# Patent

## Title
Methods and systems for scanning and processing an image using the error diffusion screening technology

## Abstract

Disclosed is a method for scanning and processing an image using the error diffusion screening technology, comprising: (1) scanning each pixel Mi of an nth line in an original image one by one and then storing a scanning result of the pixel Mi to an ith storage location; and (2) processing the stored result of the pixel Mi by using error diffusion and scanning pixels of an n+1th line in the original image until all pixels of the nth line have been processed and all pixels in the n+1th line have been scanned and stored, wherein once processing for the pixel Mi is completed, a scanning result of a pixel of the n+1th line is stored to the ith storage location previously occupied by the pixel Mi. Based on the method, the capacity for storing is only required to be able to store the data of one line in an image in the scanning direction, which saves the storage for bidirectional scanning. The method can optimize the hardware used to implement error diffusion and improve the operating efficiency. Also disclosed is a system for achieving the method.

## Background
FIELD OF THE INVENTION
The present invention relates to a method for scanning and processing data in the field of hard copying for image duplication, more particularly to a method and system for scanning and processing an image using the error diffusion screening technology.
BACKGROUND OF THE INVENTION
The conventional method for scanning and processing an image is unidirectional, which will results in textures, degrade the random characteristic based on the error diffusion, and thus impact the quality of the output. The method of bidirectional scanning and processing an image can address the problems in a certain extent. In the method of bidirectional scanning and processing, lines of the image is scanned and processed in two directions, alternatively (i.e. from the left to the right for one line and from the right to the left for the next line). Currently, many methods for scanning and processing an image on the basis of the error diffusion are bidirectional.
However, the method of bidirectional scanning and processing needs to operate an image pixel by pixel, which results in a low efficiency. The general method for solving this problem is to improve the hardware. If pixels are scanned one by one in a single direction, a pixel can be processed and output as soon as the pixel is scanned, without needing buffer storage for data of the scanned pixels. However, when a method of bidirectional scanning and processing is used, the results of error operation of the processed pixels should be weightedly added to adjacent unprocessed pixels according to the principle of error diffusion. Some of the unprocessed pixels are in the next line which is un-scanned. Therefore, if pixels are scanned and processed bidirectionally, the data of pixels scanned in two different directions must be stored and then processed one by one. Particularly, in the event of bidirectional scanning and processing, if the current line is scanned from the left to the right, the next line will be scanned from the right to the left. Before the firstly scanned pixel in the next line is processed, errors of last several scanned pixels in the current line are needed to be diffused to the firstly scanned pixel in the next line. Therefore, the value of error diffusion for a pixel in the next line can be obtained only after all pixels in the current line are scanned and processed.
The inventors have disclosed a method for screening in a previously filed Chinese patent application No. 200510068127.8 published on Sep. 14, 2005, entitled “method for frequency-modulation screening using error diffusion based on dual-feedback”, which is incorporated herein by reference. According to the method, an FM-AM mixed screening can be achieved by using a dual-feedback error diffusion technology. However, the method also needs to previously store the data of pixels scanned in two different directions and then process the stored data one by one so as to achieve the bidirectional scanning.
SUMMARY OF THE INVENTION
To overcome the shortcomings of the prior art, the present invention is to provide a method for scanning and processing an image using the error diffusion screening technology, comprising: (1) scanning each pixel Mi of an nth line in an original image one by one and then storing a scanning result of the pixel Mi to an ith storage location; and (2) processing the stored result of the pixel Mi by using error diffusion and scanning pixels of an n+1th line in the original image until all pixels of the nth line have been processed and all pixels in the n+1th line have been scanned and stored, wherein once processing for the pixel Mi is completed, a scanning result of a pixel of the n+1th line is stored to the ith storage location previously occupied by the pixel Mi.
According to another aspect, the present invention is to provide a system for scanning and processing an image using the error diffusion screening technology, comprising a scanning device, a storing device and a processing device, wherein the scanning device has an output terminal connected to an input terminal of the storing device, the storing device has an output terminal connected to an input terminal of the processing device, the scanning device scans each pixel Mi of an nth line in an original image one by one, and a scanning result of the pixel Mi is stored to the storing device; and wherein the processing device processes stored data for the pixel Mi by using error diffusion, once processing for the pixel Mi is completed, a obtained value for the pixel Mi is output so that an ith storage location in the storing device can be idle, at the same time, the scanning device scans pixels of an n+1th line in the original image, once the ith storage location in the storing device is idle, scanned data for a pixel in the n+1th line is stored to the ith storage location, and once all pixels in the nth line have been processed, all pixels in the n+1th line have been scanned and stored.
