.gtchunk files are used along with zone / gtzone and worldDir files.

They represent chunks, a playable piece of the game world.

They contain a version that must match the version the game is expecting.

A Chunk is made up of node layers and those nodes can have sub layers. Some nodes are compressed as well.

Load order of chunk from client:

• Importing chunk from serialized data...
• Imported file does not appear to be a world file
• You need to use a more current version of the software. Try SVN update with appropriate Loading chunk file v. (code built for v.)
• Invalid number of physics material ids
• Failed to read world chunk data
• Error while loading MOPP data:
• ...chunk loading succeeded.

The chunk format contains a small index of data at the top followed by several compressed blocks of data.

The index is parsed on map load, and the offsets of the compressed blocks of data are stored.

The compressed blocks represent chunk data at different levels of detail, and are loaded and unloaded on demand.

The chunks all store the data in the following order, with offsets included in the index:

Data
Root/Index
LOD-0 Shared Data
LOD-0 SubChunk 0x0 Data
LOD-1 Shared Data
LOD-1 SubChunk 0x0 Data
LOD-2 Shared Data
LOD-2 SubChunk 0x0 Data
LOD-2 SubChunk 1x0 Data
LOD-2 SubChunk 0x1 Data
LOD-2 SubChunk 1x1 Data
LOD-3 SubChunk 0x0 Data
LOD-3 SubChunk 1x0 Data
LOD-3 SubChunk 2x0 Data
LOD-3 SubChunk 3x0 Data
LOD-3 SubChunk 0x1 Data
LOD-3 SubChunk 1x1 Data
LOD-3 SubChunk 2x1 Data
LOD-3 SubChunk 3x1 Data
LOD-3 SubChunk 0x2 Data
LOD-3 SubChunk 1x2 Data
LOD-3 SubChunk 2x2 Data
LOD-3 SubChunk 3x2 Data
LOD-3 SubChunk 0x3 Data
LOD-3 SubChunk 1x3 Data
LOD-3 SubChunk 2x3 Data
LOD-3 SubChunk 3x3 Data
LOD-4 Shared Data
LOD-4 SubChunk 0x0 Data
LOD-4 SubChunk 1x0 Data
LOD-4 SubChunk 2x0 Data
LOD-4 SubChunk 3x0 Data
LOD-4 SubChunk 4x0 Data
LOD-4 SubChunk 5x0 Data
LOD-4 SubChunk 6x0 Data
LOD-4 SubChunk 7x0 Data
LOD-4 SubChunk 0x1 Data
LOD-4 SubChunk 1x1 Data
LOD-4 SubChunk 2x1 Data
LOD-4 SubChunk 3x1 Data
LOD-4 SubChunk 4x1 Data
LOD-4 SubChunk 5x1 Data
LOD-4 SubChunk 6x1 Data
LOD-4 SubChunk 7x1 Data
LOD-4 SubChunk 0x2 Data
LOD-4 SubChunk 1x2 Data
LOD-4 SubChunk 2x2 Data
LOD-4 SubChunk 3x2 Data
LOD-4 SubChunk 4x2 Data
LOD-4 SubChunk 5x2 Data
LOD-4 SubChunk 6x2 Data
LOD-4 SubChunk 7x2 Data
LOD-4 SubChunk 0x3 Data
LOD-4 SubChunk 1x3 Data
LOD-4 SubChunk 2x3 Data
LOD-4 SubChunk 3x3 Data
LOD-4 SubChunk 4x3 Data
LOD-4 SubChunk 5x3 Data
LOD-4 SubChunk 6x3 Data
LOD-4 SubChunk 7x3 Data
LOD-4 SubChunk 0x4 Data
LOD-4 SubChunk 1x4 Data
LOD-4 SubChunk 2x4 Data
LOD-4 SubChunk 3x4 Data
LOD-4 SubChunk 4x4 Data
LOD-4 SubChunk 5x4 Data
LOD-4 SubChunk 6x4 Data
LOD-4 SubChunk 7x4 Data
LOD-4 SubChunk 0x5 Data
LOD-4 SubChunk 1x5 Data
LOD-4 SubChunk 2x5 Data
LOD-4 SubChunk 3x5 Data
LOD-4 SubChunk 4x5 Data
LOD-4 SubChunk 5x5 Data
LOD-4 SubChunk 6x5 Data
LOD-4 SubChunk 7x5 Data
LOD-4 SubChunk 0x6 Data
LOD-4 SubChunk 1x6 Data
LOD-4 SubChunk 2x6 Data
LOD-4 SubChunk 3x6 Data
LOD-4 SubChunk 4x6 Data
LOD-4 SubChunk 5x6 Data
LOD-4 SubChunk 6x6 Data
LOD-4 SubChunk 7x6 Data
LOD-4 SubChunk 0x7 Data
LOD-4 SubChunk 1x7 Data
LOD-4 SubChunk 2x7 Data
LOD-4 SubChunk 3x7 Data
LOD-4 SubChunk 4x7 Data
LOD-4 SubChunk 5x7 Data
LOD-4 SubChunk 6x7 Data
LOD-4 SubChunk 7x7 Data

Chunks have 5 levels of detail. The levels of detail range from 0 to 4 with 0 being the lowest and 4 being the highest.

For each level of detail, the chunk format contains 1 "Shared" block of data, and 1 to several "SubChunks". At the highest level of detail, the chunk is segmented into 64 separate parts, while at the lowest level of detail, it is not segmented and the whole chunk is just 1 piece.

