Multi-Block Update

Packet ID	Direction	Mojang Name	MCP Name
0x34	Clientbound	ChunkTilesUpdatePacket	Packet52MultiBlockChange

This is used to update multiple blocks in one go. If only a single block is being updated, consider using the Block Update packet.

The format for the coordinates is slightly obtuse but boils down to using one 16-bit Short to store the 4-bit X, 4-bit Z and 8-bit Y value of the block coordinate within the chunk.

Bit 15 - 12	Bit 11 - 8	Bit 7 - 0
X	Z	Y

Some example code to demonstrate how to convert to and from this format.

int16_t format_multi_block(int8_t x, int8_t y, int8_t z) {
    return (
        ((int16_t(x) & 0x0F) << 12) |
        ((int16_t(z) & 0x0F) << 8 ) |
        ((int16_t(y) & 0xFF)      )
    );
}

Int3 unformat_multi_block(int16_t value) {
    return {
        ((value >> 12) & 0x0F),
        ((value >> 8 ) & 0x0F),
        ((value      ) & 0xFF)
    }
}

Clientbound

Field	Type	Description
X	Integer	The X position of the chunk
Z	Integer	The Z position of the chunk
Number of Blocks	Short	The number of changed blocks
Block Coordinates	Short Array	Array of Block coordinates in the format described above
Blocktypes	Byte Array	Array of Block types those coordinates change to
Metadata	Byte Array	Array of Block metadata those coordinates change to

Example Packet

Field	Value
X	-9
Z	12
Number of Blocks	1
Block Coordinates	0101 0111 01111111* (x: 5, z: 7, y: 127)
Blocktypes	1 (Stone)
Metadata	0

_{* Binary value is spaced out to be more readable.}