JPH0820103A - Halftone image outputting method - Google Patents

Abstract

PURPOSE:To obtain halftone images in appropriate dot percentage corresponding to density value shown by image signals. CONSTITUTION:Image signals for a standard density are generated (step 1), and after converting the image signals into halftone signals with a halftone signal generating device (step 2), standard halftone images are outputted with a n output scanner (step 3). Then, the standard halftone images to be controlled are read (step 11) with read density higher than the recording density for that halftone dot, and binary signals are obtained therefrom (step 13) with a specified threshold applied. Then, dot percentage of the standard halftone images is measured (step 14) by counting the number of signals on one of the binary signals, and correction data are produced (step 16) by comparing the counted results with the value of standard density. Conversion characteristic for the halftone signal generating device is determined, based on the correction data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a halftone dot image output method for printing with halftone dots based on an image signal.

[0002]

2. Description of the Related Art In order to control the finish of a halftone dot portion such as a photograph in a printed matter, a halftone dot area ratio indicating a ratio of an image portion of the halftone dot portion (a portion to which ink is attached in the printed matter) is defined as an original image signal. It is necessary to check whether or not the value represented by corresponds appropriately. This halftone dot management may have to be performed on the printed matter obtained in the final step, or may have to be performed at the plate used before the step or at the stage of the film for making the plate. is there.

In conventional halftone dot management, the halftone dot area ratio (density value) is obtained by measuring the transmittance or reflectance of a film, printing plate, printed matter, etc. using a densitometer, and the measured density value And the density value represented by the original image signal are compared to check whether the correspondence between the two is proper, and if not, correct the conversion characteristic from the original image signal to the halftone dot signal. It was done by the method. Here, as shown in FIG. 8, in the measurement by the densitometer, the density reading range 83 is set to the size of the halftone dot of the halftone dot image 80 formed on the film, the printing plate, the printed matter, that is, the individual halftone dot By setting the size to be sufficiently larger than the size of the image portion 81 / non-image portion 82 for each, the average density within the range 83 was measured as the density value and converted into the halftone dot area ratio.

[0004]

Image portion or non-image portion (a portion to which ink does not adhere) of a film, a printing plate, a printed matter, or the like.
Since the density varies depending on the recording medium used, there is an error in the halftone dot area ratio calculated based on the density value measured by the densitometer. In particular, in the case of a printing plate, the difference in reflectance between the image portion and the non-image portion is small, so that there is a problem in that there is a large error in the measurement with the densitometer.

The present invention has been made to solve the above problems, and an object of the present invention is to accurately measure a dot area ratio of a film, a printing plate, a printed matter, or the like. Another object of the present invention is to provide a halftone dot image output method capable of outputting halftone dots having an appropriate halftone dot area ratio according to an original image signal.

[0006]

The halftone dot image output method according to the present invention, which was made to solve the above-mentioned problems, includes (a) converting an image signal indicating a reference density into a halftone dot signal, and (b) The reference halftone dot signal is output based on the halftone dot signal corresponding to the reference density, and (c) the reference halftone dot image is read in pixel units smaller than the halftone dot and binarized with a predetermined threshold value.
The halftone dot area ratio is measured by generating a binarized signal and (d) determining the ratio of the number of signals of one value of the binarized signal, and (e) the reference density and the reference halftone corresponding to the reference density. Determine the conversion characteristics based on the relationship with the measured halftone dot area ratio of the point image, (f) the image signal representing the output target image is converted to a halftone dot signal based on the determined conversion characteristics, (g) The halftone dot signal is output based on the halftone dot signal obtained by the conversion. The predetermined threshold value may be variably set.

[0007]

The operation of the halftone dot image outputting method of the first aspect is as follows. First, when outputting a halftone image based on an image signal representing an output target image, the following preparatory process is performed. The image signal indicating the reference density is converted into a halftone dot signal, and a reference halftone dot image indicating the reference density is output based on the halftone dot signal. The halftone dot area ratio of the reference halftone dot image generally does not match the proper halftone dot area ratio to be output corresponding to the reference density. Then, the output reference halftone dot image is read in pixel units smaller than the halftone dot to generate a binarized signal binarized with a predetermined threshold value. The halftone dot area ratio is measured by obtaining the number of signals having one value of the binarized signal. This makes it possible to detect the relationship between the image signal indicating the reference density and the measured dot area ratio of the dot image output thereby. The conversion characteristic for correcting the image signal is determined based on this relationship, and the preparation process ends.

