Question

I use this to functions as a base of my tracking algorithm.

    //1. detect the features
    cv::goodFeaturesToTrack(gray_prev, // the image 
    features,   // the output detected features
    max_count,  // the maximum number of features 
    qlevel,     // quality level
    minDist);   // min distance between two features

    // 2. track features
    cv::calcOpticalFlowPyrLK(
    gray_prev, gray, // 2 consecutive images
    points_prev, // input point positions in first im
    points_cur, // output point positions in the 2nd
    status,    // tracking success
    err);      // tracking error

cv::calcOpticalFlowPyrLK takes vector of points from the previous image as input, and returns appropriate points on the next image. Suppose I have random pixel (x, y) on the previous image, how can I calculate position of this pixel on the next image using OpenCV optical flow function?

Answer 1

As you write, cv::goodFeaturesToTrack takes an image as input and produces a vector of points which it deems “good to track”. These are chosen based on their ability to stand out from their surroundings, and are based on Harris corners in the image. A tracker would normally be initialised by passing the first image to goodFeaturesToTrack and obtaining a set of features to track. These features could then be passed to cv::calcOpticalFlowPyrLK as the previous points, along with the next image in the sequence and it will produce the next points as output, which then become input points in the next iteration.

If you want to try to track a different set of pixels (rather than features generated by cv::goodFeaturesToTrack or a similar function), then simply provide these to cv::calcOpticalFlowPyrLK along with the next image.

Very simply, in code:

// Obtain first image and set up two feature vectors
cv::Mat image_prev, image_next;
std::vector<cv::Point> features_prev, features_next;

image_next = getImage();

// Obtain initial set of features
cv::goodFeaturesToTrack(image_next, // the image 
  features_next,   // the output detected features
  max_count,  // the maximum number of features 
  qlevel,     // quality level
  minDist     // min distance between two features
);

// Tracker is initialised and initial features are stored in features_next
// Now iterate through rest of images
for(;;)
{
    image_prev = image_next.clone();
    feature_prev = features_next;
    image_next = getImage();  // Get next image

    // Find position of feature in new image
    cv::calcOpticalFlowPyrLK(
      image_prev, image_next, // 2 consecutive images
      points_prev, // input point positions in first im
      points_next, // output point positions in the 2nd
      status,    // tracking success
      err      // tracking error
    );

    if ( stopTracking() ) break;
}