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112 lines
4 KiB
Python
112 lines
4 KiB
Python
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"""Parse ETC1, a terrible micro block-based image compression format.
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Please enjoy the docs.
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https://www.khronos.org/registry/gles/extensions/OES/OES_compressed_ETC1_RGB8_texture.txt
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"""
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import io
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import itertools
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three_bit_twos_complement = [0, 1, 2, 3, -4, -3, -2, -1]
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etc1_modifier_tables = [
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( 2, 8, -2, -8),
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( 5, 17, -5, -17),
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( 9, 29, -9, -29),
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(13, 42, -13, -42),
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(18, 60, -18, -60),
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(24, 80, -24, -80),
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(33, 106, -33, -106),
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(47, 183, -47, -183),
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]
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def decode_etc1(data):
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# TODO sizes are hardcoded here
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f = io.BytesIO(data)
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f.read(0x80)
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outpixels = [[None] * 128 for _ in range(128)]
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for blocky in range(0, 128, 8):
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for blockx in range(0, 128, 8):
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for z in range(4):
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row = f.read(16)
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if not row:
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raise RuntimeError
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alpha = row[:8]
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etc1 = int.from_bytes(row[8:], 'big')
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diffbit = row[12] & 2
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flipbit = row[12] & 1
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lopixelbits = int.from_bytes(row[8:10], 'little')
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hipixelbits = int.from_bytes(row[10:12], 'little')
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if diffbit:
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red1 = row[15] >> 3
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red2 = max(0, red1 + three_bit_twos_complement[row[15] & 0x7])
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green1 = row[14] >> 3
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green2 = max(0, green1 + three_bit_twos_complement[row[14] & 0x7])
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blue1 = row[13] >> 3
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blue2 = max(0, blue1 + three_bit_twos_complement[row[13] & 0x7])
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red1 = (red1 << 3) | (red1 >> 2)
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green1 = (green1 << 3) | (green1 >> 2)
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blue1 = (blue1 << 3) | (blue1 >> 2)
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red2 = (red2 << 3) | (red2 >> 2)
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green2 = (green2 << 3) | (green2 >> 2)
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blue2 = (blue2 << 3) | (blue2 >> 2)
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else:
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red1 = row[15] >> 4
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red2 = row[15] & 0xf
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green1 = row[14] >> 4
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green2 = row[14] & 0xf
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blue1 = row[13] >> 4
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blue2 = row[13] & 0xf
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red1 = (red1 << 4) | red1
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green1 = (green1 << 4) | green1
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blue1 = (blue1 << 4) | blue1
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red2 = (red2 << 4) | red2
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green2 = (green2 << 4) | green2
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blue2 = (blue2 << 4) | blue2
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base1 = red1, green1, blue1
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base2 = red2, green2, blue2
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codeword1 = row[12] >> 5
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codeword2 = (row[12] >> 2) & 0x7
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table1 = etc1_modifier_tables[codeword1]
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table2 = etc1_modifier_tables[codeword2]
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def nybbles(b):
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for byte in b:
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yield (byte & 0xf) << 4
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yield byte >> 4 << 4
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it = nybbles(alpha)
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for c in range(4):
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for r in range(4):
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x = blockx + c
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y = blocky + r
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if z in (1, 3):
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x += 4
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if z in (2, 3):
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y += 4
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if flipbit:
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# Horizontal
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whichblock = 1 if r < 2 else 2
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else:
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whichblock = 1 if c < 2 else 2
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if whichblock == 1:
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table = table1
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base = base1
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else:
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table = table2
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base = base2
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pixelbit = c * 4 + r
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idx = 2 * ((hipixelbits >> pixelbit) & 1) + ((lopixelbits >> pixelbit) & 1)
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mod = table[idx]
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color = tuple(min(255, max(0, b + mod)) for b in base) + (next(it),)
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outpixels[y][x] = color
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return 128, 128, 4, None, outpixels
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