JP3092147B2 - Image processing device - Google Patents

Description

The present invention relates to an image obtained by reading pixels in a main scanning direction by electronic scanning of a reading sensor and reading pixels in a sub-scanning direction by moving the reading sensor. The present invention relates to an image processing device that corrects a spatial frequency characteristic of data with a digital filter.

[Conventional technology]

In the digital image processing apparatus, for example, a CCD line sensor that reads a document at a resolution of 400 spi is used as a reading unit. This CCD line sensor can take out the video signal of the original image with the mixed primary colors B (blue), G (green), and R (red) by installing a color filter or by switching the light source. , And can be used for reading an original of a color digital copying machine.

FIG. 3 is a diagram showing a schematic configuration of a color digital copying machine.

In the color digital copying machine shown in FIG. 3, the IIT 41 scans and reads the original using, for example, a CCD line sensor, converts the analog signal into a digital signal by the A / D converter 42, and performs various correction processes. The IOT49 reproduces the binary image and the halftone image of the original by the halftone image. As described above, when a video signal of a document image color-separated into B, G, and R is output from the IIT 41, the analog value of the video signal is first processed.
The A / D converter 42 converts the data into an 8-bit digital value.
Then, in the shading correction circuit 43, the light amount unevenness of the light source and the CC
After correcting unevenness in sensitivity of the D line sensor, the output B
GR is divided into B, G and R colors by a color separation circuit 44,
The ghost correction circuit 45 corrects color ghosts that could not be color-separated. Further, the image data is output as a color copy by the IOT 49 through the density adjustment circuit (AE) 46, the digital filter 47, and the TRC 48. Note that the same configuration is adopted for a copy other than full color.

Generally, if the image data obtained by reading with IIT41 is processed as it is and reproduced with IOT49, the image is blurred.
The digital filter 47 performs MTF correction in accordance with the characteristics of the IOT 49. The digital filter 47 detects and enhances edges for character and line images, and removes halftone dots for halftone images. Also,
TRC48 controls the tone reproduction.

In image processing devices such as copiers and fax machines that use a CCD line sensor as described above, the CCD line sensor is moved in the sub-scanning direction by moving the CCD line sensor while reading pixels in the main scanning direction by electronic scanning. Are read to output a document image.

[Problems to be solved by the invention]

However, even if the spatial frequency characteristics are corrected using a digital filter as described above and edge enhancement is performed, the correction is not sufficient for a line image, and the line width and density cannot be output in a well-balanced manner. For this reason, when characters of different sizes are mixed in a character document, clear characters cannot be reproduced in the same manner, and a problem occurs that the image is remarkably deteriorated as copying is repeated.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has as its object to provide an image processing apparatus that improves the reproducibility of a line image. It is another object of the present invention to effectively correct the spatial frequency characteristics of a document reading sensor.

[Means for solving the problem]

For this purpose, the present invention corrects the spatial frequency characteristics of image data obtained by reading pixels in the main scanning direction by electronic scanning of the reading sensor and reading pixels in the sub-scanning direction by moving the reading sensor using a digital filter. Wherein the correction gain of the digital filter is set higher in the sub-scanning direction than in the main scanning direction, and higher in the frequency band of the line image than in other frequency bands. As a result, even when characters of different sizes are mixed in a character document, clear characters can be reproduced similarly, and the definition of horizontal lines in the sub-scanning direction in the line image can be increased.

〔Example〕

 Hereinafter, embodiments will be described with reference to the drawings.

FIG. 1 is a diagram for explaining an embodiment of an image processing apparatus according to the present invention, and FIG. 2 is a diagram for explaining an effect of improving the reproducibility of a line image.

In the IIT, which reads electronically in the main scanning direction and mechanically in the sub-scanning direction, the MTF (spatial frequency) characteristics in the sub-scanning direction are worse. Reproducibility is poor. Therefore, when the gain of the digital filter is made isotropic in the vertical and horizontal directions, the output copy is considerably weaker in the horizontal line than in the vertical line.

Therefore, the present invention is directed to the main scanning direction as shown in FIG.
The MTF gain in the sub-scanning direction is set higher than the MTF gain. The example shown in FIG. 1 is a 5 × 5 two-dimensional digital filter as shown in FIG. 1A, in which coefficients are set as a to i. Then, as shown in FIG. 3B, the horizontal axis represents the spatial frequency and the reciprocal of the sampling frequency, and the vertical axis represents the MTF gain. ".5" on the horizontal axis indicates that the sampling frequency is 400s, for example.
In the case of pi (spot per inch), the frequency band is half that of 200 spi.

In general, a character that can be reproduced at a resolution of 400 spi is a character with 6 points. On the other hand, half the frequency of 400 spi is the optical limit of the lens, and is also the so-called Nyquist limit of the sampling theorem in a region where various noises are likely to enter. For this reason, as shown in FIG. 7B, when the MTF gain is increased near “.3” and is suppressed toward “.5”, the MTF gain particularly corresponds to a horizontal line of 6-point Mincho characters. It is possible to improve the reproducibility in the area, that is, the limit at which reproduction can be performed at a resolution of 400 spi, and suppress noise in an area where noise is likely to enter. In this case, the MTF gain in the main scanning direction and the sub-scanning direction has a certain width due to the characteristics of the IIT, but the sub-scanning direction becomes `` 3.
In contrast to “0”, the main scanning direction is “1.75”.

For a 6-point Mincho font, a book with a gain of 1.78 and a gain of 3.0 in the sub-scanning direction compared to a conventional isotropic digital filter with gains of 2.0 in both the main and sub-scanning directions. The results of comparing the image quality difference of the anisotropic digital filter of the present invention with the sensory evaluation value as one index are shown as 57 for the former and 64 for the latter. From this, the introduction of the anisotropic filter eliminates the density difference between the vertical line and the horizontal line, and the reproducibility of the horizontal fine line is clearly improved.

FIG. 2 shows comparison data of reproducibility for a thin line of 100 μm and a middle line of 300 μm. The density of the input data (D _IN ) and the line peak density (LINE PEAK DENSIT
Y). The horizontal line (H) of the conventional isotropic filter is indicated by △, the vertical line (V) is indicated by ▲, the horizontal line (H) of the anisotropic filter of the present invention is indicated by 、, and the vertical line (V) is indicated by ●. The horizontal and vertical balances are greatly improved by the anisotropic filter of the present invention in both FIGS. In particular,
It can be seen that a fine line of 100 μm corresponding to the six-point Mincho font has been improved well.

〔The invention's effect〕

As is apparent from the above description, according to the present invention, the MTF in the sub scanning direction is not an isotropic filter but an anisotropic filter.
Since the MTF characteristic is corrected by setting the gain higher than that in the main scanning direction, the reproducibility of the horizontal line is improved, and the balance between the vertical line and the horizontal line can be improved in both the line width and the density.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an embodiment of an image processing apparatus according to the present invention, FIG. 2 is a diagram for explaining the effect of improving the reproducibility of a line image, and FIG. FIG. 3 is a diagram showing a configuration outline.

Continuation of front page (56) References JP-A-61-137466 (JP, A) JP-A-2-14671 (JP, A) JP-A-62-252260 (JP, A) (58) Fields investigated (Int) .Cl. ⁷ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (1)

(57) [Claims] A spatial frequency characteristic of image data obtained by reading pixels in a main scanning direction by electronic scanning of a reading sensor and reading pixels in a sub-scanning direction by moving the reading sensor is corrected using a digital filter. An image processing apparatus, wherein a correction gain of a digital filter is set higher in a sub-scanning direction than in a main scanning direction and higher in a line image frequency band than in other frequency bands.