OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Multisample Fragment Operations

By default, multisampling calculates fragment coverage values that are independent of alpha. However, if you glEnable() one of the following special modes, then a fragment’s alpha value is taken into consideration when calculating the coverage, assuming that multisampling itself is enabled and that there is a multisample buffer associated with the framebuffer. The special modes are as follows:

• GL_SAMPLE_ALPHA_TO_COVERAGE uses the alpha value of the fragment in an implementation-dependent manner to compute the final coverage value.

• GL_SAMPLE_ALPHA_TO_ONE sets the fragment’s alpha value the maximum alpha value, and then uses that value in the coverage calculation.

• GL_SAMPLE_COVERAGE uses the value set with ...