如何使用PIL确定ICO图像的透明颜色索引？

Question

具体来说，这是从.ico文件中提取的，因此没有像你在gif中获取的"透明" "信息" 属性。下面的示例演示了如何将Yahoo!的favicon转换为png，并使用正确的透明度索引"0"，我猜测得到的。如何检测ico是否实际上是透明的，并且透明度索引为0？

import urllib2
import Image
import StringIO

resp = urllib2.urlopen("http://www.yahoo.com/favicon.ico")
image = Image.open(StringIO.StringIO(resp.read()))

f = file("test.png", "w")

# I guessed that the transparent index is 0.  how to
# determine it correctly ?
image.save(f, "PNG", quality=95, transparency=0)

- zzzeek

---

好问题。+1。刚看到你的推文。可以踩（但要明智地选择）;) - VonC

Answer 1

看起来有人意识到 PIL 不能正确地读取 ICO（在研究 ICO 格式时，我可以从其源代码中发现相同的问题 - 它有一个确定透明度的 AND 位图），并提出了这个扩展：

http://www.djangosnippets.org/snippets/1287/

由于这对非 Django 应用程序非常有用，因此我在此处重新发布它，并对其异常抛出进行了一些微调：

import operator
import struct

from PIL import BmpImagePlugin, PngImagePlugin, Image


def load_icon(file, index=None):
    '''
    Load Windows ICO image.

    See http://en.wikipedia.org/w/index.php?oldid=264332061 for file format
    description.
    '''
    if isinstance(file, basestring):
        file = open(file, 'rb')

    try:
        header = struct.unpack('<3H', file.read(6))
    except:
        raise IOError('Not an ICO file')

    # Check magic
    if header[:2] != (0, 1):
        raise IOError('Not an ICO file')

    # Collect icon directories
    directories = []
    for i in xrange(header[2]):
        directory = list(struct.unpack('<4B2H2I', file.read(16)))
        for j in xrange(3):
            if not directory[j]:
                directory[j] = 256

        directories.append(directory)

    if index is None:
        # Select best icon
        directory = max(directories, key=operator.itemgetter(slice(0, 3)))
    else:
        directory = directories[index]

    # Seek to the bitmap data
    file.seek(directory[7])

    prefix = file.read(16)
    file.seek(-16, 1)

    if PngImagePlugin._accept(prefix):
        # Windows Vista icon with PNG inside
        image = PngImagePlugin.PngImageFile(file)
    else:
        # Load XOR bitmap
        image = BmpImagePlugin.DibImageFile(file)
        if image.mode == 'RGBA':
            # Windows XP 32-bit color depth icon without AND bitmap
            pass
        else:
            # Patch up the bitmap height
            image.size = image.size[0], image.size[1] >> 1
            d, e, o, a = image.tile[0]
            image.tile[0] = d, (0, 0) + image.size, o, a

            # Calculate AND bitmap dimensions. See
            # http://en.wikipedia.org/w/index.php?oldid=264236948#Pixel_storage
            # for description
            offset = o + a[1] * image.size[1]
            stride = ((image.size[0] + 31) >> 5) << 2
            size = stride * image.size[1]

            # Load AND bitmap
            file.seek(offset)
            string = file.read(size)
            mask = Image.fromstring('1', image.size, string, 'raw',
                                    ('1;I', stride, -1))

            image = image.convert('RGBA')
            image.putalpha(mask)

    return image

- zzzeek