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[英]How to compose an image with an alpha channel over another image(RGB, without alpha channel)? Using python PIL
[英]With the Python Imaging Library (PIL), how does one compose an image with an alpha channel over another image?
我有兩個圖像,都有alpha通道。 我想將一個圖像放在另一個圖像上,從而生成帶有alpha通道的新圖像,就像它們在圖層中呈現一樣。 我想用Python Imaging Library做到這一點,但是其他系統中的建議會很棒,即使原始數學也是一個好處; 我可以使用NumPy。
這似乎可以解決問題:
from PIL import Image
bottom = Image.open("a.png")
top = Image.open("b.png")
r, g, b, a = top.split()
top = Image.merge("RGB", (r, g, b))
mask = Image.merge("L", (a,))
bottom.paste(top, (0, 0), mask)
bottom.save("over.png")
Pillow 2.0現在包含一個執行此操作的alpha_composite
函數。
img3 = Image.alpha_composite(img1, img2)
我在PIL中找不到alpha復合函數,所以這是我用numpy實現它的嘗試:
import numpy as np
from PIL import Image
def alpha_composite(src, dst):
'''
Return the alpha composite of src and dst.
Parameters:
src -- PIL RGBA Image object
dst -- PIL RGBA Image object
The algorithm comes from http://en.wikipedia.org/wiki/Alpha_compositing
'''
# http://stackoverflow.com/a/3375291/190597
# http://stackoverflow.com/a/9166671/190597
src = np.asarray(src)
dst = np.asarray(dst)
out = np.empty(src.shape, dtype = 'float')
alpha = np.index_exp[:, :, 3:]
rgb = np.index_exp[:, :, :3]
src_a = src[alpha]/255.0
dst_a = dst[alpha]/255.0
out[alpha] = src_a+dst_a*(1-src_a)
old_setting = np.seterr(invalid = 'ignore')
out[rgb] = (src[rgb]*src_a + dst[rgb]*dst_a*(1-src_a))/out[alpha]
np.seterr(**old_setting)
out[alpha] *= 255
np.clip(out,0,255)
# astype('uint8') maps np.nan (and np.inf) to 0
out = out.astype('uint8')
out = Image.fromarray(out, 'RGBA')
return out
例如,鑒於這兩個圖像,
img1 = Image.new('RGBA', size = (100, 100), color = (255, 0, 0, 255))
draw = ImageDraw.Draw(img1)
draw.rectangle((33, 0, 66, 100), fill = (255, 0, 0, 128))
draw.rectangle((67, 0, 100, 100), fill = (255, 0, 0, 0))
img1.save('/tmp/img1.png')
img2 = Image.new('RGBA', size = (100, 100), color = (0, 255, 0, 255))
draw = ImageDraw.Draw(img2)
draw.rectangle((0, 33, 100, 66), fill = (0, 255, 0, 128))
draw.rectangle((0, 67, 100, 100), fill = (0, 255, 0, 0))
img2.save('/tmp/img2.png')
alpha_composite
產生:
img3 = alpha_composite(img1, img2)
img3.save('/tmp/img3.png')
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