The present invention has at least one of the following effects. Based on the method of bidirectional scanning and processing an image using the error diffusion, the capacity for storing is only required to be able to store the data of one line in an image in the scanning direction, which saves the storage for bidirectional scanning. The method and system of the present invention can optimize the hardware used to implement error diffusion, improve the operating efficiency, and save the cost of hardware.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating a system for scanning and processing an image according to an embodiment of the present invention;
FIG. 2 is a schematic diagram illustrating bidirectional scanning and processing;
FIG. 3 is a schematic diagram illustrating the scanning for the first line of an image;
FIG. 4 is a schematic diagram illustrating the processing for the first line of the image and the scanning for the second line of the image at the same time;
FIG. 5 is a schematic diagram illustrating the processing for the second line of the image and the scanning for the third line of the image at the same time;
FIG. 6 is a schematic diagram illustrating the direction for storing the scanned data and the direction for processing the stored data; and
FIG. 7 is a schematic diagram of a process according to an embodiment of the present invention.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, a detailed description of the present invention will be given with reference to the appended drawings and embodiments.
The following embodiments are based on 2-bit imaging depth and bidirectional scanning and processing for the original image as shown in FIG. 2.
FIG. 1 shows a system for scanning and processing an image using the error diffusion screening technology according to an embodiment of the present invention. As shown in FIG. 1, the system comprises a scanning device 2, a storing device 3 and a processing device 4. An original image 1 can be input to the system by an input terminal of the scanning device 2. An output terminal of the scanning device 2 is connected to an input terminal of the storing device 3 and an output terminal of the storing device 3 is connected to an input terminal of the processing device 4. The result of the processing device 4 will be the output of the system. The scanning device 2 scans, one by one, the pixels of the current line (for example, the nth line) in the original image 1 from the beginning to the end. The scanned data for each pixel Mi is stored to the ith location in the storing device 3. The stored data for each pixel Mi is processed by the processing device 4 by using error diffusion. Once the processing for a pixel is completed, the obtained value for the pixel is output so that the ith location in the storing device 3 can be idle. At the same time, the scanning device 2 scans the pixels of the next line (i.e. the n+1th line) in the original image 1. Once the ith location in the storing device 3 is idle, the scanned data for a pixel in the n+1th line is stored to this ith location. Once all the pixels in the nth line have been processed, all the pixels in the n+1th line have been scanned and stored.
In this disclosure, C is defined to indicate the direction for processing the current line (the nth line). C=0 indicates that the pixels in the current line are processed from the beginning to the end; and C=1 indicates that the pixels in the current line are processed from the end to the beginning.
H is defined to indicate the storage location in the storing device 3 of the beginning pixel of a line. H=0 indicates that the beginning pixel of a line is stored in the first location in the storing device 3; and H=1 indicates that the beginning pixel of a line is stored in the last location in the storing device 3.
As shown in FIG. 7, a method for scanning and processing an image using the error diffusion screening technology according to an embodiment of the present invention comprises the following steps.
Step 1 is to scan, one by one, the pixels of the nth line (n=1, 2, 3 ...) in the original image 1 from the beginning to the end after the C and H are initialized and then store the value g(m, t) of each pixel Mi, as shown in FIG. 3.
Step 2 is to process the stored value g(m, t) of each pixel Mi by using error diffusion according to the processing direction determined by C and H and scan the pixels of the next line (i.e. the n+1th line) in the original image 1 one by one from the beginning to the end. Once the processing for a pixel is completed, the obtained value for the pixel is output so that the ith storage location occupied by the pixel can be idle. Then, the scanned data for a scanned pixel of the n+1th line is stored to the idle ith storage location. Once all the pixels in the nth line have been processed, all the pixels in the n+1th line have been scanned and stored.
In Step 2, the C indicating the direction for processing the current line and the H indicating the storage location of the beginning pixel of the next line in the storing device 3 are recorded.
The processing direction of the processing device 4 is determined by C and H according to the following conditions.

