PIL Drawing a semi-transparent square overlay on i

问题:

from PIL import Image
from PIL import ImageDraw 
from io import BytesIO
from urllib.request import urlopen

url = "https://i.ytimg.com/vi/W4qijIdAPZA/maxresdefault.jpg"
file = BytesIO(urlopen(url).read())
img = Image.open(file)
img = img.convert("RGBA")
draw = ImageDraw.Draw(img, "RGBA")
draw.rectangle(((0, 00), (img.size[0], img.size[1])), fill=(0,0,0,127))
img.save('dark-cat.jpg')

This is giving me a giant black square. I want it to be a semi transparent black square with a cat. Any Ideas?

回答1:

Sorry, apparently it's not a bug, as I originally thought.

You can do it by creating a temporary image and using Image.alpha_composite() as shown below:

from PIL import Image
from PIL import ImageDraw
from io import BytesIO
from urllib.request import urlopen

url = "https://i.ytimg.com/vi/W4qijIdAPZA/maxresdefault.jpg"
file = BytesIO(urlopen(url).read())
img = Image.open(file)
img = img.convert("RGBA")

# Make a blank image for the rectangle, initialized to a completely
# transparent color.
tmp = Image.new('RGBA', img.size, (0,0,0,0))

# Create a drawing context for it.
draw = ImageDraw.Draw(tmp)

# Determine the bounding box of the largest possible semi-transparent
# square rectangle centered on the temporary image and draw it.
if img.size[0] > img.size[1]:
    size = img.size[1]
    llx, lly = (img.size[0] - img.size[1]) // 2, 0
else:
    size = img.size[0]
    llx, lly = 0, (img.size[1] - img.size[0]) // 2

# Add one to upper point because second point is just outside the drawn
# rectangle.
urx, ury = llx + size + 1, lly + size + 1
draw.rectangle(((llx, lly), (urx, ury)), fill=(0,0,0,127))

# Alpha composite the two images together.
img = Image.alpha_composite(img, tmp)
img = img.convert("RGB") # Remove alpha for saving in jpg format.
img.save('dark-cat.jpg')
img.show()

Here's the result with your test image:

回答2:

If you just want to dim the entire image, there's a simpler way:

img = Image.eval(img, lambda x: x/2)