JPH0720199B2 - Image processing device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image processing apparatus for supplying a recording signal to an image forming means for forming dots for each pixel such as ink jet recording, thermal recording or thermal transfer recording.

[Prior Art] Conventionally, for the purpose of improving the gradation of an image to be printed, it has been proposed to equip this type of image processing apparatus with a unit for changing the density of dots forming pixels. For example, in an ink jet printer, a density varying method using two or more kinds of inks having different densities has been proposed. In a thermal printer or a thermal transfer printer, a density varying method for changing a voltage or a pulse width applied to a thermal head is proposed. A scheme has been proposed. In a density-variable printer that performs image recording by changing the density of these dots, a method is generally used in which highlight areas are recorded with light dots and dark areas are recorded with dark dots. It was

However, in such a conventional dark and light dot output type printer, an image of an original is once converted into an image signal corresponding to the density of the original by an image input device such as a television camera or a drum scanner, and the image signal is converted into the image signal. Printing is performed in the form of dark and light dots based on this, but good image quality can be obtained for images with gradually changing gradations such as photographic images, but on the other hand, gradations such as characters and line figures change rapidly. Regarding the image, since the blur due to the blur of the optical system of the image reading device or the aperture effect of the image reading element (for example, CCD) is faithfully reproduced at the time of output, there is a drawback that the image quality becomes poor with blurred edges. It was

On the other hand, various means have been proposed for a means for discriminating an image area such as a photographic image (hereinafter referred to as a photograph area) from an image area such as a character or a line figure (hereinafter referred to as a character / line figure area). This is a technique, for example, Japanese Patent Laid-Open No. 56-149674 discloses an area discrimination method for identifying the type of image area according to the run length distribution structure of binary image information, and Japanese Patent Laid-Open No. Sho 58-58. -3374 discloses that the screen is divided into blocks each including a plurality of pixels, and the image area is determined according to whether the difference between the density levels of the maximum and minimum density levels in each block is larger or smaller than a predetermined value. A region discrimination method for identifying the type of is disclosed.

However, regarding the method of improving the image quality of the character / line figure area by combining the means for discriminating the area of the image and the means for changing the density of dots, no effective proposal has been made so far. .

[Object] In view of the above points, an object of the present invention is that when a gradation image is formed by changing the density of dots formed for each pixel in a plurality of steps, the input image signal causes the line drawing area to By controlling the selection of a plurality of recording materials for dot formation depending on whether it represents a halftone area or a halftone area, the line drawing area can produce a sharp image formation without an edge blur in one screen. The area is intended to form an image with excellent gradation.

In order to achieve such an object, the present invention supplies a multi-gradation recording signal for each pixel to an image forming unit that forms a gradation image by changing the density of dots formed for each pixel in a plurality of steps. An image processing apparatus for performing the image processing, the input means inputting an image signal for each pixel, and an image for determining for each pixel whether the image signal input by the input means represents a line drawing area or a halftone area. Area discrimination means, image processing means for processing the image signal input by the input means, and outputting a multi-tone recording signal for each pixel to be supplied to the recording means, and the image area discrimination means When it is determined that the image signal represents a line drawing area according to the determination signal, dots are formed by the first recording material, and when it is determined that the image signal represents a halftone area, The first recording agent and Serial and having a control means for selecting control of the recording material to form a dot by different second recording agent from the first recording agent.

[Examples] Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of the configuration of the circuit system of the image processing apparatus of the present invention. Here, 1 is an area discriminating circuit for discriminating whether or not the input digital image signal S1 is a signal of a photographic image area, and sends out a photographic area signal S2 when it is a photographic image area. Reference numeral 2 denotes a selection circuit, which sends the input image signal S1 as it is as an output signal S3 when the photographic region signal S2 as a selection signal is a logic "1", and outputs it as a logic "0".
Send "0" as S3. Reference numeral 3 denotes a D / A converter for forming a light dot, which converts only a signal having a predetermined level or less in the digital image signal S3 input from the selection circuit 2 into an analog signal S4 to form the light dot forming means 5 Send to.