    
    
        (1) If C=0 and H=0, the pixels in the next line are processed by the processing device 4 from the last location to the first location of the storing device 3.
        (2) If C=0 and H=1, the pixels in the next line are processed by the processing device 4 from the first location to the last location of the storing device 3.
        (3) If C=1 and H=0, the pixels in the next line are processed by the processing device 4 from the first location to the last location of the storing device 3.
        (4) If C=1 and H=1, the pixels in the next line are processed by the processing device 4 from the last location to the first location of the storing device 3.
    
    


Hereinafter, a detailed description of the processing and storing routine of the pixels according to the present method will be given with reference to the example shown in FIGS. 4 and 5.
As shown in FIG. 4, after all the pixels in the nth line (assuming n=1) are scanned and stored, the processing device 4 is started to read and process the stored data for the first pixel from the left in the nth line. After the stored data for the first pixel is processed, the stored data for the second pixel in the nth line is read and processed by the processing device 4 and the data for the first pixel in the n+1th line (i.e. the 2nd line) is stored in the first location of the storing device 3, which is just idle. The above steps are repeated until all the pixels in the 1st line are processed and all the pixels in the 2nd line are stored. Now, C=0 and H=0, which complies with the above condition (1).
As shown in FIG. 5, after the processing for the 1st line and the storing for the 2nd line are completed, the processing device 4 is started to read and process the data stored in the storing device 3 from the last location to the first location. That is to say, after the data for the pixel in the 2nd line which is stored in the last location of the storing device 3 is processed, the data stored in the second last location is read and processed by the processing device 4 and the data for the first pixel in the 3rd line is stored in the corresponding idle location of the storing device 3. The above steps are repeated until all the pixels in the 2nd line are processed and all the pixels in the 3rd line are stored Now, C=1 and H=1, which complies with the above condition (4).
The whole original image 1 can be scanned and processed by repeating the above steps. For saving the storage, the capacity of the storing device 3 is only required to be able to store the data for one line in the original image 1 in the scanning direction.
In this embodiment, the process implemented by the processing device 4 is based on the FM screening method using the dual-feedback error diffusion. The detailed process is described as below.
(a) The original image is scanned and input. An operation of a threshold comparison is carried out on the final value g′′(m, t) of the current pixel in the original image. The result of the operation is converted to the output value b(m, t) of the current pixel. The operation of a threshold comparison and the conversion for the result of the operation is conventional, which will not be described in detail herein.
(b) The output value b(m, t) of the current pixel is compared with the medium value g′(m, t) of the current pixel to obtain the error value e(m, t) of the current pixel.
(c) The error value e(m, t) is multiplied by certain weight distribution coefficients through a first diffusion filter e. Then, the results of multiplying are diffused to those unprocessed pixels around the current pixel. Each of the diffused results to the unprocessed pixels around the current pixel is weightedly added to the original value g(m, t) of the pixel (Mx) to obtain a new medium value g′(m, t) of the corresponding pixel Mx. The error diffusion filter uses a diffusion principle and weight distribution coefficients as below:


















**
d5
d3


d2
d4
d5
d4
d2


d1
d2
d3
d2
d1









wherein, ** represents the current pixel's position and each of d1 to d5 on the other positions represents the diffusion weight coefficient relative to the current pixel **. d1 to d5 are taken in [0, 1] and satisfy the following expression:

2×d1+4×d2+2×d3+2×d4+2×d5ε[0,1].

In the embodiment, the coefficients are set as:

d1=1/44, d2=2/44, d3=5/44, d4=4/44, d5=8/44.

From the above, the first diffusion feedback operation is accomplished.
(d) This step is implemented in parallel with the steps (b) and (c). Processed results are respectively diffused to corresponding pixels Mx, surrounding the current pixel, and each of the diffused processed results are weightedly added to the medium value g′(m, t) of the corresponding pixel Mx, obtained from error diffusion in the steps (b) and (c) to obtain the final value g′′(m, t) of the corresponding pixel Mx, wherein the processed results are obtained by implementing a multiplying operation on the output value b(m, t) of the current pixel using a second diffusion filter w and processing the results of the multiplying operation with the dithering algorithm. The diffusion mode of the second diffusion filter w is set as:















**
w0


w3
w2
w1









wherein, the scanning direction is from left to right, ** represents the current pixel's position and each of w0 to w3 on the other positions represents the diffusion weight coefficient relative to the current pixel. The parameters are taken in [0, 1] and satisfy the following expression:

wsum=(w0+w1+w2+w3)ε[0,1].

