function [f] = spreadForce(F, XIB, X, Y, dq, N, h);
%
% [f] = spreadForce(F, XIB, X, Y, N, h);
%
%  Spreads a force on the Lagrangian points, XIB to the fluid 
%     at pts (X,Y)
%  
%  Returns:
%     f = force on fluid 
%
%  Input:
%     F      = force at Lagrangian points
%     XIB     = location of Lagrangian points
%     X      = N*N vector of X location of fluid mesh point
%     Y      = ", but Y locations
%     dq     = lagrangian point spacing
%     N      = number of mesh points in each direction
%     h      = mesh width
%


[idxs delta] = evalDelta(XIB, X, Y, N, h);

f = zeros(N*N,2);

% for each Langrangian point calculate the force to apply:
for( l = 1:length(XIB(:,1)) )
  f(idxs(l,:),1) = f(idxs(l,:),1) + F(l,1) * dq * delta(l,:)';
  f(idxs(l,:),2) = f(idxs(l,:),2) + F(l,2) * dq * delta(l,:)';
end