COLOR TRAPPING ON A HALFTONED BI-LEVEL BITMAP

• US 20110243429A1
• Filed: 04/05/2010
• Published: 10/06/2011
• Est. Priority Date: 04/05/2010
• Status: Active Grant

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: YAO, MENG
• Granted Patent: US 8,340,396 B2
• Time in Patent Office: Days
• US Class Current: 382/162
• CPC Class Codes: H04N 1/58 Edge or detail enhancement; Noise or error suppression, e.g. colour misregistration correction H04N1/62 takes precedence

Abstract

What is disclosed is a novel system and method for determining whether a pixel resides along a two-color edge in a halftoned bi-level bitmap. In one embodiment, a bitmap having a plurality of colored pixels is received. For each pixel in the bitmap, a window of size n×m is defined centered on the current pixel. The window is partitioned into a plurality of regions with each region having an orientation direction with each orientation direction having an associated numeric value. A number of pixels are counted for each of the first and second colors in each of the associated orientation directions. A determination is made as to the respective orientation direction having the maximum and minimum pixel counts for each of the first and second colors. Thereafter, a determination is made whether the pixel resides along a two color edge based upon the max/min counts and each associated orientation direction.

Claims

1. A method for determining whether a color pixel resides along a two-color edge in a bi-level bitmap of a halftoned color image, the method comprising:

receiving a bitmap of a halftoned color image having a plurality of pixels;

defining a window of size n×

m centered on a current pixel in the bitmap;

partitioning the window into a plurality of regions, with each region having an orientation direction with each orientation direction having an associated numeric value;

counting a number of pixels of each of the first and second colors in each of the plurality of regions;

determine which region has a maximum and minimum pixel count for each of the first and second colors; and

using the maximum and minimum pixel counts and each of the associated orientation directions to determine whether the current pixel resides along a two-color edge in the bitmap.

2. The method of claim 1, wherein the defined m×

n window has a row/column format given by R_x, C_x, and the plurality of regions comprises;

DIR_N=1, where (R_x<

3), DIR_NE=2, where (C_x>

R_x), DIR_E=3, where (C_x>

3), DIR_SE=4, where (R_x+C_x>

6), DIR_S=5, where (R_x>

3), DIR_SW=6, where (C_x<

R_x), DIR_W=7, where (C_x<

3), and DIR_NW=8, where (R_x+C_x<

6).

3. The method of claim 1, further comprising:

deleting at least one row of pixels immediately adjacent to the pixel;

deleting at least one column of pixels immediately adjacent to the pixel; and

defining the window of size n×

m centered on the current pixel which does not include any pixels in each of the deleted rows and columns.

4. The method of claim 1, wherein in response to the current pixel residing along the two-color edge in the bitmap, further comprising estimating contone color values for the first and second colors of the two-color edge based upon halftone threshold values of the halftone screens used to color the area of the bitmap wherein the current pixel resides.

5. The method of claim 3, further comprising:

assigning the contone color value estimated for the color on an opposing side of the two-color edge to the pixel; and

re-halftoning the current pixel using the assigned contone color value.

6. The method of claim 1, further comprising:

subdividing the window into a plurality of estimate windows and corresponding intermediate estimate windows; and

determining values for each of the intermediate estimate windows based upon contone color values of a corresponding estimate window and estimate windows which are immediately adjacent to that estimate window.

7. The method of claim 6, further comprising:

setting c_min and m_min to their respective minimum values of the intermediate estimate windows;

determining the condition given by;

(c_min<

LOW_COVC)∥

(m_min<

LOW_COVM),

where LOW_COVC is the low cyan coverage, and LOW_COVM is the low magenta coverage; and

trapping the pixel in response to the condition having been satisfied.

8. A system for determining whether a color pixel resides along a two-color edge in a bi-level bitmap of a halftoned color image, the system comprising:

a memory;

a storage medium; and

a processor in communication with the memory and the storage medium, the processor executing machine readable instructions for performing for each of a number of pixels in the bitmap;

receiving a bitmap of a halftoned color image having a plurality of pixels;

defining a window of size n×

m centered on a current pixel in the bitmap;

partitioning the window into a plurality of regions, with each region having an orientation direction with each orientation direction having an associated numeric value;

counting a number of pixels of each of the first and second colors in each of the plurality of regions;

determine which region has a maximum and minimum pixel count for each of the first and second colors; and

using the maximum and minimum pixel counts and each of the associated orientation directions to determine whether the current pixel resides along a two-color edge in the bitmap.

