1. A method for mapping an input grayscale into an output luminance comprising: selecting a first reference grayscale, a first reference luminance, a second reference grayscale and a second reference luminance according to an input grayscale; generating a middle reference grayscale and a middle luminance by computing first and second equations of: 
 X _MID=( X 1+ X 2)/2; and 
 Y _MID=( Y 1+ Y 2)/2±Δ, wherein Δ denotes an offset, X 1 denotes the first reference grayscale, X 2 denotes the second reference grayscale, Y 1 denotes the first reference luminance, Y 2 denotes the second reference luminance, X_MID denotes the middle reference grayscale, and the Y_MID denotes the middle luminance; replacing a value of the first or second reference grayscale by a value of the middle reference grayscale, and replacing a value of the first or second reference luminance by a value of the middle luminance according to the middle reference grayscale and the input grayscale; and generating an output luminance by computing a third equation of: 
 Y=Y 1+( Y 2 −Y 1)/( X 2− X 1)*( X−X 1), wherein Y denotes the output luminance.