Using Optical Flow to warp an image
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Hi guys. I have computed the optical flow between images A and B. I wish to use this to warp image C to D. The problem is: the flow is a velocity vector with decimal values. Images are in the form of matrices for which rows and columns are integer values. Hence, C(i,j)+ optical flow(i,j) won't give me D(i,j) Please tell me how I can convert a matrix to a graph or how I can handle the decimal values. Thanks !!!
2 Comments
David Young
on 12 Dec 2011
Do you have a single optic flow vector for the whole image, or a sparse set of optic flow vectors for some features in the image, or a dense set of optic flow vectors for each pixel in the image?
Answers (1)
David Young
on 14 Dec 2011
Given a vector for every pixel, you can use interp2 to do the warping and to handle the non-integer lookup. Suppose that vx represents the x-component of the flow, such that the rightwards component of velocity at D(i,j) is x(i,j). Likewise vy is the y-component. Then you can do something similar to this:
C= imread('pout.tif'); % test image
[x, y] = meshgrid(1:size(C,2), 1:size(C,1));
% generate synthetic test data, for experimenting
vx = 0.1*y; % an arbitrary flow field, in this case
vy = 0.1*x; % representing shear
% compute the warped image - the subtractions are because we're specifying
% where in the original image each pixel in the new image comes from
D = interp2(double(C), x-vx, y-vy);
% display the result
imshow(D, []);
The bit that matters is the line in the middle with the call to interp2.
By the way, if you had a sparse set of flow vectors instead, you'd need to look at using imtransform with maketform.
5 Comments
Natesh Srinivasan
on 17 Mar 2013
@David
I think this is incorrect for non uniform flow fields. For Example, consider the following code, where A is the input image and B is the 'warped' image
function testinterpolation()
A = rand(5,5)
[X, Y] = meshgrid([1:5],[1:5]);
Mx = X/10;
My = Y/10;
xplusMx = X - Mx;
yplusMy = Y - My;
B = interp2 (A,xplusMx,yplusMy,'linear',0)
end
we cannot predict where a particular pixel has come from if by simply taking a minus sign because this could have been from a completely different pixel.
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