JP2673690B2 - Image reading device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image reading apparatus which converts binary data obtained by reading an image into image information of pseudo halftone by a dither method and outputs the image information. [Prior Art] A dither method is known in which a halftone image is obtained by pseudo halftone processing of image information which is binary data in a facsimile machine, an image filing system and the like. Generally, the scanner unit that reads an image with the above-mentioned device changes the linear density during optical reading scanning,
Alternatively, after reading, there is a device in which the linear density of the obtained image information is converted or the image information having an arbitrary linear density can be taken out. For example, as shown in FIG. 4A, assume that the vertical and horizontal sizes of the image read by the scanner unit are X and Y, and the pixel pitch is A when the linear density is set to "normal". The binary image information obtained by reading the image in this state is subjected to dither processing using, for example, a 4 × 4 dither matrix, and converted into halftone image information. With this image information, when an image is displayed or recorded, 4 × 4 pixels corresponding to the matrix size become a set of resolution units. Therefore, in the figure, a square with one side of the dither pitch d ₁ is the resolution unit, and the apparent resolution of the image is 1 /
Drop to 4. Next, for example, when the linear density is set to “coarse”, it is assumed that the pixel pitch is 2A. Conventionally, when the image is read in this state and the dither process is performed, the dither process is performed with the same matrix size regardless of the linear density at the time of reading. Therefore, in this case, as shown in FIG. 6B, the image has a square with a dither pitch d _{2 as} a resolution unit. Then, as is clear from the figure, the linear density is originally coarse, and further, the resolution is lowered by the dither processing, and the image quality is greatly lowered. Next, consider the case where the line density is set to "dense". In this case, since the image is originally a high-density image, the reduction in resolution due to dither processing does not pose a problem. From a psychological point of view when a person evaluates an image, an image with high resolution looks smooth and natural when the number of gradations of halftone is high. However, the number of gradations is determined by the dither matrix size, and has been constant in the past regardless of the linear density.
Therefore, the obtained image has an unnecessarily high number of gradations when the linear density is low, but lacks the smoothness of halftone and lacks the number of gradations when the linear density is high. As described above, conventionally, there is a problem that a good image that is natural and easy to see cannot be obtained. [Purpose] It is an object of the present invention to solve the above problems and provide an image reading apparatus that can obtain a good image that is natural and easy to see. [Structure] To this end, the present invention includes a plurality of dither matrix data whose matrix sizes are respectively proportional to a plurality of linear densities, and executes dither processing by the dither matrix data corresponding to the selected linear density. The dither pitch with respect to the image size is always constant. Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a block diagram of an image reading apparatus according to an embodiment of the present invention. In the figure, a line sensor 1 is a one-dimensional solid-state sensor for reading a document image, and an amplifier 2 is for amplifying a read image signal. The ROM 3 stores data for compensating the characteristics of the light source (not shown) and the sensitivity of the line sensor 1 and the like. The shading correction circuit 4 converts the image signal input as an analog signal into the above data. While the image information is corrected based on the above, the image information is converted into digital image information. The line density conversion unit 5 converts the image information of each line into image information having different line densities by a thinning method or a linear interpolation method. In the present embodiment, "normal", "coarse", and "dense". Three types of linear density conversion are possible. ROM6 is used for dither processing as shown in FIG.
3 types of matrix data table of × 2,4 × 4,8 × 8
D1 to D3 are stored respectively. The counter 7 is for reading each data in each of the tables T1 to T3, and the preset circuit 8 is for initializing the counter 7. The linear density / dither control circuit 9 controls the linear density and dither processing. The comparison circuit 10 outputs pseudo-halftone image information based on the read image information and the matrix data from the ROM 6. With the above configuration, when the image is read by the image reading apparatus of the present embodiment, one of the three types of "normal", "coarse", and "dense" linear density that corresponds to the read image is The dither control circuit 9 is set to start the device. When the apparatus is activated, the line sensor 1 reads an image at a constant linear density. The image signal from the line sensor 1 is amplified by the amplifier 2, and the shading correction circuit 4 performs shading correction and digital image signal 2
Converted to value image information. The image information is converted into image information having the above-mentioned linear density by the linear density conversion unit 5,
It is output to the comparison circuit 10. On the other hand, the linear density / dither control circuit 9
As shown in the following tables (a) and (c), the dither matrix data tables D1 to D3 in the ROM 6 are set according to the set linear density.
