Add doctests to convert_to_negative

Contributes to #9943

Author: adrian-cancio

digital_image_processing/convert_to_negative.py

@@ -2,10 +2,32 @@
 Implemented an algorithm using opencv to convert a colored image into its negative
 """
 
+import numpy as np
 from cv2 import destroyAllWindows, imread, imshow, waitKey
 
 
 def convert_to_negative(img):
+    """
+    Convert an image to its negative.
+
+    Args:
+        img: NumPy array representing the image (BGR format)
+
+    Returns:
+        NumPy array with inverted colors
+
+    >>> import numpy as np
+    >>> img = np.array([[[100, 150, 200]]], dtype=np.uint8)
+    >>> result = convert_to_negative(img.copy())
+    >>> result[0][0].tolist()
+    [155, 105, 55]
+    >>> img2 = np.array([[[0, 0, 0]], [[255, 255, 255]]], dtype=np.uint8)
+    >>> neg = convert_to_negative(img2.copy())
+    >>> neg[0][0].tolist()
+    [255, 255, 255]
+    >>> neg[1][0].tolist()
+    [0, 0, 0]
+    """
     # getting number of pixels in the image
     pixel_h, pixel_v = img.shape[0], img.shape[1]