What is a texture atlas? One image, many pieces

Texture atlas: the definition

A texture atlas is one image that holds many smaller images packed together. Each piece is picked out by texture coordinates: numbers that say which rectangle, or which triangle, of the big image belongs to which surface. In 2D games and on the web the same idea is called a sprite sheet.

The word is used in two related ways, and it helps to keep them apart:

• Packing atlas. Many separate textures (the crate, the barrel, the lamp) are combined into one sheet so they can be drawn together. Common in games, UI and fonts.
• UV atlas of one model. A single 3D mesh is cut into pieces (UV islands or charts), and those pieces are laid out side by side on one texture. Every textured 3D model with a proper UV map already has one of these.

Both are the same trick: one image, many regions, coordinates that map each region to the right place.

Why use a texture atlas?

• Fewer texture switches. The graphics card treats the atlas as one texture. Many objects that share it can often be drawn together, instead of stopping to swap textures between each one. NVIDIA’s 2004 whitepaper on atlases is about exactly this: better batching.
• Fewer files to load. One image instead of fifty means less disk and network overhead. This is why websites use sprite sheets too.
• One texture per model. For a single mesh, a UV atlas lets the whole object live in one base colour image, one normal map and so on. That is what most game engines, GLB files and 3D printing slicers expect.
• Easier export. A model with a dozen materials and a dozen textures is fragile to move around. One atlas is simpler to ship.

How a 3D model texture atlas is made

For a 3D model, building the atlas is mostly UV unwrapping and packing: cut the surface along seams so each piece can lie flat, flatten each piece, then fit all pieces into the square with as little wasted space as possible. Our guide to UV unwrapping explained covers the cutting and flattening part in detail.

What makes a good atlas

• No overlaps. Two pieces on the same pixels share the same paint. That is sometimes done on purpose (mirrored left and right halves save space), but it means you cannot paint the two sides differently.
• Even texel density. Pieces should get texture space in proportion to their size on the model, so a hand is not blurrier than a sleeve. Many artists then give extra space to the parts people look at most, such as the face.
• Padding between pieces. A few pixels of gap, filled with the edge colour, so that when the texture is shrunk for distance (mipmaps) the colours of neighbours do not bleed into each other.
• Few seams in visible places. Each seam is a spot where two pieces meet in 3D but not in the image, and where mistakes show.

Texture atlas generators

A texture atlas generator is any tool that does the packing for you. What it packs depends on the kind of atlas:

Blender’s UV tools include Smart UV Project (automatic cutting by angle), Lightmap Pack (every face gets its own space) and Pack Islands. xatlas is a free, MIT-licensed C++ library that, in its own words, makes unique texture coordinates for baking lightmaps or texture painting. It is used in many tools behind an "auto unwrap" button.

For merging the textures of several materials into one, the usual route is a bake: make one fresh UV atlas for the whole model, then bake each old texture onto it. What is texture baking? explains how that works.

Common texture atlas problems

How the AI 3D Texturing Studio makes an atlas

The AI 3D Texturing Studio has to paint into a texture, so every model needs a UV atlas. If yours has a good one, Keep my UVs (the default for uploads) paints straight into it. The studio makes its own atlas when:

• the file has no UVs, which is true of most STL and PLY files;
• the UVs are unusable: more than 2% of the UV points lie outside the 0 to 1 square (tiling), or the whole map is squashed into a tiny spot;
• you pick Make new UVs, for example because the studio warned that your UVs overlap or are mirrored, so one spot of texture would land on two places on the model. (With Keep my UVs the studio paints into overlapping UVs anyway and tells you that mirrored parts will share paint; samples and models opened by link get new UVs for this automatically, on meshes up to 60,000 triangles.)

Its atlas is deliberately simple, and different from what a tool like xatlas makes. Every triangle gets its own small cell in a square grid, two triangles per cell, with a gap of about two texels around each one. There are no big islands to cut or seams to choose, so it never fails, and it suits AI painting, because the colour comes from camera views projected onto the surface, not from someone painting the flat image by hand.

The trade-offs, honestly: every triangle in this atlas gets the same texture space whatever its size, so on a model with a mix of big and tiny triangles the big ones get fewer pixels per centimetre. And the flat image looks like confetti, so it is not pleasant to edit later in a 2D paint program. If you plan to hand-paint the texture afterwards, unwrap in Blender first and use Keep my UVs.

Two more details. A model with more than 8 materials gets all of them combined into one texture. And new UVs only exist inside the downloaded GLB, so use Download textured model (GLB); the Texture only (PNG) download fits your own UVs. When the texture is exported, the gaps between cells are filled with neighbouring colour, so seams do not show as dark lines.

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