Detect Narrow Line in very noise image - opencv

Question

I have performed preprocessing steps in an noisy acoustic image and now I need to detect narrow black lines.
Can you think of a better way to detect these lines?
My goal is to detect the line in the red box in this image.

Answer 1

Failed Answer: - This is not a perfect solution but will require further work to make it robust for various images. I noticed that there is very less noise in the black lines, and thus Canny does not found a lot of edges within this region. Code and results below:-
import numpy as np
import cv2
gray = cv2.imread('2.png')
edges = cv2.Canny(gray,10,60,apertureSize = 7)
cv2.imwrite('2-1-edges-10-60.jpg',edges)
kernel = np.ones((5,5),np.uint8)
closeEdges = cv2.morphologyEx(edges, cv2.MORPH_CLOSE, kernel)
cv2.imwrite('2-2-edges-10-60-dilated-1.jpg',closeEdges)
invertEdges = 255 - closeEdges
cv2.imwrite('2-3-invertedges-10-60.jpg',invertEdges)
minLineLength=100
lines = cv2.HoughLinesP(image=invertEdges,rho=1,theta=np.pi/180, threshold=200,lines=np.array([]), minLineLength=minLineLength,maxLineGap=80)
a,b,c = lines.shape
for i in range(a):
cv2.line(gray, (lines[i][0][0], lines[i][0][1]), (lines[i][0][2], lines[i][0][3]), (0, 0, 255), 1, cv2.LINE_AA)
cv2.imwrite('2-4-houghlines.jpg',gray)
Using connected component on inverse of output image and finding maximum size elements could be helpful.
Another way of approaching this is use of gradient image and directly finding area of small range of gradient magnitude. This approach would be much more flexible as it will not require using fixed threshold values - 10 and 60 as above. Threshold values can be adaptive according to image gradient/you can normalize gradient of image before using hard-coded thresholds.
Better Answer(30-40% accurate)
import numpy as np
import cv2
import os
# Store all images in this folder
path='images-1'
def autocrop(image, threshold=0):
if len(image.shape) == 3:
flatImage = np.max(image, 2)
else:
flatImage = image
rows = np.where(np.max(flatImage, 0) > threshold)[0]
if rows.size:
cols = np.where(np.max(flatImage, 1) > threshold)[0]
image = image[cols[0]: cols[-1] + 1, rows[0]: rows[-1] + 1]
else:
image = image[:1, :1]
return image
def skeleton(img):
size = np.size(img)
skel = np.zeros(img.shape,np.uint8)
element = cv2.getStructuringElement(cv2.MORPH_CROSS,(3,3))
done = False
while( not done):
eroded = cv2.erode(img,element)
temp = cv2.dilate(eroded,element)
temp = cv2.subtract(img,temp)
skel = cv2.bitwise_or(skel,temp)
img = eroded.copy()
zeros = size - cv2.countNonZero(img)
if zeros==size:
done = True
return skel
def gamma_correction(img, correction):
img = img/255.0
img = cv2.pow(img, correction)
return np.uint8(img*255)
def auto_canny(image, sigma=0.33):
# compute the median of the single channel pixel intensities
v = np.median(image)
# apply automatic Canny edge detection using the computed median
lower = int(max(0, (1.0 - sigma) * v))
upper = int(min(255, (1.0 + sigma) * v))
edged = cv2.Canny(image, lower, upper)
# return the edged image
return edged
for file in os.listdir(path):
if file.endswith(".png"):
current = os.path.join(path, file)
img = cv2.imread(current, 0)
print 'processing ' + current
img = autocrop(img, 0)
cv2.imwrite(current + '-0-cropped.jpg', img)
height, width = img.shape[:2]
img = cv2.resize(img, (width, width))
cv2.imwrite(current + '-0-resized.jpg', img)
# cv2.imwrite(current +'-2-auto_canny_default.jpg', auto_canny(img))
# img = cv2.medianBlur(img,5)
# cv2.imwrite(current +'-0-medianBlur.jpg',img)
# th3 = cv2.adaptiveThreshold(img,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY,11,2)
# cv2.imwrite(current +'-1-threshold_gaussian.jpg',th3)
# laplacian = cv2.Laplacian(img,cv2.CV_64F)
# cv2.imwrite(current + '-3-threshold_gaussian.jpg', laplacian)
#img = cv2.bilateralFilter(img, 3, 3, 5)
edges = cv2.Canny(img,10,20,apertureSize = 5)
cv2.imwrite(current +'-1-edges-10-60.jpg',edges)
kernel = np.ones((3,3),np.uint8)
edges = cv2.morphologyEx(edges, cv2.MORPH_CLOSE, kernel)
cv2.imwrite(current +'-1-edgesClosed-10-60.jpg', edges)
edges = 255-edges
cv2.imwrite(current +'-2-edgesClosedInverted-10-60.jpg', edges)
im2, contours, hierarchy = cv2.findContours(edges, cv2.RETR_TREE, cv2.CHAIN_APPROX_NONE)
imgColor = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
maxArea = 0
for cnt in contours:
if maxArea < cv2.contourArea(cnt):
maxArea = cv2.contourArea(cnt)
for cnt in contours:
rect = cv2.minAreaRect(cnt) #I have used min Area rect for better result
width = rect[1][0]
height = rect[1][1]
if cv2.contourArea(cnt) > int(maxArea/2.5) and ( width < height/2 or height < width/2):
cv2.drawContours(imgColor, cnt, -1, (0,255,0), 1)
cv2.imwrite(current+'-5-Contours.jpg',imgColor)
# edges = skeleton(255-edges)
# cv2.imwrite(current +'-2-skeleton.jpg', edges)
# edges = 255-edges
# minLineLength=int(width/4)
# threshold = 20
# maxLineGap = 1
# rho = 1
# lines = cv2.HoughLinesP(image=edges,rho=rho,theta=np.pi/180, threshold=threshold,lines=np.array([]), minLineLength=minLineLength,maxLineGap=maxLineGap)
# if lines is not None:
# a,b,c = lines.shape
# for i in range(a):
# cv2.line(img, (lines[i][0][0], lines[i][0][1]), (lines[i][0][2], lines[i][0][3]), (0, 0, 255), 1, cv2.LINE_AA)
# cv2.line(edges, (lines[i][0][0], lines[i][0][1]), (lines[i][0][2], lines[i][0][3]), (0, 0, 255), 1, cv2.LINE_AA)
# cv2.imwrite(current+'-5-houghlines.jpg',img)
# cv2.imwrite(current+'-6-houghlines.jpg',edges)
# print 'cool'
# else:
# cv2.imwrite(current+'-5-houghlines.jpg',img)
Also, do check following links:
Detection of Continuous, Smooth and Thin Edges in Noisy Images Using Constrained Particle Swarm Optimisation
http://www.imagemagick.org/discourse-server/viewtopic.php?t=14491
http://answers.opencv.org/question/3454/detecting-thick-edges/