kotopanda kotopanda - 1 month ago 41x
Python Question

How to get pixel coordinates from Feature Matching in OpenCV Python

I need to get the list of the

coordinates of the pixels that the feature matcher selects in the code provided. I'm using Python and OpenCV. Can anyone help me?




bf=cv2.BFMatcher(cv2.NORM_HAMMING, crossCheck=True)
matches=sorted(matches, key= lambda x:x.distance)


matches returns a list of structures where each structure contains several fields... among them are two important fields:

  • queryIdx - The index of the feature into kp1 that matches
  • trainIdx - The index of the feature into kp2 that matches

You'd use these to index into kp1 and kp2 and obtain the pt field to obtain the (x,y) coordinates of the matches.

All you have to do is iterate through each structure in matches, append to a list of coordinates for both kp1 and kp2 and you're done.

Something like this:

# Initialize lists
list_kp1 = []
list_kp2 = []

# For each match...
for mat in matches:

    # Get the matching keypoints for each of the images
    img1_idx = mat.queryIdx
    img2_idx = mat.trainIdx

    # x - columns
    # y - rows
    # Get the coordinates
    (x1,y1) = kp1[img1_idx].pt
    (x2,y2) = kp2[img2_idx].pt

    # Append to each list
    list_kp1.append((x1, y1))
    list_kp2.append((x2, y2))

Note that I could have just done list_kp1.append(kp1[img1_idx].pt), as well as the same for list_kp2, but I wanted to make very clear on how to interpret the spatial coordinates.

list_kp1 will contain the spatial coordinates of a feature point that matched with the corresponding position in list_kp2. In other words, element i of list_kp1 contains the spatial coordinates of the feature point from img1 that matched with the corresponding feature point from img2 in list_kp2 whose spatial coordinates are in element i.

As a minor sidenote, I used this concept when I wrote a workaround for drawMatches because for OpenCV 2.4.x, the Python wrapper to the C++ function does not exist, so I made use of the above concept in locating the spatial coordinates of the matching features between the two images to write my own implementation of it.

Check it out if you like!

module' object has no attribute 'drawMatches' opencv python