JPS61283272A - Picture recorder - Google Patents

Abstract

PURPOSE:To obtain a half tone picture having a soft gradation characteristic by adding an output from a gradation characteristic correcting means on an asynchronous signal stored at an asynchronous signal storing means by an adding means and comparing the output of the adding means with a threshold signal by a comparison means. CONSTITUTION:An adder 1 that is the adding means adds a picture element data N2 outputted from a look-up table 23 and a fluctuation signal (dither signal) R from a random number memory 2 that is the asynchronous signal storing means and outputs an adder output T22 to a digital comparator 24. The random numbers memory 2 outputs the fluctuation signal R from an address which is indicated by counting of a clock pulse RCP asynchronous with a clock pulse CP by an address counter 3. Also, the clock pulse RCP is raised higher than the read out period of a picture element data N1 and the space frequency component of increasing noise is also raised and thereby, noise against eyesight is not made prominent.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image recording apparatus that reproduces a pseudo halftone image in accordance with an input halftone image signal.

[Conventional technology]

Conventionally, the dither method, 11, has been used as a method of pseudo-reproducing halftones using a recording device that does not have very good halftone reproduction.
A pattern method has been proposed and put into practical use. Regarding these methods, including Japanese Patent Application Laid-open No. 57-7H77,
Since a large number of applications have been filed and it is described in detail in literature, etc., the explanation will be omitted. The method used to date is to compare the halftone image signal (grayscale image signal) with a threshold signal (dither signal), convert it into binary or multivalued numbers, and then convert the dots into Halftones are artificially reproduced by the size or density of dots.

For example, if the threshold signal is a 4×4 matrix,
A recording device that can only express binary values can only reproduce 17 gradations, resulting in images with large density differences per gradation and noticeable so-called false contours. In order to improve this, increasing the matrix size has contradictory characteristics in that the resolution decreases.

In addition, in recording devices that can express multi-values, it is possible to increase the number of gradations with the same matrix size by using multi-values.
For example, inkjet printers attempt to create multiple values based on dot size or ink density, and electrophotographic laser printers attempt to create multiple values by subdividing one matrix in the main scanning direction and performing pulse width modulation. We are planning to convert it into a value (see Japanese Patent Laid-Open No. 57-99884).

However, in the above example, when using an inkjet printer to perform multilevel printing, there is a limit to increasing the dot size and the number of shading types, and even with an electrophotographic laser printer, it is difficult to increase the pulse frequency. Since there is a limit in terms of practicality, it is difficult to make the difference in density of one gradation of light indistinguishable.

Next, a conventional multi-value process will be explained with reference to FIG.

FIG. 5 is a block diagram showing the configuration of a conventional electrophotographic laser beam printer, and the multilevel process is based on a dither method or a density pattern method.

The structure and operation will be explained below.

21 is an image memory, and the 0 image memory 21 stores each pixel as 6-bit halftone image data.
Pixel data N1 at an address specified by an address counter 22 that counts a clock pulse CP and a main scanning synchronization signal (horizontal synchronization signal) BD, which will be described later, is output. 23
is a look-up table that converts the gradation characteristics of the pixel data N into linearly corrected pixel data N2, which is input to one side of the digital comparator 24. The lookup table 23 associates the pixel data N1 as pixel data N2 with an address different from the pixel data Nl, and stores the pixel data N1 as pixel data N2.
Performing N2 conversion. The other input of the digital comparator 24 receives an output from the memory 31 in which the threshold matrix is stored, and the digital comparator 24 compares the two to form a binary signal V.
The semiconductor laser 26 is driven through. Semiconductor laser 2
The output light weight 6 is reflected by a rotating polygon mirror 27 rotating in the direction of the arrow, and the output light weight scans the photosensitive drum 28 in the main scan direction. The photosensitive drum 28 is rotated in the direction of the arrow to perform sub-scanning, thereby performing two-dimensional scanning. A latent image is formed on the photosensitive drum 28 by an electrophotographic process (not shown) and scanning of output light (2), and toner is developed and transferred.

Fixing is performed and image recording is performed. Further, a part of the output light beam is reflected by the mirror 29 and enters the photodetector 30 to obtain the main scanning synchronization signal BD. On the other hand, the memory 31 has 16
The column address counter 32 counts the clock pulse OF in hexadecimal, and the column address in the main scanning direction is sent to the row address counter 33.
counts the main scanning synchronizing signal BD in four notation and designates each row address in the sub-scanning direction. Therefore, the address counter 22 sequentially specifies pixels in the main scanning direction with each hexadecimal count of the clock pulse CP, and generates the main scanning synchronization signal BD.
If we sequentially specify pixels in the sub-scanning direction every 4 counts, we get
One pixel stored in the image memory 21 becomes a density pattern corresponding to the entire memory 31. Furthermore, if the address counter 22 updates the address every 8 counts of the clock pulse CP and every 2 counts of the main scanning synchronization signal BD, then 1
The pixel corresponds to % of memory 3 and is in an intermediate state between the dither method and the density pattern method. Regarding the latter, JP-A-57-1
It is described in detail in Japanese Patent No. 39887.

