicates negative, the MTF correction coefficient α2 for the negative direction is changed to the MTF correction coefficient α.
And input to the multiplier 8. As is apparent from the description of the selector 13, in this embodiment, the subtractor 7 for performing the operation of {2B- (A + C)} also serves as the edge direction detecting means, and its sign output S Indicates the detected edge direction. The MTF filter used for MTF correction is that shown in FIG. 2, but the MTF correction coefficient α is switched to α1 or α2 according to the edge direction as described above. Further, the MTF correction coefficients in each direction are switched and set according to the binarization slice level SL as shown in Table 1. The operation of the image signal processing device configured as described above will be described below with reference to the waveform diagram shown in FIG. 7 again. For example, the input image signal (solid line) shown in FIG.
When P1 becomes the pixel of interest, the sign output S of the subtractor 7 becomes negative, so α2 is selected as α by the selector 13, and MTF correction is performed using this α. here,
When the slice level SL is as high as La shown in FIG. 7, α1 = 1 and α2 = 0 are set as shown in Table 1, so that the P1 pixel is not corrected (emphasized), and the slice level L is not corrected.
a. The same applies to the pixel P2.
Therefore, pixels such as P1 and P2 which are noise components are 2 pixels.
It does not appear as black noise in a solid white portion of the digitized image. On the other hand, for the pixel at the positive edge, α1 = 1 is selected and emphasized at the same time as in the related art, so that the MTF correction works well. The slice level SL is set to L shown in FIG.
c, α1 = 0, α2 as shown in Table 1.
= 1 is set. In this case, when the pixel P3 becomes the target pixel, the sign output S of the subtractor 7 becomes positive, and α1 = 0
Is not emphasized, and does not exceed the slice level Lc. Therefore, the pixel P3 does not appear as white noise in a solid black portion of the binary image. Since α2 = 1 is selected and emphasized for the pixel at the negative direction edge, the same MTF correction effect as that of the related art is obtained, and blurring of a thin line is corrected. When the slice level L is a normal level as shown by Lb in FIG. 7 and noise is hardly generated in solid white or solid black portions, α1 = α2 = 1 is set. MTF correction is performed in both the positive and negative directions. In this embodiment, the MTF correction coefficients α1 and α2 for each edge direction are automatically switched and set to three levels according to the slice level SL so as to automatically correspond to three types of slice levels. It is also possible to make the switching setting as described above. Also. When the slice level is fixed, fixed α1 and α2 corresponding to the slice level may be set. In this embodiment, the edge direction is detected by the subtractor 7 for the MTF correction operation. However, the edge direction may be detected by providing a dedicated circuit. Further, functions such as MTF correction calculation can be realized by firmware or software. Further, in this embodiment, MTF correction in the main scanning direction is performed. However, the MTF filter as shown in FIG. 3 or FIG. 4 is used, and the edge direction detection in the sub-scanning direction or the main and sub-directions is performed. To select the MTF correction coefficient, the same MTF correction can be performed in the sub-scanning or both main and sub-scanning directions. As is apparent from the above description, the present invention detects the edge direction of an image signal, selects an MTF correction coefficient according to the edge direction, and performs MTF correction. By suppressing the generation of noise in a portion or a solid black portion, and making the MTF correction work well, there is an effect that the quality of a reproduced image can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an image signal processing apparatus according to an embodiment of the present invention. FIG. 2 is a diagram showing an MTF filter for a main scanning direction. FIG. 3 is a diagram showing an MTF filter for a sub-scanning direction. FIG. 4 is a diagram showing an MTF filter for both main and sub scanning directions. FIG. 5 is a waveform diagram for explaining the operation of MTF correction in the main scanning direction. FIG. 6 is a block diagram of a conventional image signal processing device. 7 Waveform diagram for explaining relationship between slice level and MTF correction effect [Explanation of symbols] 2, 3, 4 Latch circuit 5 Shift register 6, 10 Adder 7 Subtractor 8 Multiplier 11 Clamp circuit 12 Decoder ( MTF correction coefficient setting means for each edge direction) 13 selector (MTF correction coefficient selection means)

Claims (1)

(57) [Claims] A detecting means for edge direction of pixels to be MTF correction detects whether positive or negative direction, the edge direction different MT
Setting means for setting the F correction coefficient, and
The detection is performed from the MTF correction coefficient for each set edge direction.
MTF correction corresponding to the edge direction detected by the means
An image signal processing device comprising: a selection unit for selecting a coefficient .