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Is there a way to make the background clear of anything and leave only the biggest contour after applying adaptive threshold

This is the full code I'm using and I've included the output images. I'm trying to remove the background and then apply contour to it so that the only item left is the outline of the plane. I've attached the images from the code after applying threshold and contouring
After Threshold
After Contouring
import cv2
import numpy as np
import argparse
ap = argparse.ArgumentParser()
ap.add_argument("-i", "--input", required=True,
help="path to input image")
ap.add_argument("-o", "--output", required=True,
help="path to output image")
args = vars(ap.parse_args())
src = cv2.imread(args["input"], 1) # read input image
gray = cv2.cvtColor(src, cv2.COLOR_BGR2GRAY) # convert to grayscale
blur = cv2.blur(gray, (3, 3)) # blur the image
# Otsu's thresholding
th2 = cv2.adaptiveThreshold(blur,255,cv2.ADAPTIVE_THRESH_MEAN_C,\
cv2.THRESH_BINARY,11,2)
thresh = cv2.resize(th2, (1080 , 480))
cv2.imshow("thresh",thresh)
contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
# create hull array for convex hull points
hull = []
# calculate points for each contour
for i in range(len(contours)):
# creating convex hull object for each contour
hull.append(cv2.convexHull(contours[i], False))
# create an empty black image
drawing = np.zeros((thresh.shape[0], thresh.shape[1], 3), np.uint8)
# draw contours and hull points
for i in range(len(contours)):
color_contours = (0, 255, 0) # green - color for contours
color = (255, 0, 0) # blue - color for convex hull
# draw ith contour
cv2.drawContours(drawing, contours, i, color_contours, 1, 8, hierarchy)
# draw ith convex hull object
drawing = cv2.resize(drawing, (1080 , 480))
cv2.imshow(args["output"], drawing)
cv2.destroyAllWindows()

Here is an example of filtering on area to get the largest contour:
# get outer contours and filter to get the largest (presumably only one)
cntrs = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cntrs = cntrs[0] if len(cntrs) == 2 else cntrs[1]
# create black background image
result = np.zeros_like(src)
area_thresh = 0
for c in cntrs:
area = cv2.contourArea(c)
if area > area_thresh:
area_thresh = area
big_contour = c
# draw largest contour on black background
cv2.drawContours(result, [big_contour], -1, (0,0,255), 1)

If you want to keep more than one contour, you can simply keep any that are larger than some threshold in Python OpenCV. Here is the previous code modified to do that for a minimum area of 10000. All contours larger than 10000 will be drawn.
# get outer contours and filter to get the largest (presumably only one)
cntrs = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cntrs = cntrs[0] if len(cntrs) == 2 else cntrs[1]
# create black background image
result = np.zeros_like(src)
area_thresh = 10000
for c in cntrs:
area = cv2.contourArea(c)
if area > area_thresh:
cv2.drawContours(result, [c], -1, (0,0,255), 1)