When outputting the output target image, the image signal representing the output target image is converted into a halftone dot signal based on its conversion characteristic, and the halftone dot image is obtained based on the halftone dot signal obtained by the conversion. Is output. The halftone dot area ratio of the halftone dot image thus obtained shows a proper correspondence with the density value represented by the image signal.

Further, according to a second aspect of the present invention, a material having a threshold value for outputting a halftone dot image, a sensitivity characteristic,
By changing according to conditions such as color, it is possible to accurately measure the dot area ratio.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As one embodiment of the present invention, a halftone dot image output method according to the present invention is applied to a personal computer DTP system equipped with an input scanner for reading an image and an output scanner for outputting an image on a photosensitive material such as a film or a printing plate. An example of implementation will be described. As shown in FIG. 2, the personal computer DTP system of this embodiment is composed of a personal computer 10 having an input scanner 14 for reading an image and an output scanner 15 for outputting an image, as shown in FIG. A workstation class computer may be used instead of the personal computer 10. A display 11, a keyboard 12, a mouse 13 and the like are connected to the personal computer 10.
In terms of software, the personal computer DTP system uses the image data of the image read by the input scanner 14 or the character data input from the keyboard 12 to form an image of one page or a predetermined number of pages of printed matter. However, a DTP program for output by the output scanner 15 is provided.

A continuous tone image such as a photograph read by the input scanner 14 is expressed by halftone dots when it is output by the output scanner 15 as an intermediate step for reproducing a printed matter. Therefore, the DTP program includes a program (halftone signal generation program) for converting an image signal (for example, 8 bits) into a halftone signal (1 bit). This halftone dot signal generation program must determine the conversion characteristic so that the value obtained by converting the density value represented by the image signal into the halftone dot area ratio and the halftone dot area ratio of the output image have an appropriate relationship, for example, the same. I have to. Therefore, in the personal computer DTP system of this embodiment, the conversion characteristic from the image signal to the halftone dot signal is determined by the following method.

First, a plurality of reference halftone dot images are generated by the procedure shown in FIG. First, an image signal representing a predetermined reference density (reference density image signal) is generated (step S1). Next, by the halftone dot signal generation program,
This reference density image signal is converted into a halftone dot signal (step S2). Then, this halftone dot signal is sent to the output scanner 15 to output a halftone dot image (step S3). It is determined whether or not the halftone image has been output for all of the plurality of reference densities (step S4), and if there are still remaining reference densities, the process returns to step S1 and outputs the halftone image until step S3. repeat. When the halftone image is output for all the reference densities, this routine ends. An example of the reference halftone dot image generated by this routine is shown in FIG. In the reference halftone dot image 18 of FIG. 4, reference densities in steps of 10% from 10% to 100% are used, but this selection is arbitrary.

Next, according to the procedure shown in FIG. 1B, the reference halftone dot image 18 is read by the input scanner 14 and correction data for the halftone dot signal generation program is created. Here, the reference halftone dot image 18 read by the input scanner 14 is
Use one that complies with management standards. That is, the output medium used in the output scanner 15 may be a film for making a printing plate, a printing plate itself, or the like.
Although there are various cases, when the film for plate making is used as a control reference, the reference halftone dot image 18 formed on the film is read by the input scanner 14. When using a printing plate created from a film as a management reference, the reference halftone dot image 18 formed on the created printing plate, and when using a printed matter printed on the printing plate as a management reference, The reference halftone dot image 18 on the printed matter is read by the input scanner 14.

In the routine shown in FIG. 1B, first, the reference halftone dot image 18 on the medium to be managed thus selected is read by the input scanner 14 to generate image data (step S11). The size of the read pixel 30 here is set to be smaller than the halftone dots forming the reference halftone dot image 18, as shown in FIG. That is, the reading density is set higher than the halftone dot recording density. For example, when the halftone image is 100 LPI, the read density is 10
It is preferably 1000 to 4000 dpi, which is more than double. After the input image data of the reference halftone dot image 18 is temporarily stored in the memory, the reference halftone dot image 18 is displayed on the display 11. While observing this display, the operator sets the area ratio measurement region for each reference concentration region using the mouse 13 or the keyboard 12 (step S12). The area may be set automatically by using a well-known image recognition program.

