How to find Angle of Different Edges in an Image-CodePudding

I have an image where there are some star streaks, with the help of edge detection and non max suppression, I am able to convert it into grayscale Image with Edges. I need to get the angle w.r.t. the x-axis at which the streaks are pointing. These are my Images

From this Image I generate the Image containing edges. (The orientation is different because the ds9 software for viewing astronomical images plot it with lower origin coordinates)

I have tried a number of ways including PCA and Connected components but the results are not consistent. All I want is the mean angle at which the streaks are oriented with minimum possible error. I'd be grateful if anyone can help me with this

CodePudding user response：

Imho one option could be convert each streak into a 2d vector, and calculate the average angular coefficient

For getting the lines I suggest to look into the Hough lines method https://docs.opencv.org/3.4/d9/db0/tutorial_hough_lines.html

With some image cleanup and tuning a bit the function parameters(especially min lenght and max gap values), you should be able to get those vectors

Then, calculate for each vector the inclination (y2-y1) / (x2-x1), take the average and here you go

CodePudding user response：

Here's a possibility with following method:

threshold to black and white
find contours
iterate over contours fitting rotated rectangle to each
discard small contours
print angle of rotation of rectangle

#!/usr/bin/env python3

import cv2

# Load image as greyscale
im = cv2.imread('trails.png', cv2.IMREAD_GRAYSCALE)

# Threshold for just the brightest things
_, thr = cv2.threshold(im, 192,255,type=cv2.THRESH_BINARY)

# Find contours
contours,hierarchy = cv2.findContours(thr, 1, 2)

# Iterate over contours
for cnt in contours:
    # Get area of blob
    area = cv2.contourArea(cnt)
    # Only work with decent size blobs - ignore smallest ones
    if area > 8:
        center, size, angle = cv2.minAreaRect(cnt)
        print(f'Area: {area}, angle: {angle}')

I get these results:

Area: 10.5, angle: 11.309932708740234
Area: 11.5, angle: 18.43494987487793
Area: 12.0, angle: 14.03624439239502
Area: 74.5, angle: 9.462322235107422
Area: 9.5, angle: 14.03624439239502
Area: 35.0, angle: 11.309932708740234
Area: 30.5, angle: 18.434947967529297
Area: 11.5, angle: 18.43494987487793
Area: 16.0, angle: 18.43494987487793
Area: 9.0, angle: 11.309932708740234
Area: 27.0, angle: 26.56505012512207
Area: 15.5, angle: 21.801410675048828
Area: 9.0, angle: 14.03624439239502
Area: 10.5, angle: 11.309932708740234
Area: 16.0, angle: 18.43494987487793
Area: 32.0, angle: 14.03624439239502
Area: 14.0, angle: 14.03624439239502
Area: 46.0, angle: 11.309932708740234
Area: 25.0, angle: 15.945395469665527
Area: 14.0, angle: 14.03624439239502
Area: 18.0, angle: 90.0
Area: 24.5, angle: 11.309932708740234
Area: 10.0, angle: 14.03624439239502
Area: 36.0, angle: 9.462322235107422
Area: 16.5, angle: 18.43494987487793
Area: 23.5, angle: 11.309932708740234
Area: 11.0, angle: 18.43494987487793
Area: 11.5, angle: 14.03624439239502
Area: 16.5, angle: 14.03624439239502
Area: 12.0, angle: 14.03624439239502
Area: 18.0, angle: 18.43494987487793
Area: 11.5, angle: 14.03624439239502
Area: 23.5, angle: 11.309932708740234
Area: 14.0, angle: 14.03624439239502
Area: 28.0, angle: 11.309932708740234
Area: 18.0, angle: 18.434947967529297
Area: 16.0, angle: 15.945395469665527
Area: 9.0, angle: 15.945395469665527
Area: 10.0, angle: 14.03624439239502
Area: 10.5, angle: 14.03624439239502
Area: 11.0, angle: 14.03624439239502
Area: 8.5, angle: 14.03624439239502
Area: 15.0, angle: 18.43494987487793
Area: 19.0, angle: 14.03624439239502
Area: 20.5, angle: 14.03624439239502
Area: 37.5, angle: 90.0
Area: 34.5, angle: 90.0
Area: 11.5, angle: 14.03624439239502
Area: 27.0, angle: 14.03624439239502
Area: 22.5, angle: 90.0
Area: 8.5, angle: 18.434947967529297
Area: 13.0, angle: 14.03624439239502
Area: 10.0, angle: 15.945394515991211
Area: 12.0, angle: 14.03624439239502
Area: 25.0, angle: 14.03624439239502
Area: 16.0, angle: 90.0
Area: 9.0, angle: 14.03624439239502

I guess you could further refine the assumptions and parameters, and smooth or statistically analyse the angles further - but they seem to average around 14 degrees

I just guessed a threshold of 192 to get something sensible. Tune this.

I just plucked a minimum area of 8 from thin air so that I am only looking at decent size blobs, and disregarding tiny, direction-less blobs. Tune this.