In the above example, since one pixel is 6-bit data, pixel data Nl, N2 and output T each take a value of 0 to 63, and when the level of pixel data N2 is higher than the level of output T, a binary signal is generated. V becomes “1” and the semiconductor laser 2
6 emits light, but the density characteristics of the output image vary depending on the arrangement order of the threshold values in the memory 31, the emission intensity of the semiconductor laser 26, the electrophotographic process, etc.

[Problem that the invention seeks to solve]

FIG. 6 is a characteristic diagram showing the relationship between the pixel data Nl, N2 and the output image density. Characteristic (a) shows the output image density due to the pixel data N1 when the lookup table 23 is not used. , Characteristic (b) shows the output image density due to the pixel data N2 when the lookup table 23 is used. Note that the horizontal axis represents pixel data, and the vertical axis represents output image density. Further, the output image density for each pixel data changes stepwise for each pulse.

As can be seen from this figure, as shown in characteristic (a), when gradation correction is not applied, the output image density rises significantly in low density areas due to the large laser beam diameter, and reaches high density areas. As the density increases, the characteristics deteriorate, and the maximum density difference ΔD MAX in one stage is when the pixel data N1 goes from 5 to 6, and if each density is D l s D 2 , then ΔDMAX = 02−D+, which is the actual measured data. Then, ΔDMAx=0.08. There have been various publications regarding the discrimination density difference of the human eye, but it is said that ΔDM^X can be distinguished up to about 0.01. Therefore, since 0.08 is naturally identified as a shank, the gradation characteristics of the output image appear step-like, so-called false contours occur, and a halftone image with noticeable false contours is output, resulting in poor gradation expression. There was a problem in that the value decreased significantly. In this way, it cannot be concluded that characteristic (a) has desirable gradation characteristics. Therefore, when the gradation characteristics are linearly corrected, the characteristic (b) shown in FIG. 6 is obtained, for example. 11th~ of pixel data N1
The 14th data is the 5th data of the pixel data N2, and the pixel data N! The 15th to 18th are pixel data N? Convert it so that it becomes the sixth data.

This conversion expands the rising portions of characteristic (a) and compresses the flat portions, and the pixel data resulting from this conversion becomes pixel data N2, that is, characteristic (b).

Even if such conversion is performed, the maximum density difference ΔD in one stage
The WAX does not change, but a halftone image with noticeable false contours is output as described above, and the gradation expressivity is remarkable.
There was a problem that it deteriorated. Note that this phenomenon becomes noticeable where the rise of characteristic (a) or (b) is large, indicating that the gradation in that area is degraded.

The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide an image recording apparatus that can output a halftone image in which false contours are not noticeable by applying fluctuations to a threshold signal.

[Means for solving problems]

An image recording device according to the present invention includes a gradation processing means for correcting gradation of a halftone image signal, and an asynchronous signal that stores an asynchronous signal that changes at a small pitch compared to a pixel pitch of the halftone image signal. The apparatus is provided with a storage means and an addition means for adding the output from the gradation processing means to the asynchronous signal stored in the asynchronous signal storage means.

[Effect]

In this invention, the addition means adds the output from the gradation correction means to the asynchronous signal stored in the asynchronous signal storage means, and the comparison means compares the output of the addition means with the threshold signal.

〔Example〕

FIG. 1 is a block diagram showing the configuration of an image recording apparatus showing an embodiment of the present invention, and the same components as in FIG. 3 are given the same reference numerals.

In this figure, 1 is an age device which is an additional means of this invention.
Then, the pixel data N2 outputted from the lookup table 23 and the fluctuation signal (dither signal) R from the random number memory 2 serving as an asynchronous signal storage means are added, and an adder output T22 is outputted to the digital comparator 24. The random number memory 2 outputs a fluctuation signal R from an address designated by the address counter 3 counting clock pulses RCP asynchronous to the clock pulse CP. Further, the clock pulse RCP is set higher than the readout period of the pixel data N1, and the spatial frequency component of the increasing noise is made high, thereby making the noise less noticeable to the human eye.

Next, the operation of FIG. 1 will be explained with reference to FIGS. 2(a) to 2(C).

FIGS. 2(a) to 2(C) are time charts showing the signal timing of each part in FIG.
l+ indicates the threshold output, T21 indicates the adder output when the fluctuation signal R is not added, and T22 indicates the adder output when the fluctuation signal is added. In the same figure (b), vl indicates the fluctuation signal R This shows the comparator output without adding. In the same figure (C), v2 indicates the humbrate output when the fluctuation signal R is added.

