An adaptive image tone mapping curve based on perceptual preference guidelines
is generated as a sigmoidal function, in which the sigmoidal function parameters
(slope and shift) are determined by original image statistics. Tone curves 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:
##EQU1##
where is the slope parameter and is the shift parameter. The
input value x in the sigmoidal function varies in the range [0, 100], because the
tone curve is generated on an L* scale, which has values from 0 to 100. The sigmoidal
tone curve calculation can be implemented efficiently using simple arithmetic operations
by pre-calculating and storing various factors used in the calculation of
and and by pre-generating a pair of fixed tone curves with two extreme
slopes and interpolating between the curves.