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I couldn’t find any information about the new biome storage format introduced in Bedrock 1.18, so I did some reverse engineering, and share the results to help others.

Bedrock 1.18 assigns a biome for each block, unlike Java, which assigns one to a 4x4x4 cube. Biome data for a whole chunk is stored in one LevelDB entry with the chunk key type 43 (0x2b), containing biome information for each subchunk.

Entry format:

• 512 bytes: seems to be a heightmap
• Paletted biome data for each subchunk from bottom to top
  • 1 byte header
    • bit 0: always 1 (?)
    • bits 7..1: bits per value for the following array (can be 0 if the palette contains only one biome)
    • 0xff indicates non-existent subchunk, in this case the following array and palette data are omitted
  • n bytes: biome data for each block in the 16x16x16 subchunk in XZY order using the same aligned bit-packed int32 array format used for paletted block data. This array is omitted if the palette contains only one biome.
  • 1 byte: biome palette length
  • n * int32: biome palette entries (numeric biome IDs)