Method of color halftoning using space filling curves

• US 5,855,433 A
• Filed: 02/10/1997
• Issued: 01/05/1999
• Est. Priority Date: 04/28/1992
• Status: Expired due to Fees

• Current Assignee: Jonas M. Gomes, Luiz C. Velho
• Original Assignee: Jonas M. Gomes, Luiz C. Velho
• Inventors: Gomes, Jonas M., Velho, Luiz C.
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): Williams, Kimberly A.
• Application Number:
  US08/798,583
• Time in Patent Office: 694 Days
• Field of Search: 358/455, 358/456, 358/429, 358/298, 358/536, 358/448, 358/457, 358/458, 358/459, 358/534, 395/102, 395/109, 382/266, 382/267, 382/269, 382/316, 382/162
• US Class Current: 382/162
• 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 1/4052 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 of color halftoning using stochastic screening includes clustering, performing error diffusion and applying an adaptive criteria to change the cluster size according to the variation of the image color values. The method incorporates features from the traditional amplitude modulated (AM) digital halftoning methods, along with the advantages of frequency modulated (FM) techniques, recently introduced into the raster image processors of high resolution phototypesetters.

Claims

1. A halftoning method for reproducing a color image in electronic display and printing systems, the color image being composed of the primary color components cyan, magenta, yellow and black, said method comprising the steps of:

subdividing the color image into regions based on a trace of a space filling curve,

further subdividing each of said regions into equally sized pixels, the region defining a pixel cluster, each of said regions including at least one pixel,

scanning said color image along the trace of said space filling curve to visit all pixels of said color image in a sequential order,

determining a norm of a directional derivative along the space filling curve,

calculating an average image intensity for each region of the color image,

measuring a variation of image intensity along the space filling curve to determine a rate of change of image intensity,

establishing a range of pixel cluster intensity levels based on a number of said pixels in said cluster, a particular intensity level of said pixel cluster being defined by the ratio of the number of pixels in said cluster being in an ON state and the total number of pixels in said cluster, the spatial configuration of said ON state pixels producing a dot pattern having an intensity level corresponding to the average image intensity for that region,

varying the cluster size inversely relative to the rate of change of image intensity, and

generating said dot pattern for each of said pixel clusters by activating individual ones of said pixels to said ON state in an order corresponding with said sequential order along the trace of said space filling curve through each of said pixels of said region with the option of distributing said ON pixels around a point of highest intensity of the color image in the region.

2. A halftoning method as recited in claim 1 wherein said subdivision of the color image into regions and said further subdivision of each of said regions into pixels is determined by the trace of said space filling curve over the color image.

3. A halftoning method as recited in claim 1 wherein each of said pixel clusters are comprised of a block of contiguous image elements along the trace of said space filling curve, said pixel clusters having a fixed size for a given source image with the pixel cluster size starting from one pixel in said region and increasing by increments of one pixel.

4. A halftoning method as recited in claim 1 wherein the size of said pixel clusters varies in accordance with a given color image so as to adapt to particular properties of the color image, including, but not limited to, the directional derivative of the intensity value along said space filling curve and relative contrast of color component values.

5. A halftoning method as recited in claim 1 wherein the size of said pixel clusters is derived from characteristics of the display system, including, but not limited to, display resolution, tone reproduction and color space.

6. A halftoning method as recited in claim 1 wherein said step of calculating an average intensity for each region further includes the steps of:

accumulating intensity values of all pixels in said pixel cluster,

calculating a quantized value and corresponding quantization error for said region, and

distributing the quantization error among subsequent pixel clusters along the trace of the space filling curve.

7. A halftoning method as recited in claim 1 wherein a rendition of the color image in electronic display systems is created by separately executing the steps of the method as recited in claim 1 for each of said color components of the color image.

8. A halftoning method as recited in claim 7 wherein relative positions of said dot patterns for each color component are offset within each pixel cluster proportionally to the number of color components and the size of the region.

9. A halftoning method as recited in claim 1 further including the steps of:

applying a digital filter to the color image to change the average image intensity of one or more of said regions.