An example. If you enable alpha test, you can reject fragments based on it’s alpha value. Say you decide to reject fragments with alpha less than 0.5. Now, if you set the clear color to (1, 1, 1, 0.1), the alpha value of the clear color will be 0.1 which is less than 0.5. So, one can ask him/herself, will all fragments generated by glClear be rejected becasue of the alpha function? No it won’t, becuse the specifications says that the alpha function is ignored.