I am trying to perform 3D Reconstruction(Structure From Motion) from Multiple Images of Planar Markers. I very new to MVG and openCV.
As far I have understood I have to do the following steps:
- Identify corresponding 2D corner points in the one images.
- Calculate the Camera Pose of the first image us cv::solvePNP(assuming the
origin to be center of the marker).
- Repeat 1 and 2 for the second image.
- Estimate the relative motion of the camera by Rot_relative = R2 - R1,
Trans_relative = T2-T1.
- Now assume the first camera to be the origin construct the 3x4 Projection
Matrix for both views, P1 =[I|0]*CameraMatrix(known by Calibration) and P2 =
[Rot_relative |Trans_relative ].
- Use the created projection matrices and 2D corner points to triangulate the
3D coordinate using cv::triangulatePoints(P1,P2,point1,point2,OutMat)
- The 3D coordinate can be found by dividing the each rows of OutMat by the 4th
- I was hoping to keep my "First View" as my origin and iterate
through n views repeating steps from 1-7(I suppose its called Global SFM).
I was hoping to get (n-1)3D points of the corners with "The first View as origin" which we could optimize using Bundle Adjustment.
But the result I get is very disappointing the 3D points calculated are displaced by a huge factor.
These are questions:
1.Is there something wrong with the steps I followed?
2.Should I use cv::findHomography() and cv::decomposeHomographyMat() to find the
relative motion of the camera?
3.Should point1 and point2 in cv::triangulatePoints(P1,P2,point1,point2,OutMat)
be normalized and undistorted? If yes, how should the "Outmat" be interpreted?
Please anyone who has insights towards the topic, can you point out my mistake?
P.S. I have come to above understanding after reading "MultiView Geometry in Computer Vision"
Please find the code snippet below:
cv::Mat rVectFirstView, tVecFristView;
cv::Mat rVectSecondView, tVecSecondView;
cv::Mat RotMatFirstView = cv::Mat(3, 3, CV_64F);
cv::Mat RotMatSecondView = cv::Mat(3, 3, CV_64F);
cv::solvePnP(RealWorldPoints, ImagePointsFirstView, cameraMatrix, distortionMatrix, rVectFirstView, tVecFristView);
cv::solvePnP(RealWorldPoints, ImagePointsSecondView, cameraMatrix, distortionMatrix, rVectSecondView, tVecSecondView);
cv::Mat RelativeRot = RotMatFirstView-RotMatSecondView ;
cv::Mat RelativeTrans = tVecFristView-tVecSecondView ;
cv::hconcat(RelativeRot, RelativeTrans, RelativePose);
cv::Mat ProjectionMatrix_0 = cameraMatrix*cv::Mat::eye(3, 4, CV_64F);
cv::Mat ProjectionMatrix_1 = cameraMatrix* RelativePose;
cv::undistortPoints(ImagePointsFirstView, ImagePointsFirstView, cameraMatrix, distortionMatrix, cameraMatrix);
cv::undistortPoints(ImagePointsSecondView, ImagePointsSecondView, cameraMatrix, distortionMatrix, cameraMatrix);
cv::triangulatePoints(ProjectionMatrix_0, ProjectionMatrix_1, ImagePointsFirstView, ImagePointsSecondView, X);
X.row(0) = X.row(0) / X.row(3);
X.row(1) = X.row(1) / X.row(3);
X.row(2) = X.row(2) / X.row(3);