The invention discloses a Gamma transform unit. The Gamma transform unit has a lookup table. The lookup table stores a plurality of difference values corresponding to a plurality of predetermined input values. Each of the difference values substantially equals a difference between an ideal output value and a Gamma function output value, both of which correspond to one of the predetermined input values. After the Gamma transform unit has received an input value, it generates a required Gamma function output value corresponding to the received input value through referring to the lookup table.