imagePoints matrix and boardSize is different for two images for same checkerboard.

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Naseeb Gill
Naseeb Gill on 12 Feb 2016
Commented: Dima Lisin on 13 Feb 2016
I have two images picb3 and picb6. My code is like this
I1 = imread('C:\Users\Naseeb\Desktop\new st vs\left cam\picb3.png');
load('C:\Users\Naseeb\Desktop\cameraParams.mat');
im = undistortImage(I1, cameraParams);
[imagePoints, boardSize] = detectCheckerboardPoints(im);
squareSize = 21; % in millimeters
worldPoints = generateCheckerboardPoints(boardSize, squareSize);
[rotationMatrix, translationVector] = extrinsics(imagePoints, worldPoints, cameraParams)
When I used image picb3, matlab shows boardSize of [8,9] and imagePoints are of matrix [56x2] but when I used image picb6, matlab shows boardsize of [7,10] and imagePoints are of matrix [54x2]. Both images are taken from same camera (left camera or camera 1) and checkerboard is also same. Translation vector also shows in unconventional way as I'm attaching snapshot of that. I'm attaching my cameraParams, images and checkerboard pdf and translation vector. Can someone please explain why this happening and if it lead to any error? Thanks.

Accepted Answer

Dima Lisin
Dima Lisin on 12 Feb 2016
Edited: Dima Lisin on 12 Feb 2016
Hi Naseeb,
The problem here is that detectCheckerboardPoints does not detect the checkerboard correctly 100% of the time. The correct size for this particular checkerboard is [7 10], and the points you got for picb3 are wrong. You can check visually whether the checkerboard was detected correctly as follows:
im = imread('picb3.png');
[imagePoints, boardSize] = detectCheckerboardPoints(im);
imshow(im);
hold on
plot(imagePoints(:, 1), imagePoints(:, 2), 'r*-');
Here's the result for for picb3. As you can see, the board was not detected correctly.
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