title: OpenCV feature matching multiple objects
body: <p>How can I find multiple objects of one type on one image.
  I use ORB feature finder and brute force matcher (opencv = 3.2.0).</p>
  
  <p>My source code:</p>
  
  <pre><code>import numpy as np
  import cv2
  from matplotlib import pyplot as plt
  
  MIN_MATCH_COUNT = 10
  
  img1 = cv2.imread('box.png', 0)  # queryImage
  img2 = cv2.imread('box1.png', 0) # trainImage
  
  #img2 = cv2.cvtColor(img1, cv2.COLOR_BGR2GRAY)
  
  # Initiate ORB detector
  # 
  orb = cv2.ORB_create(10000, 1.2, nlevels=9, edgeThreshold = 4)
  #orb = cv2.ORB_create()
  
  # find the keypoints and descriptors with SIFT
  kp1, des1 = orb.detectAndCompute(img1, None)
  kp2, des2 = orb.detectAndCompute(img2, None)
  
  FLANN_INDEX_KDTREE = 0
  index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
  search_params = dict(checks = 50)
  
  flann = cv2.FlannBasedMatcher(index_params, search_params)
  
  des1 = np.float32(des1)
  des2 = np.float32(des2)
  
  # matches = flann.knnMatch(des1, des2, 2)
  
  bf = cv2.BFMatcher()
  matches = bf.knnMatch(des1, des2, k=2)
  
  # store all the good matches as per Lowe's ratio test.
  good = []
  for m,n in matches:
      if m.distance &lt; 0.7*n.distance:
          good.append(m)
  
  if len(good)&gt;3:
      src_pts = np.float32([ kp1[m.queryIdx].pt for m in good ]).reshape(-1,1,2)
      dst_pts = np.float32([ kp2[m.trainIdx].pt for m in good ]).reshape(-1,1,2)
  
      M, mask = cv2.findHomography(src_pts, dst_pts, cv2.RANSAC, 2)
  
      if M is None:
          print ("No Homography")
      else:
          matchesMask = mask.ravel().tolist()
  
          h,w = img1.shape
          pts = np.float32([ [0,0],[0,h-1],[w-1,h-1],[w-1,0] ]).reshape(-1,1,2)
          dst = cv2.perspectiveTransform(pts,M)
  
          img2 = cv2.polylines(img2,[np.int32(dst)],True,255,3, cv2.LINE_AA)
  
  else:
      print ("Not enough matches are found - %d/%d" % (len(good),MIN_MATCH_COUNT))
      matchesMask = None
  
  draw_params = dict(matchColor = (0,255,0), # draw matches in green color
                     singlePointColor = None,
                     matchesMask = matchesMask, # draw only inliers
                     flags = 2)
  
  img3 = cv2.drawMatches(img1,kp1,img2,kp2,good,None,**draw_params)
  
  plt.imshow(img3, 'gray'),plt.show()
  </code></pre>
  
  <p>But it can find only one instance of query image.</p>
  
  <p><strong>Query Image</strong></p>
  
  <p><a href="https://i.stack.imgur.com/iwxcd.png" rel="noreferrer"><img src="https://i.stack.imgur.com/iwxcd.png" alt="Query Image"></a></p>
  
  <p><strong>Test Image</strong>
  <a href="https://i.stack.imgur.com/GQyEj.png" rel="noreferrer"><img src="https://i.stack.imgur.com/GQyEj.png" alt="Test Image"></a></p>
  
  <p><strong>Result</strong>
  <a href="https://i.stack.imgur.com/togFv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/togFv.png" alt="Result"></a></p>
  
  <p>So its found only one image from two.
  What I am doing wrong?</p>