% optimality criteria update
function [xnew]=oc(nelx,nely,x,volfrac,dc)
l1=0;l2=100000;move=0.2;
while (l2-l1>1e-4)
    lmid=0.5*(l2+l1);
    xnew =max(0.001,max(x-move,min(1.,min(x+move,x.*sqrt(-dc./lmid)))));
    if sum(sum(xnew))-volfrac*nelx*nely>0;
        l1=lmid;
    else
        l2=lmid;
    end
end