--- /dev/null
+#!/usr/bin/env python
+
+# Generate Mac OS X .icns files, or at least the simple subformats
+# that don't involve JPEG encoding and the like.
+#
+# Sources: https://en.wikipedia.org/wiki/Apple_Icon_Image_format and
+# some details implicitly documented by the source code of 'libicns'.
+
+import sys
+import struct
+import subprocess
+
+# The file format has a typical IFF-style (type, length, data) chunk
+# structure, with one outer chunk containing subchunks for various
+# different icon sizes and formats.
+def make_chunk(chunkid, data):
+ assert len(chunkid) == 4
+ return chunkid + struct.pack(">I", len(data) + 8) + data
+
+# Monochrome icons: a single chunk containing a 1 bpp image followed
+# by a 1 bpp transparency mask. Both uncompressed, unless you count
+# packing the bits into bytes.
+def make_mono_icon(size, rgba):
+ assert len(rgba) == size * size
+
+ # We assume our input image was monochrome, so that the R,G,B
+ # channels are all the same; we want the image and then the mask,
+ # so we take the R channel followed by the alpha channel. However,
+ # we have to flip the former, because in the output format the
+ # image has 0=white and 1=black, while the mask has 0=transparent
+ # and 1=opaque.
+ pixels = [rgba[index][chan] ^ flip for (chan, flip) in [(0,0xFF),(3,0)]
+ for index in range(len(rgba))]
+
+ # Encode in 1-bit big-endian format.
+ data = ""
+ for i in range(0, len(pixels), 8):
+ byte = 0
+ for j in range(8):
+ if pixels[i+j] >= 0x80:
+ byte |= 0x80 >> j
+ data += chr(byte)
+
+ # This size-32 chunk id is an anomaly in what would otherwise be a
+ # consistent system of using {s,l,h,t} for {16,32,48,128}-pixel
+ # icon sizes.
+ chunkid = { 16: "ics#", 32: "ICN#", 48: "ich#" }[size]
+ return make_chunk(chunkid, data)
+
+# Mask for full-colour icons: a chunk containing an 8 bpp alpha
+# bitmap, uncompressed. The RGB data appears in a separate chunk.
+def make_colour_mask(size, rgba):
+ assert len(rgba) == size * size
+
+ data = "".join(map(lambda pix: chr(pix[3]), rgba))
+
+ chunkid = { 16: "s8mk", 32: "l8mk", 48: "h8mk", 128: "t8mk" }[size]
+ return make_chunk(chunkid, data)
+
+# Helper routine for deciding when to start and stop run-length
+# encoding.
+def runof3(string, position):
+ return (position < len(string) and
+ string[position:position+3] == string[position] * 3)
+
+# RGB data for full-colour icons: a chunk containing 8 bpp red, green
+# and blue images, each run-length encoded (see comment inside the
+# function), and then concatenated.
+def make_colour_icon(size, rgba):
+ assert len(rgba) == size * size
+
+ data = ""
+
+ # Mysterious extra zero header word appearing only in the size-128
+ # icon chunk. libicns doesn't know what it's for, and neither do
+ # I.
+ if size == 128:
+ data += "\0\0\0\0"
+
+ # Handle R,G,B channels in sequence. (Ignore the alpha channel; it
+ # goes into the separate mask chunk constructed above.)
+ for chan in range(3):
+ pixels = "".join([chr(rgba[index][chan])
+ for index in range(len(rgba))])
+
+ # Run-length encode each channel using the following format:
+ # * byte 0x80-0xFF followed by one literal byte means repeat
+ # that byte 3-130 times
+ # * byte 0x00-0x7F followed by n+1 literal bytes means emit
+ # those bytes once each.
+ pos = 0
+ while pos < len(pixels):
+ start = pos
+ if runof3(pixels, start):
+ pos += 3
+ pixval = pixels[start]
+ while (pos - start < 130 and
+ pos < len(pixels) and
+ pixels[pos] == pixval):
+ pos += 1
+ data += chr(0x80 + pos-start - 3) + pixval
+ else:
+ while (pos - start < 128 and
+ pos < len(pixels) and
+ not runof3(pixels, pos)):
+ pos += 1
+ data += chr(0x00 + pos-start - 1) + pixels[start:pos]
+
+ chunkid = { 16: "is32", 32: "il32", 48: "ih32", 128: "it32" }[size]
+ return make_chunk(chunkid, data)
+
+# Load an image file from disk and turn it into a simple list of
+# 4-tuples giving 8-bit R,G,B,A values for each pixel.
+#
+# My icon-building makefile already depends on ImageMagick, so I use
+# identify and convert here in place of more sensible Python libraries
+# so as to add no build dependency that wasn't already needed.
+def load_rgba(filename):
+ size = subprocess.check_output(["identify", "-format", "%wx%h", filename])
+ width, height = map(int, size.split("x"))
+ assert width == height
+ data = subprocess.check_output(["convert", "-depth", "8",
+ filename, "rgba:-"])
+ assert len(data) == width*height*4
+ rgba = [map(ord, data[i:i+4]) for i in range(0, len(data), 4)]
+ return width, rgba
+
+data = ""
+
+# Trivial argument format: each argument is a filename prefixed with
+# "mono:", "colour:" or "output:". The first two indicate image files
+# to use as part of the icon, and the last gives the output file name.
+# Icon subformat chunks are written out in the order of the arguments.
+for arg in sys.argv[1:]:
+ kind, filename = arg.split(":", 2)
+ if kind == "output":
+ outfile = filename
+ else:
+ size, rgba = load_rgba(filename)
+ if kind == "mono":
+ data += make_mono_icon(size, rgba)
+ elif kind == "colour":
+ data += make_colour_icon(size, rgba) + make_colour_mask(size, rgba)
+ else:
+ assert False, "bad argument '%s'" % arg
+
+data = make_chunk("icns", data)
+
+with open(outfile, "w") as f:
+ f.write(data)
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