Blendfile thumbnail extractor, so icons can be automatically extracted from blend files as PNG's and displayed in a file browser.

Runs with python 2.6 and 3.1 with no special deps.

To have nautilus display thumbnails automatically.
 gconftool --type boolean --set /desktop/gnome/thumbnailers/application@x-blender/enable true
 gconftool --type string --set /desktop/gnome/thumbnailers/application@x-blender/command "blender-thumbnailer.py %i %o"

release/freedesktop/blender-thumbnailer.py

@@ -0,0 +1,132 @@
+#!/usr/bin/python
+
+# ##### BEGIN GPL LICENSE BLOCK #####
+#
+#  This program is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU General Public License
+#  as published by the Free Software Foundation; either version 2
+#  of the License, or (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software Foundation,
+#  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# ##### END GPL LICENSE BLOCK #####
+
+"""
+Thumbnailer runs with python 2.6 and 3.x.
+To run automatically with nautilus:
+   gconftool --type boolean --set /desktop/gnome/thumbnailers/application@x-blender/enable true
+   gconftool --type string --set /desktop/gnome/thumbnailers/application@x-blender/command "blender-thumbnailer.py %i %o"
+"""
+
+import os
+import struct
+import sys
+
+def blend_extract_thumb(path):
+    # def MAKE_ID(tag): ord(tag[0])<<24 | ord(tag[1])<<16 | ord(tag[2])<<8 | ord(tag[3])
+    REND = 1145980242 # MAKE_ID(b'REND')
+    TEST = 1414743380 # MAKE_ID(b'TEST')
+
+    blendfile = open(path, 'rb')
+
+    head = blendfile.read(7)
+
+    if head[0:2] == b'\x1f\x8b': # gzip magic
+        import gzip
+        blendfile.close()
+        blendfile = gzip.open(path, 'rb')
+        head = blendfile.read(7)
+
+    if head != b'BLENDER':
+        blendfile.close()
+        return None, 0, 0
+
+    is_64_bit = (blendfile.read(1) == b'-')
+
+    # true for PPC, false for X86
+    is_big_endian = (blendfile.read(1) == b'V')
+
+    # Now read the bhead chunk!!!
+    blendfile.read(3) # skip the version
+
+    sizeof_pointer = 8 if is_64_bit else 4
+
+    sizeof_bhead = 24 if is_64_bit else 20
+    
+    int_endian = '>i' if is_big_endian else '<i'
+    int_endian_pair = '>ii' if is_big_endian else '<ii'
+    
+    while True:
+        try:
+            code, length = struct.unpack(int_endian_pair, blendfile.read(8)) # 8 == sizeof(int) * 2
+        except IOError:
+            return None, 0, 0
+        
+        # finally read the rest of the bhead struct, pointer and 2 ints
+        blendfile.seek(sizeof_bhead - 8, os.SEEK_CUR)
+
+        if code == REND:
+            blendfile.seek(length, os.SEEK_CUR)
+        else:
+            break
+    
+    if code != TEST:
+        return None, 0, 0
+
+    try:
+        x, y = struct.unpack(int_endian_pair, blendfile.read(8)) # 8 == sizeof(int) * 2
+    except struct.error:
+        return None, 0, 0
+
+    length -= 8 # sizeof(int) * 2
+    
+    if length != x * y * 4:
+        return None, 0, 0
+    
+    image_buffer = blendfile.read(length)
+    
+    if len(image_buffer) != length:
+        return None, 0, 0
+
+    return image_buffer, x, y
+
+
+def write_png(buf, width, height):
+    import zlib
+
+    # reverse the vertical line order and add null bytes at the start
+    width_byte_4 = width * 4
+    raw_data = b"".join([b'\x00' + buf[span:span + width_byte_4] for span in range((height - 1) * width * 4, -1, - width_byte_4)])
+    
+    def png_pack(png_tag, data):
+        chunk_head = png_tag + data
+        return struct.pack("!I", len(data)) + chunk_head + struct.pack("!I", 0xFFFFFFFF & zlib.crc32(chunk_head))
+
+    return b"".join([
+        b'\x89PNG\r\n\x1a\n',
+        png_pack(b'IHDR', struct.pack("!2I5B", width, height, 8, 6, 0, 0, 0)),
+        png_pack(b'IDAT', zlib.compress(raw_data, 9)),
+        png_pack(b'IEND', b'')])
+
+
+if __name__ == '__main__':
+    if len(sys.argv) < 2:
+        print("Expected 2 arguments <input.blend> <output.png>")
+    else:
+        file_in = sys.argv[-2]
+
+        buf, width, height = blend_extract_thumb(file_in)
+        
+        if buf:
+            file_out = sys.argv[-1]
+
+            f = open(file_out, "wb")
+            f.write(write_png(buf, width, height))
+            f.close()