Clean up and comment ETC1 decoder

pokedex/extract/lib/etc1.py

@@ -1,15 +1,21 @@
-"""Parse ETC1, a terrible micro block-based image compression format.
+"""Parse ETC1, a terrible 4x4 block-based image compression format.
 
 Please enjoy the docs.
 https://www.khronos.org/registry/gles/extensions/OES/OES_compressed_ETC1_RGB8_texture.txt
+
+The format supported here isn't actually ETC1, but a Nintendo-flavored variant
+that decodes four 4x4 blocks one 8x8 block at a time, because of course it is.
+(I believe the 3DS operates with 8x8 tiles, so this does make some sense.)
 """
 import io
-import itertools
 
+# Easier than doing math
+THREE_BIT_TWOS_COMPLEMENT = [0, 1, 2, 3, -4, -3, -2, -1]
 
-three_bit_twos_complement = [0, 1, 2, 3, -4, -3, -2, -1]
+# Table of magic numbers.  Note that the columns aren't in the same order as
-
+# they appear in the docs, because the order of columns in the docs doesn't
-etc1_modifier_tables = [
+# match how the format actually picks them!
+ETC1_MODIFIER_TABLES = [
     (2,  8,   -2,  -8),
     (5,  17,  -5,  -17),
     (9,  29,  -9,  -29),
@@ -20,32 +26,82 @@ etc1_modifier_tables = [
     (47, 183, -47, -183),
 ]
 
+
+def iter_alpha_nybbles(b):
+    """Iterates nybbles from a string of bytes, in little-endian order."""
+    for byte in b:
+        nybble = byte & 0x0f
+        yield (nybble << 4) | nybble
+        nybble = byte >> 4
+        yield (nybble << 4) | nybble
+
+
+def clamp_to_byte(n):
+    return max(0, min(255, n))
+
+
 def decode_etc1(data):
     # TODO sizes are hardcoded here
+    width = 128
+    height = 128
+
+    # TODO this seems a little redundant; could just ask for a stream
     f = io.BytesIO(data)
+    # Skip header
     f.read(0x80)
-    outpixels = [[None] * 128 for _ in range(128)]
+
-    for blocky in range(0, 128, 8):
+    outpixels = [[None] * width for _ in range(height)]
-        for blockx in range(0, 128, 8):
+    # ETC1 encodes as 4x4 blocks.  Normal ETC1 arranges them in English reading
+    # order, right and down.  This Nintendo variant groups them as 8x8
+    # superblocks, where the four blocks in each superblock are themselves
+    # arranged right and down.  So we read block offsets 8 at a time, and 'z'
+    # is our current position within a superblock.
+    # TODO this may do the wrong thing if width/height is not divisible by 8
+    for blocky in range(0, height, 8):
+        for blockx in range(0, width, 8):
             for z in range(4):
                 row = f.read(16)
                 if not row:
-                    raise RuntimeError
+                    raise EOFError
 
+                # Each block is encoded as 16 bytes.  The first 8 are a 4-bit
+                # alpha channel; the latter 8 are color data and flags.
                 alpha = row[:8]
-                etc1 = int.from_bytes(row[8:], 'big')
+                # A block is encoded in two halves.  This bit determines
-                diffbit = row[12] & 2
+                # whether the split is vertical (0) or horizontal (1).
                 flipbit = row[12] & 1
+                # Each half-block has a base color, and its palette is computed
+                # relative to that color.  If this bit is 0, the halves use
+                # "individual" mode, where each gets its own 4-bit base color;
+                # if 1, use "differential" mode, where the first half has a
+                # 5-bit base color and the other is given by a 3-bit offset.
+                diffbit = row[12] & 2
+                # Each half-block also uses one of the predefined tables of
+                # four modifiers listed above.  There are eight such tables,
+                # thus three bits to pick one.
+                codeword1 = row[12] >> 5
+                codeword2 = (row[12] >> 2) & 0x7
+                table1 = ETC1_MODIFIER_TABLES[codeword1]
+                table2 = ETC1_MODIFIER_TABLES[codeword2]
+                # Finally, each pixel uses one each of these bits to get an
+                # index into the modifier table, then adds that modifier to the
+                # base color.  (Note that no pixel can be the base color.)
                 lopixelbits = int.from_bytes(row[8:10], 'little')
                 hipixelbits = int.from_bytes(row[10:12], 'little')
 
+                # Read the base color for each half-block, depending on mode
                 if diffbit:
+                    # Differential mode: first half uses 5-bit color, second
+                    # half is relative to it
                     red1 = row[15] >> 3
-                    red2 = max(0, red1 + three_bit_twos_complement[row[15] & 0x7])
                     green1 = row[14] >> 3
-                    green2 = max(0, green1 + three_bit_twos_complement[row[14] & 0x7])
                     blue1 = row[13] >> 3
-                    blue2 = max(0, blue1 + three_bit_twos_complement[row[13] & 0x7])
+                    red2 = clamp_to_byte(
+                        red1 + THREE_BIT_TWOS_COMPLEMENT[row[15] & 0x7])
+                    green2 = clamp_to_byte(
+                        green1 + THREE_BIT_TWOS_COMPLEMENT[row[14] & 0x7])
+                    blue2 = clamp_to_byte(
+                        blue1 + THREE_BIT_TWOS_COMPLEMENT[row[13] & 0x7])
 
                     red1 = (red1 << 3) | (red1 >> 2)
                     green1 = (green1 << 3) | (green1 >> 2)
@@ -70,17 +126,8 @@ def decode_etc1(data):
                 base1 = red1, green1, blue1
                 base2 = red2, green2, blue2
 
-                codeword1 = row[12] >> 5
+                # Now deal with individual pixels
-                codeword2 = (row[12] >> 2) & 0x7
+                it = iter_alpha_nybbles(alpha)
-                table1 = etc1_modifier_tables[codeword1]
-                table2 = etc1_modifier_tables[codeword2]
-
-                def nybbles(b):
-                    for byte in b:
-                        yield (byte & 0xf) << 4
-                        yield byte >> 4 << 4
-                it = nybbles(alpha)
-
                 for c in range(4):
                     for r in range(4):
                         x = blockx + c
@@ -90,12 +137,7 @@ def decode_etc1(data):
                         if z in (2, 3):
                             y += 4
 
-                        if flipbit:
+                        if (flipbit and r < 2) or (not flipbit and c < 2):
-                            # Horizontal
-                            whichblock = 1 if r < 2 else 2
-                        else:
-                            whichblock = 1 if c < 2 else 2
-                        if whichblock == 1:
                             table = table1
                             base = base1
                         else:
@@ -103,9 +145,12 @@ def decode_etc1(data):
                             base = base2
 
                         pixelbit = c * 4 + r
-                        idx = 2 * ((hipixelbits >> pixelbit) & 1) + ((lopixelbits >> pixelbit) & 1)
+                        hibit = (hipixelbits >> pixelbit) & 0x1
-                        mod = table[idx]
+                        lobit = (lopixelbits >> pixelbit) & 0x1
-                        color = tuple(min(255, max(0, b + mod)) for b in base) + (next(it),)
+                        mod = table[hibit * 2 + lobit]
+                        color = tuple(clamp_to_byte(b + mod) for b in base)
+                        color += (next(it),)
                         outpixels[y][x] = color
 
-    return 128, 128, 4, None, outpixels
+    # 4 is the bit depth; None is the palette
+    return width, height, 4, None, outpixels