Method and apparatus for minimizing artifacts in images produced by error diffusion halftoning utilizing ink reduction processing

• US 5,592,592 A
• Filed: 10/11/1994
• Issued: 01/07/1997
• Est. Priority Date: 07/01/1994
• Status: Expired due to Fees

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Shu, Joseph S.
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): LEGREE, TRACY MICHELLE
• Application Number:
  US08/320,537
• Time in Patent Office: 819 Days
• Field of Search: 395/109, 395/101, 358/454, 358/455, 358/456, 358/463, 358/447, 358/298, 358/534, 358/502, 382/252, 382/498, 382/499, 382/275, 382/270, 382/269, 382/304
• US Class Current: 358/1.9
• CPC Class Codes: H04N 1/4052 by error diffusion, i.e. transferring the binarising error to neighbouring dot decisions

Abstract

An image preprocessing method and apparatus processes image data line-by-line using an error diffusion or dithering process to generate a halftoned image in which the radius of a dot representing a pixel is /dpi≦r≦√ 2/dpi The method and apparatus selectively performs the halftone process on every other pixel to reduce in consumption and computation time while maintaining a high resolution.

Claims

1. A method of converting a continuous-tone image, represented as an array of electronically encoded n×

m pixels comprising n rows of m pixels each specifying a gray-scale value, into a binary raster suitable for electronic printing, n and m being positive integers, the method comprising the steps of;

A. identifying a series of pixels corresponding to a linear segment of the image, the identified pixels comprising one of

(1) odd ones of the m pixels on corresponding odd ones of the n rows of pixels and even ones of the m pixels on corresponding even ones of the n rows of pixels; and

(2) even ones of the m pixels on corresponding odd ones of the n rows of pixels and odd ones of the m pixels on corresponding even ones of the n rows of pixels;

B. processing the identified pixels, in a sequence corresponding to movement along the segment in a predetermined direction, to convert the pixels into binary raster values; and

C. repeating the identification and processing steps until the image has been fully processed.

2. The method of claim 1, wherein step B comprises of steps of:

B1. comparing the value of the (i,j) one of the identified pixels with a threshold value, wherein 1≦

i≦

n and 1≦

j≦

m;

B2. calculating an error value defined as a difference between the value of the (i,j) identified pixel and the threshold value; and

B3. adjusting the value at least one succeeding identified pixel to be processed in accordance with at least a portion of the error value.

3. The method of claim 2, wherein the at least one succeeding identified pixel comprises at least one identified pixel on the i+1 row and at least one identified pixel on the i+2 row.

4. The method of claim 2, wherein the at least one succeeding identified pixel comprise a j+2 pixel on the row i, a j-1 pixel on a row i+1, a j+1 pixel on the row i+1, a j+2 pixel on the row i+2, j pixel on the row i+2 and a j-2 pixel on the row i+2.

5. The method of claim 1, further comprising the step of outputting a dot for each of the binary raster values, the dot having a radius r in the range of /dpi≦

r≦

√

2/dpi, and dpi is a resolution of the pixels in dots per inch.

6. An image-processing system for converting a continuous-tone image into a binary raster suitable for printing, the apparatus comprising:

memory means for storing an array of electronically encoded n×

m pixels each representing a gray-scale value, wherein n and m are positive integers;

reading means for reading every other electronically encoded pixel from said memory means; and

halftoning means for iteratively processing series of pixels read by said reading means each corresponding to a linear segment of the image, in a sequence representative of movement along the segment in a predetermined direction, so as to produce binary raster values therefrom.

7. The system of claim 6, wherein said halftoning means comprises:

a comparator for comparing the values of the (i,j) one of the identified pixels with a threshold value, wherein 1≦

i≦

n and 1≦

j≦

m;

a calculating circuit for calculating an error value defined as a difference between the value of the (i,j) identified pixel and the threshold value; and

adjusting circuit for adjusting the value of at least one succeeding identified pixel to be processed in accordance with at least a portion of the error value.

8. The system of claim 7, wherein the at least one succeeding identified pixels comprises at least one identified pixel on the i+1 row and at least one identified pixel on the i+2 row.