The dithering algorithm in the embodiment uses the following Formula:

fRand=(R(m,t)/R_MAX−0.5)×cDither

dw0=w0−fRand

dw2=w2+fRand

dw1=w1+fRand

dw3=w3−fRand

In this Formula, fRand is a parameter for fine adjusting dithering; R(m, t) is a parameter with random value for scanning the current dot; R_MAX is the maximum of the random parameter R(i); cDither is a parameter for adjusting amplitude of dithering and determines the performance of amplitude-modulation; and dw0 ̃dw3 are the diffusion weight coefficients of the filter w in different directions after dithering.
This step accomplishes the second diffusion feedback operation and implements the amplitude-modulation characteristic of the frequency-modulation screen.
In the embodiment, the coefficients are set as:

w0=w2=0.175, w1=w3=0.025, then wsum=0.4

cDither=0.2

During screening, the size of the frequency-modulation halftone dots is changed by adjusting Wsum and the shapes of the frequency-modulation halftone dots is controlled by adjusting the values of w0 ̃w3.
(e) The steps (a)-(d) are repeated until the original input values g(m, n) of all the pixels are processed.
According to the method for scanning and processing of the present invention, the directions shown in FIG. 6 can achieve the bidirectional scanning and processing shown in FIG. 2. Furthermore, the capacity of the storing device is only required to be able to store the data in one line in the original image 1 in the scanning direction, which can optimize the used hardware system.
The present invention is not limited to the embodiments mentioned above. The method of the present invention also can be based on the conventional bidirectional scanning and processing by using screening technology with one-time error diffusion. Other embodiments obtained by the skilled in the art according to the technical solutions in the present invention should be within the scope of the technical innovation of the present invention.

## Claims

1. A method for scanning and processing an image using the error diffusion screening technology, comprising:
(1) scanning each pixel Mi of an nth line in an original image one by one and then storing a scanning result of the pixel Mi to an ith storage location; and
(2) processing the stored result of the pixel Mi by using error diffusion and scanning pixels of an n+1th line in the original image until all pixels of the nth line have been processed and all pixels in the n+1th line have been scanned and stored,
wherein once processing for the pixel Mi is completed, a scanning result of a pixel of the n+1th line is stored to the ith storage location previously occupied by the pixel Mi and
wherein the step (2) further comprises:
recording a processing direction parameter C of the nth line and a storing location parameter H for a beginning pixel of the n+1th line; and
determining the processing direction of the n+1th line based on values of the parameters C and H,
wherein C=0 indicates that pixels in the nth line are processed from beginning to end; C=1 indicates that pixels in the nth line are processed from end to beginning; H=0 indicates that the beginning pixel of the n+1th line is stored in a first location of a storing device; and H=1indicates that the beginning pixel of the n+1th line is stored in a last location of the storing device.

2. The method of claim 1, wherein the processing direction is determined by the parameters C and H according to following conditions:
(1) If C=0 and H=0, pixels in the n+1th line are processed by a processing device from the last location of a storing device;
(2) If C=0 and H=1, the pixels in the n+1th line are processed by the processing device from the first location of the storing device;
(3) If C=1 and H=0, the pixels in the n+1th line are processed by the processing device from the first location of the storing devices;
(4) If C=1 and H=1, the pixels in the n+1th line are processed by the processing device from the last location of the storing device.

