SFDerivatives_Surf.m

function [dndx,dndy,detj,ele_area] = SFDerivatives_Surf(iele,nnps,nqpts,swts,nps,x,y,z,dnda,dndb)
% compute surface shape function derivatives in global coordinates

sdetj = zeros(1,nqpts);
dndx = zeros(nnps,nqpts);
dndy = zeros(nnps,nqpts);
ele_area;

for i=1:1:nqpts  
    
    tjac=zeros(2,2);
         
  for j=1:1:nnps  
	
      j1=nps(iele,j) ;  
	  
      tjac(1,1)=tjac(1,1)+dnda(j,i)*x(j1);                           
      tjac(1,2)=tjac(1,2)+dndb(j,i)*x(j1);	                           
      tjac(2,1)=tjac(2,1)+dnda(j,i)*y(j1);                          
      tjac(2,2)=tjac(2,2)+dndb(j,i)*y(j1);   	 
	  
  end                                                         
                                                                    
      detj(i)=det(tjac);
      
      ele_area = ele_area + detj(i)*swts(i);
	  
      tjaci = inv(tjac);                                      
                                                                     
  for  j=1:1:nnpe
	
      dndx(j,i)=tjaci(1,1)*dnda(j,i)+tjaci(2,1)*dndb(j,i);            
      dndy(j,i)=tjaci(1,2)*dnda(j,i)+tjaci(2,2)*dndb(j,i);  
  end      
  

end