Is specified. The preset circuit 8 and the counter 7 cause the ROM 6 to output each data in the selected dither matrix data tables D1 to D3 at a predetermined timing corresponding to the image information output from the linear density conversion unit 5. The comparator circuit 10 outputs a data bit "1" when the value of the input image information data is larger than the value of the dither matrix data. As a result, the predetermined dither processing is executed, and the halftone image information subjected to the pseudo halftone processing is output from the comparison circuit 10. Here, as shown in the above table, it is assumed that the pixel pitch is A when the linear density is “normal”, similarly 2A when “coarse” and A / 2 when “fine”. Now, assuming that an image is read with a linear density of "normal", dither processing of 4 × 4 dither matrix data is performed as shown in the above table. In this case, as in the case of FIG. 4A, the halftone image with the dither pitch d ₁ is taken out. Next, assuming that the image is read with the linear density "coarse", as shown in the above table, the dither processing is performed on the 2 × 2 dither matrix data. In this case, as shown in FIG. 3 (a), the dither matrix size becomes smaller as the pixel pitch becomes larger, and the halftone image having the same dither pitch d ₁ as described above is taken out. Also, when the image is read with the linear density "dense",
As shown in the above table, dither processing is performed on 8 × 8 dither matrix data. In this case, FIG. 3 (b)
As shown in (3), the dither matrix size increases as the pixel pitch decreases, and a halftone image having the same dither pitch d ₁ as described above is extracted. As described above, according to the present embodiment, the dither processing is executed for the selected linear density by the dither matrix data in which the dither pitch with respect to the image size is always constant. Therefore, in the above-described embodiment, since the square having the dither pitch d ₁ as _one piece is always the resolution unit of the image, the higher the resolution of the image, the higher the number of gradations of the halftone, and a good image that is natural and easy to see. You will get it. In the above-described embodiment, the line sensor 1 reads an image at a constant linear density, and then the linear density conversion unit 5 performs linear density conversion. The present invention can be similarly applied to the case of changing. Also, one dither matrix size is prepared for each linear density, but the dither matrix size is
It is also possible to prepare a certain number and arbitrarily select an appropriate linear density. As described above, it goes without saying that the type of linear density and the dither matrix size can be set arbitrarily. [Effect] As described above, according to the present invention, a plurality of dither matrix data whose matrix sizes are respectively proportional to a plurality of linear densities are provided, and dither processing is performed using the dither matrix data corresponding to the selected linear density. By executing this, the dither pitch with respect to the image size is always made constant. Therefore, the higher the resolution of the image, the higher the number of gradations of the halftone, so that a good image that is natural and easy to see can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block configuration diagram of an image reading device according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of a matrix data table, and FIGS. 3 (a) and 3 (b) are obtained. FIGS. 4 (a) and 4 (b) are explanatory views of the dither pitch of the obtained image in the related art. 1 ... Line sensor, 2 ... Amplifier, 3 ... ROM, 4 ...
... Shading correction circuit, 5 ... Linear density converter, 6 ...
... ROM, 7 ... Counter, 8 ... Preset circuit, 9 ...
… Linear density / dither control circuit, 10 …… Comparison circuit.

Claims (1)

(57) [Claims] An image reading unit for reading an image to obtain image information of one linear density selected from a plurality of linear densities, and a dither processing unit for converting the image information into halftone image information by dither processing are provided. In the image reading device, by using a plurality of dither matrix data whose matrix size is proportional to each of the plurality of linear densities and dither matrix data corresponding to the selected linear density, the dither pitch is always set to the image size. An image reading apparatus comprising: a dither matrix data selecting unit that executes a dither process at a constant value.