GAT

This document describes the GAT file format used in the Ragnarok Online client.

Contents

GAT files contain the following information:

• Altitude data, encoded as height vectors, for each tile of a map
• Terrain type flags to identify the terrain on a given tile

The file contents can be interpreted as a 2-dimensional navigation map, similar to 3D navigation meshes in concept.

Layout

Version 1.2

The vast majority of maps in the RO client use this version, including those found in alpha clients (and even Arcturus)¹.

Field	Offset	Length	Type	Description
Header	0	4	string	"GRAT" as an ASCII-encoded, fixed-size string
MajorVersion	4	1	byte	Versioning information
MinorVersion	5	1	byte	Versioning information
Width	6	4	int	The horizontal size of the map, given in tiles
Height	10	4	int	The vertical size of the map, given in tiles
Tiles	14+	20	array	Contains the navigation properties for Width * Height tiles

Tile Properties

Each entry in the Tiles array represents a surface block ("tile") of the given map's terrain, and is structured as follows:

Field	Offset	Length	Type	Description
SouthWestAltitude	0	4	float	Altitude at the bottom left corner, i.e., at (0, 0) relative coordinates
SouthEastAltitude	4	4	float	Altitude at the bottom right corner, i.e., at (1, 0) relative coordinates
NorthWestAltitude	8	4	float	Altitude at the top left corner, i.e., at (0, 1) relative coordinates
NorthEastAltitude	12	4	float	Altitude at the top right corner, i.e., at (1, 1) relative coordinates
TerrainType	16	4	uint	Identifier for the tile's terrain type

Terrain Types

info

More research is needed. If you know anything about the topic, please help fill in the blanks!

The following terrain types are known to have practical applications:

Value	Walkable	Interpretation
0	YES	Regular (walkable) terrain
1	NO	Obstructed (impassable) terrain
5	NO	Impassable snipeable terrain ("cliff")

There are several other terrain types, but it's unclear whether they affect clientside collision checks at all.

Version 1.3

info

More research is needed. If you know anything about the topic, please help fill in the blanks!

This version adds a twist to the TerrainType values: A special flag marks water tiles, though the layout itself hasn't changed. Effectively, two flags are seemingly embedded in the existing terrain type field. Consider the following example:

• A water tile might use a TerrainType of 1 (obstructed), which in GAT 1.2 would be encoded as 0x01000000 (LE int, value is 1)
• In GAT 1.3, the sameTerrainType property would however be encoded as 0x01000080 (LE int, value is now -2147483647)
• The IsWaterTile flag can then be extracted by masking the TerrainType, leaving only the 0x08 component (binary: 0000 1000)

What exactly should be done with that information is unclear, but all water tiles in GAT 1.3 files have been assigned such a flag.

References

• Rendering/Coordinate Systems describes the GAT-based coordinate system ("map coordinates")

Footnotes

1. Technically, old versions have a zero-byte prefix shifting the layout. It's still GAT 1.2 otherwise. ↩