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# astc_writer.py
#
# Minimal ASTC writer that mirrors the C/C++ write_astc_file() logic from example_capi.c.
# Writes a valid single-slice 2D ASTC texture file (no array slices, no 3D, no mips).
#
# Usage:
# from astc_writer import write_astc_file
# write_astc_file("output.astc", blocks, block_width, block_height, width, height)
#
# "blocks" must be a bytes-like object containing the full ASTC block data
# using 16 bytes per block (standard ASTC block size).
def write_astc_file(
filename: str,
blocks: bytes,
block_width: int,
block_height: int,
width: int,
height: int
) -> None:
"""
Write an ASTC file to disk.
Parameters:
filename : Output filename ("something.astc")
blocks : Bytes-like object containing ASTC blocks (16 bytes per block)
block_width : ASTC block width (e.g. 4-12)
block_height : ASTC block height (e.g. 4-12)
width : Original image width in pixels
height : Original image height in pixels
Notes:
- ASTC files use 2D blocks; depth is always 1.
- Block layout goes row-major: (num_blocks_y num_blocks_x) blocks.
- No mipmaps are stored in this format.
"""
# Validate block dimensions
if block_width < 4 or block_width > 12:
raise ValueError(f"ASTC block_width {block_width} out of range (412)")
if block_height < 4 or block_height > 12:
raise ValueError(f"ASTC block_height {block_height} out of range (412)")
# Compute block grid
num_blocks_x = (width + block_width - 1) // block_width
num_blocks_y = (height + block_height - 1) // block_height
total_blocks = num_blocks_x * num_blocks_y
expected_size = total_blocks * 16 # 16 bytes per ASTC block (always)
if len(blocks) != expected_size:
raise ValueError(
f"ASTC block buffer incorrect size: expected {expected_size}, got {len(blocks)}"
)
# Write file
with open(filename, "wb") as f:
# ASTC magic number (0x13AB A15C)
f.write(bytes([0x13, 0xAB, 0xA1, 0x5C]))
# Block dims: x, y, z (z=1)
f.write(bytes([
block_width & 0xFF,
block_height & 0xFF,
1
]))
# ASTC stores width/height/depth as 24-bit LE
def write_24bit_le(v: int):
f.write(bytes([
v & 0xFF,
(v >> 8) & 0xFF,
(v >> 16) & 0xFF
]))
write_24bit_le(width)
write_24bit_le(height)
write_24bit_le(1) # depth
# Write actual block payload
f.write(blocks)
print(f"[ASTC Writer] Wrote: {filename} ({width}x{height}, {block_width}x{block_height} blocks)")
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