JPH04104576A - Picture reader - Google Patents

Abstract

PURPOSE:To obtain reproduced picture without jaggy by switching the cuttoff frequency of a low-pass filter according to reading resolution. CONSTITUTION:The picture signal photoelectrically converted by a CCD20 is inputted to an A/D converter 22 after its high-frequency component is cut by a low-pass filter 21. In this case, the low-pass filter 21 is composed of plural filters with different cut-off frequency, and these filters are designed to be selected by an analog switch 27. When the reading is performed by the resolution lower than the one set by an optical system or when a half tone original or the like is read, the cut-off frequency of the low-pass filter inserted to the circuit dealing with the picture signal is switched. Thus, the jaggy generated by picture thinning or the like can be reduced by adjusting the frequency of the picture signal in an analog part according to the reading resolution.

Description

[Detailed description of the invention] [Industrial application fields] The present invention relates to an image reading device.

[Prior Art] Conventionally, this type of image reading device has an optical path, an optical system such as a lens, an image sensor, an analog section such as an amplifier, an A/D
It consists of a digital section such as a converter and memory, and when reading at a resolution other than that set by the optical system, processing such as pixel thinning was performed in the digital section.

Furthermore, when reading halftone originals, etc., moiré interference occurs, so countermeasures have been taken such as inserting an optical low-pass filter into the optical path.

[Problems to be Solved by the Invention] However, when reading at a resolution lower than that set in the optical system due to thinning processing, etc., if there are lines or edges of the image diagonal to the scanning direction, jaggedness may appear in the reproduced image. There is a problem in that this occurs and the image quality deteriorates. Here, to explain the process of occurrence of jaggies using Fig. 4, Fig. 4(a) shows a document image, and Fig. 4(b) shows an image reading device with a resolution that is half of the maximum resolution. In the figure, black circles indicate pixels used by the image reading device to read a document and reproduce an image, and white circles indicate pixels discarded due to thinning processing. Moreover, FIG. 4(C) shows a reproduced image in this case. In this way, when thinning processing is performed, the values of discarded pixels are not taken into account, which causes judder.

Furthermore, when an optical low-pass filter or the like is inserted in the optical path as a measure against moiré, there is a problem in that the reading resolution is unnecessarily lowered even for documents such as photographs where moiré interference is less likely to occur.

The present invention has been made to solve the above-mentioned problems, and reduces judder caused by pixel thinning by adjusting the frequency of the image signal in the analog section according to the reading resolution. The purpose is to

Another purpose is to efficiently prevent moiré interference by removing high frequency components of image signals in the analog portion in accordance with the original.

[Means for Solving the Problems] In order to achieve this object, the image reading device of the present invention includes a low-pass filter with a variable cutoff frequency in a circuit that handles image signals.

The apparatus also includes means for switching the cutoff frequency of the low-pass filter depending on the original to be read and the resolution to be read.

[Function] The image reading device of the present invention having the above configuration can be inserted into a circuit that handles image signals when reading at a resolution lower than the resolution set by the optical section or when reading halftone originals, etc. Switch the cutoff frequency of the low-pass filter.

[Example 1] Hereinafter, an example embodying the present invention will be described with reference to the drawings.

First, the overall configuration of the reading device will be explained with reference to FIGS. 2 and 3.

A guide shaft 2 and a guide plate 3 are attached to a frame 1 in parallel with a predetermined interval.

A reading unit 4 is arranged between the guide shaft 2 and the guide plate 3, and one side of the reading unit 4 is mounted on a bearing 5.
The reading unit 4 is supported on the opposite side by the guide shaft 2 via the guide plate 3 with a guide plate 3 sandwiched therebetween to prevent the reading unit 4 from floating. Further, the reading unit 4 is movable along the guide shaft 2.

A drive motor 7 is attached to the frame 1.
The reading unit 4 is moved by a drive motor 7 via a wire lobe 6. Further, a glass cover 8 is attached to the frame 1, and a document table glass 9 is fixed on the glass cover 8.

The control board 10 is for controlling the operation of the reading device, and the reading unit 4 is connected to the flat cable (
(not shown). Further, the control board 10 is attached to the frame 1 together with the protection plate 11. The frame 1, the upper cover 12, and the lower cover 13 are attached to each other via rubber 14, which is fitted into the frame 1. A document presser 15 is attached to the upper cover 12 for holding the document so that the document does not shift during reading.

Next, the configuration of the reading unit 4 will be explained with reference to FIG. The reading unit 4 includes a light source 17 that illuminates a document 16 placed on a document table glass 9, and three mirrors 19 arranged at predetermined positions to guide reflected light from the document 16 to an imaging lens 18. has been done. Further, an imaging lens 18 for imaging the reflected light from the original 16 on the CCD 20 is provided inside, and a lens bracket is arranged to support the CCD 20 at a predetermined position. In addition, a light source 17, a mirror 1
9, the lens 18 and the CCD 20 are integrally constructed as a unit, and are scanned under the document table glass 9 by the drive motor 7.

