Render chunk

A render chunk, or simply "chunk", is the encapsulated data required to issue a single* draw call. This includes a material, a vertex buffer, an index buffer and a small amount of metadata. A 3D mesh is comprised of a number of render chunks. The render chunk's material may be shared with other render chunks in the same mesh or other meshes in the same asset.

Render chunks are a data packet (ie. noun) rather than an action on the GPU (ie. verb). The act of submitting a render chunk to the GPU for rendering is known as a draw call.

There are numerous cases where the use of a single render chunk may technically result in multiple draw calls; eg.

• where the chunk's parent mesh is present multiple times in the scene.
• where a multi-pass material is used (in Trainz, this is currently limited to a depth prepass used for some materials, the details of which are trainz-version specific).
• for shadow rendering.
• when the scene is rendered from multiple views (for example, for true reflection rendering).

There are also some cases where multiple render chunks may be rendered in a single draw call. This can only occur in cases where the chunks all share a single material:

• mesh stitching
• hardware instancing
• import-time or load-time optimization of the chunks' parent mesh.