System and method for digital image tone mapping using an adaptive sigmoidal function based on perceptual preference guidelines

摘要

An adaptive image tone mapping curve (170) based on perceptual preference guidelines is generated as a sigmoidal function, in which the sigmoidal function parameters (150) (slope and shift) are determined by original image statistics (149). Tone curves (170) generated for different images each have a smooth sigmoidal shape, so that the tone mapping process does not change the image histogram shape drastically. The sigmoidal function has the form: <DF>t(x) = 100 DIVIDED 1 + exp(- alpha (x/100 - beta )), </DF> where alpha is the slope parameter (150a) and beta is the shift parameter (150b). The input value x in the sigmoidal function varies in the range Ä0, 100Ü, because the tone curve (170) is generated on an L* scale, which has values from 0 to 100. The sigmoidal tone curve (170) calculation can be implemented efficiently using simple arithmetic operations by pre-calculating and storing various factors used in the calculation of alpha and beta and by pre-generating a pair of fixed tone curves (158) with two extreme slopes and interpolating between the curves (158). <IMAGE>