OpenCV  4.10.0-pre
Open Source Computer Vision
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Feature Matching + Homography to find Objects

Goal

In this chapter,

  • We will mix up the feature matching and findHomography from calib3d module to find known objects in a complex image.

Basics

So what we did in last session? We used a queryImage, found some feature points in it, we took another trainImage, found the features in that image too and we found the best matches among them. In short, we found locations of some parts of an object in another cluttered image. This information is sufficient to find the object exactly on the trainImage.

For that, we can use a function from calib3d module, ie cv.findHomography(). If we pass the set of points from both the images, it will find the perspective transformation of that object. Then we can use cv.perspectiveTransform() to find the object. It needs at least four correct points to find the transformation.

We have seen that there can be some possible errors while matching which may affect the result. To solve this problem, algorithm uses RANSAC or LEAST_MEDIAN (which can be decided by the flags). So good matches which provide correct estimation are called inliers and remaining are called outliers. cv.findHomography() returns a mask which specifies the inlier and outlier points.

So let's do it !!!

Code

First, as usual, let's find SIFT features in images and apply the ratio test to find the best matches.

import numpy as np
import cv2 as cv
from matplotlib import pyplot as plt
MIN_MATCH_COUNT = 10
img1 = cv.imread('box.png', cv.IMREAD_GRAYSCALE) # queryImage
img2 = cv.imread('box_in_scene.png', cv.IMREAD_GRAYSCALE) # trainImage
# Initiate SIFT detector
sift = cv.SIFT_create()
# find the keypoints and descriptors with SIFT
kp1, des1 = sift.detectAndCompute(img1,None)
kp2, des2 = sift.detectAndCompute(img2,None)
FLANN_INDEX_KDTREE = 1
index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
search_params = dict(checks = 50)
flann = cv.FlannBasedMatcher(index_params, search_params)
matches = flann.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 < 0.7*n.distance:
good.append(m)
Flann-based descriptor matcher.
Definition features2d.hpp:1294
CV_EXPORTS_W Mat imread(const String &filename, int flags=IMREAD_COLOR)
Loads an image from a file.

Now we set a condition that at least 10 matches (defined by MIN_MATCH_COUNT) are to be there to find the object. Otherwise simply show a message saying not enough matches are present.

If enough matches are found, we extract the locations of matched keypoints in both the images. They are passed to find the perspective transformation. Once we get this 3x3 transformation matrix, we use it to transform the corners of queryImage to corresponding points in trainImage. Then we draw it.

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 = cv.findHomography(src_pts, dst_pts, cv.RANSAC,5.0)
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)
img2 = cv.polylines(img2,[np.int32(dst)],True,255,3, cv.LINE_AA)
else:
print( "Not enough matches are found - {}/{}".format(len(good), MIN_MATCH_COUNT) )
matchesMask = None
Mat findHomography(InputArray srcPoints, InputArray dstPoints, int method=0, double ransacReprojThreshold=3, OutputArray mask=noArray(), const int maxIters=2000, const double confidence=0.995)
Finds a perspective transformation between two planes.
void perspectiveTransform(InputArray src, OutputArray dst, InputArray m)
Performs the perspective matrix transformation of vectors.
void polylines(InputOutputArray img, InputArrayOfArrays pts, bool isClosed, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
Draws several polygonal curves.

Finally we draw our inliers (if successfully found the object) or matching keypoints (if failed).

draw_params = dict(matchColor = (0,255,0), # draw matches in green color
singlePointColor = None,
matchesMask = matchesMask, # draw only inliers
flags = 2)
img3 = cv.drawMatches(img1,kp1,img2,kp2,good,None,**draw_params)
plt.imshow(img3, 'gray'),plt.show()
void drawMatches(InputArray img1, const std::vector< KeyPoint > &keypoints1, InputArray img2, const std::vector< KeyPoint > &keypoints2, const std::vector< DMatch > &matches1to2, InputOutputArray outImg, const Scalar &matchColor=Scalar::all(-1), const Scalar &singlePointColor=Scalar::all(-1), const std::vector< char > &matchesMask=std::vector< char >(), DrawMatchesFlags flags=DrawMatchesFlags::DEFAULT)
Draws the found matches of keypoints from two images.

See the result below. Object is marked in white color in cluttered image:

image

Additional Resources

Exercises