This is not difficult in principle, since the image viewer is capable of showing overlay images in the RGBA format, where A=opacity defined separately for every pixel. The thing that requires mental effort (hard on a Monday) is the transformation from threshold value to opacity, and what kind of controls the user gets to control this transformation. And where to put them in the AFNI GUI -- probably in some popup hidden menu, since the window space in the GUI is pretty jammed up nowadays.