Next, the image data in the set area is read out from the memory, and for each pixel, it belongs to the image portion 181 or the non-image portion 182 of the reference halftone dot image 18 (FIG. 6A). In order to distinguish (reference), it binarizes (step S13). Specifically, a value between the density value of the image portion 181 and the density value of the non-image portion is set as the threshold value,
A distinction is made depending on whether the value of the image data of each pixel is greater than or less than this threshold value (see FIG. 6B).

When the output medium to be managed is always the same (for example, when it is determined that the final printed matter is always managed), the densities of the image part and the non-image part of the halftone image are almost unchanged. , You can always use a constant threshold value. However, if there is a possibility that the halftone image media to be managed may be different, such as managing the final printed matter and the printing plate directly output by the output scanner, the image part of each medium or Since the non-image areas have different densities, it is desirable to make the threshold value variable. As one of the threshold value setting methods in this case, a method of using an intermediate density between the maximum density and the minimum density of the image data obtained in step S11 can be used. Step S
A histogram relating to the density (see FIG. 6C) is created on the basis of the image data obtained in 11, and two peaks (non-image part 182 and image part 18 respectively) appearing therein are created.
It is also possible to use a method in which the threshold value is a density value having a local minimum value (corresponding to 1). Of course, other methods may be used.

Based on the binarized signal output in step S13, the number of pixels (number of signals) belonging to the image portion and pixels belonging to the non-image portion is counted, and the number of pixels belonging to the image portion is calculated with respect to the total number of pixels. The ratio, that is, the halftone dot area ratio in the set area of the reference halftone dot image 18 is obtained (step S1).
4). Then, it is determined whether or not the calculation of the halftone dot area ratios has been completed for all the reference darkness areas (step S15). If there is a reference darkness area for which the halftone dot area ratios have not been calculated yet, the procedure proceeds to step S12. Then, the same process is repeated. When the calculation of the halftone dot area ratios for all the reference density regions is completed, the halftone dot area ratio values measured by the process of step S14 for each of the reference density regions,
The density value represented by the image signal is converted into the halftone dot area ratio by comparing the halftone dot area ratio of the halftone dot image to be managed with the reference density value (density%) used when generating the standard halftone dot image. The correction data is created so as to have an appropriate relationship with the calculated value (step S16).

The basic idea of creating correction data is as follows. First, the halftone dot area ratio D converted based on each reference density value used when generating the reference halftone dot image is measured based on the halftone dot image S output based on the reference halftone density value. The function (mapping) to be mapped to is obtained as a function g represented by a curve 41 as shown in FIG. 7A, for example. Then, an inverse function (inverse mapping) g ⁻¹ of the function g is obtained, and this inverse function g ⁻¹ is used as correction data. The correction data thus created is stored in the memory. Then, when the halftone image is output to the output medium based on the image signal representing the image to be actually output, the halftone dot area ratio converted based on the image signal as shown in FIG. 7B. DD is converted into a correction signal y by correction data, that is, an inverse function g ⁻¹ , the correction signal y is converted into a halftone dot signal, and a halftone image is output. The halftone dot area ratio SS of the halftone dot image thus output is determined by the image signal DD.
Since the correction signal y obtained by mapping is obtained by the function (mapping) g ⁻¹ is further copied by the function (mapping) g, it becomes equal to the halftone dot area ratio DD. By thus correcting the image signal before converting it into a halftone dot signal, a halftone dot image having an appropriate halftone dot area ratio with respect to the density value represented by the image signal can be obtained.

Further, in order to create other correction data, the difference or ratio between the straight line (S = D) 40 and the curve 41 showing the ideal characteristic shown in FIG. May be obtained for each density value and stored as correction data in the memory. In this case, when actually outputting an image signal indicating an image to be output, a correction signal is first obtained by adding or multiplying the image signal based on the correction data, and a halftone dot signal is obtained based on this correction signal. It suffices to perform conversion and then output a halftone image.

