pyFollowWhiteLine

# Script to find a white line, and draw a red blob that follows the line
# Mike Kerry Jan 2022
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):
    # Draw a red spot of the desired size. y, x is the top left corner.
    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

    # block the return path by setting already visited matrix positions negative
    for dy in range(0, BLOCKSIZE):
        for dx in range(0, BLOCKSIZE):
            matrix[y + dy][x + dx] = -666       # make that point blacker than black!

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

# Try and follow the line without retracing
while True:
    cv2.imshow("img", img)
    cv2.waitKey(30)             # pause for effect
    # scan around the current location, in 8 directions, looking for the whitest adjacent block of pixels
    best, besty, bestx = 0, 0, 0
    for incy in range(-BLOCKSIZE, BLOCKSIZE+1, BLOCKSIZE):                # y becomes -5, 0, 5
        for incx in range(-BLOCKSIZE, BLOCKSIZE+1, BLOCKSIZE):            # x becomes -5, 0, 5
            y2 = ypos + incy
            x2 = xpos + incx
            v = get_block_avg(y2, x2, BLOCKSIZE)
            if v > best:
                best, besty, bestx = v, y2, x2      # save the coords at top left of white block

    ypos, xpos = besty, bestx   # top left of the whitest adjacent block of pixels
    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 best < 670:          # 670 was found empirically to be a good value to end the loop
        break               # No adjacent white block found. We have reached the end of the line

cv2.waitKey(0)