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

Question

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)

Answer 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