What is terrain?

Most of the world in Terra Diem is represented by terrain. In the game, terrain is a fairly general concept covering everything from rock, water, trees, vegetation, and even buildings. There are three fundamental properties of terrain:

1. Terrain has a fixed 3D location in the world (in meters). It does not move (although it can change)
2. All terrain can be subdivided into individual cubic meter pieces. For instance, a tree is made out of individual pieces of trunk, branch, and leaves within each cubic meter of space in the world.
3. Terrain is made out of materials. Materials have various properties that define how the terrain looks and behaves in the world. For instance, a material may be liquid, loose, or solid. It also may be transparent, translucent, or opaque.

Terrain representation

Terrain is organized hierarchically:

• Column: The top level division of terrain is the column. Columns cover a 16x16 meter area and the entire vertical span of the world (which in turn is set via generation parameters). Columns are largely an optimization to aid culling and light propagation.
• Chunk: A chunk is a 16x16x16 meter cube of space in the world. This is the smallest unit of generation, serialization, and simulation for the world. A chunk stores all the terrain and local simulation state for the chunk. At a minimum each chunk consumes a little over 8K of memory when loaded (8K for block storage and some miscellaneous book keeping).
• Block: A block represents a single cubic meter of space in the world. It may be as small as 2 bytes (and usually is) or very large depending on what types of terrain exist in the block.
• Block Instance: Each block contains a list of instances which define what is in the block. A block with no instances is pure air. Each instance has a type, material, and other parameters which together defines what terrain is in the block. A chunk may contain up to 16,384 block instances.

Block volume

Each block represents a finite amount of space, and thus material. The smallest unit of volume in Terra Diem is a “bit”, and each block may contain up to 64 bits of material. Each block instance represents a certain volume in bits, but the total volume of all block instances will never exceed 64. Any unused space in a block is implicitly filled with air.

Block instance types

There are currently five types of block instances:

• Single material: This is the simplest form of instance. It specifies a material that completely fills the remaining space in the block.
• Partial block: This subdivides a block into 8 smaller cubes (2x2x2) which may or may not contain material. Partial blocks are made of a solid material, and may also define a secondary solid material which consumes the rest of the space.
• Partial slope: This subdivides a block by chopping off one or more corners of the block. The remaining corners are wrapped giving a convex solid which defiines the space consumed by the instance material. Like partial blocks, partial slopes are made of solid material and may also define a secondary solid material which consumes the rest of the space.
• Partial amount: This instance type defines only how much of a material is in the block (in bits). Partial amount materials pile up in the block in the space not consumed by other instances.
• Tree: This represents a trunk or branch, with zero or more child branches leading to neighboring blocks (directly adjacent or diagonal). Each branch is an octagonal prism. Tree instances are made of a solid material (usually wood), and consume space based on the diameter of the branches.