Detect objects with OpenCV Python

Question

I'm really new with OpenCV. I'm reading some tutorials and documentation in order to make my first little script.

I have an image and I want to detect objects on this one : street lamps, trashcans ...

My image looks like :

I wrote this script :

import cv2

img_filt = cv2.medianBlur(cv2.imread('ville.jpg',0), 5)
img_th = cv2.adaptiveThreshold(img_filt,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C,cv2.THRESH_BINARY,11,2)
img_filt, contours, hierarchy = cv2.findContours(img_th, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)

display = cv2.imshow("Objects",img_filt)
wait_time = cv2.waitKey(0)

But How I can display the picture result with rectangles ?

Thank you so much !

U.Swap · Accepted Answer

I guess for object detection , which I personally use and recommend to all, is by using SIFT(Scale-Invariant Feature Transform) or SURF algorithm , but note that these algorithms are now patented , and no longer included in OpenCV 3, still availble in openCV2 , as good alternative to this I prefer to make use of ORB which is opensource implementaition of SIFT/SURF.

Brute-Force Matching with SIFT Descriptors and Ratio Test

here we use BFMatcher.knnMatch() to get k best matches. In this example, we will take k=2 so that we can apply ratio test explained by D.Lowe in his paper.

import numpy as np
import cv2
from matplotlib import pyplot as plt

img1 = cv2.imread('box.png',0)          # queryImage
img2 = cv2.imread('box_in_scene.png',0) # trainImage

# Initiate SIFT detector
sift = cv2.SIFT()

# find the keypoints and descriptors with SIFT
kp1, des1 = sift.detectAndCompute(img1,None)
kp2, des2 = sift.detectAndCompute(img2,None)

# BFMatcher with default params
bf = cv2.BFMatcher()
matches = bf.knnMatch(des1,des2, k=2)

# Apply ratio test
good = []
for m,n in matches:
    if m.distance < 0.75*n.distance:
        good.append([m])

# cv2.drawMatchesKnn expects list of lists as matches.
img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,flags=2)

plt.imshow(img3),plt.show()

Moving ahead with FLANN based Matcher

FLANN stands for Fast Library for Approximate Nearest Neighbors. It contains a collection of algorithms optimized for fast nearest neighbor search in large datasets and for high dimensional features. It works more faster than BFMatcher for large datasets. We will see the second example with FLANN based matcher.

For FLANN based matcher, we need to pass two dictionaries which specifies the algorithm to be used, its related parameters etc. First one is IndexParams. For various algorithms, the information to be passed is explained in FLANN docs. As a summary, for algorithms like SIFT, SURF etc.

Sample code by using FLANN with SIFT:

import numpy as np
import cv2
from matplotlib import pyplot as plt

img1 = cv2.imread('box.png',0)          # queryImage
img2 = cv2.imread('box_in_scene.png',0) # trainImage

# Initiate SIFT detector
sift = cv2.SIFT()

# find the keypoints and descriptors with SIFT
kp1, des1 = sift.detectAndCompute(img1,None)
kp2, des2 = sift.detectAndCompute(img2,None)

# FLANN parameters
FLANN_INDEX_KDTREE = 0
index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
search_params = dict(checks=50)   # or pass empty dictionary

flann = cv2.FlannBasedMatcher(index_params,search_params)

matches = flann.knnMatch(des1,des2,k=2)

# Need to draw only good matches, so create a mask
matchesMask = [[0,0] for i in xrange(len(matches))]

# ratio test as per Lowe's paper
for i,(m,n) in enumerate(matches):
    if m.distance < 0.7*n.distance:
        matchesMask[i]=[1,0]

draw_params = dict(matchColor = (0,255,0),
                   singlePointColor = (255,0,0),
                   matchesMask = matchesMask,
                   flags = 0)

img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,matches,None,**draw_params)



plt.imshow(img3,),plt.show()

See the result below:

But what I recommend is, Brute-Force Matching with ORB Descriptors

In this example I used ORB with Bruteforce matcher, this code captures frames from camera at realtime and computes the keypoints,descriptors from input frames and compares it with the stored query image, by doing the same , and returns the matching keypoint lengths, the same can be applied on above code which uses SIFT algorithm instead of ORB.

import numpy as np
import cv2
from imutils.video import WebcamVideoStream
from imutils.video import FPS

MIN_MATCH_COUNT = 10

img1 = cv2.imread('input_query.jpg', 0)


orb = cv2.ORB()
kp1, des1 = orb.detectAndCompute(img1, None)

webcam = WebcamVideoStream(src=0).start()
fps = FPS().start()

while True:
    img2 = webcam.read()
    key = cv2.waitKey(10)
    cv2.imshow('',img2)
    if key == 1048603:
        break
    kp2, des2 = orb.detectAndCompute(img2, None)

    bf = cv2.BFMatcher(cv2.NORM_HAMMING, crossCheck=True)

    matches = bf.match(des1, des2)
    matches = sorted(matches, key=lambda x: x.distance)  # compute the descriptors with ORB

    if not len(matches) > MIN_MATCH_COUNT:
        print "Not enough matches are found - %d/%d" % (len(matches), MIN_MATCH_COUNT)
        matchesMask = None

    #simg2 = cv2.polylines(img2,[np.int32(dst)],True,255,3, cv2.LINE_AA)

    print len(matches)
    #img3 = cv2.drawMatches(img1, kp1, img2, kp2, matches[:10], None, flags=2)

    fps.update()

fps.stop()

More descriptive video tutorial on this will be found here, https://www.youtube.com/watch?v=ZW3nrP2OyLQ and one more good thing is it's opensource : https://gitlab.com/josemariasoladuran/object-recognition-opencv-python.git

Detect objects with OpenCV Python

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