function [Irec,ind_new,ind_1,ind_2,ind_3,ind_4,a1,a2,a3,a4] = rect_index(I,R,f,c,k,alpha,KK_new);


if nargin < 5,
   k = [0;0;0;0;0];
   if nargin < 4,
      c = [0;0];
      if nargin < 3,
         f = [1;1];
         if nargin < 2,
            R = eye(3);
            if nargin < 1,
               error('ERROR: Need an image to rectify');
               break;
            end;
         end;
      end;
   end;
end;


if nargin < 7,
   if nargin < 6,
		KK_new = [f(1) 0 c(1);0 f(2) c(2);0 0 1];
   else
   	KK_new = alpha; % the 6th argument is actually KK_new   
   end;
   alpha = 0;
end;



% Note: R is the motion of the points in space
% So: X2 = R*X where X: coord in the old reference frame, X2: coord in the new ref frame.


if ~exist('KK_new'),
   KK_new = [f(1) alpha_c*fc(1) c(1);0 f(2) c(2);0 0 1];
end;


[nr,nc] = size(I);

Irec = 255*ones(nr,nc);

[mx,my] = meshgrid(1:nc, 1:nr);
px = reshape(mx',nc*nr,1);
py = reshape(my',nc*nr,1);

rays = inv(KK_new)*[(px - 1)';(py - 1)';ones(1,length(px))];


% Rotation: (or affine transformation):

rays2 = R'*rays;

x = [rays2(1,:)./rays2(3,:);rays2(2,:)./rays2(3,:)];


% Add distortion:
xd = apply_distortion(x,k);


% Reconvert in pixels:

px2 = f(1)*(xd(1,:)+alpha*xd(2,:))+c(1);
py2 = f(2)*xd(2,:)+c(2);


% Interpolate between the closest pixels:

px_0 = floor(px2);
py_0 = floor(py2);


good_points = find((px_0 >= 0) & (px_0 <= (nc-2)) & (py_0 >= 0) & (py_0 <= (nr-2)));

px2 = px2(good_points);
py2 = py2(good_points);
px_0 = px_0(good_points);
py_0 = py_0(good_points);

alpha_x = px2 - px_0;
alpha_y = py2 - py_0;

a1 = (1 - alpha_y).*(1 - alpha_x);
a2 = (1 - alpha_y).*alpha_x;
a3 = alpha_y .* (1 - alpha_x);
a4 = alpha_y .* alpha_x;

ind_1 = px_0 * nr + py_0 + 1;
ind_2 = (px_0 + 1) * nr + py_0 + 1;
ind_3 = px_0 * nr + (py_0 + 1) + 1;
ind_4 = (px_0 + 1) * nr + (py_0 + 1) + 1;

ind_new = (px(good_points)-1)*nr + py(good_points);


Irec(ind_new) = a1 .* I(ind_1) + a2 .* I(ind_2) + a3 .* I(ind_3) + a4 .* I(ind_4);



return;


% Convert in indices:

fact = 3;

[XX,YY]= meshgrid(1:nc,1:nr);
[XXi,YYi]= meshgrid(1:1/fact:nc,1:1/fact:nr);

%tic;
Iinterp = interp2(XX,YY,I,XXi,YYi); 
%toc

[nri,nci] = size(Iinterp);


ind_col = round(fact*(f(1)*xd(1,:)+c(1)))+1;
ind_row = round(fact*(f(2)*xd(2,:)+c(2)))+1;

good_points = find((ind_col >=1)&(ind_col<=nci)&(ind_row >=1)& (ind_row <=nri));