function Est = Poisson_KNNQuadfit(AP,DV,Expr)

% Finds K nearest neighbors, then makes quadratic fit to the expression
% data.

K = 20;

% Rescale coordinates.
AP2 = AP/100;
DV2 = DV/(100*2.5);

% Go through each point
Est = zeros(length(Expr),1);
for QueryPt = 1:length(Expr)
    % Find Euclidean distances
    EucDistSqr = (AP2(QueryPt)-AP2).^2+(DV2(QueryPt)-DV2).^2;
    
    % Find K nearest nuclei
    [Z,I] = sort(EucDistSqr);
    Near = I(2:(K+1));
    
    % Set up and solve the quadratic fit
    Xnear=[AP2(Near) AP2(Near).^2 DV2(Near) DV2(Near).^2 AP2(Near).*DV2(Near) ones(K,1)];
    Ynear=Expr(Near);
    W = inv(Xnear'*Xnear)*Xnear'*Ynear;
    
    Est(QueryPt) = W(1)*AP2(QueryPt) + ...
        W(2)*AP2(QueryPt)^2 + ...
        W(3)*DV2(QueryPt) + ...
        W(4)*DV2(QueryPt)^2 + ...
        W(5)*AP2(QueryPt)*DV2(QueryPt) + ...
        W(6);
end