Reference numeral 4 denotes a D / A converter for forming a dark dot, which converts only a signal having a predetermined level or higher in the input digital image signal S1 into an analog signal S5 and sends it to the dark dot forming means 6. .

In the figure, the image signal S1 is read from an original by an image input device such as a television camera or a drum scanner, and is subjected to image signal correction processing such as shading correction or gamma correction to be a 4-bit, 16-step digital signal. It is a digital signal corresponding to the density of the document that has been read. The brightest part of the document has a value of 0 and the darkest part has a value of 15. The image signal S1 is supplied to the area discriminating circuit 1, and the area discriminating circuit 1 discriminates whether or not the image signal S1 belongs to the photograph area. From the area discrimination circuit 1,
Logic "1" when the image signal S1 belongs to the photo area,
If not, the photographic area signal S2 which becomes logical "0" is output.

The photo area signal S2 sent from the area discrimination circuit 1 is supplied to the selection circuit 2 as a selection signal. Therefore, the selection circuit 2 outputs the image signal S1 to the D / A converter 3 only when the image signal S1 to be output belongs to the photographic area, and otherwise outputs 0 to the D / A converter 3. On the other hand, D
The image signal S1 is directly supplied to the / A converter 4.

The D / A converters 3 and 4 receive the supplied digital image signals S3 and S
In response to 1, output predetermined analog voltage signals S4, S5.
The digital-analog conversion characteristics of the D / A converters 3 and 4 are the recording characteristics of the light dot forming means 5 and the dark dot forming means 6, that is, the relationship between the applied voltage and the density of the print image to be printed, as described later. Preset from.

Examples of the light dot forming means include an inkjet head that ejects light ink from a nozzle, and examples of the dark dot forming means include an ink jet head that ejects dark ink from a nozzle.

FIG. 2 shows an example of the relationship between the level value (0 to 15) corresponding to the density of the original of the image signal S1 and the density of the print recorded by the light and dark dots. A curve 7 in FIG. 2 shows a recording area with light dots, and a curve 8 shows a recording area with dark dots. As shown in the figure, when the level of the image signal S1 is 8 levels or less, light dots are used for recording. When the signal S1 is at level 9 or higher, recording is performed using dark dots.
The relationship between the input digital image signal S1 and the print density recorded by the dot forming means 5 and 6 should be a substantially linear relationship by setting the digital-analog conversion characteristics of the D / A converters 3 and 4 described above. Has become. Therefore, if recording is performed by the circuit of FIG. 1 based on the image signal S1 that is input by the image input device and is corrected to have a linear relationship with the density of the original through processing such as shading correction and gamma correction, And the print density have a linear relationship, and faithful gradation reproduction is performed.

FIGS. 3 (A) to (C) and FIGS. 4 (A) to (D) show how the image signals of the photograph area and the character / line figure area are reproduced by the apparatus of the present invention shown in FIG. Indicates.

First, FIG. 3 (A) shows an example of the relationship between the density and the position of a photographic image on a document in which the document density changes gently. In the figure, the horizontal axis χ represents the position on the document along one scanning line of an image input device such as a television camera or a drum scanner, and the vertical axis represents the density of the document at that position.

FIG. 3B is a position on the original image as shown in FIG. 3A, and a digitized image signal read by the image input device and subjected to shading correction and gamma correction. The relationship with the level value of S1 is shown, and the sampling pitch during image input is 0.1 mm. FIG. 3C shows the relationship between the print position χ ′ and the print density when recording is performed by the apparatus of the present invention shown in FIG. 1 based on the image signal S1 shown in FIG. 3B. Here, it is assumed that the print is made in the same size as the original, and the positional relationship between χ and χ ′ is matched. The relationship between the image signal S1 and the print density is based on the characteristics shown in FIG.

As is clear from FIGS. 3 (A) to 3 (C), in the photographic area where the gradation changes smoothly, dot printing is performed with both light dots and dark dots, so that from the low density area to the high density area. The gradation is reproduced smoothly.

