how to determine the transparent color index of ICO image with PIL?

问题

Specifically, this is from an .ico file, so there is no "transparent" "info" attribute like you would get in a gif. The below example illustrates converting Yahoo!'s favicon to a png using the correct transparency index of "0", which I guessed. how to detect that the ico is in fact transparent and that the transparency index is 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)

回答1:

looks like someone recognized that PIL doesn't really read ICO correctly (I can see the same thing after reconciling its source code with some research on the ICO format - there is an AND bitmap which determines transparency) and came up with this extension:

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

since this is useful for non-django applications, I've reposted here with a few tweaks to its exception throws:

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

来源：https://stackoverflow.com/questions/987916/how-to-determine-the-transparent-color-index-of-ico-image-with-pil

标签

png

transparency

python-imaging-library

ico