pyChangeBalloonColour

# Given a JPG image of coloured balloons, find the yellow balloon and change it to black
import cv2

img = cv2.imread("Balloons.jpg")
cv2.imshow("img", img)      # Show the original image
cv2.waitKey(0)              # Wait for a key press

yellow = (0, 255, 255)       # Colour code of Yellow, in Blue, Green, Red sequence (as per CV2)

for y, line in enumerate(img):          # y is line counter
    for x, bgr in enumerate(line):      # x is horizontal pixel counter
        # find a pixel that is yellowish by computing a numerical difference from yellow
        diffred = abs(yellow[2] - bgr[2])
        diffgrn = abs(yellow[1] - bgr[1])
        diffblu = abs(yellow[0] - bgr[0])
        diff = diffred + diffgrn + diffblu
        if diff < 180:                   # 180 was found empirically, to be a value that worked well
            img[y][x] = (0, 0, 0)        # Set this pixel to black

cv2.imshow("img", img)                  # Show the modified image
cv2.waitKey(0)                          # Wait for a key press