Image projection (homography with 4 points)

Hello,

I have to project an image taken from an angle into a specific plane, so that that I can look straight at that plane.

Following How to find projective transformation with 4 points? - MATLAB Answers - MATLAB Central (mathworks.com) I wrote the code here below, but do not get any reasonable image as output.

I am not sure what I do wrong. Is it the reference system or?

img_in = imread('Test_D9.jpg');

% 4 3D points on the laser sheet plane
pA = [0 2.1 0 1]'; % [m]
pB = [4.2 6 0 1]'; % [m]
pC = [4.111 3.929 0 1]'; % [m]
pD = [2.056 3.015 0 1]'; % [m]

worldPoints = [pA pB pC pD]';

% modify the ref system to agree with the pin --> needed??
worldPoints(:,1) = (5-worldPoints(:,1));
worldPoints(:,2) = (7-worldPoints(:,2));

pA_projected = [2300 1332]'; % px center light
pB_projected = [560 272]'; % px higher point
pC_projected = [536 1484]'; % px probe tip
pD_projected = [1548 1376]'; % px probe half

imagePoints = [pA_projected(1:2) pB_projected(1:2) pC_projected(1:2) ...
     pD_projected(1:2)]';

% Homography
H = fitgeotrans(imagePoints,worldPoints(:,1:2),'projective');
xWorldLimits = [0.5   5.5];
yWorldLimits = [0.5   5.5];
imref = imref2d([diff(yWorldLimits), diff(xWorldLimits)]*1e3, xWorldLimits, yWorldLimits);
% This way, each px is 1mm

%Attempt
[img_out, img_out_size] = imwarp(img_in,(H),'OutputView',imref);

figure()
imshow(img_out,img_out_size)

If you could help, I'd be very glad :)

Answer 1

[Pratyush Swain]

Hi Magheritta,

I understand you want to perform projective transformation and you are using the 'fitgeotrans' function for the same.Please refer to this matlab answer - https://in.mathworks.com/matlabcentral/answers/1817630-i-am-having-a-query-in-image-transformation-on-fitgeotform2d-function-can-anyone-please-explain-wha , where it has been demonstrated how to perform projective tranformation on a sample image.

Please note 'fitgeotrans' is not recommended to use for MATLAB version since R2022b, it has been replaced by 'fitgeoform2d'.

Hope this helps.