 # Find homography for stitching

Hi, I’m working on the following task:

I have 6 fisheye cameras and would like to produce a 360 degree stitched image.

After carrying out the calibration procedure with findChessboardCorners, calibrateCamera, I obtained the intrinsic and extrinsic matrix.
Starting from the 6 images with fish-eye effect, through the fisheye.initUndistortRectifyMap function, I obtained the 6 planar images.
The two planar images from above:

Now I should do the stitching to get a 360 degree image.

I tried to do this using the cv2.createStitcher function, but this doesn’t always work, also I would like to have access to the homography matrix to determine the static matrices of the system.

So I tried to calculate the homography matrix, identifying through the SIFT algorithm, the common keypoints between two images and keeping the keypoints that best match.

I then stitched the two images using the warpPerspective function.

I believe that the procedure is correct up to the calculation of the keypoints, but I do not understand why the final result is not good.

In fact, in an attempt to stitch the second image is completely deformed / changed in perspective with a loss of right image.

Here there is the code:

``````import cv2
import numpy as np

def cvshow(name, img):
cv2.imshow(name, img)
cv2.waitKey(0)
cv2.destroyAllWindows()
cv2.destroyAllWindows()

def sift_kp(image):
gray_image = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
sift = cv2.xfeatures2d.SIFT_create()
sift = cv2.xfeatures2d.SIFT_create()
kp, des = sift.detectAndCompute(image, None)
kp_image = cv2.drawKeypoints(gray_image, kp, None)
return kp_image, kp, des

def get_good_match(des1, des2):
bf = cv2.BFMatcher()
matches = bf.knnMatch(des1, des2, k=2)  # des1 is the template image, des2 is the matching image
matches = sorted(matches, key=lambda x: x.distance / x.distance)
good = []
for m, n in matches:
if m.distance < 0.55 * n.distance:
good.append(m)
return good

def drawMatches(imageA, imageB, kpsA, kpsB, matches, status):
# Initialize the visualization picture, connect the A and B pictures left and right together
(hA, wA) = imageA.shape[:2]
(hB, wB) = imageB.shape[:2]
vis = np.zeros((max(hA, hB), wA + wB, 3), dtype="uint8")
vis[0:hA, 0:wA] = imageA
vis[0:hB, wA:] = imageB

# Joint traversal, draw matching pairs
for ((trainIdx, queryIdx), s) in zip(matches, status):
# When the point pair is matched successfully, draw it on the visualization
if s == 1:
# Draw matching pairs
ptA = (int(kpsA[queryIdx]), int(kpsA[queryIdx]))
ptB = (int(kpsB[trainIdx]) + wA, int(kpsB[trainIdx]))
cv2.line(vis, ptA, ptB, (0, 255, 0), 1)

# Return visualization results
return vis

# Panorama stitching
def siftimg_rightlignment(img_right, img_left):
_, kp1, des1 = sift_kp(img_right)
_, kp2, des2 = sift_kp(img_left)
goodMatch = get_good_match(des1, des2)
# When the matching pairs of the filter items are greater than 4 pairs: calculate the perspective transformation matrix
if len(goodMatch) > 4:
# Get the point coordinates of the matching pair
ptsA = np.float32([kp1[m.queryIdx].pt for m in goodMatch]).reshape(-1, 1, 2)
ptsB = np.float32([kp2[m.trainIdx].pt for m in goodMatch]).reshape(-1, 1, 2)
ransacReprojThreshold = 4
H, status = cv2.findHomography(ptsA, ptsB, cv2.RANSAC, ransacReprojThreshold)

print(H)
#H = np.array([[-3.95002617e-01,-7.49813070e-02, 4.43642683e+02], [-4.06655962e-01,5.27365057e-01, 1.20636875e+02],[-1.60149798e-03, -3.69708507e-05, 1.00000000e+00]])

# The function of this function is to first use RANSAC to select the best four sets of pairing points, and then calculate the H matrix. H is a 3*3 matrix

# Change the angle of view to the right of the picture, result is the transformed picture
result = cv2.warpPerspective(img_right, H, (img_right.shape + img_left.shape, img_right.shape))
cvshow('result_medium', result)
# Pass the picture left to the left end of the result picture
result[0:img_left.shape, 0:img_left.shape] = img_left
return result

# Feature matching + panoramic stitching
import numpy as np
import cv2

# Read the stitched pictures (note the placement of the left and right pictures)
# Is to transform the graphics on the right

img_right = cv2.resize(img_right, None, fx=0.5, fy=0.3)
# Ensure that the two images are the same size
img_left = cv2.resize(img_left, (img_right.shape, img_right.shape))

kpimg_right, kp1, des1 = sift_kp(img_right)
kpimg_left, kp2, des2 = sift_kp(img_left)

# Display the original image and the image after key point detection at the same time
cvshow('img_left', np.hstack((img_left, kpimg_left)))
cvshow('img_right', np.hstack((img_right, kpimg_right)))
goodMatch = get_good_match(des1, des2)
all_goodmatch_img = cv2.drawMatches(img_right, kp1, img_left, kp2, goodMatch, None, flags=2)

# goodmatch_img Set the first goodMatch[:10]
goodmatch_img = cv2.drawMatches(img_right, kp1, img_left, kp2, goodMatch[:10], None, flags=2)

cvshow('Keypoint Matches1', all_goodmatch_img)
cvshow('Keypoint Matches2', goodmatch_img)

# Stitch the picture into a panorama
result = siftimg_rightlignment(img_right, img_left)
cvshow('result', result)`````````

given your code, the results look as expected.

what you do is simply not applicable to real panoramas, which span 180 degrees or more. and if the data even approaches 180 degrees, you get exactly those “weird” results.

homographies transform plane to plane. a plane can’t span 180 degrees, and even approaching 180 degrees, the picture will look as weird as you just saw.

the correct camera model for a panorama can’t be a flat image plane. it’s a cylinder or a sphere. by “camera model” I also mean the resulting pictures you calculate. the camera model needn’t be physically possible/feasible.

you need to read up on the theory behind panorama stitching and discard anything that doesn’t mention those projection types.

anything that merely finds a homography and applies a perspective warp… is useless for your application.