3. The method of claim 1, wherein the error diffusion further comprises:
(a) scanning and inputting the original image, carrying out an operation of a threshold comparison on a final value g′′(m, t) of a current pixel in the original image, and then converting a result of the operation to an output value b(m, t) of the current pixel;
(b) comparing the output value b(m, t) of the current pixel with a medium value g′(m, t) of the current pixel to obtain an error value e(m, t) of the current pixel;
(c) multiplying the error value e(m, t) by preset weight distribution coefficients through a first diffusion filter e and then diffusing results of the multiplying to unprocessed pixels around the current pixel, wherein each of the diffused results to the unprocessed pixels around the current pixel is weightedly added to an original value g(m, t) of the pixel Mx to obtain a new medium value g′(m, t) of the pixel Mx;
(d) diffusing processed results to corresponding pixels Mx surrounding the current pixel, respectively, and weightedly adding each of the diffused processed results to the medium value g′(m, t) of the corresponding pixel Mx obtained from error diffusion in the step (c) to obtain the final value g′′(m, t) of the corresponding pixel Mx, wherein the processed results are obtained by implementing a multiplying operation on the output value b(m, t) of the current pixel using a second diffusion filter w and processing results of the multiplying operation with a dithering algorithm, and the step (d) is implemented in parallel with the steps (b) and (c); and
(e) repeating the steps (a)-(d) until the original input values g(m, t) of all pixels are processed.

4. The method of claim 3, wherein the first error diffusion filter e in the step (c) uses a diffusion principle and weight distribution coefficients as below:


















**
d5
d3


d2
d4
d5
d4
d2


d1
d2
d3
d2
d1








wherein, ** represents the current pixel's position, each of d1 to d5 on other positions represents the diffusion weight coefficient relative to the current pixel **, and d1 to d5 are taken in [0, 1] and satisfy the following expression:


2×d1+4×d2+2×d3+2×d4+2×d5ε[0,1].

5. The method of claim 4, wherein a diffusion mode of the second diffusion filter w is set as:


















**
w0



w3
w2
w1









wherein, a scanning direction is from left to right, ** represents the current pixel's position, each of w0 to w3 on other positions represents the diffusion weight coefficient relative to the current pixel, and w0 to w3 are taken in [0, 1] and satisfy the following expression:


w0+w1+w2+w3ε[0,1].

6. The method of claim 5, wherein the dithering algorithm for the second diffusion filter w in the step (d) is as below:


fRand=(R(m,t)/R_MAX−0.5)×cDither



dw0=w0−fRand



dw2=w2+fRand



dw1=w1+fRand



dw3=w3−fRand

wherein, fRand is a parameter for fine adjusting dithering; R(m, t) is a parameter with random value for scanning a current dot; R_MAX is a maximum of a random parameter R(i); cDither is a parameter for adjusting amplitude of dithering; and dw0 ̃dw3 are the diffusion weight coefficients of the second diffusion filter w in different directions after dithering.

7. A system for scanning and processing an image using the error diffusion screening technology, comprising a scanning device, a storing device and a processing device, wherein the scanning device has an output terminal connected to an input terminal of the storing device, the storing device has an output terminal connected to an input terminal of the processing device, the scanning device scans each pixel Mi of an nth line in an original image one by one, and a scanning result of the pixel Mi is stored to the storing device; and
wherein the processing device processes stored data for the pixel Mi by using error diffusion, once processing for the pixel Mi is completed, a obtained value for the pixel Mi is output so that an ith storage location in the storing device can be idle, at the same time, the scanning device scans pixels of an n+1th line in the original image, once the ith storage location in the storing device is idle, scanned data for a pixel in the n+1th line is stored to the ith storage location, and once all pixels in the nth line have been processed, all pixels in the n+1th line have been scanned and stored, a processing direction parameter C of the nth line and a storing location parameter H for a beginning pixel of the n+1th line are recorded, and the processing direction of the n+1th line is determined based on values of the parameters C and H, and wherein C=0 indicates that pixels in the nth line are processed from beginning to end; C=1 indicates that pixels in the nth line are processed from end to beginning; H=0 indicates that the beginning pixel of the n+1th line is stored in a first location of a storing device; and H=1 indicates that the beginning pixel of the n+1th line is stored in a last location of the storing device.

