Tone dependent threshold perturbation error diffusion halftone

• US 8,760,725 B1
• Filed: 11/12/2009
• Issued: 06/24/2014
• Est. Priority Date: 11/12/2008
• Status: Expired due to Fees

• Current Assignee: Marvell International Limited (Marvell Technology Group Limited)
• Original Assignee: Marvell International Limited (Marvell Technology Group Limited)
• Inventors: Keithley, Douglas G.
• Primary Examiner(s): ZHU, RICHARD Z
• Application Number:
  US12/617,397
• Time in Patent Office: 1,685 Days
• Field of Search: 358/1.9, 358/3.06, 358 303- 32, 358533-536
• US Class Current: 358/3.04
• CPC Class Codes: H04N 1/405 Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels
• CPC Class Codes: H04N 1/4051 producing a dispersed dots halftone pattern, the dots having substantially the same size different sizes H04N1/4057
• CPC Class Codes: H04N 1/4052 by error diffusion, i.e. transferring the binarising error to neighbouring dot decisions
• CPC Class Codes: H04N 1/52 Circuits or arrangements for halftone screening

Abstract

A method and system for generating a halftone image from an input image is disclosed. The method and system use error distribution and threshold perturbation that is based on the incoming tone value. A threshold bias table may be generated in real time for each incoming tone value. The values of the threshold bias tables are selected so that certain positions are forced to include a dot, in the case of light tone values, or not include a dot, in the case of dark tone values. A halftone process that uses threshold bias tables based on the incoming tone can reduce unnatural patterns in the image apparent to the casual observer, which results in a more pleasing image.

Claims

1. A method of generating a halftone image from an input image represented as a plurality of tone values, the method comprising:

comparing a preceding tone value and a corresponding preceding output pixel;

determining a first error distribution based on the comparison of the preceding tone value and the corresponding preceding output pixel;

determining a distribution range for applying the first error distribution across pixels of the input image based on whether the preceding tone value is within a first range of tone color values or within a second range of tone color values that is different from the first range of tone color values, wherein the distribution range is determined to be distributed over a larger number of pixels of the input image when the preceding tone value is within the first range of tone color values compared to when the preceding tone value is within the second range of tone color values;

receiving, at a controller, a first tone value of the input image that has been adjusted by at least a portion of the first error distribution, wherein the first tone value corresponds to a pixel that is within the distribution range of the first error distribution;

calculating, by the controller, a first threshold value based on whether the first tone value is determined to correspond to the first range of tone color values or the second range of tone color values;

comparing, by the controller, the first tone value to the first threshold value;

generating, by the controller, a first output pixel of the halftone image based on the comparison of the first tone value to the first threshold value; and

outputting the first output pixel of the halftone image.

2. The method of claim 1, further comprising:

calculating a second error distribution based on at least a portion of a difference between the first tone value and a scaled output value;

receiving a second tone value of the input image;

calculating a second threshold value as a function of the second tone value; and

generating a second output pixel of the halftone image based on a comparison of the second tone value to the second threshold value.

3. The method of claim 2, further comprising:

varying an allocation of the second error distribution among subsequent pixels as a function of the first tone value.

4. The method of claim 2, wherein the first tone value is equal to the second tone value.

5. The method of claim 1, wherein the first threshold value is calculated differently when the first tone value is determined to correspond to the first range of tone color values and when the first tone value is determined to correspond to the second range of tone color values.

6. The method of claim 1, wherein the first threshold value is calculated based on a first slope when the first tone value is greater than a critical tone value and a second slope when the first tone value is less than the critical tone value.

7. The method of claim 6, wherein the first threshold value is calculated based on a third slope when the first tone value is greater than a second critical tone value.

8. The method of claim 1, wherein the calculation of the first threshold value introduces a constant to threshold values when the first tone value is greater than a critical tone value.

9. The method of claim 1, wherein the calculation of the first threshold value introduces a constant to threshold values when the first tone value is less than a critical tone value.