When the fluctuation signal R is not added, only the adder output T21 is output from the adder 1, and the adder output T2+ is compared with the threshold signal Tll as shown in FIG. A comparator output V] as shown in Figure (b) is output.

On the other hand, when the fluctuation signal R is added, the adder output T22 is output from the adder 1, and the adder output T22 and the threshold signal T11 are compared by the digital comparator 24, as shown in FIG. A fluctuating comparator output v2 is output.

FIG. 3 is a waveform diagram showing the correlation between pixel data and output image density when fluctuation is added before gradation correction.

As can be seen from this figure, if the amount of fluctuation is adjusted and the pixel data N1 is fluctuated by ΔN1.

The density fluctuation is ΔDI, and since the pixel data Nl is on a linear scale, the amount of fluctuation ΔN2 in the high density area is ΔN
becomes equal to 1. Therefore, the density fluctuation ΔD2 is also ΔDI
is equal to For this reason, as shown in Figure 6, the gradation expression ability is high in the high density area, so the density fluctuations may be small, but the gradation expression ability is subject to fluctuations equivalent to the low density area, and the entire gradation expression ability is high. The S/N similarly decreases in the concentration region.

However, in this invention, since the adder 1 is inserted after the lookup table 3, the density characteristics shown in FIG. 4 are obtained.

FIG. 4 is a characteristic diagram showing the correlation between pixel data and output image density according to the present invention.

In this figure, characteristic I indicates pixel data N2 to which fluctuation signal R is added.

As shown in this figure, the amount of fluctuation in the low density area is ΔN
21, the density fluctuation is ΔD3, and when the amount of fluctuation in the high density area is ΔN22, the density fluctuation is ΔD.
4, and the pixel data N2 has a scale expanded in the low density area and compressed in the high density area, so the fluctuation amount Δ
N21 is compressed to the amount of fluctuation ΔN22 in high-density areas, and the relationship between density fluctuations is ΔD3>>ΔD4, so the S/N decreases in low-density areas with low gradation expression ability, but with high gradation expression ability. The S/N ratio in high-density areas does not decrease, and a halftone image with inconspicuous false contours can be obtained.

In the above embodiment, the fluctuation signal is generated from the random number memory 2, but it may also be generated by amplifying thermal noise, but if it is added digitally, A/D conversion It is necessary to provide a container. Further, in the above embodiment, a case has been described in which the fluctuation signal R is added by the adder 1, but it may be done by decrementing, but addition is simpler from the viewpoint of the hardware configuration. Furthermore, when changing the fluctuation signal R, the contents of the random number memory 2 may be changed, but it may also be achieved by bit shifting before adding the fluctuation signal R to the 7-dar 1. , clock pulse CP
, we have explained the case where RCP is asynchronous, but this is due to the periodic pattern that occurs when RCP is synchronized, that is,
This is to prevent the occurrence of texture.

〔Effect of the invention〕

As described above, the present invention provides a gradation processing means for correcting the gradation of a halftone image signal, and an asynchronous signal that stores an asynchronous signal that changes at a small pitch compared to the pixel pitch of the halftone image signal. Since the storage means and the addition means for adding the output from the gradation processing means to the asynchronous signal stored in the asynchronous signal storage means are provided, the conventional density pattern method,
Even with the dithering method, remaining false contours can be significantly improved, and a halftone image with smooth gradation can be obtained. It has the excellent advantage of being limited to

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an image recording apparatus showing an embodiment of the present invention, FIGS. 2(a) to (C) are time charts showing the signal timing of each part of the $1 diagram, and FIG. A characteristic diagram showing the correlation between pixel data and output image density when fluctuation is added before gradation correction, FIG. 4 is a characteristic diagram showing the correlation between pixel data and output image density according to the present invention,
FIG. 5 is a block diagram showing the configuration of a conventional electrophotographic laser beam printer, and FIG. 6 is a characteristic diagram showing the relationship between pixel data and output image density. In the figure, 1 is an adder, 2 is a random number memory, 3 is an address counter, 24 is a digital counter, N, , N2 are pixel data, R is a fluctuation signal, CRP is a clock pulse, Tl
1 is a threshold output, and T22 is an adder output. Figure 2 Figure 3 Figure 4 Figure 6

Claims (2)

[Claims] (1) It has a comparison means for comparing the halftone image signal and the threshold signal, and according to the output of this comparison means, it is multivalued to a limited number of gradations of two or more values to create pseudo halftones. In an image recording device that reproduces an image, a gradation processing means for correcting gradation of the halftone image signal and an asynchronous signal that changes at a small pitch compared to a pixel pitch of the halftone image signal are stored. an asynchronous signal storage means and an addition means for adding the output from the gradation processing means to the asynchronous signal stored in the asynchronous signal storage means, and comparing the output of the addition means and the threshold signal. An image recording device characterized by comparing by means. (2) The image recording apparatus according to claim (1), wherein the asynchronous signal is generated from a random number and read out asynchronously with respect to a threshold signal.