% every micro ele in contourf
function [elesOffsetx,elesOffsety, nodesOffsetx, nodesOffsety] = offsetSIMP...
                                (eles, nodes, microx, microy, nelx, nely)

    elesOffsetx = zeros(nely, nelx, microx);
    elesOffsety = zeros(nely, nelx, microy);
    nodesOffsetx = zeros(nely, nelx, microx+1);
    nodesOffsety = zeros(nely, nelx, microy+1);
    
    for i = 1:nelx
        for j = 1:nely  
            ele = j + (i-1)*nely;
            pos = nodes(eles(ele,:),1:2);
            elex = min(pos(:,1)); % begin with 0
            eley = max(pos(:,2));

            elesOffsetx(j,i,:) = elex*microx+1:(elex+1)*(microx);

            % matrix idx in matlab:  y-coordinate:
            %         1                 10
            %         |                 |
            %         y                 9
            elesOffsety(j,i,:) = (nely-eley)*microy+1:(nely-eley+1)*(microy); 
            
            nodesOffsetx(j,i,:) = elex*microx+1:(elex+1)*(microx)+1;
            nodesOffsety(j,i,:) = (nely-eley)*microy+1:(nely-eley+1)*(microy)+1;
        end
    end
end