I made some objects with a blinn material that has a refraction map called Thin Wall Refraction. The objects' opacity is at 50% (they're supposed to be like translucent glass). When the refraction map is turned off, I can render an alpha channel that ranges from white for the brightest parts of the objects, to black for the transparent/darkest parts of the image (is maybe based on RGB intensity?). When I turn the map back on, the alpha channel renders only as solid white for the combined shapes of the objects and solid black for everywhere else. How do I get both a refraction effect and a graded alpha channel with grays?