Patent ID: 6631209
Filing Date: 2003-10-07
Classification: H04N

Abstract:
An image processing method comprising:preparing a density histogram from input image data; calculating a first white reference value and a first black reference value for adjusting density of the input image data based on the density histogram, the first white reference value WK1 and the first black reference value BK1 being of higher precision than the density value of the density histogram; the first white reference value WK1 being calculated using H(WP), H(WPâˆ’1) and H(WP+1), where WP represents a density value having a maximum frequency in a background density region, H(WP) represents a frequency of pixels having density value WP, H(WPâˆ’1) represents a frequency of pixels having density value (WPâˆ’1), and H(WP+1) represents a frequency of pixels having density value (WP+1), the first black reference value BK1 being calculated using H(BP), H(BP+1). and H(BP+1), where BP represents a density value having a maximum frequency in a character density region, H(BP) represents a frequency of pixels having density value BP, H(BP+1) represents a frequency of pixels having density value (BP+1), and H(BP+1) represents a frequency of pixels having density value (BP+1); providing a final white reference value and a final black reference value by correcting at least one of the first white reference value WK1 and the first black reference value BK1 such that the first black reference value BK1 is at least a predetermined value greater than the first white reference value WK1; and adjusting the density of the input image data based on the final white reference value and the final black reference value, wherein the adjusting the density adjusts the density of the image data based on the following equation: Dâ€²=(Dâˆ’WK3)/(BK3âˆ’WK3),â€ƒâ€ƒâ€ƒwherein Dâ€² is the density of the image data after being adjusted by the density adjusting means, D is the density of the image data prior to being adjusted by the density adjusting means, WK3 is the final white reference value, and BK3 is the final black reference value.