How to detect edge if edges of an object has same colour as background

I am tasked to build a license plate detection system and my code does not work if the plate has the same colour of the paint of car (background).
Take a look at this picture below.
I have tried a variety of edge detection technique and my findings are they hardly work.
Here is my image processing pipeline:
Extract the gray channel from the image.
Reduce noise with Iterative Bilaterial Filtering
Detect edges with Adaptive Thresholding
Dilate the edges slightly
Locate contours based on some heuristics.
The edge detection part performed miserably around the plate region.
The pipeline works good and I am able to detect license plates if the car is has a different paint colour than the plate.
Code
def rectangleness(hull):
rect = cv2.boundingRect(hull)
rectPoints = np.array([[rect[0], rect[1]],
[rect[0] + rect[2], rect[1]],
[rect[0] + rect[2], rect[1] + rect[3]],
[rect[0], rect[1] + rect[3]]])
intersection_area = cv2.intersectConvexConvex(np.array(rectPoints), hull)[0]
rect_area = cv2.contourArea(rectPoints)
rectangleness = intersection_area/rect_area
return rectangleness
def preprocess(image):
image = imutils.resize(image, 1000)
# Attenuate shadows by using H channel instead of converting to gray directly
imgHSV = cv2.cvtColor(image, cv2.COLOR_BGR2HSV)
_, _, gray = cv2.split(imgHSV)
# Reduce noise while preserve edge with Iterative Bilaterial Filtering
blur = cv2.bilateralFilter(gray, 11, 6, 6)
# Detect edges by thresholding
edge = cv2.adaptiveThreshold(blur, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY_INV, 11, 5)
# Dilate edges, kernel size cannot be too big as some fonts are very closed to the edge of the plates
kernel = cv2.getStructuringElement(cv2.MORPH_ELLIPSE, (2, 2))
dilated = cv2.dilate(edge, kernel)
# Detect contours
edge, contours, _ = cv2.findContours(dilated, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
# Loop through contours and select the most probable ones
contours = sorted(contours, key = cv2.contourArea, reverse=True)[:10]
for contour in contours:
perimeter = cv2.arcLength(contour, closed=True)
approximate = cv2.approxPolyDP(contour, 0.02*perimeter, closed=True)
if len(approximate) == 4:
(x, y, w, h) = cv2.boundingRect(approximate)
whRatio = w / h
# Heuristics:
# 1. Width of plate should at least be 2x greater than height
# 2. Width of contour should be more than 5 (eliminate false positive)
# 3. Height must not be too small
# 4. Polygon must resemble a rectangle
if (2.0 < whRatio < 6.0) and (w > 5.0) and (h > 20):
hull = cv2.convexHull(approximate, returnPoints=True)
if rectangleness(hull) > 0.75:
print("X Y {} {}".format(x, y))
print("Height: {}".format(h))
print("Width : {}".format(w))
print("Ratio : {}\n".format(w/h))
cv2.drawContours(image, [approximate], -1, (0, 255, 0), 2)
cv2.imshow("Edge", edge)
cv2.imshow("Frame", image)
cv2.waitKey(0)
cv2.destroyAllWindows()

You can use cv2.morphologyEx for making the plate region become more visible. Next step is to find contours and set reasonable conditions to extract the contour that contains the plate. If you want, you can have a look at this github repository where my friend and I show detailed steps about license plate detection and recognition.
import cv2
import numpy as np
img = cv2.imread("a.png")
imgBlurred = cv2.GaussianBlur(img, (7, 7), 0)
gray = cv2.cvtColor(imgBlurred, cv2.COLOR_BGR2GRAY) # convert to gray
sobelx = cv2.Sobel(gray, cv2.CV_8U, 1, 0, ksize=3) # sobelX to get the vertical edges
ret,threshold_img = cv2.threshold(sobelx, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)
morph_img_threshold = threshold_img.copy()
element = cv2.getStructuringElement(shape=cv2.MORPH_RECT, ksize=(22, 3))
cv2.morphologyEx(src=threshold_img, op=cv2.MORPH_CLOSE, kernel=element,
dst=morph_img_threshold)
cv2.imshow("img", img)
cv2.imshow("sobelx", sobelx)
cv2.imshow("morph_img_threshold", morph_img_threshold)
cv2.waitKey()
cv2.destroyAllWindows()

Highlight all possible circles ( Bubble sheet choices ) in opencv

I am working on automatically correcting a bubble-sheet tests that are scanned.
Currently, I can extract the solutions part of the sheet and fix its rotation.
So I have this image.
The output image with detected contours
Running the following code yields in the output image
def get_answers(image):
display_normal("Just image",image)
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blurry = cv2.GaussianBlur(gray, (3, 3), 1)
thresh = cv2.threshold(blurry, 225, 255,
cv2.THRESH_BINARY_INV)[1]
display_normal("Binary", thresh)
# find contours in the thresholded image, then initialize
# the list of contours that correspond to questions
cnts = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL,
cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[1]
questionCnts = []
# loop over the contours
for c in cnts:
# compute the bounding box of the contour, then use the
# bounding box to derive the aspect ratio
(x, y, w, h) = cv2.boundingRect(c)
ar = w / float(h)
# in order to label the contour as a question, region
# should be sufficiently wide, sufficiently tall, and
# have an aspect ratio approximately equal to 1
if w >= 18 and h >= 18 and 0.9 <= ar and ar <= 1.2:
questionCnts.append(c)
cv2.drawContours(image, questionCnts, -1, (255, 0, 0), 1)
display_normal("Image with contours",image.copy())
if(questionCnts < 45*4):
raise Exception("Didn't found all possible answers")
Here is the problem : I convert the input image to binary and try to find contours that looks like a circle, but I can't find the whole possible 45*4 choices.. I fail to detect some of these circles..
So is there any better idea/algorithm to do this specific task ?

