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

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.

Answer 1

[Dima Lisin]

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.

[Naseeb Gill]

Thanks Dima,

I have many images in which checkerboard not detected correctly. I want to know what are the factors responsible fro this and what I should consider to avoid this?

[Dima Lisin]

In this particular case I would suggest using a larger checkerboard.