Drawing semi-transparent polygons in PIL

Question

How do you draw semi-transparent polygons using the Python Imaging Library?

Answer 1

Can you draw the polygon on a separate RGBA image then use the Image.paste(image, box, mask) method?

Edit: This works.

from PIL import Image
from PIL import ImageDraw
back = Image.new('RGBA', (512,512), (255,0,0,0))
poly = Image.new('RGBA', (512,512))
pdraw = ImageDraw.Draw(poly)
pdraw.polygon([(128,128),(384,384),(128,384),(384,128)],
              fill=(255,255,255,127),outline=(255,255,255,255))
back.paste(poly,mask=poly)
back.show()

http://effbot.org/imagingbook/image.htm#image-paste-method

Answer 2

Using the Image.paste(image, box, mask) method will convert the alpha channel in the pasted area of the background image into the corresponding transparency value of the polygon image.

The Image.alpha_composite(im1,im2) method utilizes the alpha channel of the "pasted" image, and will not turn the background transparent. However, this method again needs two equally sized images.