How to use the writeCloud() OpenCV function to contruct point cloud given 3D point coordinates?

Question

I'm beginner in OpenCV and currently I'm using Visual Studio 2013 (64-bit) and OpenCV 3.2 (C++) to construct a two view geometry and try to display those matched 3D points in MeshLab. I use the triangulatePoints() to get the Points4D, which is a 4*N matrix that contains the coordinates of the matched points from two images. This the documentation of writeCloud() .

triangulatePoints(CameraMatrix_1, CameraMatrix_2, matchpoints_1, matchpoints_2, Points4D);
writeCloud("twoview.ply", cloud, noArray(), noArray(), false);

My question is, what should be the cloud input of writeCloud() so that I could save those 3D points into a .ply file and display them? Assume that I do not assign color to the point cloud first.

Also, I have tried to use the MATLAB to generate a pointcloud.ply file and analyse it with the readCloud() , then I find out the following code successfully read a point cloud and save it into another one. But strangely, the cv::Mat twoviewcloud here is a 1*N matrix, how could you construct a point cloud form one dimensional array? I am totally confused.

Mat twoviewcloud = readCloud("pointcloud.ply");
writeCloud("trial.ply", twoviewcloud, noArray(), noArray(), false);

I would sincerely thank you if someone could give me some hint!

Answer 1

Ok, so I am still confused to use the original OpenCV function writeCloud() , however, I could just implement my own function to write the .ply file. Here is the code, it is quite simple actually and you could read the wiki page for the detailed .ply format.

struct dataType { Point3d point; int red; int green; int blue; };
typedef dataType SpacePoint;
vector<SpacePoint> pointCloud;

ofstream outfile("pointcloud.ply");
outfile << "ply\n" << "format ascii 1.0\n" << "comment VTK generated PLY File\n";
outfile << "obj_info vtkPolyData points and polygons : vtk4.0\n" << "element vertex " << pointCloud.size() << "\n";
outfile << "property float x\n" << "property float y\n" << "property float z\n" << "element face 0\n";
outfile << "property list uchar int vertex_indices\n" << "end_header\n";
for (int i = 0; i < pointCloud.size(); i++)
{
    Point3d point = pointCloud.at(i).point;
    outfile << point.x << " ";
    outfile << point.y << " ";
    outfile << point.z << " ";
    outfile << "\n";
}
outfile.close();