In this context MIP stands for Maximum Intensity Projection. It is used in medical imaging to visualize blood flow. MIP finds the brightest texel alpha from all the texture slices at each pixel location. MIP is a contrast enhancing operator; structures with higher alpha values tend to stand out against the surrounding data.
MIP can be implemented with OpenGL using the blend function and the blend minmax extension:
glBlendFunc(GL_ONE, GL_ONE) glBlendEquationEXT(GL_MAX_EXT)