colour contour on grayscale image

Question

I made a simple code to find the contour of an object in an image.

    img = cv2.imread(file_path, cv2.IMREAD_GRAYSCALE)

    blur = cv2.GaussianBlur(img_enhanced,(25,25),0) # apply blur for contour
    ret, binary = cv2.threshold(blur,1,255,cv2.THRESH_BINARY) # apply threshold to blur image

    contours, hierarchy = cv2.findContours(binary, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE) # find countour
    obj_index = contours.index(max(contours, key=len)) # find index of largest object
    contour_img = cv2.drawContours(img, contours, obj_index, (0,255,0), 3) # draw coutour on original image

    plt.imshow(contour_img)
    plt.show()

The original image is already grayscale but anyway I applied cv2.IMREAD_GRAYSCALE when I import the image.

And I thought if I apply the grayscale image when I 'Draw' contour, with below syntax,

contour_img = cv2.drawContours(img, contours, obj_index, (0,255,0), 3)

I can have a grayscale image with colored contour, but it shows weird colored image.

How can I draw a colored contour line on the grayscale image?

Thanks in advance

image sample: black background and white object

Answer 1

Based on my comment:

Try something like this and tell me if it works or not

Edit: I have changed the threshold range. cv2.threshold(blur,25,255,cv2.THRESH_BINARY)

img = cv2.imread(file_path, 1) 
print(img.shape) # this should give you (img_h, img_w, 3)
img2 = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

blur = cv2.GaussianBlur(img2,(25,25),0) # apply blur for contour
ret, binary = cv2.threshold(blur,25,255,cv2.THRESH_BINARY) # apply threshold to blur image

contours, hierarchy = cv2.findContours(binary, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE) # find countour
obj_index = contours.index(max(contours, key=len)) # find index of largest object
contour_img = cv2.drawContours(img, contours, obj_index, (0,255,0), 3) # draw coutour on original image

plt.imshow(contour_img, cmap='gray')
plt.show()

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plt.hist(blur.ravel(), bins=50)

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