Texture atlases

A texture atlas is one texture that holds the textures of several different objects, each in its own region. Because all those objects now read from the same texture, they can share one material. Unity's manual describes combining the textures of otherwise identical materials into one large texture as "texture atlasing", and explains why it matters: "Only GameObjects sharing the same Material can be batched together" (Unity Manual: Draw call batching).

Atlases save materials and draw calls more than they save pixels. The total texture area is roughly the same as before, but it is held in one file.

#Atlas or trim sheet?

The two are often confused because both are "one texture, many meshes". The difference is how the space is used.

Many kits use both. The free Modular Dungeon Props kit on Sketchfab, made to learn modular texturing, uses "a brick trim sheet" plus "a texture atlas" alongside a few dedicated materials (Sketchfab: Modular Dungeon Props). Commercial courses teach atlases next to modular and weighted-normal workflows (FlippedNormals: Sci-Fi Game Environment in Blender & UE4).

#Building an atlas

There are two common routes:

1. Atlas from the start. Unwrap a group of props together into one UV space and texture them as one set. Simple, and texel density is easy to match. See UV packing and padding and texel density.
2. Atlas afterwards. Texture objects separately, then pack their finished textures into one sheet and rescale each object's UVs into its region. Useful when assets come from different artists.

Either way, group objects that appear together (one room, one market stall) and share a shader. An atlas mixing a glass bottle and a wooden crate still needs two shaders, so the saving is smaller.

#Padding and mip bleeding

Atlas regions sit right next to each other, so colours from one region can leak into another. Two things cause it:

• Filtering. When the GPU samples near a region's edge, bilinear filtering blends in neighbouring texels. Ashaman73 explains on Polycount that linear filtering causes the bleed and that it recurs at every mip level (Polycount: manual mip maps? or how to avoid texture…).
• Mipmaps. Each smaller mip level averages blocks of pixels. At distance, a border pixel may be an average of two regions. Ashaman73's example: at mip level 3 of a 2 × 2 atlas, "around 43% of the texture will have bleeding" (Polycount: manual mip maps? or how to avoid texture…).

The fix is padding (also called edge dilation or bleed): extend each region's edge colours outward into the gap around it. "Padding is pretty much the universal solution to this unfortunately", as marks puts it on Polycount (Polycount: manual mip maps? or how to avoid texture…). Lower mips need more of it. In the trim sheet context, Epitaph notes that lower-resolution versions need more padding to avoid bleeding (Polycount: The 'ultimate trim' technique from Sunset…).

#Decal atlases

Decals are a natural fit for atlases: dozens of small stains, cracks, labels and bolts, all used on top of other surfaces. DECALmachine for Blender packs "individual Decal textures into a single texture sheet" so the decals "use a single material and set of textures", which it calls "necessary for efficient rendering in game engines like Unity, Unreal Engine etc." (machin3.io: Atlasing). Decals that share an atlas material can render more efficiently still when joined into a single object (machin3.io: Atlasing). See also mesh decals.

#Common mistakes

• Too little padding, so props show coloured fringes at a distance.
• Atlasing objects that never appear together, which loads texture memory you do not need.
• Different texel densities between regions, so one prop looks sharper than its neighbour.
• Mixed shaders in one atlas (opaque, masked, translucent), which still need separate materials.
• Trying to tile an atlas region with normal UV wrapping.

#Further reading and watching