Reproducing Camera Reprojection Error

Question

I loaded some images into the camera calibration app and exported the resulting camera parameters into a structure called "oldCameraParams". What I want to do now is to reproduce the reprojection error for one of the images used during calibration. I aim to do this like this:

I = imread(imagePath);
[imagePoints,boardSize] = detectCheckerboardPoints(I);
worldPoints = generateCheckerboardPoints(boardSize, squareSize);
squareSize = 30; 
camMatrix = cameraMatrix(oldCameraParams,oldCameraParams.RotationMatrices(:,:,1),oldCameraParams.TranslationVectors(1,:));
projectedPoints = [worldPoints zeros(size(worldPoints,1),1) ones(size(worldPoints,1),1)] * camMatrix;
projectedPoints = bsxfun(@rdivide,    projectedPoints(:,1:2),projectedPoints(:,3));
euclideanDistances = sqrt(sum((imagePoints - projectedPoints) .^2, 2));
meanReprojErrors(ii) = mean(euclideanDistances);

I should use the exact same parameters (camera intrinsics, rotation matrix and translation vector) but still my projectedPoints are different than the ones created during calibration and therefore my meanReprojErrors is higher. Any idea why?

Answer 1

You are getting different reprojection errors, because you are not accounting for distortion. If you call undistortImage before calling detectCheckrboardPoints, then you should get values that are closer to the reprojection errors computed during calibration.