1. Quake,
http://www.idsoftware.com/
2. Half-life,
http://orange.half-life2.com/
3. Mao, W., Tang, M.: Three-dimensional Game Design Book-OpenGL, pp. 60–65. Electronic audiovisual publishing house in Sichuan, Chendu (2005)
4. Wang, R.: Computer Graphics, pp. 180–210. Posts & Telecom Press, Beijing (2009)
5. Trent, P.: Focus On 3D Terrain Programming, pp. 179–181. Premier Press (2003)