10. The method of claim 1, wherein the first tone value of the input image comprises an 8-bit value and the first output pixel of the halftone image comprises a I-bit value.

11. The method of claim 1, wherein the calculation of the first threshold value modifies a threshold table including all possible tone values.

20. The method of claim 1, wherein the first range of tone color values corresponds to a light tone and the second range of tone color values corresponds to a dark tone.

12. A printer controller for generating a halftone image from an input image represented as a plurality of tone values, the printer controller comprising:

an interface configured to receive a first tone value of the input image that has been adjusted by at least a portion of a first error distribution from a preceding tone value comparison between a preceding tone value and a corresponding preceding output pixel, wherein the first tone value corresponds to a pixel that is within a distribution range of the first error distribution, wherein the distribution range is determined to be distributed over a larger number of pixels of the input image when the preceding tone value is a first tone color compared to when the preceding tone value corresponds to a second tone color, the first tone color being identified within a first range of tone color values and the second tone color being identified within a second range of tone color values that is different from the first range of tone color values; and

a processor configured to;

calculate a first threshold value based on whether the first tone value is determined to correspond to the first tone color or the second tone color;

compare the first tone value to the first threshold value;

generate a first output pixel of the halftone image based on the comparison of the first tone value to the first threshold value; and

output the first output pixel of the halftone image based on the comparison.

13. The printer controller of claim 12, wherein the interface is further configured to receive a second tone value of the input image, and the processor is further configured to calculate a second threshold value as a function of the second tone value such that a second output pixel of the halftone image is generated based on a comparison of the second tone value to the second threshold value.

14. The printer controller of claim 12, wherein the first threshold value is calculated differently for when the first tone value is determined to correspond to the first tone color and when the first tone value is determined to correspond to the second color.

15. The printer controller of claim 12, wherein the processor is configured to calculate the first threshold value based on a first slope when the first tone value is greater than a critical tone value and a second slope when the first tone value is less than the critical tone value.

16. The printer controller of claim 12, wherein the processor is configured to calculate the first threshold value by adding a first constant to threshold values that correspond to tone values greater than a critical tone value or subtracts a second constant to threshold values that correspond to tone values less than a second critical tone value.

17. The printer controller of claim 12, wherein the processor is further configured to vary an allocation of the first error distribution among subsequent pixels as a function of the first tone value.

18. The printer controller of claim 17, wherein the processor is configured to calculate the first threshold value by modifying a threshold table including 2ⁿ-1 tone values where n is equal to a number of bits per pixel of the input image.

19. A method of generating a halftone image from an input image, the method comprising:

receiving, at a controller, the input image as a plurality of tone values including at least a first tone value and a second tone value;

calculating, by the controller, a first threshold bias table as a function of the first tone value and a second threshold bias table as a function of the second tone value, wherein threshold value entries included in the first threshold bias table and the second threshold bias table are based on whether the first tone value is determined to be within a first range of tone color values or within a second range of tone color values that is different from the first range of tone color values, and whether the second tone value is determined to correspond to the first tone color or the second tone color, respectively;

comparing, by the controller, the first tone value to an entry in the first threshold bias table;

generating, by the controller, a first output pixel of the halftone image based on the comparison of the first tone value to the entry in the first threshold bias table;

calculating, by the controller, an error distribution based on at least a portion of a difference between the first tone value and a scaled output value based on the first tone value;

determining a distribution range of the error distribution across pixels of the input image to be distributed over a larger number of pixels of the input image when the preceding tone value is within the first range of tone color values compared to when the preceding tone value is within the second range of tone color values;

comparing, by the controller, the second tone value to an entry in the second threshold bias table;

generating, by the controller, a second output pixel of the halftone image based on the comparison of the second tone value and the entry in the second threshold bias table, the second tone value being within the distribution range of the error distribution such that the second tone value is adjusted by the error distribution; and

outputting at least the first output pixel and the second output pixel of the halftone image.