Drawing semi-transparent polygons in PIL

How 开发者_如何转开发do you draw semi-transparent polygons using the Python Imaging Library?

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

I think @Nick T's answer is good, but you need to be careful when using his code as written with a very large background image, especially in the case that you may be annotating several polygons on said image. This is something I do when processing huge satellite images with some object detection code and annotating the detections using a transparent rectangle. To make the code efficient no matter the size of the background image, I make the following suggestion.

I would modify the solution to specify that the polygon image that you will paste be only as large as required to hold the polygon, not the same size as the back image. The coordinates of the polygon are specified with respect to the local bounding box, not the global image coordinates. Then you paste the polygon image at the offset in the larger background image.

import Image
import ImageDraw
img_size = (512,512)
poly_size = (256,256)
poly_offset = (128,128) #location in larger image
back = Image.new('RGBA', img_size, (255,0,0,0) )
poly = Image.new('RGBA', poly_size )
pdraw = ImageDraw.Draw(poly)
pdraw.polygon([ (0,0), (256,256), (0,256), (256,0)], 
              fill=(255,255,255,127), outline=(255,255,255,255))
back.paste(poly, poly_offset, mask=poly)
back.show()

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.