Patent ID: 7440139

Claim:
A method for controlling a tone reproduction curve of a halftone digital image, the method comprising: receiving the halftone digital image; selecting a target pixel and a neighborhood area of the selected target pixel; estimating a local gray value of the selected target pixel and the selected neighborhood area; determining a modification value for the selected target pixel based on a relationship between the estimated local gray value and a desired gray level; modifying a pixel value of the selected target pixel using the determined modification value; determining a quantization error for the selected target pixel, wherein when the determined quantization error is not equal to zero, the method further comprising: selecting a diffusion mask, the mask relating to the target pixel and pixels neighboring thereto; diffusing the determined quantization error using a rank-ordered error diffusion to at least one pixel based on the selected diffusion mask, the rank-order error diffusion acting on only four pixels of the neighboring area and comprising: receiving a first pair of adjacent pixel values selected from the neighboring area for a first pixel pair including a first adjacent pixel and a second adjacent pixel; receiving a second pair of different adjacent pixel values selected from the neighboring area for a second pixel pair including a third adjacent pixel and a fourth adjacent pixel; determining a ranking order of the first, second, third and fourth adjacent pixels by comparing pixel values of the first, second, third and fourth adjacent pixels; determining which of the first and second pair is the closest neighbor to the target pixel; and diffusing the determined quantization error using only the first, second, third and fourth adjacent pixels based on the determined ranking order of the first, second, third and fourth adjacent pixels, the error being diffused first to the determined closest neighbor pair by diffusing the error to the adjacent pixel of the four pixels in the determined closest pair having the highest ranking order, followed by diffusion of a portion of any remaining error to the remaining pixel of the closest pair, any remaining error then being diffused to the remaining pair of the first and second pair, the remaining error being diffused first to the adjacent pixel in the remaining pair having the highest ranking order, and then diffused to the other pixel of the remaining pair; and performing a blurring operation about neighboring pixels prior to performing rank-order error diffusion when the received halftone digital image is a binary image.