You could have tried using adaptive threshold:
adapt_thresh = cv2.adaptiveThreshold(equ, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY_INV, 11, 2)
cv2.imshow('adapt_thresh.jpg', adapt_thresh)
(I resized the original image to keep it smaller)
UPDATE:
Another approach that I just performed.......
I equalized the gray scale image using histogram equalization:
equalized_img = cv2.equalizeHist(gray)
cv2.imshow('Equalized Image.jpg', equalized_img )
I then obtained the median of the equalized image using np.median(equalized_img) and applied a binary threshold by selecting all pixel values below [0.6 * median]
ret, thresh = cv2.threshold(equalized_img, lower, 255, 1)
cv2.imwrite("Final Image.jpg", thresh)
Now you can go ahead and find your desired contours on this image.
Hope it helps .. :)

How do I validate OpenCV gives me contours?

I followed along at http://www.pyimagesearch.com/2016/10/03/bubble-sheet-multiple-choice-scanner-and-test-grader-using-omr-python-and-opencv/
now I am trying to make this function in real time. My end goal is to detect a solid colored circle so this looked like a good start.
My crash:
~/py:.python test_grader.py
Traceback (most recent call last):
File "test_grader.py", line 82, in <module>
questionCnts = contours.sort_contours(questionCnts,
AttributeError: 'list' object has no attribute 'sort_contours'
questionCnts = contours.sort_contours(questionCnts,
method="top-to-bottom")[0] << line 82 crashing
questionCnts is defined as [].. I dont understand how appending the contour is supposed to add this method.
Full source
from imutils.perspective import four_point_transform
from imutils import contours
import numpy as np
import argparse
import imutils
import cv2
import copy
cap = cv2.VideoCapture(0)
ANSWER_KEY = {0: 1, 1: 4, 2: 0, 3: 3, 4: 1}
while(True):
ret, image = cap.read()
clone = copy.copy(image)
gray = cv2.cvtColor(clone, cv2.COLOR_BGR2GRAY)
blurred = cv2.GaussianBlur(gray, (5, 5), 0)
edged = cv2.Canny(blurred, 75, 200)
ret, thresh = cv2.threshold(gray,127,255,1) #only black squares?
contours, h = cv2.findContours(thresh,1,1) #was 2
# find contours in the edge map, then initialize
# the contour that corresponds to the document
cnts = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
# cnts = cv2.findContours(edged.copy(), cv2.RETR_EXTERNAL,
# cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if imutils.is_cv2() else cnts[1]
docCnt = None
# ensure that at least one contour was found
if len(cnts) > 0:
# sort the contours according to their size in
# descending order
cnts = sorted(cnts, key=cv2.contourArea, reverse=True)
# loop over the sorted contours
for c in cnts:
# approximate the contour
peri = cv2.arcLength(c, True)
approx = cv2.approxPolyDP(c, 0.02 * peri, True)
# if our approximated contour has four points,
# then we can assume we have found the paper
if len(approx) == 4:
docCnt = approx
break
# apply a four point perspective transform to both the
# original image and grayscale image to obtain a top-down
# birds eye view of the paper
paper = four_point_transform(image, docCnt.reshape(4, 2))
warped = four_point_transform(gray, docCnt.reshape(4, 2))
# apply Otsu's thresholding method to binarize the warped
# piece of paper
thresh = cv2.threshold(warped, 0, 255,
cv2.THRESH_BINARY_INV | cv2.THRESH_OTSU)[1]
# find contours in the thresholded image, then initialize
# the list of contours that correspond to questions
cnts = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL,
cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if imutils.is_cv2() else cnts[1]
questionCnts = []
# loop over the contours
for c in cnts:
# compute the bounding box of the contour, then use the
# bounding box to derive the aspect ratio
(x, y, w, h) = cv2.boundingRect(c)
ar = w / float(h)
# in order to label the contour as a question, region
# should be sufficiently wide, sufficiently tall, and
# have an aspect ratio approximately equal to 1
if w >= 20 and h >= 20 and ar >= 0.9 and ar <= 1.1:
questionCnts.append(c)
# sort the question contours top-to-bottom, then initialize
# the total number of correct answers
questionCnts = contours.sort_contours(questionCnts,
method="top-to-bottom")[0]
correct = 0
# each question has 5 possible answers, to loop over the
# question in batches of 5
for (q, i) in enumerate(np.arange(0, len(questionCnts), 5)):
# sort the contours for the current question from
# left to right, then initialize the index of the
# bubbled answer
cnts = contours.sort_contours(questionCnts[i:i + 5])[0]
bubbled = None
# loop over the sorted contours
for (j, c) in enumerate(cnts):
# construct a mask that reveals only the current
# "bubble" for the question
mask = np.zeros(thresh.shape, dtype="uint8")
cv2.drawContours(mask, [c], -1, 255, -1)
# apply the mask to the thresholded image, then
# count the number of non-zero pixels in the
# bubble area
mask = cv2.bitwise_and(thresh, thresh, mask=mask)
total = cv2.countNonZero(mask)
# if the current total has a larger number of total
# non-zero pixels, then we are examining the currently
# bubbled-in answer
if bubbled is None or total > bubbled[0]:
bubbled = (total, j)
# initialize the contour color and the index of the
# *correct* answer
color = (0, 0, 255)
k = ANSWER_KEY[q]
# check to see if the bubbled answer is correct
if k == bubbled[1]:
color = (0, 255, 0)
correct += 1
# draw the outline of the correct answer on the test
cv2.drawContours(paper, [cnts[k]], -1, color, 3)
# grab the test taker
score = (correct / 5.0) * 100
print("[INFO] score: {:.2f}%".format(score))
cv2.putText(paper, "{:.2f}%".format(score), (10, 30),
cv2.FONT_HERSHEY_SIMPLEX, 0.9, (0, 0, 255), 2)
cv2.imshow("Original", image)
cv2.imshow("Exam", paper)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
cv2.waitKey(0)

