Texture arrays

A texture array is a stack of 2D textures that share one size and format, stored as a single texture object. The shader chooses a layer, or slice, by number. Unity defines it as "a collection of 2D textures with the same size, format, and flags", where "you can use an index to sample each slice" (Unity Manual: Introduction to texture arrays).

For artists, arrays matter in two places: terrain and other layered materials that need many surface types in one shader, and any custom shader that picks a texture per object or per pixel. You will rarely make one by hand, but you will often author the textures that go into one, and the rules are strict.

#Why use one

• Many surfaces, one material. Unity calls arrays "an efficient way to access many textures of the same size and format" and singles out "custom terrain rendering systems" as a typical use (Unity Manual: Introduction to texture arrays). A terrain shader that blends grass, rock, mud and snow can read all four from one array, choosing slices from a splat map or index map (see terrain layers and splat maps).
• Clean tiling. Each slice is a complete texture with its own 0–1 UV space, so it can repeat like any tileable material. A region of an atlas cannot, because normal wrapping repeats the whole atlas.
• No bleeding between layers. Godot notes that an array is like a 3D texture "but without interpolation between layers" (Godot documentation: Importing images), so filtering and mips never mix the rock slice into the grass slice.

#Format rules

Every slice must match. Unity requires "the same size, format and flags" (Unity Manual: 2D texture arrays), so:

• All slices share one resolution. A 4K cliff and a 1K pebble texture cannot share an array; scale them to one size first.
• All slices share one compression format and colour space. Keep base colours (sRGB) and normal maps (linear, BC5) in separate arrays (see colour space: sRGB and linear).
• Slices are chosen by number, so keep a written order (slice 0 grass, slice 1 rock…) that the material and the artists agree on.

#Costs and limits

• Platform support. Unity lists DirectX 11 and 12, Metal, OpenGL Core, OpenGL ES 3.0 and Vulkan, and offers SystemInfo.supports2DArrayTextures to check at runtime (Unity Manual: Introduction to texture arrays).
• Streaming. As above, Unity does not stream arrays (Unity Manual: Introduction to mipmap streaming). Unreal's UTexture2DArray has stream-in and stream-out functions, "Only usable if the asset is streamable" (Unreal Engine documentation: UTexture2DArray). See texture streaming.
• Memory per slice. An array of eight 2K BC7 slices uses eight times the memory of one slice, so budget it as eight textures.

#In different tools

• Unreal Engine: a Texture2DArray asset holds a list of source textures (shown as Add Textures), and can also come from a multi-slice DDS file (Unreal Engine documentation: UTexture2DArray). It has separate address modes for the X, Y and Z axes (Unreal Engine documentation: UTexture2DArray).
• Unity: import a source image divided into cells as a 2D Array, build one from script with the Texture2DArray API, or render into one (Unity Manual: 2D texture arrays). Sample it in a custom shader (Unity Manual: 2D texture arrays).
• Godot: set the import type to Texture2DArray, then sample it in a custom spatial shader (Godot documentation: Importing images). Godot's importer has a Normal Map (RG Channels) option available only for layered textures such as arrays (Godot documentation: Importing images).

#Further reading and watching

Related: shaders for artists, master materials and instances, terrain and landscapes, vertex painting and blending, texture atlases.