Styjun

I'm trying to measure water level in a glass channel using OpenCV and Python. I've decided to use HaughLines in a selected ROI and find the midpoints of the said lines so I can calculate the difference between the ones that I want and multiply it with a set reference size that I'll get later on. So far the part where I find the lines look like this:

import cv2
import numpy as np

def midpoint(ptA, ptB, ptC, ptD):
 return ((ptA + ptC) * 0.5, (ptB + ptD) * 0.5)

img = cv2.imread("b2924.JPG")
img = cv2.resize(img, None, fx=3/10, fy=3/10)

r = cv2.selectROI("main", img, False, False)
cropped = img[r[1]:(r[1]+r[3]), r[0]:(r[0]+r[2])]
cv2.destroyWindow("main")

imgray = cv2.cvtColor(cropped, cv2.COLOR_BGR2GRAY)
edges = cv2.Canny(imgray, 35, 75)

lines = cv2.HoughLinesP(edges, 1, np.pi/180, 75, maxLineGap=1000)

midPoint = []

for line in lines:
 x1, y1, x2, y2 = line[0]
 cv2.line(cropped, (x1, y1), (x2, y2), (0, 0, 255), 1)
 mP = midpoint(x1, y1, x2, y2)
 midPoint.append(mP)

midPoint.sort(key = lambda x: x[1])

img[r[1]:(r[1]+r[3]), r[0]:(r[0]+r[2])] = cropped

print(lines)
print(midPoint)

cv2.imshow("img", img)
cv2.waitKey()
cv2.destroyAllWindows()

Depending on the image and the ROI I select I find inconsistent results. Image examples and where I select the ROIs:

Note that base of the channel starts where the duct tape reaches. It looks like I can almost never find that exact line because how noisy it is at the base. Right now these threshold values with no morphology seem to give the better results. I tried to use sobel derivative aswell instead of canny but got worse results.

Is it even possible to get exact measurements in this enviroment? Is it a matter of coding or changing the way I take the pictures or both? In the future I will possibly need to map the water profile during heavy turbulance, should I simply move away from OpenCV for that, since the noise is too much? Any help is appreciated.

I would not invest in any image processing with that setup.

If you insist on image processing (if you are only interested in the level at a few positions you might be better off using conventional level sensors)

Add LED panels or any other kind of homogeneous background illumination to the back of the basin. Add dye to the water to get some contrast.
Get rid of the window reflections. Clean the glass.
Alternatively make the background dark and add something to the water that makes it stray light or fluorescent.

You could also add stuff that floats on the surface and is either retroreflective or self-illuminated. That way you would get a bright surface level indicator that is easily detected in an image.