blocks|key|1374277|text|img.info是关于整个图像的--+RGBA图像中的alpha值是以像素为单位的，所以它当然不会在img.info中。给定一个坐标作为参数，图像对象的getpixel方法将返回一个元组，其中包含该像素的(在本例中为四个)波段的值--元组的最后一个值将是A，即Alpha值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1374278|entityMap^0|0|8|1E|8|24|8|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

The <code>img.info</code> is about the image as a whole -- the alpha-value in an RGBA image is per-pixel, so of course it won't be in <code>img.info</code>. The <code>getpixel</code> method of the image object, given a coordinate as argument, returns a tuple with the values of the (four, in this case) bands for that pixel -- the tuple's last value will then be A, the alpha value.

blocks|key|1374286|text|#+python+2.6%2B

import+operator,+itertools

def+get_alpha_channel(image):
++++"Return+the+alpha+channel+as+a+sequence+of+values"

++++#+first,+which+band+is+the+alpha+channel?
++++try:
++++++++alpha_index=+image.getbands().index('A')
++++except+ValueError:
++++++++return+None+#+no+alpha+channel,+presumably

++++alpha_getter=+operator.itemgetter(alpha_index)
++++return+itertools.imap(alpha_getter,+image.getdata())|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1374287|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code># python 2.6+

import operator, itertools

def get_alpha_channel(image):
 "Return the alpha channel as a sequence of values"

 # first, which band is the alpha channel?
 try:
 alpha_index= image.getbands().index('A')
 except ValueError:
 return None # no alpha channel, presumably

 alpha_getter= operator.itemgetter(alpha_index)
 return itertools.imap(alpha_getter, image.getdata())
</code></pre>

blocks|key|1159100|text|要获得RGBA图像的alpha层，您需要做的全部工作是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1159101|red,+green,+blue,+alpha+=+img.split()|code-block|syntax|javascript|1159102|或|1159103|alpha+=+img.split()[-1]|1159104|有一种方法可以设置alpha层：|1159105|img.putalpha(alpha)|1159106|透明键仅用于在调色板模式(P)中定义透明度索引。如果您还想涵盖调色板模式的透明度情况，并涵盖所有情况，可以这样做|1159107|if+img.mode+in+('RGBA',+'LA')+or+(img.mode+==+'P'+and+'transparency'+in+img.info):
++++alpha+=+img.convert('RGBA').split()[-1]|1159108|注意:当image.mode为LA时，需要使用convert方法，因为在PIL中有一个bug。|1159109|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|14|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

To get the alpha layer of an RGBA image all you need to do is:

<pre><code>red, green, blue, alpha = img.split()
</code></pre>

or

<pre><code>alpha = img.split()[-1]
</code></pre>

And there is a method to set the alpha layer:

<pre><code>img.putalpha(alpha)
</code></pre>

The transparency key is only used to define the transparency index in the palette mode (P). If you want to cover the palette mode transparency case as well and cover all cases you could do this

<pre><code>if img.mode in ('RGBA', 'LA') or (img.mode == 'P' and 'transparency' in img.info):
 alpha = img.convert('RGBA').split()[-1]
</code></pre>

Note: The convert method is needed when the image.mode is LA, because of a bug in PIL.

blocks|key|1159174|text|你可以通过'A‘模式将图像转换为字符串，一次性地从整个图像中获取alpha数据。例如，这个示例从图像中获取alpha数据，并将其保存为灰度图像:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1159175|from+PIL+import+Image

imFile="white-arrow.png"
im+=+Image.open(imFile,+'r')
print+im.mode+==+'RGBA'

rgbData+=+im.tostring("raw",+"RGB")
print+len(rgbData)
alphaData+=+im.tostring("raw",+"A")
print+len(alphaData)

alphaImage+=+Image.fromstring("L",+im.size,+alphaData)
alphaImage.save(imFile%2B".alpha.png")|code-block|syntax|javascript|1159176|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can get the alpha data out of whole image in one go by converting image to string with 'A' mode e.g this example get alpha data out of image and saves it as grey scale image :)

<pre><code>from PIL import Image

imFile="white-arrow.png"
im = Image.open(imFile, 'r')
print im.mode == 'RGBA'

rgbData = im.tostring("raw", "RGB")
print len(rgbData)
alphaData = im.tostring("raw", "A")
print len(alphaData)

alphaImage = Image.fromstring("L", im.size, alphaData)
alphaImage.save(imFile+".alpha.png")
</code></pre>

blocks|key|1374466|text|我试过这个：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1374467|from+PIL+import+Image
import+operator,+itertools

def+get_alpha_channel(image):+
+++try:+
++++++alpha_index+=+image.getbands().index('A')
+++except+ValueError:
++++++#+no+alpha+channel,+so+convert+to+RGBA
++++++image+=+image.convert('RGBA')
++++++alpha_index+=+image.getbands().index('A')
+++alpha_getter+=+operator.itemgetter(alpha_index)
+++return+itertools.imap(alpha_getter,+image.getdata())|code-block|syntax|javascript|1374468|这返回了我所期望的结果。然而，我做了一些计算来确定平均值和标准差，结果与imagemagick的fx:mean函数略有不同。|offset|length|style|CODE|1374469|也许转换改变了一些值？我不确定，但它似乎相对微不足道。|1374470|entityMap^0|0|0|1C|7|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

I tried this:

<pre><code>from PIL import Image
import operator, itertools

def get_alpha_channel(image): 
 try: 
 alpha_index = image.getbands().index('A')
 except ValueError:
 # no alpha channel, so convert to RGBA
 image = image.convert('RGBA')
 alpha_index = image.getbands().index('A')
 alpha_getter = operator.itemgetter(alpha_index)
 return itertools.imap(alpha_getter, image.getdata())
</code></pre>

This returned the result that I was expecting. However, I did some calculation to determine the mean and standard deviation, and the results came out slightly different from imagemagick's <code>fx:mean</code> function. 

Perhaps the conversion changed some of the values? I'm unsure, but it seems relatively trivial.

How to detect if a PNG image has transparent alpha channel or not using PIL?

<pre><code>img = Image.open('example.png', 'r')
has_alpha = img.mode == 'RGBA'
</code></pre>

With above code we know whether a PNG image has alpha channel not not but how to get the alpha value?

I didn't find a 'transparency' key in img.info dictionary as described at <a href="http://www.pythonware.com/library/pil/handbook/format-png.htm" rel="noreferrer">PIL's website</a>

I'm using Ubuntu and zlib1g, zlibc packages are already installed.

How to get alpha value of a PNG image with PIL?

Ubuntu

如何使用PIL检测PNG图像是否有透明的alpha通道？img = Image.open('example.png', 'r')has_alpha = img.mode == 'RGBA'通过上面的代码，我们知道PNG图像是否有alpha通道，不是没有，但如何获得alpha值？我在img.info字典中找不到在中描述的...

问如何用PIL获取PNG图像的Alpha值？
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