16.2.4.4 Under

Volume slices rendered front to back with the under operator give the same result as the over operator blending slices from back to front. Unfortunately, OpenGL doesn't have an exact equivalent for the under operator, although using glBlendFuncGL_ONE_MINUS_DST, GL_DST(GL_ONE_MINUS_DST, GL_DST) is a good approximation. Use the over operator and back to front rendering for best results. See Section 8.1 for more details.