function [phi,dphix,dphiy] = shape(gpos,dmax,x,v,dm)
% EFG shape function and it's derivative with linear base
L = length(v);
won = ones(1,L);
nv = x(1:2,v);
nvo=nv(1,:)+nv(2,:);
nv1=nv(1,:)-nv(2,:);
p = [won;nvo;nv1];
dif = gpos*won-nv;
t = dm(1:2,v)/dmax;

% WEIGHTS--W and dw are vectors
[w,dwdx,dwdy] = cubwgt(dif,t,v,dmax,dm);
B = p.*[w;w;w];
pp = zeros(3);
aa = zeros(3);
daax = zeros(3);
daay = zeros(3);
for i=1:L
pp = p(1:3,i)*p(1:3,i)';
aa = aa+w(1,i)*pp;
daax = daax+dwdx(1,i)*pp;
daay = daay+dwdy(1,i)*pp;
end

gpos1=gpos(1,1)+gpos(2,1);
gpos2=gpos(1,1)-gpos(2,1);
pg = [1 gpos1 gpos2];
Ainv = inv(aa);
phi = pg*Ainv*B;
dbx = p.*[dwdx;dwdx;dwdx];
dby = p.*[dwdy;dwdy;dwdy];
dax = -Ainv *(daax*Ainv);
day = -Ainv *(daay*Ainv);
dphix = [0,1,1]*Ainv*B + pg*(dax*B + Ainv*dbx);
dphiy = [0,1,-1]*Ainv*B + pg*(day*B + Ainv*dby);