Finding edges of road using opencv

I am interested in finding the edges of the road. The output image should have only edges marked.
Here is one of my input image:
But the edges in output are either distorted or have lot of noise.
Here is its output:
I have tried applying watershed algorithm, but it does not detect the roads properly.
Here is my code:
import cv2
import numpy as np
img = cv2.imread('road2.jpg',0)
ret,thresh1 = cv2.threshold(img,127,255,cv2.THRESH_BINARY)
kernel = np.ones((5,5),np.uint8)
erosion = cv2.erode(thresh1,kernel,iterations = 1)
#Removing noise from image
blur = cv2.blur(img,(5,5))
#finding edges using edge detection
edges = cv2.Canny(blur, 100 ,200)
laplacian = cv2.Laplacian(edges, cv2.CV_8UC1)
sobely = cv2.Sobel(laplacian,cv2.CV_8UC1, 0, 1, ksize=5)
im2, contours, hierarchy = cv2.findContours(sobely,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
frame = cv2.drawContours(im2, contours, -1, (255,0,0), 3)
cv2.imshow('window',frame)
cv2.waitKey(0)
cv2.destroyAllWindows()
What should do inoder to mark only the edges. I want only the edges in the output beacuse I would later need these edges to find the middle lane of the road.

Result can be seen here. Not perfect, but best I could do. Idea taken from here
The code works for this image on the assumption that:
If there are no cars on the road, then the two markings(left and right), will meet at a vanishing point on the horizon and form a triangle. So, I keep only the largest contour that can be approximated by a triangle.
import cv2
import numpy as np
img = cv2.imread('road2.jpg',0)
ret,thresh1 = cv2.threshold(img,127,255,cv2.THRESH_BINARY)
kernel = np.ones((5,5),np.uint8)
erosion = cv2.erode(thresh1,kernel,iterations = 1)
#Removing noise from image
blur = cv2.blur(img,(5,5))
#finding edges using edge detection
edges = cv2.Canny(blur, 100 ,200)
laplacian = cv2.Laplacian(edges, cv2.CV_8UC1)
sobely = cv2.Sobel(laplacian,cv2.CV_8UC1, 0, 1, ksize=5)
# Do a dilation and erosion to accentuate the triangle shape
dilated = cv2.dilate(sobely,kernel,iterations = 1)
erosion = cv2.erode(dilated,kernel,iterations = 1)
im2, contours, hierarchy = cv2.findContours(erosion,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
#keep 10 largest contours
cnts = sorted(contours, key = cv2.contourArea, reverse = True)[:10]
screenCnt = None
for c in cnts:
# approximate the contour
peri = cv2.arcLength(c, True)
approx = cv2.approxPolyDP(c, 0.05 * peri, True)
# if our approximated contour has three points, then
# it must be the road markings
if len(approx) == 3:
screenCnt = approx
break
cv2.drawContours(img, [screenCnt], -1, (0, 255, 0), 3)
cv2.imshow("Road markings", img)
cv2.waitKey(0)