diff options
Diffstat (limited to 'magick.py')
| -rwxr-xr-x | magick.py | 47 |
1 files changed, 46 insertions, 1 deletions
@@ -71,6 +71,51 @@ def palettize(img, pal): return result +# we cannot use zlib.compress() because different compressors may compress the +# same data differently, for example by using different optimizations on +# different architectures: +# https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/R7GD4L5Z6HELCDAL2RDESWR2F3ZXHWVX/ +# +# to make the compressed representation of the uncompressed data bit-by-bit +# identical on all platforms we make use of the compression method 0, that is, +# no compression at all :) +def compress(data): + # two-byte zlib header (rfc1950) + # common header for lowest compression level + # bits 0-3: Compression info, base-2 logarithm of the LZ77 window size, + # minus eight -- 7 indicates a 32K window size + # bits 4-7: Compression method -- 8 is deflate + # bits 8-9: Compression level -- 0 is fastest + # bit 10: preset dictionary -- 0 is none + # bits 11-15: check bits so that the 16-bit unsigned integer stored in MSB + # order is a multiple of 31 + result = b"\x78\x01" + # content is stored in deflate format (rfc1951) + # maximum chunk size is the largest 16 bit unsigned integer + chunksize = 0xFFFF + for i in range(0, len(data), chunksize): + # bits 0-4 are unused + # bits 5-6 indicate compression method -- 0 is no compression + # bit 7 indicates the last chunk + if i * chunksize < len(data) - chunksize: + result += b"\x00" + else: + # last chunck + result += b"\x01" + chunk = data[i : i + chunksize] + # the chunk length as little endian 16 bit unsigned integer + result += struct.pack("<H", len(chunk)) + # the one's complement of the chunk length + # one's complement is all bits inverted which is the result of + # xor with 0xffff for a 16 bit unsigned integer + result += struct.pack("<H", len(chunk) ^ 0xFFFF) + result += chunk + # adler32 checksum of the uncompressed data as big endian 32 bit unsigned + # integer + result += struct.pack(">I", zlib.adler32(data)) + return result + + def write_png(data, path, bitdepth, colortype, palette=None): with open(path, "wb") as f: f.write(b"\x89PNG\r\n\x1A\n") @@ -124,7 +169,7 @@ def write_png(data, path, bitdepth, colortype, palette=None): raw += struct.pack(">B", val) else: raise Exception() - compressed = zlib.compress(raw) + compressed = compress(raw) block = b"IDAT" + compressed f.write( struct.pack(">I", len(compressed)) |