1. A method comprising: receiving a perceptual curve digital code value D for an image; determining a normalized perceptual curve signal value V from a perceptual curve digital code value D, the perceptual curve digital code value D excludes reserved code values; determining a normalized component value Y based on a functional model of Y = ( max [ ( V 1 / m - c 1 ) , 0 ] c 2 - c 3 V 1 / m ) 1 / n ; and determining an absolute value L from the normalized component value Y, wherein parameters n, m, c 1 , c 2 , and c 3 are predetermined values.