JP2859333B2 - Image processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus, and in particular, binarizes a pixel of interest based on an average density value of a predetermined area, and generates a binarization error that occurs in peripheral pixels. The present invention relates to an image processing apparatus that can diffuse image data and store image density.

[Related Art] In recent years, in an image processing apparatus such as a digital printer or a facsimile apparatus, an error diffusion method for dispersing an error generated in the binarization processing to peripheral pixels as a binarization technique for reproducing halftones. Attention is being paid to this technique.

[Problem to be Solved by the Invention] As a similar technique, a technique called an average density preservation method has been proposed. In this average density storage method, a target pixel is binarized using an average density calculated from the arrangement of dots in an already processed area around the target pixel as a threshold, and a binarization error generated at this time is assigned to peripheral pixels. This is a method of preserving the concentration by dispersing. However, when the binarization error is larger than a predetermined threshold, the error diffusion processing is stopped (adaptive error correction).

However, in the above-described average density storage method, whether or not to perform the error diffusion processing is determined based on a predetermined threshold value. Therefore, for example, in a portion where the document density is low, dots are generated close to each other in a reproduced image. There is a drawback that they are connected in a linear manner and markedly degrade image quality. On the other hand, in a portion where the document density is high, there is also a disadvantage that white noise is generated because no dot is formed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem. By making a threshold value for stopping error diffusion variable according to an input pixel density, dots generated in a portion having a low image density are close to each other. It is an object of the present invention to provide an image processing apparatus which can prevent the phenomenon of being hit by a dot and can also prevent white noise caused by no dot being hit in a portion where the image density is high.

[Means for Solving the Problems] In order to achieve the above object, an image processing apparatus according to the present invention comprises: hand,
Setting means for setting a threshold value for determining whether or not a binarization error generated when binarizing the image data of the target pixel is diffused into image data of a newly input pixel around the target pixel The image data of the pixel of interest is calculated using the average density value of binarized pixels around the pixel of interest as a threshold value.
Binarizing means for binarizing the value data, and comparing the binarization error generated by the binarization processing of the binarization means with a threshold value set by the setting means, thereby performing the binarization. Control means for controlling whether or not the error is diffused into the image data of the newly input pixel.

Further, the setting means variably sets a threshold value for determining whether or not to diffuse the binarized pixel according to a density value of the image data.

Further, when the binarization error is larger than the set threshold value, the control means controls so as not to diffuse the binarization error, and the binarization error is set to the set threshold value. If it is smaller than the threshold value, control is performed to diffuse the binarization error.

[Operation] In this configuration, a binarization error generated when the image data of the target pixel is binarized is newly input in the vicinity of the target pixel according to the input density value of the image data of the target pixel. Set a threshold for determining whether to diffuse to the image data of the pixel, the image data of the pixel of interest,
By using the average density value of the binarized pixels around the target pixel as a threshold, the binarization processing is performed on the binary data, and the binarization error generated by the binarization processing is compared with the set threshold. It controls whether or not the binarization error is diffused into the image data of the new pixel.

Hereinafter, a preferred embodiment according to the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic block diagram showing the configuration of the image processing apparatus according to the present embodiment.

First, image data read by the input device 1 including a photoelectric conversion element such as a CCD and a driving system for scanning the same is sequentially
It is sent to the A / D converter 2. The converter 2 converts the analog data of each pixel into, for example, 8-bit digital data, and quantizes the data into data having 256 levels of gradation. Next, in the correction circuit 3, correction such as shading correction is performed by digital arithmetic processing in order to correct sensitivity unevenness of the CCD sensor and illuminance unevenness due to the illumination light source. Then, the corrected signal (corrected signal) 100 is output to the threshold setting circuit 4 for stopping error diffusion and the binarization processing circuit 5 at the same time. The threshold setting circuit 4 sets a threshold for stopping error diffusion based on the corrected signal 100 from the correction circuit 3 and outputs a threshold signal 200.

On the other hand, the binarization circuit 5 binarizes (quantizes) the corrected signal (8-bit other-value image data) 100 output from the correction circuit 3 into a 1-bit binary signal 300. ,
An error diffusion process at the time of binarization is performed based on the threshold signal 200 output from the threshold setting circuit 4. In the output device 6 constituted by a laser beam printer, an ink jet printer, or the like, the binary signal 300 from the binarizing circuit 5 is turned on and off to form an image.

FIG. 2 is a block diagram showing details of the error diffusion stop threshold setting circuit 4 described above.

In the figure, the corrected signal 10 output from the correction circuit 3 is shown.
0 is input to the ROM 7, and the corresponding value is output as the threshold signal 200. Table 1 is an example showing the relationship between input and output.

Here, four threshold values are used by the ROM 7 as described above, but threshold values in more stages may be used. then,
It is better to lower the threshold value in a portion where the image density is low.

FIG. 3 is a block diagram showing the configuration of the above-described binarization circuit 5.

In the figure, the corrected signal 10 input from the correction circuit 3 is shown.
0 (image density of interest) is added to the binarization error E _ij (sum of errors distributed to the pixel of interest) stored in the error buffer memory 9 by the adder 8, and as a result, the error corrected A signal 210 is output.

