Warping an image using control points

Question

I want to convert an image using control points according to this scheme extracted from here:

A and B contains the coordinates of

Answer 1

You have to "adapt" the control points to the size of the image you're working with. The way I did this is by computing an affine transformation between the corners of the control points in A and the corners of the source image (preferrably you want to make the points are in the same clockwise order).

One thing I should point out is that the order of points in your matrix A does not match the picture you've shown, so I fixed that in the code below...

Here is the code to estimate the homography (tested in MATLAB):

% initial control points
A = [51 228;  51 127; 191 127; 191 228];
B = [152 57; 219 191;  62 240;  92 109];
A = circshift(A, [-1 0]);  % fix the order of points to match the picture

% input image
%I = imread('peppers.png');
I = im2uint8(checkerboard(32,5,7));
[h,w,~] = size(I);

% adapt control points to image size
% (basically we estimate an affine transform from 3 corner points)
aff = cp2tform(A(1:3,:), [1 1; w 1; w h], 'affine');
A = tformfwd(aff, A);
B = tformfwd(aff, B);

% estimate homography between A and B
T = cp2tform(B, A, 'projective');
T = fliptform(T);
H = T.tdata.Tinv

I get:

>> H
H =
   -0.3268    0.6419   -0.0015
   -0.4871    0.4667    0.0009
  324.0851 -221.0565    1.0000

Now let's visualize the points:

% check by transforming A points into B
%{
BB = [A ones(size(A,1),1)] * H;        % convert to homogeneous coords
BB = bsxfun(@rdivide, BB, BB(:,end));  % convert from homogeneous coords
%}
BB = tformfwd(T, A(:,1), A(:,2));
fprintf('error = %g\n', norm(B-BB));

% visually check by plotting control points and transformed A
figure(1)
subplot(121)
plot(A([1:end 1],1), A([1:end 1],2), '.-', 'MarkerSize',20, 'LineWidth',2)
line(BB([1:end 1],1), BB([1:end 1],2), 'Color','r', 'Marker','o')
text(A(:,1), A(:,2), num2str((1:4)','a%d'), ...
    'VerticalAlign','top', 'HorizontalAlign','left')
title('A'); legend({'A', 'A*H'}); axis equal ij
subplot(122)
plot(B([1:end 1],1), B([1:end 1],2), '.-', 'MarkerSize',20, 'LineWidth',2)
text(B(:,1), B(:,2), num2str((1:4)','b%d'), ...
    'VerticalAlign','top', 'HorizontalAlign','left')
title('B'); legend('B'); axis equal ij

Finally we can apply the transformation on the source image:

% transform input image and show result
J = imtransform(I, T);
figure(2)
subplot(121), imshow(I), title('image')
subplot(122), imshow(J), title('warped')

Answer 2

Your problem is that you accidentally cropped the output image when you specified your XData and YData in imtransform. One option would be to use tformfwd with to transform A to compute the valid XData and YData ranges.

[U,V] = tformfwd(T, A(:,1), A(:,2));

Z = imtransform(X,T,'XData',[min(U) max(U)], 'YData', [min(V) max(V)]);