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