Volume rendering has proven to be an effective and flexible visualization method for large three dimensional data sets from scientific or medical applications perspective. Transfer functions are particularly important to the quality of volume rendered images because their role is essentially to make the data visible by assigning optical properties like color and opacity to the original values of the data set being visualized. Good transfer functions reveal the important structures in the data without obscuring them with unimportant regions. Unfortunately

specifying a good transfer function can be a difficult and tedious task due to the lack of intuitive interface and guiding information. Recent research has focused on automatic and semiautomatic techniques for creating transfer functions. A comprehensive survey of the up to date design approaches and strategies for transfer function is presented. After introducing the mathematical definition of the transfer function

different data and optical properties that can be used in the domain of transfer function are discussed. The proposed approaches of designing transfer functions can be classified into three categories;image centric

data centric and object centric methods according to the extent of automation and interaction. Their characteristics and relationship also are analyzed and reviewed in detail. Along existing methods

some potential directions for future research are predicted finally.