# matcher.knnMatch() error: unsupported format or combination of formats

**URL:** <https://forum.opencv.org/t/matcher-knnmatch-error-unsupported-format-or-combination-of-formats/11676>\
**Category:** Python\
**Tags:** flann\
**Created:** [January 18, 2023, 2:21am UTC](https://forum.opencv.org/t/matcher-knnmatch-error-unsupported-format-or-combination-of-formats/11676 "2023-01-18T02:21:17Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![yawnmoth](https://avatars.discourse-cdn.com/v4/letter/y/0ea827/32.png) [@yawnmoth](https://forum.opencv.org/u/yawnmoth)\
**Post date:** [January 18, 2023, 2:21am UTC](https://forum.opencv.org/t/matcher-knnmatch-error-unsupported-format-or-combination-of-formats/11676/1 "2023-01-18T02:21:17Z")

</div>

I was trying to simplify [opencv/find\_obj.py at 4.x · opencv/opencv · GitHub](https://github.com/opencv/opencv/blob/4.x/samples/python/find_obj.py) and make it use `warpPerspective()` and am now getting an error.

Before I share the error here’s the code:

```auto
import sys
import cv2

FLANN_INDEX_KDTREE = 1

def filter_matches(kp1, kp2, matches, ratio = 0.75):
  mkp1, mkp2 = [], []
  for m in matches:
    if len(m) == 2 and m[0].distance < m[1].distance * ratio:
      m = m[0]
      mkp1.append( kp1[m.queryIdx] )
      mkp2.append( kp2[m.trainIdx] )
  p1 = np.float32([kp.pt for kp in mkp1])
  p2 = np.float32([kp.pt for kp in mkp2])
  kp_pairs = zip(mkp1, mkp2)
  return p1, p2, list(kp_pairs)

def alignImages(im1, im2):
  detector = cv2.AKAZE_create()
  flann_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
  matcher = cv2.FlannBasedMatcher(flann_params, {})

  kp1, desc1 = detector.detectAndCompute(im1, None)
  kp2, desc2 = detector.detectAndCompute(im2, None)

  print('img1 - %d features, img2 - %d features' % (len(kp1), len(kp2)))

  raw_matches = matcher.knnMatch(desc1, trainDescriptors = desc2, k = 2)
  p1, p2, kp_pairs = filter_matches(kp1, kp2, raw_matches)
  if len(p1) < 4:
    print('%d matches found, not enough for homography estimation' % len(p1))
    sys.exit()

  height, width = im2.shape

  imResult = cv2.warpPerspective(im1, H, (width, height))

  return imResult

im = cv2.imread('ref.png', cv2.IMREAD_GRAYSCALE)
imRef = cv2.imread('new.png', cv2.IMREAD_GRAYSCALE)

imNew = alignImages(im, imRef)

cv2.imwrite('output.png', imNew)

```

Here’s the error:

```auto
Traceback (most recent call last):
  File "align.py", line 47, in <module>
    imNew = alignImages(im, imRef)
  File "align.py", line 32, in alignImages
    raw_matches = matcher.knnMatch(desc1, trainDescriptors = desc2, k = 2)
cv2.error: OpenCV(4.4.0) /tmp/pip-req-build-sw_3pm_8/opencv/modules/flann/src/miniflann.cpp:315: error: (-210:Unsupported format or combination of formats) in function 'buildIndex_'
> type=0
>

```

Any ideas?

The problem images are as follows:

htt­ps://www.terrafrost.com/new.png  
htt­ps://www.terrafrost.com/ref.png

(copy / paste the URL from the www and on; this forum won’t let post the link as a clickable URL so I added a soft hyphen between the ht and tps; also, I can only upload one image - I can’t upload two)

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<div class="post-metadata">

**Author:** ![berak](https://avatars.discourse-cdn.com/v4/letter/b/85f322/32.png) [@berak](https://forum.opencv.org/u/berak)\
**Post date:** [January 18, 2023, 7:51am UTC](https://forum.opencv.org/t/matcher-knnmatch-error-unsupported-format-or-combination-of-formats/11676/2 "2023-01-18T07:51:19Z")

</div>

have another look at the sample code:

> <https://github.com/opencv/opencv/blob/c63d79c5b16fcbbec46f1b8bb871dab2274e2b01/samples/python/find_obj.py#L49-L55>

`FLANN_INDEX_KDTREE` is for SIFT/SURF (L2 norm).  
for AKAZE/ORB/BRIEF features(hamming distance) you need `FLANN_INDEX_LSH` …

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<div class="post-metadata">

**Author:** ![Kevin\_Patel](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/kevin_patel/32/10134_2.png) [@Kevin\_Patel](https://forum.opencv.org/u/Kevin_Patel)\
**Post date:** [April 4, 2024, 7:36pm UTC](https://forum.opencv.org/t/matcher-knnmatch-error-unsupported-format-or-combination-of-formats/11676/3 "2024-04-04T19:36:31Z")

</div>

Thanks for this, it worked for me. NOTE: just don’t forget to add `FLANN_INDEX_LSH=6` before this code block.