Assert thrown by OpenCV checkVector() on otherwise valid vector<vector<Point3f>> - c++

Question

Been chasing this bug all night, so please forgive any incoherence.
I'm attempting to use the OpenCV's calibrateCamera() to extract intrinsic and extrinsic parameters from a set of fifteen pictures whose object points and world points are given. From what I can tell from debugging, I'm grabbing valid points from the input files and placing them in a vector<Point3f>, which is itself placed into another vector.
I pass the whole shebang to calibrateCamera(),
double rms = calibrateCamera(worldPoints, pixelPoints, src.size(), intrinsic, distCoeffs, rvecs, tvecs);
which throws Assertion failed (ni >= 0) in unknown function, file ...\calibration.cpp, line 3173
Pulling up this file gives us
static void collectCalibrationData( InputArrayOfArrays objectPoints,
InputArrayOfArrays imagePoints1,
InputArrayOfArrays imagePoints2,
Mat& objPtMat, Mat& imgPtMat1, Mat* imgPtMat2,
Mat& npoints )
{
int nimages = (int)objectPoints.total();
int i, j = 0, ni = 0, total = 0;
CV_Assert(nimages > 0 && nimages == (int)imagePoints1.total() &&
(!imgPtMat2 || nimages == (int)imagePoints2.total()));
for( i = 0; i < nimages; i++ )
{
ni = objectPoints.getMat(i).checkVector(3, CV_32F);
CV_Assert( ni >= 0 );
total += ni;
}
...
So far as I know, a Point3f is of CV_32F depth, and I can see good data in the double vector just before calibrateCamera is called.
Any ideas what might be happening here? calibrateCamera() requires a vector<vector<Point3f>>, as said by http://aishack.in/tutorials/calibrating-undistorting-with-opencv-in-c-oh-yeah/ and the documentation; hopefully getMat(i) isn't failing due to that.
Could it possibly have been called on the vector<vector<Point2f>> of pixel points just after it? I have been over so many errors I am willing to believe anything.
Edit:
Consequently, checkVector()'s documentation was not really helpful
int cv::Mat::checkVector (int elemChannels, int depth = -1, bool RequireContinuous = true) const
returns N if the matrix is 1-channel (N x ptdim) or ptdim-channel (1 x N) or (N x 1); negative number otherwise

Answer 1

The problem is possibly in one of your InputArrayOfArrays arguments (in worldPoints precisely, if the assertion is thrown from the line pasted in your question). Mat:s should work just fine here.
I solved the same assertion error in my code by making all the 3 InputArrayOfArrays (or vector > and vector > in my case) same length vectors with fully populated entries. So my problem was in my architecture: my objectPoints vector was containing empty entries (even though the existing data was valid), and calibrate.cpp requires that no empty entries are present in any of the 3 InputArrayOfArrays. Btw I am using greyscale images for calibration so single channel data.
In calib3d source the most probable reason for throwing error is a null value if you have checked that data types match. You might try double-checking your valid input data:
1) count the # of valid calibration images from your chosen structure
validCalibImages = (int)goodCalibrationImages.size()
2) define worldPoints as vector<vector<Point3f> > worldPoints
3) IMPORTANT: resize to accommodate for data for each calibration entry
worldPoints.resize(validCalibImages)
4) populate with data e.g.
for(int k = 0; k < (int)goodCalibImages.size(); k++){
for(int i = 0; i < chessboardSize.height; i++){
for(int j = 0; j < chessboardSize.width; j++){
objectPoints[k].push_back(Point3f(i*squareSize, j*squareSize, 0));
}
}
}
'
Hope it helps!

Answer 2

I agree with FSaccilotto - call checkVector and make sure you are passing a vector of size n of Mat 1x1:{3 channel} and not vector of Mat 1 x n:{3 channel} or worse Mat 1 x n:{2 channel} which is what MatOfPoint spits out. That usually fixes 90% of assert failed issues. Explicitly declare the Mat yourself.
The object pattern is somewhat strange in that the x y z coords are in the channels not in the Mat dimensions.