JPS61118065A - Picture processing device - Google Patents

Abstract

PURPOSE:To obtain a reproduced picture with fidelity for a picture signal modulated in the main scanning direction by forming the shape of a beam spot of light beam so that the beam diameter in the main scanning direction is smaller than the beam diameter in the sub-scanning direction. CONSTITUTION:The beam spot in which the beam diameter LH in the main scanning direction is smaller than the beam diameter LV in the sub-scanning direction is used and the beam diameter LH in the main scanning direction is selected as the width of 1/2 picture element. Pulse width modulation is applied in the main scanning direction by the beam diameter LH in the main scanning direction to obtain a reproduced picture with fidelity to the picture signal modulated in the main scanning direction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an image processing apparatus that performs image processing using a dither method or the like and records an image using a light beam.

[Prior Art] In recent years, digital copying devices that read an original image using a solid-state image pickup device such as a CCD line sensor and output it using, for example, a laser beam printer have become widespread due to their high speed and high image quality. In such devices, the input original images include continuous tone photographs (hereinafter referred to as photographs), characters and line drawings (hereinafter referred to as line drawings), and printed matter composed of halftone dots (hereinafter referred to as photographs).
(hereinafter referred to as halftone photographs), etc. are often mixed together.

In laser beam printers based on electrophotography, it is well known that the dot concentration type dither matrix output method (hereinafter referred to as halftone dotting) provides excellent halftone expression. In this method, when an image containing a mixture of line drawings and photographs is converted into halftone dots, a smooth halftone expression is achieved in the photographic part, but the line drawing becomes sharp due to the halftone conversion. It is especially difficult to decipher complex words such as kanji.

This is because the dither method creates density using the number of dots per unit area, and is therefore excellent in gradation expression, but it also applies gradation processing to line drawings, etc., which results in a decrease in resolution. be. Therefore, one dot of the laser beam is pulse-width modulated (so-called multilevel) to express gradation by expressing 3 to 4 gradations in one pixel, and by reducing the dither matrix size, resolution deterioration is prevented. I tried. This is a so-called multivalued dither method.

However, in order to pulse width modulate one dot, the pulse width modulation device also becomes complicated. Furthermore, since the oscillation characteristics of a laser change depending on conditions such as temperature, pulse width modulation has been limited to three-level or four-level conversion. In such multi-value conversion such as ternarization or tetra-value conversion, it is not possible to reduce the dither matrix size very much in order to obtain gradation, and it is not possible to prevent resolution deterioration.

〔the purpose〕

An object of the present invention is to provide an image processing device equipped with a beam spot of a light beam capable of reproducing an image signal subjected to multilevel pulse width modulation in the main scanning direction of one pixel with higher gradation and resolution. It's about doing.

[First example] Hereinafter, the present invention will be explained in detail based on Examples.

First, FIG. 1 shows an example of a schematic configuration of an image recording apparatus to which the present invention is applied. In the illustrated configuration example, a light beam modulated by an image signal from a semiconductor laser 11 is incident on a rotating polygon mirror 12 through a collimating lens 10 and deflected, and the deflected light beam is passed through an imaging lens 13 to a photosensitive drum 3. An image is formed on the photosensitive layer and the photosensitive layer is scanned. During the light beam scanning, a photodetector 15 detects reflected light from a mirror 14 placed at the tip of the line scan to form a synchronization signal for the line scan.

FIG. 2 schematically shows an image input device to which the present invention is applied. In the illustrated configuration example, a document 21 illuminated by a light source 22 is transferred to a CCD line sensor 24 by a lens 23.
An image is formed on the object and its output signal is obtained. This is the direction of the CCD line sensor 24 or the main scanning direction. C for manuscript 21
Sub-scanning is performed by relative movement of OD line sensor z4.
Obtain dimensional image output.

