OpenCV image Stiching

Posted February 19, 2019 by Rokas Balsys

Image stiching second part

Welcome to our second tutorial series, where we'll finish our first tutorial part and we'll receive our stitched image.

So here is the list of steps from our first tutorial what we should do to get our final stitched result:

1. Compute the sift-key points and descriptors for left and right images.
2. Compute distances between every descriptor in one image and every descriptor in the other image.
3. Select the top best matches for each descriptor of an image.
4. Run RANSAC to estimate homography.
5. Warp to align for stitching.
6. Finally stitch them together.

In our previous tutorial we finished three first steps, so last 3 steps left to do. In our first tutorial we did the most job, what is left is just a several lines of code.

So, once we have obtained best matches between the images, our next step is to calculate the homography matrix. As we described before, the homography matrix will be used with best matching points, to estimate a relative orientation transformation within the two images.

To estimate the homography in OpenCV is a simple task, it's a one line of code:

H, __ = cv2.findHomography(srcPoints, dstPoints, cv2.RANSAC, 5)


Before starting coding stitching algorithm we need to swap image inputs. So "img_" now will take right image and "img" will take left image.

So lets jump into stiching coding:

MIN_MATCH_COUNT = 10
if len(good) > MIN_MATCH_COUNT:
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,5.0)

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)
cv2.imshow("original_image_overlapping.jpg", img2)
else:
print ("Not enough matches are found - %d/%d" % (len(good),MIN_MATCH_COUNT))


So at first we set our minimum match condition count to 10 (defined by MIN_MATCH_COUNT), and we only do stitching if our good matched exceeds our required matches. Otherwise simply show a message saying not enough matches are present.

So in if statement we are converting our Keypoints (from a list of matches) to an argument for findHomography() function. I can't explain this in details, because didn't had time to chatter this and there is no use for that.

Simply talking in this code line cv2.imshow("original_image_overlapping.jpg", img2) we are showing our received image overlapping area:

So, once we have established a homography we need to to warp perspective, essentially change the field of view, we apply following homography matrix to the image:

warped_image = cv2.warpPerspective(image, homography_matrix, dimension_of_warped_image)


So we use this as following:

dst = cv2.warpPerspective(img_,M,(img.shape[1] + img_.shape[1], img.shape[0]))
dst[0:img.shape[0], 0:img.shape[1]] = img


In above two lines of code we are taking overlapping area from two given images. Then in "dst" we have received only right side of image which is not overlapped, so in second line of code we are placing our left side image to final image. So at this point we have fully stitched image:

So from this point what is left is to remove dark side of image, so we'll write following code to remove black font from all image borders:

def trim(frame):
#crop top
if not np.sum(frame[0]):
return trim(frame[1:])
#crop bottom
elif not np.sum(frame[-1]):
return trim(frame[:-2])
#crop left
elif not np.sum(frame[:,0]):
return trim(frame[:,1:])
#crop right
elif not np.sum(frame[:,-1]):
return trim(frame[:,:-2])
return frame


And here is the final defined function we call to trim borders and at the same time we show that mage in our screen. If you want you can also write it to disk:

cv2.imshow("original_image_stiched_crop.jpg", trim(dst))
#cv2.imwrite("original_image_stiched_crop.jpg", trim(dst))


With above code we'll receive original image as in first place:

Here is the full final code:

import cv2
import numpy as np

#img_ = cv2.resize(img_, (0,0), fx=1, fy=1)
img1 = cv2.cvtColor(img_,cv2.COLOR_BGR2GRAY)

#img = cv2.resize(img, (0,0), fx=1, fy=1)
img2 = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)

sift = cv2.xfeatures2d.SIFT_create()
# find key points
kp1, des1 = sift.detectAndCompute(img1,None)
kp2, des2 = sift.detectAndCompute(img2,None)

#cv2.imshow('original_image_left_keypoints',cv2.drawKeypoints(img_,kp1,None))

#FLANN_INDEX_KDTREE = 0
#index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
#search_params = dict(checks = 50)
#match = cv2.FlannBasedMatcher(index_params, search_params)
match = cv2.BFMatcher()
matches = match.knnMatch(des1,des2,k=2)

good = []
for m,n in matches:
if m.distance < 0.03*n.distance:
good.append(m)

draw_params = dict(matchColor=(0,255,0),
singlePointColor=None,
flags=2)

img3 = cv2.drawMatches(img_,kp1,img,kp2,good,None,**draw_params)
#cv2.imshow("original_image_drawMatches.jpg", img3)

MIN_MATCH_COUNT = 10
if len(good) > MIN_MATCH_COUNT:
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, 5.0)

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)
#cv2.imshow("original_image_overlapping.jpg", img2)
else:
print("Not enought matches are found - %d/%d", (len(good)/MIN_MATCH_COUNT))

dst = cv2.warpPerspective(img_,M,(img.shape[1] + img_.shape[1], img.shape[0]))
dst[0:img.shape[0],0:img.shape[1]] = img
cv2.imshow("original_image_stitched.jpg", dst)

def trim(frame):
#crop top
if not np.sum(frame[0]):
return trim(frame[1:])
#crop top
if not np.sum(frame[-1]):
return trim(frame[:-2])
#crop top
if not np.sum(frame[:,0]):
return trim(frame[:,1:])
#crop top
if not np.sum(frame[:,-1]):
return trim(frame[:,:-2])
return frame

cv2.imshow("original_image_stitched_crop.jpg", trim(dst))
#cv2.imsave("original_image_stitched_crop.jpg", trim(dst))


In this tutorial post we learned how to perform image stitching and panorama construction using OpenCV and wrote a final code for image stitching.

Our image stitching algorithm requires four main steps: detecting keypoints and extracting local invariant descriptors; get matching descriptors between images; apply RANSAC to estimate the homography matrix; apply a warping transformation using the homography matrix.

This algorithm works well in practice when constructing panoramas only for two images.

I hope you enjoyed these two tutorials, but this is not the end!