Menu
I tried to use the following function in OpenCV (C++)
calcOpticalFlowPyrLK(prev_frame_gray, frame_gray, points[0], points[1], status, err, winSize, 3, termcrit, 0, 0.001);
and I get this error
OpenCV Error: Assertion failed ((npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0) in calcOpticalFlowPyrLK,
file /home/rohit/OpenCV_src/opencv-2.4.9/modules/video/src/lkpyramid.cpp, line 845
terminate called after throwing an instance of 'cv::Exception'
what(): /home/rohit/OpenCV_src/opencv-2.4.9/modules/video/src/lkpyramid.cpp:845:
error: (-215) (npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0 in function calcOpticalFlowPyrLK
Both of the following return -1
frame_gray.checkVector(2, CV_32F, true)
prev_frame_gray.checkVector(2, CV_32F, true)
I wanted to know what checkVector actually does because it is leading to the assertion error as you can see above.
957
The Mat class of OpenCV library is used to store the values of an image. It represents an n-dimensional array and is used to store image data of grayscale or color images, voxel volumes, vector fields, point clouds, tensors, histograms, etc.
Mat is basically a class with two data parts : the matrix header (containing information such as the size of the matrix, the method used for storing, at which address is the matrix stored, and so on) and a pointer to the matrix containing the pixel values (taking any dimensionality depending on the method chosen for ...
Vec3b is the abbreviation for "vector with 3 byte entries" Here those byte entries are unsigned char values to represent values between 0 .. 255. Each byte typically represents the intensity of a single color channel, so on default, Vec3b is a single RGB (or better BGR) pixel.
That is, image of type CV_64FC1 is simple grayscale image and has only 1 channel: image[i, j] = 0.5. while image of type CV_64FC3 is colored image with 3 channels: image[i, j] = (0.5, 0.3, 0.7) (in C++ you can check individual pixels as image.at<double>(i, j) ) CV_64F is the same as CV_64FC1 .
The official OpenCV's doc says:
cv::Mat::checkVector()returns N if the matrix is 1-channel (N x ptdim) or ptdim-channel (1 x N) or (N x 1); negative number otherwise
OpenCV considers some data types equivalent in case of some functions i.e. objectPoints of cv::solvePnP() can be:
cv::Mat cv::Mat std::vector<cv::Point3f>With checkVector you can make sure that you are passing the correct representation of your data.
70
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