FIG. 3 is a block diagram showing a signal processing system for image signals obtained from the image input device. An analog image signal from the CCD reading section 30 is subjected to density conversion by an analog-LOG conversion circuit 31. Such signal is then sent to A/D converter 32.
The signal is converted into a 6- to 8-bit digital signal, and subjected to addition/subtraction shading correction by the next shading correction circuit 33. Such signal processing is performed by storing shading data of a white board in advance in a RAM (random access memory) and subtracting this shading data from the obtained image data.

Next, the image is γ-converted by a γ-conversion circuit 34, and multi-valued dithered by a gradation processing circuit 35, which is an embodiment of the present invention, to form a multi-valued dithered image 37.

FIG. 4 shows the configuration of the gradation processing circuit, and FIG. 5 shows its timing chart. In the embodiment shown in FIG. The signal is input to the adjustment processing circuit 35. Further, the gradation processing circuit 35 of this embodiment divides one pixel clock into two small pixels with m=2 and n=4, that is, one pixel clock is doubled, and the small pixel is divided into four small pixels.
2X (4-1)+1 by performing value pulse width modulation
= Achieved 7-value conversion. The reason why 1 is added is because the O output is also included. Details will be described later.

In FIG. 4, 41, 42, and 43 are 8-bit comparators that compare the A input and B input, and make the output true when A2B. 44, 45, and 46 are ROMs (read-only memories) in which dither threshold values are stored.These dither threshold values are shown in FIGS. 8(A) to 8(C). Figure (A) is in the ROM 44,
FIG. 8(B) corresponds to the ROM 45, and FIG. 8(C) corresponds to the ROM 46.

In the figure, the output of the ROM is expressed in 8-bit hexadecimal notation.As can be seen from the figure, each ROM has 72 threshold values, but since the 72 threshold values correspond to 6 x 6 = 36 pixels, 1 The pixels are compared to two ROM thresholds.

The structure of the dither threshold matrix is determined as shown in FIG. The numbers in FIG. 7 are numbers for determining matrix elements in order while gradually blackening the threshold value. Therefore, matrix elements assigned the same number are determined simultaneously, and two 3×3 dither matrices are simultaneously darkened diagonally from the center of each matrix. In this way, the threshold value of the ROM 44 in FIG. 8(A) is determined, and the ROM
45, the threshold values of ROM 46 are determined according to the same determination method, and further their respective corresponding threshold values are
It is determined as follows: ≦ROM45≦ROM46.

FIG. 9 diagrammatically shows a comparison between such a dither threshold value and the input image signal 36. In FIG. 9, the four pixels of the image signal are expressed in hexadecimal notation, for example, "BC", '36',
It is shown that when the data has the values "DC"* and 'B 4 ++, it is compared with the dither threshold values of the ROMs 44, 45, and 46 at the same time, and a 7-level dither image 37 is obtained.

Next, multilevel pulse width modulation of an image signal will be explained in detail using FIGS. 4 and 5.

The input image signal 36 is sent to a comparator 41 in 8-bit units.
It is simultaneously input to the A inputs of 42 and 43. Further, the outputs of the ROMs 44, 45, and 46 mentioned above are connected to the B input of each comparator, and when A2B, the outputs [51, 53, and 55 each become true.

The input image signal 36 is input to each comparator in synchronization with the pixel clock shown in FIG.
6 outputs threshold A1 in synchronization with the double pixel clock having twice the frequency of the pixel clock.

Therefore, as can be seen from FIG. 5, the image signal 36 is compared with two threshold values within one pixel. In this way, first, binarization of the image signal is achieved.

Next, the output 51 of the comparator 41 is sent to the gate 47 as a pulse signal φ^, and the output 53 of the comparator 42 is sent to the gate 4.
At step 8, the respective signals are logically ANDed with the pulse signal φB. As shown in FIG. 5, the pulse widths of φ^ and φB are each 1/3.2/3 of the pulse width of the pixel clock.

The output signal of the gate 47, the output signal of the gate 48, and the output 55 of the comparator 43 are logically summed by an OR gate 49 to obtain a dithered image 37 as shown in FIG. The image signal binarized by the double pixel clock is thus further φ^.

