pyFollowLineByLongestPath

# Script to find a white line, and draw a red blob that follows the line
# Mike Kerry Jan 2022. Version 2 - follow the direction with longest path available
import cv2
import numpy as np

SPOTSIZE = 8
BLOCKSIZE = 5

def get_block_avg(y, x, dim):
    block = matrix[y: y + dim, x: x + dim]      # Extract a block of integers from matrix
    return block.sum() // (dim * dim)           # Find the average whiteness of the block

def draw_red(y, x):
    matrix[y][x] = -666         # Block the current pixel from any revisit
    # Draw a red spot of the desired size. y, x is the centre.
    y -= 2
    x -= 2
    for dy in range(0, SPOTSIZE):
        for dx in range(0, SPOTSIZE):
            img[y + dy][x + dx] = (0, 0, 255)       # make the image pixels red

def find_start():
    # Scan down and right looking for a block of pixels that are basically white
    for y in range(0, height - BLOCKSIZE, 3):
        for x in range(0, width - BLOCKSIZE, 3):
            v = get_block_avg(y, x, 5)
            if v > 650:             # 650 was found empirically to be a good value to suggest white
                return y, x         # Return the coords of the top left of a white block
    return -1, -1

orig = cv2.imread('pyFollowLineMed.jpg')
img = orig.copy()
dims = img.shape
height, width = dims[0], dims[1]
matrix = img.sum(axis=2, dtype=np.int16)      # Reduce number of axes (?) by summing BGR

ypos, xpos = find_start()           # Get the coordinates of the first block of white pixels found

# Define y and x increments per compass direction, and permitted next directions
#           NW      N         NE      E        SE        S        SW       W
dirs =  [(-1,-1),  (-1,0),  (-1,1),  (0,1),   (1,1),   (1,0),   (1,-1),  (0,-1)]
allows = [(0,1,7), (0,1,2), (1,2,3), (2,3,4), (3,4,5), (4,5,6), (5,6,7), (0,6,7)]

allownow = (0,1,2,3,4,5,6,7)  # Initially, allow movement in any direction. Thereafter 3 directions

# Try to follow the line without retracing
while True:
    cv2.imshow("img", img)
    cv2.waitKey(10)             # pause for effect
    # Look in all permitted directions for the longest white path available
    bestpathlen, bestidx = 0, 0

    for diridx, dr in enumerate(dirs):
        if diridx not in allownow:
            continue
        y, x = ypos, xpos
        ctr = 0
        while True:
            y += dr[0]                  # Adjust y and x for the current direction of travel
            x += dr[1]
            if matrix[y][x] < 650:
                break                   # end of path in current direction
            ctr += 1                    # count white path length
        if ctr > bestpathlen:
            bestpathlen = ctr
            bestidx = diridx            # remember the best direction so far

    bestdir = dirs[bestidx]
    ypos += bestdir[0]                  # move by one pixel in best direction
    xpos += bestdir[1]
    allownow = allows[bestidx]

    img = orig.copy()           # get the original image again (without any red spots)
    draw_red(ypos, xpos)        # draw the new red spot and block the return path
    if bestpathlen < 4:         # 4 was found empirically to be a good value to end the loop
        break                   # No good path found. We have reached the end of the line

cv2.waitKey(0)