Variable on Off-lattice boundary

Dear Communities,

I am doing some post process on my simulation. In the simulation, I use a stl file and off-lattice boundary condition. However, when I process the data, I realize that I cannot get the variable on geometry surface by interpolation. This is because the point in the geometry have no value.

I also read the code in gridrefinement3d, but it seems no this part.
Is this possible that I can get the variable on the surface in Palabos to make contour on surface?