The level of detail partitions are as follows:

LOD	SubChunks
0	1
1	1
2	4
3	16
4	64

As you traverse the game world, the game will dynamically load and replace chunks and SubChunks based on your distance and their bounding boxes.

At LOD 4, SubChunks are referred to in the following way in-game, with 0x0 representing the bottom left corner:

	0	1	2	3	4	5	6	7
7	0x7	1x7	2x7	3x7	4x7	5x7	6x7	7x7
6	0x6	1x6	2x6	3x6	4x6	5x6	6x6	7x6
5	0x5	1x5	2x5	3x5	4x5	5x5	6x5	7x5
4	0x4	1x4	2x4	3x4	4x4	5x4	6x4	7x4
3	0x3	1x3	2x3	3x3	4x3	5x3	6x3	7x3
2	0x2	1x2	2x2	3x2	4x2	5x2	6x2	7x2
1	0x1	1x1	2x1	3x1	4x1	5x1	6x1	7x1
0	0x0	1x0	2x0	3x0	4x0	5x0	6x0	7x0

Every node starts with the layer header described on the zone page: the marker 0x12ED5A12ED5B12ED, a uint32 id and a uint32 length. The fields below follow that header.

CHUNK, the root node:

Field	Type	Notes
version	uint32	8 in 1962
timestamp	uint64	Unix time in milliseconds
numLods	uint32	5
lods	CHUNK_LOD[numLods]

CHUNK_LOD:

Field	Type	Notes
lodIdx	uint32
subChunkExponent	int32	The number of SubChunks is 4 to the power of this, 1 << 2 * subChunkExponent
dataOffset	uint32	Offset of the shared data block
compressedSize	int32	Size of the block, including its id
uncompressedSize	int32
subChunks	CHUNK_SUBCHUNK[]	One per SubChunk

CHUNK_SUBCHUNK:

Field	Type	Notes
dataOffset	uint32	Offset of the SubChunk data block
compressedSize	int32	Size of the block, including its id and the LZMA properties
uncompressedSize	int32
boundsMin	float[3]
boundsMax	float[3]

Note that the data offsets are relative to the end of the root node, i.e. 16 bytes of header plus the root node's length, not to the start of the file.

The data blocks are compressed, in 1962 the shared blocks use zlib and the SubChunk blocks LZMA. The id at the start of each block tells which one it is, so pick the decompressor by the id and not by the position of the block:

Id	Layout
DATA	char[4] id, then a zlib stream (2 byte header, deflate, Adler-32)
DAT2	char[4] id, byte[5] LZMA properties, then a raw LZMA stream

The LZMA stream has no size in front of it, it decodes to the uncompressedSize of the index node.

Within the data blocks are layer/nodes parsed in the same manner as the root/index and zone layer, described in the next section.

The following nodes only occur at the top of the file and describe where to find the rest of the data.

Name	ID	Parent	Description
CHUNK	0x40000 (262144)		Root node of the index.
CHUNK_LOD	0x40001 (262145)	CHUNK	Represents a LOD, contains offset and size for Shared Data.
CHUNK_SUBCHUNK	0x40002 (262146)	CHUNK_LOD	Represents a SubChunk, contains its offset, size and bounds.

The following nodes occur as part of the data blocks.

Name	ID	Description
CHUNK_TERRAIN	0x040100 (262400)	Contains child nodes for the visual representation of the base terrain.
CHUNK_COLLISION1	0x040101 (262401)	Static geometry collision. Only appears on LOD 3. Collision data for terrain and static objects. One node per SubChunk.
CHUNK_SUBZONEGRID	0x040102 (262402)	Sub zone grid
CHUNK_COLLISION3	0x040103 (262403)	Movement blocker collision. Only appears on LOD 3. Additional collision data, possibly for invisible blockers.
CHUNK_PROP_ENCOUNTER_NAME_REGISTRY2	0x040104 (262404)	Encounter name registry
CHUNK_COLLISION2	0x040105 (262405)	Water collision. Only appears on LOD 3. Additional collision data, for water volumes and maybe more.
CHUNK_PROP	0x040200 (262656)	Contains child nodes for additional elements to add such as lights, particle effects, audio, statically placed monster models, etc.
CHUNK_PROP_ENCOUNTER_NAME_REGISTRY	0x040204 (262660)	Prop encounter name registry
CHUNK_VEGETATION	0x040205 (262661)	Vegetation
CHUNK_OVERLAY	0x040206 (262662)	Overlay
CHUNK_SECTORS	0x040207 (262663)	Sectors, the children are 0x50013 sector layers
CHUNK_WATER	0x040208 (262664)	Visual representation of certain water elements local to the chunk.
CHUNK_VEGETATION2	0x040209 (262665)	Vegetation, second variant
CHUNK_GEOMETRY	0x040210 (262672)	Visual representation of static objects. Data within the nodes is split into 64 segments representing the full chunk area. At low LOD a single node will contain all segments while at higher LOD the nodes are split out over the SubChunks only containing their respective segments.
CHUNK_GEOMETRY2	0x40203 (262659)	Like CHUNK_GEOMETRY with some minor differences

• gtchunk.bt
• gtchunk.payload.bt
• _common_gtlayer.bt

• Reader and Writer have been implemented in FauFau: GtChunk.cs. Its GtLayer reads the node tree of the decompressed blocks.