OpenCV - Variable value range of trackbar

Question

I have a set of images and want to make a cross matching between all and display the results using trackbars using OpenCV 2.4.6 (ROS Hydro package). The matching part is done using a vector of vectors of vectors of cv::DMatch-objects:

image[0] --- image[3] -------- image[8] ------ ...
   |             |                 |
   |         cv::DMatch-vect   cv::DMatch-vect
   |
image[1] --- ...
   |
image[2] --- ...
   |
  ...
   |
image[N] --- ...

Because we omit matching an image with itself (no point in doing that) and because a query image might not be matched with all the rest each set of matched train images for a query image might have a different size from the rest. Note that the way it's implemented right I actually match a pair of images twice, which of course is not optimal (especially since I used a BruteForce matcher with cross-check turned on, which basically means that I match a pair of images 4 times!) but for now that's it. In order to avoid on-the-fly drawing of matched pairs of images I have populated a vector of vectors of cv::Mat-objects. Each cv::Mat represents the current query image and some matched train image (I populate it using cv::drawMatches()):

image[0] --- cv::Mat[0,3] ---- cv::Mat[0,8] ---- ...
   |
image[1] --- ...
   |
image[2] --- ...
   |
  ...
   |
image[N] --- ...

Note: In the example above cv::Mat[0,3] stands for cv::Mat that stores the product of cv::drawMatches() using image[0] and image[3].

Here are the GUI settings:

• Main window: here I display the current query image. Using a trackbar - let's call it TRACK_QUERY - I iterate through each image in my set.
• Secondary window: here I display the matched pair (query,train), where the combination between the position of TRACK_QUERY's slider and the position of the slider of another trackbar in this window - let's call it TRACK_TRAIN - allows me to iterate through all the cv::Mat-match-images for the current query image.

The issue here comes from the fact that each query can have a variable number of matched train images. My TRACK_TRAIN should be able to adjust to the number of matched train images, that is the number of elements in each cv::Mat-vector for the current query image. Sadly so far I was unable to find a way to do that. The cv::createTrackbar() requires a count-parameter, which from what I see sets the limit of the trackbar's slider and cannot be altered later on. Do correct me if I'm wrong since this is exactly what's bothering me. A possible solution (less elegant and involving various checks to avoid out-of-range erros) is to take the size of the largest set of matched train images and use it as the limit for my TRACK_TRAIN. I would like to avoid doing that if possible. Another possible solution involves creating a trackbar per query image with the appropriate value range and swap each in my secondary windows according to the selected query image. For now this seems to be the more easy way to go but poses a big overhead of trackbars not to mention that fact that I haven't heard of OpenCV allowing you to hide GUI controls. Here are two example that might clarify things a little bit more:

Example 1: In main window I select image 2 using TRACK_QUERY. For this image I have managed to match 5 other images from my set. Let's say those are image 4, 10, 17, 18 and 20. The secondary window updates automatically and shows me the match between image 2 and image 4 (first in the subset of matched train images). TRACK_TRAIN has to go from 0 to 4. Moving the slider in both directions allows me to go through image 4, 10, 17, 18 and 20 updating each time the secondary window.

Example 2: In main window I select image 7 using TRACK_QUERY. For this image I have managed to match 3 other images from my set. Let's say those are image 0, 1, 11 and 19. The secondary window updates automatically and shows me the match between image 2 and image 0 (first in the subset of matched train images). TRACK_TRAIN has to go from 0 to 2. Moving the slider in both directions allows me to go through image 0, 1, 1 and 19 updating each time the secondary window.

If you have any questions feel free to ask and I'll to answer them as well as I can. Thanks in advance!

PS: Sadly the way the ROS package is it has the bare minimum of what OpenCV can offer. No Qt integration, no OpenMP, no OpenGL etc.

Answer 1

After doing some more research I'm pretty sure that this is currently not possible. That's why I implemented the first proposition that I gave in my question - use the match-vector with the most number of matches in it to determine a maximum size for the trackbar and then use some checking to avoid out-of-range exceptions. Below there is a more or less detailed description how it all works. Since the matching procedure in my code involves some additional checks that does not concern the problem at hand, I'll skip it here. Note that in a given set of images we want to match I refer to an image as object-image when that image (example: card) is currently matched to a scene-image (example: a set of cards) - top level of the matches-vector (see below) and equal to the index in processedImages (see below). I find the train/query notation in OpenCV somewhat confusing. This scene/object notation is taken from http://docs.opencv.org/doc/tutorials/features2d/feature_homography/feature_homography.html. You can change or swap the notation to your liking but make sure you change it everywhere accordingly otherwise you might end up with a some weird results.

