[Chimera-users] mask using volume contour surface

Hi All, I have a question about volume contour surface in chimera. I have a density volume of a shell shape. I plan to use the outer contour surface of the volume to mask out all density encased by the outer surface in a tomogram. But instead, I could only mask out the density within the shell. I am wondering if I can make a surface model only using the outer surface. Thanks a lot. Gongpu