Then, the error corrected signal 210 is input to the comparator 10,
The average density signal 220 calculated by the average density calculator 12 is compared with the average density signal 220. Here, the error corrected signal 210 is the average density signal 220
If it is larger than "1", if smaller than "0", the binary signal 300
Is output as The binary signal 300 is input to the output device 6 and to a predetermined pixel position in the binary memory 11.

Next, the average density calculator 12 performs weighting (α _KL ) on the binary memory 11 to calculate an average density signal 220. The average density is the sum of binary data and a weighting coefficient. FIG. 4 shows an example of the weight coefficient (α _KL ).
However, “*” corresponds to the target pixel position (I, J).

The average density signal 220 is input to the above-described comparator 10 and also to the arithmetic unit 13. Further, the above-described error corrected signal 210 is input to the arithmetic unit 13. Here, the difference between these two signals is calculated and the signal 230
(ΔE _ij ). Next, the signal 230 is input to the comparator 14 and compared with the threshold signal 200 output from the threshold setting circuit 4 described above. As a result, signal 230 becomes signal 20
If it is larger than 0, “0” (error diffusion stop) is equal or smaller, and the value is output as the signal 240 as it is. This signal 240 is input to the weighting circuit 15,
Here, after the weighting (β _KI ) is performed, it is added to the error at a predetermined pixel position in the error buffer memory 9. FIG. 5 shows an example of the weight coefficient (β _KI ). Note that “*” corresponds to the target pixel position (I, J).

By repeating the above operation, the binarization processing by the average density storage method is performed.

The reason why dot connection can be prevented in this way can be considered as follows.

For example, as an extreme example, the threshold value for stopping error diffusion is “0”.
Consider the case Then, since error diffusion is not performed,
For a pixel having a low image density, binary-applied data applied to the weight coefficient around the target pixel is checked, and if there is a signal for turning on the dot in the data, the target pixel is always "0". This means that the ratio of the role played by the weight coefficient to the binarization has been increased. In other words, in such a configuration, in a low density portion, no dot is formed around the dot.

For the above reasons, by lowering the threshold value for stopping the error diffusion, it is possible to prevent the dot from approaching in a portion where the image density is low. Further, it is possible to prevent white noise caused by a dot not being formed in a portion having a high image density.

Next, another embodiment according to the present invention will be described below with reference to the drawings.

In this embodiment, a case where a part of the threshold value setting circuit 4 is changed will be described.

The signal 100 output from the correction circuit 3 is input to the ROM 7. The ROM 7 outputs a threshold signal 200 according to the following equation.

(Signal 200) = (Signal 100) / (L1 + L2)… {If (Signal 100) ≤ 128} (Signal 200) = (225-(Signal 100)) / (L1 + L2) ... (Signal 100)> 128 Case 1 Here, L1 = 2 and L2 = 40.

FIG. 6 shows the relationship between the signal 100 and the signal 200. This is an example, and as the signal 100 approaches “0” or “255”,
The present invention is not limited to this as long as the relationship that the signal 200 becomes small is satisfied.

With the above configuration, it is possible to set a threshold value for stopping error diffusion as in the case of the above-described embodiment.

[Effects of the Invention] As described above, according to the present invention, by setting a threshold value for stopping error diffusion in accordance with an input pixel density, dots generated in a low image density portion are close to each other. The phenomenon of being hit can be prevented.

Further, it is also possible to prevent white noise that occurs due to the fact that dots are not formed in a portion where the image density is high.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an image processing apparatus according to the present embodiment, FIG. 2 is a block diagram showing a configuration of a threshold setting circuit 4, and FIG. 3 is a block diagram showing a detailed configuration of a binarization circuit 5. FIG. 4 is a diagram showing a weighting factor of the average density calculation, FIG. 5 is a diagram showing a weighting factor of the error diffusion calculation, and FIG. 6 is a diagram showing a relationship between the signal 100 and the signal 200 in another embodiment. is there. In the figure, 1 ... an input device, 2 ... an A / D converter, 3 ... a correction circuit, 4 ... a threshold setting circuit, 5 ... a binarization circuit, 6 ...
Output device.

Continuation of the front page (56) References JP-A-3-11882 (JP, A) JP-A-3-88570 (JP, A) JP-A-1-115272 (JP, A) JP-A-3-18178 (JP , A) JP-A-63-155955 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (3)

(57) [Claims] An input unit for inputting image data; and a binary value generated when the image data of the pixel of interest is binarized according to a density value of the image data of the pixel of interest input by the input unit. Setting means for setting a threshold value for determining whether or not the binarization error is to be diffused into image data of a newly input pixel around the pixel of interest; and binarizing the image data of the pixel of interest around the pixel of interest. A binarizing unit that binarizes the binary data into binary data using the average density value of the pixels as a threshold value, and a threshold value set by the setting unit for a binarization error generated by the binarizing process of the binarizing unit. Control means for controlling whether or not the binarization error is diffused into image data of a pixel to be newly inputted by comparing with the image data. 2. The apparatus according to claim 1, wherein said setting means variably sets a threshold value for determining whether or not to diffuse the binarized pixel according to a density value of the image data. The image processing apparatus according to any one of the preceding claims. 3. The control means controls so as not to diffuse the binarization error when the binarization error is larger than the set threshold value, and controls the binarization error according to the setting. The image processing apparatus according to claim 1, wherein when the threshold value is smaller than the threshold value, control is performed to diffuse the binarization error.