// stores all the images that we want to cross-match
std::vector<cv::Mat> processedImages;
// stores keypoints for each image in processedImages
std::vector<std::vector<cv::Keypoint> > keypoints;
// stores descriptors for each image in processedImages 
std::vector<cv::Mat> descriptors;

// fill processedImages here (read images from files, convert to grayscale, undistort, resize etc.), extract keypoints, compute descriptors
// ...

// I use brute force matching since I also used ORB, which has binary descriptors and HAMMING_NORM is the way to go
cv::BFmatcher matcher;
// matches contains the match-vectors for each image matched to all other images in our set
// top level index matches.at(X) is equal to the image index in processedImages
// middle level index matches.at(X).at(Y) gives the match-vector for the Xth image and some other Yth from the set that is successfully matched to X
std::vector<std::vector<std::vector<cv::DMatch> > > matches;
// contains images that store visually all matched pairs
std::vector<std::vector<cv::Mat> > matchesDraw;

// fill all the vectors above with data here, don't forget about matchesDraw
// stores the highest count of matches for all pairs - I used simple exclusion by simply comparing the size() of the current std::vector<cv::DMatch> vector with the previous value of this variable
long int sceneWithMaxMatches = 0;
// ...

// after all is ready do some additional checking here in order to make sure the data is usable in our GUI. A trackbar for example requires AT LEAST 2 for its range since a range (0;0) doesn't make any sense
if(sceneWithMaxMatches < 2)
  return -1;

// in this window show the image gallery (scene-images); the user can scroll through all image using a trackbar
cv::namedWindow("Images", CV_GUI_EXPANDED | CV_WINDOW_AUTOSIZE);
// just a dummy to store the state of the trackbar 
int imagesTrackbarState = 0;
// create the first trackbar that the user uses to scroll through the scene-images
// IMPORTANT: use processedImages.size() - 1 since indexing in vectors is the same as in arrays - it starts from 0 and not reducing it by 1 will throw an out-of-range exception
cv::createTrackbar("Images:", "Images", &imagesTrackbarState, processedImages.size() - 1, on_imagesTrackbarCallback, NULL);
// in this window we show the matched object-images relative to the selected image in the "Images" window
cv::namedWindow("Matches for current image", CV_WINDOW_AUTOSIZE);
// yet another dummy to store the state of the trackbar in this new window
int imageMatchesTrackbarState = 0;
// IMPORTANT: again since sceneWithMaxMatches stores the SIZE of a vector we need to reduce it by 1 in order to be able to use it for the indexing later on
cv::createTrackbar("Matches:", "Matches for current image", &imageMatchesTrackbarState, sceneWithMaxMatches - 1, on_imageMatchesTrackbarCallback, NULL);

while(true)
{
  char key = cv::waitKey(20);
  if(key == 27)
    break;

  // from here on the magic begins
  // show the image gallery; use the position of the "Images:" trackbar to call the image at that position
  cv::imshow("Images", processedImages.at(cv::getTrackbarPos("Images:", "Images")));

  // store the index of the current scene-image by calling the position of the trackbar in the "Images:" window
  int currentSceneIndex = cv::getTrackbarPos("Images:", "Images");
  // we have to make sure that the match of the currently selected scene-image actually has something in it
  if(matches.at(currentSceneIndex).size())
  {
    // store the index of the current object-image that we have matched to the current scene-image in the "Images:" window
    int currentObjectIndex = cv::getTrackbarPos("Matches:", "Matches for current image");
    cv::imshow(
            "Matches for current image",
            matchesDraw.at(currentSceneIndex).at(currentObjectIndex < matchesDraw.at(currentSceneIndex).size() ? // is the current object index within the range of the matches for the current object and current scene
            currentObjectIndex : // yes, return the correct index
            matchesDraw.at(currentSceneIndex).size() - 1));  // if outside the range show the last matched pair!
  }
}

// do something else
// ...

The tricky part is the trackbar in the second window responsible for accessing the matched images to our currently selected image in the "Images" window. As I've explained above I set the trackbar "Matches:" in the "Matches for current image" window to have a range from 0 to (sceneWithMaxMatches-1). However not all images have the same amount of matches with the rest in the image set (applies tenfold if you have done some additional filtering to ensure reliable matches for example by exploiting the properties of the homography, ratio test, min/max distance check etc.). Because I was unable to find a way to dynamically adjust the trackbar's range I needed a validation of the index. Otherwise for some of the images and their matches the application will throw an out-of-range exception. This is due to the simple fact that for some matches we try to access a match-vector with an index greater than it's size minus 1 because cv::getTrackbarPos() goes all the way to (sceneWithMaxMatches - 1). If the trackbar's position goes out of range for the currently selected vector with matches, I simply set the matchDraw-image in "Matches for current image" to the very last in the vector. Here I exploit the fact that the indexing can't go below zero as well as the trackbar's position so there is not need to check this but only what comes after the initial position 0. If this is not your case make sure you check the lower bound too and not only the upper.

Hope this helps!