Difference between revisions of "Chunk Format"

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This page gives an overview of the SMP map format used in the [[Protocol|protocol]].
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This article describes the '''Chunk Section''' format used in the [[Protocol#Chunk Data|Chunk Data]] packet ([[Protocol#Play|Play]], 0x20, clientbound).
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== Packets ==
 
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Two packets use this format:
 
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* [[Protocol#Chunk_Data|0x21 chunk data]] - single column - used when player movement requires new chunks to be loaded. Also used to unload chunks.
 
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* [[Protocol#Map_Chunk_Bulk|0x26 map chunk bulk]] - multiple columns - used when first spawning into a world and when player is teleported.
 
  
 
== Concepts ==
 
== Concepts ==
  
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=== Storage ===
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* Chunk Section: a 16×16×16 area, sometimes also called chunk.
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* Chunk Data: an array, either 2048 or 4096 bytes
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* Chunk Column: 16 chunks aligned vertically (totalling 16×256×16).
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* Chunk: a 16x16x16 area, logically made up of multiple Chunk Data arrays, storing things like block ids and skylight
 
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* Chunk Column: 16 chunks aligned vertically (totalling 16x256x16). Chunks can be uninitialised (e.g. set to <code>null</code>)
 
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* World: a bunch of chunk columns
 
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=== Bitmasks ===
 
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You will sometimes get a blob of data and a bitmask. Each bit in the 16-bit short represents a chunk. The least significant bit represents the chunk from Y=0 to Y=15, and so forth. If it's <code>0</code>, the chunk is entirely air and there's no data to read.  
 
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You need a loop like this:
 
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chunks = array[16];
 
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for (i=0; i<16; i++) {
 
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    if (bitmask & (1 << i)) {
 
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        // read a chunk data array
 
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    }
 
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}
 
  
 
== Format ==
 
== Format ==
  
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=== Chunk Column Metadata ===
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A Chunk Section is defined in terms of other [[data types]]:
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The packet contains important metadata which you'll need to pull out. For bulk packets, you'll need to loop over the metadata area. Specifically:
 
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* Chunk X
 
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* Chunk Z
 
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* Section Bitmask
 
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* Continuous? (only in 0x21 - assumed true in 0x26)
 
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* Skylight? (only in 0x26 - only if overworld in 0x21)
 
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=== Data ===
 
  
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In 0x21 the data describes of a single chunk column in the format below. In 0x26, the format below is repeated for each chunk column.
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{| class="wikitable"
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|-
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! Field Name
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! Field Type
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! Notes
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|-
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| Bits Per Block
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| Unsigned Byte
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| How many bits per block in Data Array. If 0, the Palette Length and Palette fields are omitted and the global palette (with 13 bits per block) is used.
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|-
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| Palette Length
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| Optional VarInt
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| Length of the following array
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|-
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| Palette
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| Optional Array of VarInt
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| Mapping of block state IDs in the global palette to indices of this array
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|-
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| Data Array Length
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| VarInt
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| Number of bytes in the following array, divided by 8 (given as such because Notchian implements Data Array as an Array of Long)
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|-
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| Data Array
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| Byte Array
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| List of 4096 indices pointing to state IDs in the Palette, followed by padding to round the length up to the next multiple of 8 bytes
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|-
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| Block Light
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| Byte Array
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| Half byte per block
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|-
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| Sky Light
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| Optional Byte Array
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| Only if in the Overworld; half byte per block
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|}
  
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* Repeat for each chunk specified in the section bitmask:
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Data Array, Block Light, and Sky Light are given for each block with increasing x coordinates, within rows of increasing z coordinates, within layers of increasing y coordinates.
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** Block types, two bytes per block (LE, type = x >> 4, meta = x & 15)
 
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** Block light, half byte per block
 
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** If skylight: Sky light array, half byte per block
 
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* If continuous: biome array, byte per XZ coordinate, 256 bytes total.
 
  
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Each section (except for biome) contains packed chunk data for zero or more 16x16x16 chunks. You need to refer to the primary bitmask for the chunk column to see which chunks are sent.
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The global palette encodes a block as 13 bits. It uses the {{Minecraft Wiki|Data values#Block IDs|block ID}} for the first 9 bits, and the block damage value for the last 4 bits. For example, diorite (block ID <code>1</code> for <code>minecraft:stone</code> with damage <code>3</code>) would be encoded as <code>000000001 0011</code>.
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Chunks are sent bottom-to-top, i.e. the first chunk, if sent, extends from Y=0 to Y=15. Blocks are ordered Y, Z, X, i.e. the 'X' coordinate changes fastest.
 
  
 
In half-byte arrays, two values are packed into each byte. Even-indexed items are packed into the ''high bits'', odd-indexed into the ''low bits''.
 
In half-byte arrays, two values are packed into each byte. Even-indexed items are packed into the ''high bits'', odd-indexed into the ''low bits''.
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The 'biome' array is always 256 bytes when sent. Values above 22 will cause the client to crash.
 
  
 
== Implementations ==
 
== Implementations ==
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* [https://github.com/GlowstoneMC/Glowstone/blob/d3ed79ea7d284df1d2cd1945bf53d5652962a34f/src/main/java/net/glowstone/GlowChunk.java#L640 Java, 1.8]
 
* [https://github.com/GlowstoneMC/Glowstone/blob/d3ed79ea7d284df1d2cd1945bf53d5652962a34f/src/main/java/net/glowstone/GlowChunk.java#L640 Java, 1.8]
 
* [https://github.com/barneygale/smpmap Python, 1.4]
 
* [https://github.com/barneygale/smpmap Python, 1.4]
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* [https://github.com/PrismarineJS/prismarine-chunk Node.js, 1.8]

Revision as of 17:57, 29 February 2016

This article describes the Chunk Section format used in the Chunk Data packet (Play, 0x20, clientbound).

Concepts

  • Chunk Section: a 16×16×16 area, sometimes also called chunk.
  • Chunk Column: 16 chunks aligned vertically (totalling 16×256×16).

Format

A Chunk Section is defined in terms of other data types:

Field Name Field Type Notes
Bits Per Block Unsigned Byte How many bits per block in Data Array. If 0, the Palette Length and Palette fields are omitted and the global palette (with 13 bits per block) is used.
Palette Length Optional VarInt Length of the following array
Palette Optional Array of VarInt Mapping of block state IDs in the global palette to indices of this array
Data Array Length VarInt Number of bytes in the following array, divided by 8 (given as such because Notchian implements Data Array as an Array of Long)
Data Array Byte Array List of 4096 indices pointing to state IDs in the Palette, followed by padding to round the length up to the next multiple of 8 bytes
Block Light Byte Array Half byte per block
Sky Light Optional Byte Array Only if in the Overworld; half byte per block

Data Array, Block Light, and Sky Light are given for each block with increasing x coordinates, within rows of increasing z coordinates, within layers of increasing y coordinates.

The global palette encodes a block as 13 bits. It uses the block ID for the first 9 bits, and the block damage value for the last 4 bits. For example, diorite (block ID 1 for minecraft:stone with damage 3) would be encoded as 000000001 0011.

In half-byte arrays, two values are packed into each byte. Even-indexed items are packed into the high bits, odd-indexed into the low bits.

Implementations

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