9. The system of claim 8, wherein the defined m×

n window has a row/column format given by R_x, C_x, and the plurality of regions comprises;

DIR_N=1, where (R_x<

3), DIR_NE=2, where (C_x>

R_x), DIR_E=3, where (C_x>

3), DIR_SE=4, where (R_x+C_x>

6), DIR_S=5, where (R_x>

3), DIR_SW=6, where (C_x<

R_x), DIR_W=7, where (C_x<

3), and DIR_NW=8, where (R_x+C_x<

6).

10. The system of claim 8, further comprising:

deleting at least one row of pixels immediately adjacent to the pixel;

deleting at least one column of pixels immediately adjacent to the pixel; and

defining the window of size n×

m centered on the current pixel which does not include any pixels in each of the deleted rows and columns.

11. The system of claim 8, wherein in response to the current pixel residing along the two-color edge in the bitmap, further comprising estimating contone color values for the first and second colors of the two-color edge based upon halftone threshold values of the halftone screens used to color the area of the bitmap wherein the current pixel resides.

12. The system of claim 11, further comprising:

assigning the contone color value estimated for the color on an opposing side of the two-color edge to the pixel; and

re-halftoning the current pixel using the assigned contone color value.

13. The system of claim 8, further comprising:

subdividing the window into a plurality of estimate windows and corresponding intermediate estimate windows; and

determining values for each of the intermediate estimate windows based upon contone color values of a corresponding estimate window and estimate windows which are immediately adjacent to that estimate window.

14. The system of claim 13, further comprising:

setting c_min and m_min to their respective minimum values of the intermediate estimate windows;

determining the condition given by;

(c_min<

LOW_COVC)∥

(m_min<

LOW_COVM),

where LOW_COVC is the low cyan coverage, and LOW_COVM is the low magenta coverage; and

trapping the pixel in response to the condition having been satisfied.

15. A method for determining whether a color pixel resides along a two-color edge in a bi-level bitmap of a halftoned color image, the method comprising:

receiving a bitmap of a halftoned color image having a plurality of pixels; and

for each of a number of pixels in the bitmap;

partitioning the window into a plurality of regions, with each region having an orientation direction with each orientation direction having an associated numeric value;

counting a number of pixels of each of the first and second colors in each of the plurality of regions;

determine which region has a maximum and minimum pixel count for each of the first and second colors; and

determining whether the current pixel resides along a two-color edge in the bitmap based upon the maximum and minimum pixel counts and each of the associated orientation direction.

16. The method of claim 15, further comprising:

assigning the contone color value estimated for the color on an opposing side of the two-color edge to the pixel; and

re-halftoning the current pixel using the assigned contone color value.

17. The method of claim 15, further comprising:

subdividing the window into a plurality of estimate windows and corresponding intermediate estimate windows; and

determining values for each of the intermediate estimate windows based upon contone color values of a corresponding estimate window and estimate windows which are immediately adjacent to that estimate window.

18. The method of claim 17, further comprising:

setting c_min and m_min to their respective minimum values of the intermediate estimate windows;

determining the condition given by;

(c_min<

LOW_COVC)∥

(m_min<

LOW_COVM),

where LOW_COVC is the low cyan coverage, and LOW_COVM is the low magenta coverage; and

trapping the pixel in response to the condition having been satisfied.

19. The method of claim 15, further comprising:

deleting at least one row of pixels immediately adjacent to the pixel;

deleting at least one column of pixels immediately adjacent to the pixel; and

defining the window of size n×

m centered on the current pixel which does not include any pixels in each of the deleted rows and columns.

20. The method of claim 15, wherein in response to the current pixel residing along the two-color edge in the bitmap, further comprising estimating contone color values for the first and second colors of the two-color edge based upon halftone threshold values of the halftone screens used to color the area of the bitmap wherein the current pixel resides.