9. The system of claim 7, wherein the at least one succeeding identified pixel comprises a j+2 pixel on the row i, a j-1 pixel on a row i+1, a j+1 pixel on the row i+1, a j+2 pixel on a row i+2, a j pixel on the row i+2 and a j-2 pixel on the row i+2.

10. The system of claim 6, further comprising a binary output means for outputting a dot for each of the binary raster values, the dot having a radius r in the range of /dpi≦

r≧

√

2/dpi, and dpi is a resolution of the pixels in dots per inch.

11. A printer for printing a continuous-tone image as patterns of monochrome dots on a print medium, the printer comprising:

digitizing means responsive to the continuous-tone image for generating a stream of electronically m×

n encoded pixel values, each representing a grayscale value of a portion of the continuous-tone image;

memory means responsive to the stream of electronically encoded pixel values for storing the electronically encoded pixel values in a plurality of linear segments, each linear segment having a start and an end and comprising pixel values that represent contiguous portions of the continuous-tone image, wherein n and m are positive integers;

reading means for reading every other electronically encoded pixel from said memory means;

halftoning means controllable to process each pixel value corresponding to the electronically encoded pixels read by said reading means in each of the plurality of linear segments in a start to end sequence and in an end to start sequence to generate output results; and

a print mechanism responsive to the output results for generating the patterns of monochrome dots on the print medium.

12. The printer of claim 11, wherein said halftoning means comprises:

a comparator for comparing the values of the (i,j) one of the identified pixels with a threshold value, wherein 1≦

i≦

n and 1≦

j≦

m;

a calculating circuit for calculating an error value defined as a difference between the value of the (i,j) identified pixel and the threshold value; and

adjusting circuit for adjusting the value of at least one succeeding identified pixel to be processed in accordance with at least a portion of the error value.

13. The printer of claim 12, wherein the at least one succeeding identified pixel comprises at least one identified pixel on the i+1 row and at least one identified pixel on the i+2 row.

14. The printer of claim 12, wherein the at least one succeeding identified pixel comprises a j+2 pixel on the row i, a j-1 pixel on a row i+1, a j+1 pixel on the row i+1;

a j+2 pixel on a row i+2, a j pixel on the row i+2 and a j-2 pixel on the row i+2.

15. The printer of claim 12, wherein each of the monochrome dots has a radius r in the range of /dpi≦

r≦

√

2/dpi, and dpi is a resolution of the pixels in dots per inch.

16. A computer system comprising:

a memory for storing data and programs;

a central processing unit responsive to programs stored in the memory for controlling and coordinating the operation of the computer system;

digitizing means responsive to a continuous-tone image for generating a stream of electronically encoded pixel values, each representing a gray-scale value of a portion of the continuous-tone image;

memory means responsive to the stream of electronically encoded pixel values for storing the electronically encoded pixel values in a plurality of linear segments, each linear segment having a start and an end and comprising pixel values that represent contiguous portions of the continuous-tone image;

reading means for reading every other pixel from said memory means;

halftoning means controllable to process each pixel value corresponding to the electronically encoded pixel read by said reading means in each of the plurality of linear segments in a start to end sequence and in an end to start sequence to generate output results; and

a printer responsive to the output results for printing patterns of monochrome dots on a print medium to generate a halftoned image.

17. The computer system of claim 16, wherein said halftoning means comprises:

a comparator for comparing the values of the (i,j) one of the identified pixels with a threshold value, wherein 1≦

j≦

n and 1≦

j≦

m;

a calculating circuit for calculating an error value defined as a difference between the value of the (i,j) identified pixel and the threshold value; and

adjusting circuit for adjusting the value of at least one succeeding identified pixel to be processed in accordance with at least a portion of the error value.

18. The computer system of claim 17, wherein the at least one succeeding identified pixel comprises at least one identified pixel on the i+1 row and at least one identified pixel on the i+2 row.

19. The computer system of claim 17, wherein the at least one succeeding identified pixel comprises a j+2 pixel on the row i, a j-1 pixel on a row i+1, a j+1 pixel on the row i+1, a j+2 pixel on a row i+2, a j pixel on the row i+2 and a j-2 pixel on the row i+2.

20. The printer of claim 16, wherein each of the monochrome dots has a radius r in the range of /dpi≦

r≦

√

2/dpi, and dpi is a resolution of the pixels in dots per inch.