Next, the circuit configuration of the image reading device of this embodiment will be explained using FIG. 1(a). First, the image signal photoelectrically converted by the CCD 20 is passed through a low-pass filter 20.
After high frequency components are cut by the A/D converter 22
is input. Here, the low-pass filter 21 is composed of a plurality of filters having different cutoff frequencies, as shown in FIG. 1(b), and these filters can be selected by an analog switch 27. It is also possible to bypass the filter so that it does not have the same electrical effect.

Next, the image signal converted into digital data by the A/D converter 22 is subjected to processing such as pixel thinning in the processing section 23, and then outputted to the outside by the input/output circuit 24.

Furthermore, the low-pass filter 21, the processing section 23, and the input/output circuit 24 are controlled by the CPU 25, and the resolution, type of document to be read, etc. can be specified from an external computer 26.

Next, the operation of the present image reading apparatus will be explained with reference to the flowchart shown in FIG. First, this image input device is set to an initial state after power is turned on (step 1, hereinafter S1
). The initial state here refers to a state in which the reading resolution is set to the resolution set by the optical system, and the low-pass filter is set to a bypass state that does not affect it. Also,
A normal mode and several halftone dot document modes are provided as document type settings, and the normal mode is selected as an initial state. The normal mode and halftone original mode mentioned here are setting values for switching the cutoff frequency of the low-pass filter, and details will be described later.

Next, it communicates with an external computer to set the resolution, type of document to be read, etc. (S2). Here, the resolution,
Or, if there is a change in the settings of the scanned original (S3.
5) After switching the cutoff frequency of the low-pass filter (S4, S6), start reading (S
7).

Next, countermeasures against jaggies and moiré, which are the objects of the present invention, will be explained. In FIG. 5, (a) is the output waveform of the CCD, and the numbers correspond to each pixel. (b) is the output of the reading device when even-numbered pixels are discarded due to pixel thinning, but as shown in the problem section, the values of the discarded pixels are not taken into account, which causes jaggies. (C) is the waveform when a low-pass filter is inserted after the CCD, and (d) is the output of the reading device after performing A/D conversion and thinning processing based on this. As you can see from this waveform,
Since the values of even-numbered pixels that were discarded are also taken into account,
A reproduced image with less jaggedness can be formed. However, inserting a low-pass filter will result in lowering the resolution, so the cutoff frequency of the low-pass filter must be changed according to the reading resolution. In other words, it is necessary to lower the cutoff frequency according to the specified low resolution. In this embodiment, the CPU performs this process and selects a low-pass filter according to the reading resolution, so that jaggies can be efficiently reduced.

Furthermore, when reading a halftone dot document, if the input pixel pitch is smaller than the halftone dot period of the document, moiré occurs. In this case, by lowering the cutoff frequency of the low-pass filter and lowering the frequency of the image signal, it is possible to increase the apparent input pixel pitch and prevent the occurrence of moiré.

Since the image reading device of this embodiment is equipped with several types of low-pass filters, the cutoff frequency can be switched in accordance with the halftone dot period of the document.

Here, the above-mentioned normal mode and halftone original mode will be explained. Normal mode is a mode in which the low-pass filter set by the reading resolution is selected as is, and halftone original mode is a mode in which a filter with a lower cutoff frequency than the set filter is selected. , has several modes. When actually reading, if moiré occurs in the reproduced image after reading, specify the halftone original mode and select a filter with a cutoff frequency lower than the low-pass filter set in resolution specification to prevent moiré interference. This process is performed.

Moiré can be removed efficiently by lowering the cutoff frequency as the halftone period increases. Further, for documents such as photographs, the normal mode can be specified and reading can be performed without lowering the resolution more than necessary and deteriorating the image quality.

[Effects of the Invention] As is clear from the detailed description above, according to the present invention, the cutoff frequency of the low-pass filter is switched in accordance with the reading resolution, and a reproduced image without jaggedness can be obtained. Further, by switching the cutoff frequency depending on the original, it is possible to prevent moire interference from occurring.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of the reading device, FIG. 2 is a diagram of the entire device, FIG. 3 is an explanatory diagram of the reading unit, and FIG. 4 is a diagram showing the process by which jags occur. It is. Further, FIG. 5 is a diagram showing a process of reducing jaggies and moire disturbances, and FIG. 6 is a flowchart. In the figure, 20 is a CCD, 21 is a low-pass filter, 25 is a CPU, 24 is an input/output circuit, and 26 is an external computer. Figure 2 IN1 Figure (a) Figure 1 (b) Figure 3/16 1st+Figure (a) 1+Figure (b)@4th) (C) ゛5th Zaka 6th

Claims (1)

[Scope of Claims] 1. In an image reading device using a solid-state image sensor formed by arranging a plurality of light-receiving elements, a low-pass filter with a variable cut-off frequency is inserted into a circuit that handles image signals. Characteristic image reading device. 2. The image reading device according to claim 1, wherein the cutoff frequency of the low-pass filter is switched in accordance with the reading resolution. 3. The image reading device according to claim 1, wherein the cutoff frequency of the low-pass filter is switched depending on the type of document.