How to find lines in a 2d numpy array?-CodePudding

I have a 2D numpy array and I want to find boundary points of both horizontal and vertical lines.

gray_img = np.array([[0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 255, 255, 255, 255, 255, 0],
                     [0, 255, 255, 255, 255, 255, 255, 255, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 255, 255, 255, 255],
                     [0, 255, 255, 0, 0, 255, 255, 255, 255],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0]])

desired_outcome = [ [[1,1],[10,1]],
                    [[1,2],[10,2]],
                    [[1,3],[2,3]], ...]

Here are the lines I want to find:

Later, I want to remove the smaller lines to keep only those with more than 2 points distance.

CodePudding user response：

Here's an algorithm for horizontal lines based on prefix sums:

# Do a prefix sum to get the lengths of each line.
gray_cumsum = np.cumsum(gray_img / 255, axis=1)
gray_cumsum[:, 1:] = gray_cumsum[:, 1:] * (gray_cumsum[:, 1:] != gray_cumsum[:, :-1])
# Reindex all the points so each line starts at 1.
start_num = gray_cumsum.copy()
a = start_num[:,1:-1] != 0
b = start_num[:,:-2] == 0
c = start_num[:,2:] != 0
start_num[:,1:-1] = start_num[:,1:-1] * np.logical_and(np.logical_and(a, b), c)
start_num[:, -1] = 0
start_num = np.maximum.accumulate(start_num, axis=1)
gray_cumsum = np.maximum(gray_cumsum - start_num, 0)
# Detect only the ends of each line.
gray_cumsum[:,:-1] = gray_cumsum[:,:-1] * (gray_cumsum[:,1:] == 0)
# Get the starting and endings points of each line.
end_points = np.stack(gray_cumsum.nonzero(), axis=-1)
lengths = gray_cumsum[gray_cumsum.nonzero()]
start_points = end_points.copy()
start_points[:, 1] = start_points[:, 1] - lengths
print(start_points)
print(end_points)

Just change the indexing to get vertical lines. You can use the lengths array to filter out which lines you want.

CodePudding user response：

Is this what you want?

import pandas as pd
import numpy as np

a = np.array([[0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 255, 255, 255, 255, 255, 0],
                     [0, 255, 255, 255, 255, 255, 255, 255, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 255, 255, 255, 255],
                     [0, 255, 255, 0, 0, 255, 255, 255, 255],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 255, 255, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0],
                     [0, 0, 0, 0, 0, 0, 0, 0, 0]])

a = a[a != 0]
list = a.tolist()
print(list)