Hacktoberfest 2020: Added computer vision algorithm (#2946)

* Create meanthresholding.py

* Rename meanthresholding.py to meanthreshold.py

* Update meanthreshold.py

* Update computer_vision/meanthreshold.py

Verified this part works, thanks.
Co-authored-by: NChristian Clauss <cclauss@me.com>

* Update computer_vision/meanthreshold.py
Co-authored-by: NChristian Clauss <cclauss@me.com>
Co-authored-by: NChristian Clauss <cclauss@me.com>

computer_vision/meanthreshold.py

+from PIL import Image
+
+"""
+Mean thresholding algorithm for image processing
+https://en.wikipedia.org/wiki/Thresholding_(image_processing)
+"""
+
+
+def mean_threshold(image: Image) -> Image:
+    """
+    image: is a grayscale PIL image object
+    """
+    height, width = image.size
+    mean = 0
+    pixels = image.load()
+    for i in range(width):
+        for j in range(height):
+            pixel = pixels[j, i]
+            mean += pixel
+    mean //= width * height
+
+    for j in range(width):
+        for i in range(height):
+            pixels[i, j] = 255 if pixels[i, j] > mean else 0
+    return image
+
+
+if __name__ == "__main__":
+    image = mean_threshold(Image.open("path_to_image").convert("L"))
+    image.save("output_image_path")