It was converted into four values by φB, and the entire signal including the 0 output was subjected to seven-value pulse width modulation.

FIG. 6 diagrammatically shows a beam spot in which one pixel is modulated with a seven-level pulse width of O, l/8 to 6/6.

FIG. 5 also shows that the input image signal 36 is expressed in hexadecimal notation, for example.
Seven-level beam spots are drawn at times of "BC", "36", "DC", and "84".

In the first embodiment, the case of 7-value conversion was explained, but m-times pixel clock is used as the ROM synchronization signal, and φ^,
The pulse width of φB is set as 1/(n-
1), the circuit configuration remains almost the same as m・(n
-1) It is easy to see that an image processing device that performs +1 value conversion can be obtained.

In this way, multivalued dithered images can be obtained with a small-scale circuit configuration, which is excellent for expressing continuous gradations in photographs, etc.
On the other hand, it is possible to perform multivalue conversion beyond conventional ternary and quaternary conversion, and gradation expression can be obtained without increasing the size of the dither matrix, so the resolution does not deteriorate and is suitable for expressing line drawings.

[Second Embodiment] In the first embodiment, an image processing apparatus that multivalues pixels in the main scanning direction has been described. Therefore, as shown in FIG. 6, the spot of the light beam of the semiconductor laser of the image recording apparatus shown in FIG. 1 is vertically elongated. FIG. 10(A) shows a conventional laser beam spot. Conventionally, main scanning and sub-scanning were performed in the directions shown by arrows using a circular beam spot.The present invention uses a 10th pulse width modulation method to perform pulse width modulation of one pixel in the main scanning direction.
As shown in Figure CB), a beam spot is used where the beam diameter LH in the main scanning direction is smaller than the beam diameter Lv in the sub-scanning direction.For example, L)l is set to be 1/2 pixel width.

By making the beam spot shape elliptical in this way, it is possible to further improve the gradation during pulse width modulation, and it is also possible to follow changes in the laser oscillation characteristics due to temperature changes.

In addition, 1st. In the second embodiment, the case where a semiconductor laser is used has been described, but it goes without saying that the same applies to other lasers such as gas lasers, LEDs, etc.

Effects] As explained above, by using the image processing device equipped with the beam spot of the present invention, a faithful reproduced image can be obtained even for an image signal modulated in the main scanning direction.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an image recording section of an image processing apparatus to which the present invention is applied, and FIG. 2 is a schematic diagram of an image input section of an image processing apparatus to which the present invention is applied. Fig. 3 is a main block diagram of the image input section of the image processing device of the present invention, Fig. 4 is a circuit diagram of the gradation processing section of the invention, Fig. 5 is a timing chart of the gradation processing section, and Fig. 6 is a A schematic diagram of a pulse width modulated beam spot, FIG. 7 is a diagram showing the order in which each element of the dither matrix applied to the present invention is determined, and FIGS. 8 (A) to (C)
9 is a diagram showing the contents of a ROM that stores a dither matrix, and FIG. 9 is a schematic diagram showing how an image signal is multivalued pulse modulated by a dither matrix.
B) is a diagram showing the shape of a beam spot of a laser beam or the like used in the image recording section of the image processing apparatus of the present invention. In the figure, 35... gradation processing circuit, 41, 42, 43...
・Comparator, 44.45.46...ROM, 47
．． 48...AND gate, 49...OR gate, Figure 1, Figure 2, Figure 8 (A) Figure 8 (B) Figure 8 (C) Figure 10 (A) Main scanning row 1 pixel Figure 10 (8) Bar-〒-1

Claims (2)

[Claims] (1) In an image processing device that uses a light beam to record a multilevel pulse width modulated image signal in the main scanning direction of a pixel, the shape of the beam spot of the light beam is such that the beam diameter in the main scanning direction is equal to the sub-scanning direction. An image processing device characterized by making the beam diameter smaller than the beam diameter in the direction. (2) The image processing device according to claim 1, wherein the beam spot of the light beam has a substantially elliptical shape.