JPS6141465B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a halftone processing method, and more particularly to a halftone processing method that can reproduce an image with a greater number of gradation levels than the number of input pixel density levels.

A method called a systematic dither method has been proposed as a method for reproducing images with gradation using binary values. This is a display screen consisting of a matrix-like dot arrangement, as shown in Figure 1 a or b.
It is covered with a sub-matrix consisting of ×n dots, and a predetermined dither
Pattern (0 to 3 in Figure 1 a, 0 to 15 in b)
is the threshold of each dot.
Therefore, in this method, each pixel position on the screen corresponds to a predetermined threshold, the value of which is completely independent of the brightness of that point. Then, each pixel density value is compared with the corresponding value of the dither pattern of the sub-matrix, and if the pixel density values are greater or equal, black is output as the reproduced pixel output, and if the pixel density value is small, the reproduced pixel output is Gives white as output.

In this way, in the dither method, tone levels can be reproduced using binary values, but when the number of input pixel density levels is n levels (n is any integer), the number of pixel density levels to be reproduced is also There are n levels, and a reproduced image with a number of levels greater than n cannot be obtained. In other words, when the input pixel density level is 4 levels, the dither pattern shown in Figure 1a is given to reproduce it, and when the input pixel density level is 16 levels, the dither pattern shown in Figure 1a is given to reproduce it. gives the dither pattern shown in Figure b. However, when the input pixel density level is 4 levels, the number of gradations is 1
6 cannot be obtained.

On the other hand, a thermal recording method using a multi-thermal head has been proposed, which performs gradation recording by changing the density of dots. This uses the characteristic of thermal recording paper that the color density changes when the applied temperature is changed to control the current (voltage) value of the multi-head or its application time according to the density information of the image signal. It is.

That is, the recording device, as shown in FIG.
After the input image signal IN is demodulated by the demodulator DEM, it is converted to 3 by 7 comparators 1 and analog-to-digital converter 2.
Convert to parallel 320 dot digital signal,
Create gradation control information. Writing and reading are performed alternately using the two-scanning line memory 3, and the data is transferred to the driver 5 via the memory 4 in units of 32 dots. Thermal
The head 6 has a 10 block configuration with 32 dots per block, and three drivers 5 having different collector resistances are connected in parallel so that the current in each head changes according to the image signal. This method can also be applied to electrostatic recording and ink jet recording.

In this way, when the number of input pixel density levels is n levels, the output dot density can be adjusted by dividing the applied voltage into n levels when using applied voltage control, and by dividing the application time into n levels when using application time control. By changing this, an output image with n gradations is obtained.

FIG. 3 is a diagram showing the state of the applied voltage V when the input pixel density level is 4 levels L.

However, even in this method, when the number of input pixel density levels is n, the resulting output image is
It is not possible to reproduce the gradation at a higher level.

Further, the halftone resolution of a typical low-cost image sensor (scanner) is 8 to 10 levels, and only low-level gradation input images can be obtained. On the other hand, image sensors with halftone resolution of 32 or more gray levels are extremely expensive.

An object of the present invention is to solve such problems, and when the number of input pixel density levels is n,
It is possible to obtain a reproduced image with a larger number of tone levels, and even when using an image sensor (scanner) with a low level of halftone resolution, it is possible to obtain a reproduced image containing a high level of halftones. The purpose of this invention is to provide a processing method.

When the input pixel density level is n levels, the present invention calculates the corrected density of the pixel by weighting the input pixel density and the pixel density surrounding the pixel, and outputs it from this corrected density. This method obtains a reproduced image with a number of gradation levels in which the pixel density level is greater than n.

Embodiments of the present invention will be described below with reference to the drawings.

To simplify the explanation, it is assumed that an output image with 16 gradations is obtained from an input signal having 4 input pixel density levels.

Assume now that the pixel density values of an input image obtained by scanning a document have a distribution as shown in FIG.

Now, the quantization of a certain pixel and surrounding pixels is
When quantizing with one-dimensional three pixels, the input pixel density is x _i,j , the surrounding pixel density is x _i-1,j , and x
When _i+1,j , the corrected pixel density X _i,j at that point is given by the following equation.

X _i,j =a·x _i-1,j +b·x _i,j +c·x _i+1,j (1) where a, b, and c are weighting coefficients for each pixel, respectively. Now, a=1, b=3, c=
1, the corrected density X _1,1 of _the first pixel 2 _1,1 of the input image shown in FIG _. 4 is as follows.

X ₁ , ₁ = 2 + 3 × 2 + 1 = 9 ... (2) In the same way, X ₂ , ₁ and X ₃ , ₁ become as follows.

