I didn't have the edge-only volume. I had to create it to make the transparent .png files.
As was pointed out before, I think the problem really came from not using a greyscale lut. I was overlaying a statistical volume with the anatomical MNI atlas so I needed some variant on the Red-Yellow lut.
Once I decided that slicer was not going to do what I wanted (no matter how many ways I tried), using imageMagick and transparent .png's worked out just fine.
As always, thanks for the help,
* ba
|