Why is the whole picture turning red?

Question

We’ve been trying to fix this program for hours, yet nothing seems to work, we just can’t figure out what the problem is. It is supposed to make the whole picture black white, besides the red pixels. (https://i.stack.imgur.com/tXJYE.jpg should look like that afterwards)

Here is the result after the using the program, the whole picture is turning red:

https://i.stack.imgur.com/cfBZZ.jpg

How can I fix this?

from image_helper import *

img = load_rgb_image("ara.jpg")
w, h = img.size
pixels = load_rgb_pixels("ara.jpg")
#img.show()


for i in range(w*h):
    r,g,b = pixels[i]

    new_r = 2*r
    new_g = g // 2
    new_b = b + 10

    pixels[i] = (new_r, new_g, new_b)
         
new_img = new_rgb_image(w, h, pixels)                    
new_img.show()

"Why is the whole picture turning red?", because you're doubling the amount of red in each pixel — Shoejep, Mar 19 '22 at 19:37
Something like if pixel is red then leave alone, else convert to black and white — Shoejep, Mar 19 '22 at 19:41

score 0 · Answer 1 · answered Mar 19 '22 at 22:42

There is an excellent solution implemented in MATLAB.

I was tempting to translate the code from MATLAB to Python (using OpenCV).

Convert the image to HSV color space.
Select "non-red" pixels. In HSV color space, the first channel is the hue - there is a range of hue for pixels that considered to be red.
Set the selected pixel saturation channel to 0. (Pixels with zero saturation are gray).
Convert the image back from HSV to BGR color space.

Here is the Python code (conversation of the original MATLAB code):

import cv2

# The following code is a Python conversion to the following MATLAB code:
# Original MATLAB code: https://stackoverflow.com/questions/4063965/how-can-i-convert-an-rgb-image-to-grayscale-but-keep-one-color
img = cv2.imread('roses.jpg')  # Load image.
hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)  # Convert the image to HSV color space.
h = hsv[:, :, 0]  # Note: in OpenCV hue range is [0,179]) The original MATLAB code is 360.*hsvImage(:, :, 1), when hue range is [0, 1].
s = hsv[:, :, 1]  # Get the saturation plane.
non_red_idx = (h > 20//2) & (h < 340//2)  # Select "non-red" pixels (divide the original MATLAB values by 2 due to the range differences).
s[non_red_idx] = 0   # Set the selected pixel saturations to 0.
hsv[:, :, 1] = s  # Update the saturation plane.
new_img = cv2.cvtColor(hsv, cv2.COLOR_HSV2BGR)  # Convert the image back to BGR space

# Show images for testing:
cv2.imshow('img', img)
cv2.imshow('new_img', new_img)
cv2.waitKey()
cv2.destroyAllWindows()

cv2.imwrite('new_img.jpg', new_img)

Result:

Notes:

For the method used for selecting the color range, refer to the original post.
The reference post has a simpler solution, using simple for loop (with inferior results), that more resembles your code.
Consider using this code as reference.

Why is the whole picture turning red?

1 Answers1

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