X ₂ , ₁ = 2 + 3 × 1 + 0 = 5 ... (3) X ₃ , ₁ = 1 + 3 × 0 + 1 = 2 ... (4) As is clear from equation (2) above, when there are no adjacent pixels In this case, the pixel density is treated as the neighboring pixel density.

When the corrected density X _i,j is determined for each pixel in FIG. 4 in this manner, a density distribution as shown in FIG. 5 is obtained.

Corrected pixel density X _i,j of each point shown in FIG.
When reproducing halftones with multiple values using multi-head thermal recording, electrostatic recording, etc., this corrected density X _i,j is used as it is, or its normalized value X′ _i,j is used. By doing this, 16 levels of voltage values as shown in FIG. 7 can be provided.

Also, the corrected pixel density X of each point shown in FIG.
When _i,j is to be reproduced as a binary halftone using the dither method, for example, the corrected density X _i,j is normalized to X′ _i,j , and the pixel by the dither pattern shown in Figure 1b is By comparing the size of each image, a reproduced image as shown in FIG. 6 can be obtained.

Note that the values of the weighting coefficients a, b, and c can be set arbitrarily, but in that case, the weighting coefficient b of the pixel is set to the maximum value compared to the other coefficients a, c.

In the case of Fig. 5, the quantization was performed using 3 pixels in one dimension, but by increasing the surrounding pixels by 5 pixels in 1 dimension, 6 pixels in 2 dimensions, etc. over multiple rows before and after the input pixel. Of course, quantization can be performed using all the surrounding pixels that have been increased.

FIG. 8 is a block diagram of a halftone processing method showing an embodiment of the present invention.

When an input pixel density x _i,j is input from an image sensor such as a scanner, it is corrected to a modified pixel density x _i,j by a quantizer 10 and transferred to a normalizer 11 .

The quantizer 10 has m levels (m is the input level n
In order to obtain a reproduced image that includes gradations of gradations (larger arbitrary integers), weighting coefficients a, b, c of surrounding pixels are used.
Select appropriately. For example, all surrounding pixels are n
The corrected density may be set so that it becomes the maximum density of the m level when the density of the level is the maximum.

In addition, the normalizer 11 uses, for example, a value obtained by dividing each corrected density value X _i,j by the maximum density value (m-1).
Find X′ _i,j .

Thermal recording and ink that reproduce halftones with multiple values
In the case of jet recording or electrostatic recording, the corrected density value X _{i, j} or the standardized density value It is possible to obtain a reproduced image with gradations.

Also, the area within the chain line is a processing unit for reproducing halftones with binary values, which compares the normalized density value X′ _i,j with the dither pattern D _k,1 by the comparator 12. Then, when the density value X′ _i,j is large or equal, black is given as the output pixel 2LOUT, and when the density value X′ _i,j is small, white is given as the output pixel 2LOUT, thereby obtaining m-level gradations.

In the present invention, a microprocessor, ROM,
It is also possible to configure a halftone processing device using RAM and perform the above processing under program control, in which case the reproduced images obtained will be exactly the same.

As explained above, according to the present invention, by selecting the weighting coefficient of the quantizer, it is possible to obtain a reproduced image having gradations with a greater number of levels than the number of input pixel density levels. It becomes possible to use an image sensor.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a dither pattern in the dither method, FIG. 2 is a configuration diagram of a thermal recording device using a multithermal head, and FIG. 3 is a diagram of the relationship between pixel density level and applied voltage in FIG. 2. Figure 4 is a distribution diagram of pixel density values of the input image from the scanner,
Fig. 5 is a distribution diagram of corrected pixel density values showing an embodiment of the present invention, Fig. 6 is a display diagram of an image reproduced by a dither pattern of Fig. 5, and Fig. 7 is a level diagram obtained by normalizing Fig. 5. Comparison diagram of voltage values shown in correspondence, No. 8
The figure is a block diagram of a halftone processing method showing an embodiment of the present invention. 10: Quantizer, 11: Standardizer, 12: Comparator, x _i,j : input pixel density value, X _i,j : corrected pixel density value, X′ _i,j : standardized density value, Dk _,1 :
Dither pattern, MLOUT: Multi-value halftone output, 2LOUT: Two-value halftone output.

Claims (1)

[Claims] 1. Means for quantizing the input pixel density to a gradation level different from the input pixel density level based on the input pixel density and the pixel density surrounding the input pixel, and using the quantized pixel density to determine the current value of the recording head. Alternatively, a halftone processing method characterized by having means for controlling the application time or means for providing a binary output pixel by normalizing the quantized pixel density and comparing it with a dither pattern.