I'm trying to use cv::calcOpticalFlowPyrLK
but sometimes an internal assertion in that function fails. The assertion is npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0
. I'm using OpenCV 2.3.1. The source code for this function is available here.
It's hard to wrap my head around their code, especially because of my limited experience with computer graphics and their lack of comments. Why is this assertion being triggered and what does it say about my problem?
Edit: I call the function as follows:
cv::calcOpticalFlowPyrLK(curBwFrame, prvFrame, features, newFeatures, trackingStatus, errors);
I found out that the features
vector, which was obtained by calling cv::goodFeaturesToTrack(curBwFrame, features, 5, 0.2, 0.5, skinMask);
with a non-empty mask that appears to be sufficiently big and a valid image, doesn't contain any features. How can this happen?
curBwFrame
skinMask
I'm able to reproduce the problem using the following code snippet:
#include <vector>
#include <cassert>
#include <opencv2\opencv.hpp>
using std::vector;
using namespace cv;
int main() {
vector<Point2f> features;
cv::Mat curBwFrame = imread("curBwFrame.png");
cv::cvtColor(curBwFrame, curBwFrame, CV_RGB2GRAY);
imwrite("test.png", curBwFrame);
cv::Mat skinMask = imread("skinMask.png");
cv::cvtColor(skinMask, skinMask, CV_RGB2GRAY);
imwrite("test.png", skinMask);
cv::goodFeaturesToTrack(curBwFrame, features, 5, 0.2, 0.5, skinMask);
assert(features.size() > 0);
return 0;
}
The main problem are your parameters.In the OpenCV 2.3.2 documentation (no compatibility change between 2.3.1) this is the description of the method parameters:
void goodFeaturesToTrack(InputArray image, OutputArray corners, int maxCorners, double qualityLevel, double minDistance, InputArray mask=noArray(), int blockSize=3, bool useHarrisDetector=false, double k=0.04 )
Parameters:
I recommend you to play a little with qualityLevel and minDistance to suffice your needs.