In the above-described embodiment, the personal computer 10 performs all the generation of the halftone dot signal and the pixel count for the measurement of the halftone dot area ratio by software, but as shown in FIGS. It can also be configured with a hardware circuit. First, in the portion for outputting the reference halftone dot image 18, as shown in FIG.
1, the image signal correction circuit 22 and the halftone dot signal generation circuit 23 can be used. The operation of these circuits is as follows. The reference density image signal generation circuit 21 sends an image signal having various reference density values to the halftone dot signal generation circuit 23 of the output scanner 55 via the image signal correction circuit 22. The halftone dot signal generation circuit 23 generates a halftone dot signal corresponding to the reference density based on the image signal from the image signal correction circuit 22, and the reference halftone dot image 18 on the output medium 151 as in the above embodiment. Is output. At this stage, the image signal correction circuit 22 does not correct the image signal.

On the other hand, in the portion for measuring the halftone dot area ratio of the reference halftone dot image 18, as shown in FIG. 5, a binarization circuit 25 and an area ratio measuring circuit 26 can be used. The image signal generated by the photoelectric conversion unit 141 of the input scanner 14 is sent to the binarization circuit 25. The binarization circuit 25 binarizes the image signal according to the threshold value provided from the threshold setting circuit 27, and the area ratio measuring circuit 26
Send to The area ratio measuring circuit 26 counts the number of image signals of each value of the binarized signal, generates data representing the value of the dot area ratio of each part of the reference dot image 18, and the personal computer 10
To the CPU 24. The procedure for creating the correction data from the halftone dot area ratio data is as in the above embodiment. The created correction data is given from the CPU 24 to the image signal correction circuit 22, as shown in FIG. Therefore, thereafter, the image signal read from the image memory 20 is sent to the halftone dot signal generation circuit 23 after being corrected by the image signal correction circuit 22. As a result, a halftone dot image having an appropriate halftone dot area ratio corresponding to the value of the image signal is created on the printing plate, printed matter, or the like.

[0022]

According to the halftone image output method of the first aspect,
The halftone dot area ratio of the output halftone dot image is read in pixel units smaller than the halftone dot, binarized, and the halftone dot area ratio is obtained by obtaining the ratio of the number of signals of one value of the binarized signal. Therefore, it is possible to accurately measure the dot area ratio, and as a result, it is possible to determine accurate correction data according to an appropriate relationship between the image signal and the dot area ratio of the output dot image.

In the halftone dot image outputting method according to the second aspect, since the threshold value is variable depending on conditions such as the material of the medium on which the halftone dot image is output, the sensitivity characteristic, the color, etc. The point area ratio can be measured.

[Brief description of drawings]

FIG. 1 is a flowchart showing a procedure for generating correction data for dot signal conversion in a personal computer DTP system that is an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a personal computer DTP system according to an embodiment.

FIG. 3 is a circuit diagram showing another configuration example of an image output unit.

FIG. 4 is a plan view of a reference halftone image.

FIG. 5 is a circuit diagram showing another configuration example of the image reading unit.

FIG. 6 is an explanatory diagram of a procedure for reading a reference halftone dot image and binarizing it.

FIG. 7A is a graph showing the relationship between the reference density value and the actually measured halftone dot area ratio. (B) A graph showing the relationship between the image signal, the corrected image signal, and the halftone dot area ratio.

FIG. 8 is a plan view illustrating a conventional halftone image density measuring method.

[Explanation of symbols]

 10 ... Personal computer 14 ... Input scanner 15 ... Output scanner 18 ... Reference halftone image 181 ... Image portion 182 ... Non-image portion 20 ... Image memory 21 ... Reference density image signal generation circuit 22 ... Image signal correction circuit 23 ... Halftone signal generation Circuit 24 ... CPU 25 ... Binarization circuit 26 ... Area ratio measuring circuit 27 ... Threshold setting circuit 30 ... Read pixel

Claims (2)

[Claims] 1. An image signal indicating a reference density is converted into a halftone dot signal, (b) a reference halftone dot signal is output based on a halftone dot signal corresponding to the reference density, and (c) a reference halftone dot signal is output. The halftone dot image is read in pixel units smaller than the halftone dot, and a binarized signal binarized with a predetermined threshold value is generated, and (d) the ratio of the number of signals of one value of the binarized signal is obtained. By measuring the dot area ratio by (e) determining the conversion characteristic based on the relationship between the reference density and the measured dot area ratio of the reference dot image corresponding to the reference density, (f) ) An image signal representing an output target image is converted into a halftone dot signal based on the determined conversion characteristic, and (g) a halftone dot image output that outputs a halftone dot signal based on the halftone dot signal obtained by the conversion. Method. 2. The halftone image output method according to claim 1, wherein the predetermined threshold value can be variably set.