Patent ID: 6870955

Claim:
A method for deciding a semi-S curve for processing tone scale of a digital image, comprising steps of: a) analyzing tone scale distribution of said image to decide a weighting value of high occurrence color; b) analyzing tone scale of said image and deciding a transition point TP for the semi-S curve through steps of: making a histogram of tone scale, making summation of samples probability F n (n=O˜Mg, in which Mg is a maximum tone scale value of the image format) of pixel sample number locating in each tone scale (O˜Mg), making a summation of samples probability of image from the darkest level (tone scale 0) till the sum Σ F n (n=0˜N) reaching a value of 1/P, in which P is a predetermined parameter, and obtaining the position of transition point TP at the tone scale N; c) deciding a shadow portion color enhancement curve O-TP of the semi-S curve locating before the transition point TP, the curve O-TP passing through origin point O(0, 0), the transition point TP and a maximum downward offset point determined by a maximum downward offset D 1 calculated as follows: D 1 =C 1 (2 PF L )−1) when (2 PF L −1<0, and D 1 =0 when (2 PF L )−1≧0 in which F L =Σ F n (n=O˜(N/2)), C 1 is a predetermined constant; and d) deciding a light portion color enhancement curve TP-E of the semi-S curve locating after the transition point TP, the curve TP-E passing through an end point, the transition point TP and a maximum downward offset point determined by a maximum upward offset D 2 calculated as follows: D 2 =C 2 (1−2 PF H )/( P− 1) when (1−2 PF H −1>0, and D 2 =0 when (1−2 PF H )/( P− 1<0 in which F H =Σ F n (n=(N+Mg)/2−Mg), C 2 is a predetermined constant; whereby, the semi-S curve composed of the curve O-TP and the curve TP-E is obtained.