On the other hand, FIGS. 4 (A) to 4 (D) show a character / line figure image in which the document density changes abruptly, and as in FIGS. 3 (A) and 3 (B), FIG. 4A shows the relationship between the position on the document and the document density, and FIG. 4B shows the relationship between the position on the document and the level value of the image signal S1. Further, FIG. 4C shows a case where the device of the present invention as shown in FIG. 1 is not carried out, that is, the character / line figure area is not darkened without selecting the dark and light dots as in the conventional case. FIG. 4D shows the relationship between the print position and the print image density when dot printing is performed with both dots and light dots, and FIG. 4D shows the print when the apparatus of the present invention in FIG. 1 is executed. The relationship between the upper position and the print image density is shown.

The character / line figure image has an edge portion that rapidly changes from low density to high density as shown in FIG. 4 (A). However, the image signal input by the image input device
When converted to S1, the edges are blurred as shown in FIG. 4 (B) due to the blurring of the optical system and the aperture effect, and if this signal S1 is recorded as is using both light dots and dark dots, Since the low-density area to the high-density area are faithfully expressed according to the image signal S1, the print output has a blurred edge portion as shown in FIG. 4 (C).

However, in the present example, for the character / line figure area, only the high density level image signal S1 having a predetermined value or more is selected by the D / A converter 4 and the dark dot forming means 6 records only the dark dots. Since the low-density level image signal S1 is cut off by the area discrimination circuit 1 and the selection circuit 2, even if the edge-blurred image signal S1 as shown in FIG. The density range is cut, and a sharp edge as shown in FIG. 4D can be reproduced.

The area discriminating circuit 1 can be easily realized by a conventional publicly known technique, and therefore its detailed configuration is omitted.

In the above-described embodiment, the dot forming means 5 and 6 have been described by using those in which the recording density can be changed in an analog manner by the applied voltage, but the present invention is not limited to this, and whether dots are hit or not hit It can also be applied to the case of using a dot forming means that can only perform binary recording. When such a binary recording type dot forming means is used, the number of dots to be recorded in a dot matrix generally known as a dither method or a density patterning method is changed according to the density range. It is sufficient to express the low density area by a matrix of light dots and the high density area by a matrix of dark dots without gradation recording. Then, the D / A converters 3 and 4 in FIG. 1 may be replaced with circuits that generate dot matrices in their respective density regions.

As described above, the present invention is an image forming apparatus that forms an image by dots, and any type of apparatus, for example, an apparatus that only displays an image, can be used as long as it can change the density of dots. can do.

[Effect] As described above, according to the present invention, when a gradation image is formed by changing the density of dots formed for each pixel in a plurality of steps, the input image signal causes the line drawing area to By controlling the selection of a plurality of recording materials for dot formation depending on whether it represents a halftone area or a halftone area, the line drawing area can produce a sharp image formation without an edge blur in one screen. An image can be formed in the area with excellent gradation.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of the circuit configuration of the image processing apparatus of the present invention, and FIG. 2 is a characteristic diagram showing the relationship between the image signal S1 and the recording density when recording is performed with light and shade dots in the apparatus of FIG. 3 (A) to 3 (C) are characteristic diagrams for explaining the reproduction operation of the photographic area, and FIGS. 4 (A) to 4 (D) are each for explaining the reproduction operation of the character / line figure area. It is a characteristic diagram. 1 ... Area discrimination circuit, 2 ... Selection circuit, 3, 4 ... D / A converter, 5 ... Light dot forming means, 6 ... Dark dot forming means.

Claims (1)

[Claims] 1. An image processing apparatus for supplying a multi-tone recording signal for each pixel to an image forming means for forming a gradation image by changing the density of dots formed for each pixel in a plurality of steps. An input unit for inputting an image signal for each pixel; an image region determination unit for determining for each pixel whether the image signal input by the input unit represents a line drawing region or a halftone region; Image processing means for processing the image signal input by the means and outputting a multi-gradation recording signal for each pixel for supplying to the recording means; and, in accordance with the discrimination signal from the image area discrimination means, When it is determined that the image signal represents a line drawing area, dots are formed by the first recording material, and when it is determined that the image signal represents a halftone area, the first recording material and Different from the first recording agent An image processing apparatus comprising: a control unit that controls the selection of the recording agent to form dots with the second recording agent.