8. The method of claim 1, wherein the error diffusion further comprises:
(a) scanning and inputting the original image, carrying out an operation of a threshold comparison on a final value g′′(m, t) of a current pixel in the original image, and then converting a result of the operation to an output value b(m, t) of the current pixel;
(b) comparing the output value b(m, t) of the current pixel with a medium value g′(m, t) of the current pixel to obtain an error value e(m, t) of the current pixel;
(c) multiplying the error value e(m, t) by preset weight distribution coefficients through a first diffusion filter e and then diffusing results of the multiplying to unprocessed pixels around the current pixel, wherein each of the diffused results to the unprocessed pixels around the current pixel is weightedly added to an original value g(m, t) of the pixel Mx, to obtain a new medium value g′(m, t) of the pixel Mx;
(d) diffusing processed results to corresponding pixels Mx surrounding the current pixel, respectively, and weightedly adding each of the diffused processed results to the medium value g′(m, t) of the corresponding pixel Mx obtained from error diffusion in the step (c) to obtain the final value g′′(m, t) of the corresponding pixel Mx, wherein the processed results are obtained by implementing a multiplying operation on the output value b(m, t) of the current pixel using a second diffusion filter w and processing results of the multiplying operation with a dithering algorithm, and the step (d) is implemented in parallel with the steps (b) and (c); and
(e) repeating the steps (a)-(d) until the original input values g(m, t) of all pixels are processed.

9. The method of claim 8, wherein the first error diffusion filter e in the step (c) uses a diffusion principle and weight distribution coefficients as below:


















**
d5
d3


d2
d4
d5
d4
d2


d1
d2
d3
d2
d1








wherein, ** represents the current pixel's position, each of d1 to d5 on other positions represents the diffusion weight coefficient relative to the current pixel **, and d1 to d5 are taken in [0, 1] and satisfy the following expression:


2×d1+4×d2+2×d3+2×d4+2×d5ε[0,1].

10. The method of claim 9, wherein a diffusion mode of the second diffusion filter w is set as:















**
w0


w3
w2
w1








wherein, a scanning direction is from left to right, ** represents the current pixel's position, each of w0 to w3 on other positions represents the diffusion weight coefficient relative to the current pixel, and w0 to w3 are taken in [0, 1] and satisfy the following expression:


w0+w i+w2+w3ε[0, 1].

11. The method of claim 10, wherein the dithering algorithm for the second diffusion filter w in the step (d) is as below:


fRand=(R(m,t)/R_MAX−0.5)×cDither



dw0=w0−fRand



dw2=w2+fRand



dw1=w1+fRand



dw3=w3−fRand

wherein, fRand is a parameter for fine adjusting dithering; R(m, t) is a parameter with random value for scanning a current dot; R_MAX is a maximum of a random parameter R(i); cDither is a parameter for adjusting amplitude of dithering; and dw0 ̃dw3 are the diffusion weight coefficients of the second diffusion filter w in different directions after dithering.

12. The method of claim 1, wherein the error diffusion further comprises:
(a) scanning and inputting the original image, carrying out an operation of a threshold comparison on a final value g′′(m, t) of a current pixel in the original image, and then converting a result of the operation to an output value b(m, t) of the current pixel;
(b) comparing the output value b(m, t) of the current pixel with a medium value g′(m, t) of the current pixel to obtain an error value e(m, t) of the current pixel;
(c) multiplying the error value e(m, t) by preset weight distribution coefficients through a first diffusion filter e and then diffusing results of the multiplying to unprocessed pixels around the current pixel, wherein each of the diffused results to the unprocessed pixels around the current pixel is weightedly added to an original value g(m, t) of the pixel Mx to obtain a new medium value g′(m, t) of the pixel Mx;
(d) diffusing processed results to corresponding pixels Mx surrounding the current pixel, respectively, and weightedly adding each of the diffused processed results to the medium value g′(m, t) of the corresponding pixel Mx obtained from error diffusion in the step (c) to obtain the final value g′′(m, t) of the corresponding pixel Mx, wherein the processed results are obtained by implementing a multiplying operation on the output value b(m, t) of the current pixel using a second diffusion filter w and processing results of the multiplying operation with a dithering algorithm, and the step (d) is implemented in parallel with the steps (b) and (c); and
(e) repeating the steps (a)-(d) until the original input values g(m, t) of all pixels are processed.

13. The method of claim 12, wherein the first error diffusion filter e in the step (c) uses a diffusion principle and weight distribution coefficients as below:


















**
d5
d3


d2
d4
d5
d4
d2


d1
d2
d3
d2
d1








wherein, ** represents the current pixel's position, each of d1 to d5 on other positions represents the diffusion weight coefficient relative to the current pixel **, and d1 to d5 are taken in [0, 1] and satisfy the following expression:


2×d1+4×d2+2×d3+2×d4+2×d5ε[0,1].

