function [ downgrid, frontgrid ,leftgrid, rightgrid, backgrid, upgrid ] = preplot_dislayer( r,k )
%PREPLOT 此处显示有关此函数的摘要
%   此处显示详细说明
% k表示从上面数的层数
nx=r.nx;
ny=r.ny;
nz=r.nz;
downgrid=[];
frontgrid=[];
leftgrid=[]; 
rightgrid=[]; 
backgrid=[]; 
upgrid=[];
xiang=nz+1-k;
    for j=1:ny
        for i=1:nx
            rao=i+(j-1)*nx+(xiang-1)*nx*ny;
         
             downgrid=[downgrid rao];
         
         
             upgrid=[upgrid rao];
         
         if i==1
             leftgrid=[leftgrid rao];
         end
         if i==nx
             rightgrid=[rightgrid rao];
         end
         if j==1
             frontgrid=[frontgrid rao];
         end
         if j==ny
             backgrid=[backgrid rao];
         end
        end
    end

         
end