Higher order error diffusion of digital halftoning

• US 6,753,978 B1
• Filed: 09/22/1999
• Issued: 06/22/2004
• Est. Priority Date: 09/22/1999
• Status: Expired due to Term

• Current Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Original Assignee: Sharp Laboratories of America Incorporated (Hon Hai Precision Industry Co., Ltd.)
• Inventors: Chang, William Ho
• Primary Examiner(s): Lee, Thomas D.
• Assistant Examiner(s): BRINICH, STEPHEN M
• Application Number:
  US09/405,421
• Time in Patent Office: 1,735 Days
• Field of Search: 358/1.9, 358/2.99, 358/3.01, 358/3-3-, 358/3-1-, 358/3.28, 382/237, 382/238
• US Class Current: 358/3.04
• CPC Class Codes: H04N 1/4053 with threshold modulated relative to input image data or vice versa
• CPC Class Codes: H04N 1/52 Circuits or arrangements for halftone screening

Abstract

In the error diffusion technique for digital halftoning the output color for a first pixel is selected by comparing an adjusted input color signal to a boundary separating output colors in a pallette for a display device. The error or difference between the selected output color and the input color signal is determined and portions of that difference are added to the input color signals for neighboring pixels diffusing the error throughout the area neighboring the first pixel. Error diffusion can create artifacts in the image as rendered by the display device. Error diffusion artifacts are reduced or eliminated by a higher order error diffusion method where the boundary separating output colors in the device'"'"'s pallette is adjusted as a function of the error in a previous pixel.

Claims

1. A method of error diffusion for digital halftoning comprising:

(a) disassociating an input image into a plurality of input signals representing a color of each of a plurality of input image pixels;

(b) altering said input signal for a subject pixel with a diffusion function of an error signal for another pixel;

(c) modifying said diffusion function of said error signal with a transformation function;

(d) altering said modified diffusion function with a boundary adjustment function;

(e) adjusting a color space boundary in a color pallette for an output device by said diffusion function as modified by said transformation function and said boundary adjustment function; and

(f) selecting an output color from said color pallette for said subject pixel by comparing said altered input signal for said subject pixel to said adjusted color space boundary.

2. The error diffusion method of claim 1 further comprising selecting said transformation function as a function of said input signal.

3. The error diffusion method of claim 1 further comprising selecting said boundary adjustment function as a function of said input signal.

4. The method of claim 1 wherein said output color is a color to be displayed on a monitor.

5. The method of claim 1 wherein said output color is a color to be produced by a printer.