Why does imwarp returns abrupt uint8 arrays on applying a Projective Transform?

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Akella Kartik 2022-12-25

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https://ww2.mathworks.cn/matlabcentral/answers/1884122-why-does-imwarp-returns-abrupt-uint8-arrays-on-applying-a-projective-transform

评论： Akella Kartik 2022-12-26

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After performing some math on a few images, I obtained Homography matrix as below:

H =

1.0e+05 *

0.0023 0.0018 -4.4192

0.0018 0.0013 -3.3300

0.0000 0.0000 -0.0105

The image that I use is of dimension 612 * 816 * 3. After using the below code, I get the warped image dimension as 2*2*3.

tform = projective2d(H')
J = imwarp(img,tform);

Can someone please help me with this.

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DGM 2022-12-26

编辑：DGM 2022-12-26

在 MATLAB Online 中打开

It makes no sense to compare this use of warp() to imwarp(). Warp() is for graphical texturemapping. The output K is a copy of refImage. It would be a lot more obvious if the image weren't improperly scaled for its class.

H= [0.0023 0.0018 -4.4192 % this is Z-data

0.0018 0.0013 -3.3300

0 0 -0.0105] * 1e5;

refImage = imread('peppers.png'); % this is a uint8 image

% refImage is mapped onto the surface defined by H

% image is correctly scaled for class 'double', so it actually shows up

K = warp(H, im2double(refImage)).CData;

So we can ignore warp() here. It's not relevant.

EDIT: I went ahead and ran OCR on the screenshot. I kind of had a hunch that full precision was necessary for the issue to arise.

% this is H to full precision, class double
H = [234.40055498025358815539220813662, 176.16693542091721269571280572563, -441919.76452097168657928705215454;
    175.87313731614469247688248287886, 133.74617803109279634554695803672, -332998.0214978544972836971282959;
    0.55541916052038531326218162575969, 0.41922221571270390505503655731445, -1050.6303467652635390550130978227];
% this is a uint8 RGB image (the class doesn't matter here)
inpict = im2uint8(rand(612,816,3));
% apply the transformation
outpict = imwarp(inpict, projective2d(H'));
size(outpict)
ans = 1×3
     2     2     3

... and I was right. As to why that's the case, I'll defer to someone who can intuit their way around spatial transformations.

Akella Kartik 2022-12-26

@DGM, thanks a lot for the response.

The image is of 40MB and the forum isn't accepting that. Anyways, I got answers for what I was seeking. Guess downscaling isn't an issue here with the H I use.

Akella Kartik 2022-12-26