14. The method of claim 13, wherein a diffusion mode of the second diffusion filter w is set as:















**
w0


w3
w2
w1








wherein, a scanning direction is from left to right, ** represents the current pixel's position, each of w0 to w3 on other positions represents the diffusion weight coefficient relative to the current pixel, and w0 to w3 are taken in [0, 1] and satisfy the following expression:


w0+w1+w2+w3ε[0, 1].

15. The method of claim 14, wherein the dithering algorithm for the second diffusion filter w in the step (d) is as below:


fRand=(R(m,t)/R_MAX−0.5)×cDither



dw0=w0−fRand



dw2=w2+fRand



dw1=w1+fRand



dw3=w3−fRand

wherein, fRand is a parameter for fine adjusting dithering; R(m, t) is a parameter with random value for scanning a current dot; R_MAX is a maximum of a random parameter R(i); cDither is a parameter for adjusting amplitude of dithering; and dw0 ̃dw3 are the diffusion weight coefficients of the second diffusion filter w in different directions after dithering.

16. The method of claim 2, wherein the error diffusion further comprises:
(a) scanning and inputting the original image, carrying out an operation of a threshold comparison on a final value g′′(m, t) of a current pixel in the original image, and then converting a result of the operation to an output value b(m, t) of the current pixel;
(b) comparing the output value b(m, t) of the current pixel with a medium value g′(m, t) of the current pixel to obtain an error value e(m, t) of the current pixel;
(c) multiplying the error value e(m, t) by preset weight distribution coefficients through a first diffusion filter e and then diffusing results of the multiplying to unprocessed pixels around the current pixel, wherein each of the diffused results to the unprocessed pixels around the current pixel is weightedly added to an original value g(m, t) of the pixel Mx, to obtain a new medium value g′(m, t) of the pixel Mx;
(d) diffusing processed results to corresponding pixels Mx surrounding the current pixel, respectively, and weightedly adding each of the diffused processed results to the medium value g ′(m, t) of the corresponding pixel Mx obtained from error diffusion in the step (c) to obtain the final value g′′(m, t) of the corresponding pixel Mx, wherein the processed results are obtained by implementing a multiplying operation on the output value b(m, t) of the current pixel using a second diffusion filter w and processing results of the multiplying operation with a dithering algorithm, and the step (d) is implemented in parallel with the steps (b) and (c); and
(e) repeating the steps (a)-(d) until the original input values g(m, t) of all pixels are processed.

17. The method of claim 16, wherein the first error diffusion filter e in the step (c) uses a diffusion principle and weight distribution coefficients as below:


















**
d5
d3


d2
d4
d5
d4
d2


d1
d2
d3
d2
d1








wherein, ** represents the current pixel's position, each of d1 to d5 on other positions represents the diffusion weight coefficient relative to the current pixel **, and d1 to d5 are taken in [0, 1] and satisfy the following expression:


2×d1+4×d2+2×d3+2×d4+2×d5ε[0,1].

18. The method of claim 17, wherein a diffusion mode of the second diffusion filter w is set as:















**
w0


w3
w2
w1








wherein, a scanning direction is from left to right, ** represents the current pixel's position, each of w0 to w3 on other positions represents the diffusion weight coefficient relative to the current pixel, and w0 to w3 are taken in [0, 1] and satisfy the following expression:


w0+w1+w2+w3ε[0, 1].

19. The method of claim 18, wherein the dithering algorithm for the second diffusion filter w in the step (d) is as below:


fRand=(R(m,t)/R_MAX−0.5)×cDither



dw0=w0−fRand



dw2=w2+fRand



dw1=w1+fRand



dw3=w3−fRand

wherein, fRand is a parameter for fine adjusting dithering; R(m, t) is a parameter with random value for scanning a current dot; R_MAX is a maximum of a random parameter R(i); cDither is a parameter for adjusting amplitude of dithering; and dw0 ̃dw3 are the diffusion weight coefficients of the second diffusion filter w in different directions after dithering.