Projection of an Image

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Logan Davis
Logan Davis on 13 Dec 2017
Commented: Masoud Heidari on 5 Mar 2021
If I know the four corners of a rectangle that has been projected, how can I obtain a top-down view of that image using image projection. I tried to use fitgeotrans and imwarp but did not obtain the correct results. Below is the code I used:
U = double([topLeft; topRight; bottomRight; bottomLeft]);
topLeftNew = [1 1];
topRightNew = [1 width];
bottomLeftNew = [height 1];
bottomRightNew = [height width];
X = double([topLeftNew; topRightNew; bottomRightNew; bottomLeftNew]);
tform = fitgeotrans(U, X, 'projective');
B = imwarp(orig, tform);
Does anyone know what I could do to fix this or a different method? The image I'm trying to transform is:
The four corners I've obtained are shown as well. The result of the transformation is.
I'd like for the result to only contain the notecard and orient it correctly upright as well. Is this possible and how can I go about achieving this?

Answers (2)

Ben Drebing
Ben Drebing on 21 Dec 2017
I think that the orderings of your fixedpoints and movingpoints do not match up. Try
topLeft = [360, 546];
topRight = [609, 109];
botRight = [873, 240];
botLeft = [645, 703];
U = [topLeft; topRight; botRight; botLeft];
width = size(I,2);
height = size(I,1);
topLeftNew = [1 1];
topRightNew = [1 width];
bottomLeftNew = [height 1];
bottomRightNew = [height width];
X = double([topLeftNew;bottomLeftNew; bottomRightNew; topRightNew ]);
tform = fitgeotrans(U, X, 'projective');
B = imwarp(I, tform);
You can use imcrop to crop out the rest of the image.
  1 Comment
Masoud Heidari
Masoud Heidari on 5 Mar 2021
Thank you very much Ben! it was a big help for me too.

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Image Analyst
Image Analyst on 21 Dec 2017
See attached demo.

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