UV unwrapping explained (and why it matters for AI texturing)

What are UVs?

A texture is a flat 2D image. A model is a 3D surface. Something has to say which pixel of the image goes on which part of the surface. That "something" is the UV map.

Every corner (vertex) of every triangle on the model gets two extra numbers, U and V. They are a position on the texture image: U goes left to right, V goes bottom to top, both from 0 to 1. The letters U and V are used simply because X, Y and Z are already taken by 3D space.

When the model is drawn, each triangle looks up its patch of the image using those numbers. If the UVs are good, the texture wraps neatly. If they are missing or broken, the texture has no address to go to.

What does "unwrapping" mean?

Making a UV map is called unwrapping because it is like peeling an orange and pressing the peel flat on a table. You cannot flatten a sphere without cutting it, and you cannot flatten most 3D shapes without cuts either.

A good everyday picture is a cardboard box: cut along some edges and it folds out into a flat cross shape. The flat cross is the UV layout; the box is the model.

Seams and islands

Seams are the edges where you cut. Along a seam, two neighbouring triangles on the model end up in different places on the texture image. Where seams go matters:

• Put them where people rarely look: under the arms, the inside of legs, the back of the head, along natural lines like clothing hems or panel edges.
• Avoid seams across the face or in the middle of a logo, because a slight colour mismatch along a seam is easy to spot.

Islands are the flat pieces you get after cutting, like the cross of the box. A character might have islands for the head, torso, each arm and each leg. Islands are packed into the square texture with a little gap between them, called padding, so colour from one island does not bleed into its neighbour when the texture is shrunk for distance (mipmapping).

Stretching and distortion

Flattening a curved surface always bends it a little. When a UV island is squashed or stretched compared with the real surface, the texture looks squashed or stretched on the model too: circles become ovals, checker squares become long rectangles.

Artists check this with a checker texture: a grid of squares applied to the model. If the squares stay square and roughly the same size everywhere, the unwrap is good.

Texel density

A texel is a pixel of a texture as it sits on the model. Texel density is how many texels cover a given area of surface, for example texels per metre.

What matters most is that the density is consistent. If the head gets a large area of the texture and the body gets a tiny corner, the head is sharp and the body is blurry, and the difference is obvious. A common exception: faces and hero details sometimes get a bit more space on purpose, and hidden parts like the soles of shoes get less.

Texel density also explains why bigger textures look sharper: a 4096 by 4096 image has four times as many pixels as 2048 by 2048, so each part of the model gets more detail.

UV problems that break AI texturing

AI texturing paints views of the model and then writes the colours into the texture using the UVs (the whole process is in how projection texturing works). So UV problems turn directly into texture problems:

Overlapping and mirrored UVs are common in game assets on purpose: they save texture space when both sides of a character really look the same. For AI texturing, where each view paints unique details, they usually cause trouble.

Do you need to unwrap by hand?

For hand painting and for production game assets, a careful manual unwrap is still worth it. You control the seams, the space each part gets, and how the layout is organised.

For AI texturing, an automatic unwrap is often fine, because the AI paints across the model in 3D and you rarely edit the texture image directly. Blender has Smart UV Project for this, and many tools have similar options.

In the AI 3D Texturing Studio you do not have to unwrap. When a model arrives without UVs (most STL and many PLY files), the studio builds a UV atlas for it. When it has UVs, a UV map choice appears with Keep my UVs selected: the AI paints into your own layout and materials. If more than 5% of the painted area overlaps, a yellow warning says mirrored parts will share paint and points you to Make new UVs, a fresh layout where every part of the model gets its own texels. Switching after you have painted starts the texture over, so choose before you press Auto texture.

When the studio has made new UVs, download the GLB, not just the PNG. The GLB contains the new UVs. The PNG alone only fits UVs your original file does not have. The note under the Download box tells you which case you are in.

If you keep your UVs, the studio paints into your own UV layout, one texture per material (a model with very many materials is combined into one texture), at your file’s own texture size when it has one. The PNG then drops straight into the base colour slot of your existing material in Blender, Unity or Unreal. The tutorial keep your own UV map when texturing with AI walks through the choice.

A 2-minute UV check before you texture

1. Open the model in Blender and switch to the UV Editing workspace. If the UV editor is empty, the model has no UVs.
2. Select all faces in the 3D view and look at the layout. Pieces stacked exactly on top of each other mean overlapping or mirrored UVs.
3. Apply a checker texture (Blender can generate one as a new image). Look for stretched rectangles and very small squares.
4. Check that islands stay inside the 0 to 1 square. Islands outside it repeat the texture, which is fine for tiling materials but not for a unique painted texture.

Do not have Blender handy? Our GLB viewer and OBJ viewer let you open a model in the browser to see what you are working with.

Questions