Always let the fragment through.
Let the fragment through if the incoming alpha value equals the reference alpha value
Let the fragment through if the incoming alpha value is greater than or equal to the reference alpha value.
Let the fragment through if the incoming alpha value is greater than the reference alpha value
Let the fragment through if the incoming alpha value is less than or equal to the reference alpha value
Let the fragment through if the incoming alpha value is less than the reference alpha value
Never let the fragment through.
Let the fragment through if the incoming alpha value does not equal the reference alpha value
Alpha testing happens before blending primitives with the framebuffer and gives an opportunity to discard fragments based on a comparison with the incoming alpha value and a reference alpha value. The #CoglPipelineAlphaFunc determines how the comparison is done.