I am trying to do some customized coverage test in fragment shader, and in order to make it work with MSAA, I want to know the sample location within the pixel. So I use glGetMultisamplefv with GL_SAMPLE_POSITION, here are the locactions returned for 4xMSAA:
I try them but see some artifact, then I go ahead and run fragment shader per sample and output the gl_SamplePosition to a buffer and I get:
It seems like all the y-coordinate are flipped. Is this a driver problem?
OS: windows 8
Card: GTX 780