UDIM in Blender: spreading textures across several tiles

A character, a vehicle or a detailed set often needs more detail than a single texture can hold. Pushing a single image to 8K or 16K costs a lot of memory and wastes resolution on the less visible parts. UDIMs solve this problem by spreading the UVs across several images. Blender handles them without an add-on.

What is a UDIM?

UDIM, for "U Dimension", is a tile system. Instead of a single UV space from 0 to 1 tied to one image, the object has a grid of squares: the square from 0 to 1, the one from 1 to 2 to its right, and so on. Each square, or tile, has its own UV space and its own image. For the material, the whole set remains a single texture.

The benefit, according to the Blender manual, is choosing a resolution per area: a 4K tile for the main details, 2K or 1K tiles for less important areas. A single texture would be too low in resolution for the large UV islands and too high for the small ones. The manual cites three textures as a good minimum: one 4K, one 2K and one 1K.

Tile numbering

The main tile is numbered 1001. The next one, 1002, sits to its right. A row has ten tiles, from 1001 to 1010; the eleventh, 1011, starts a new row just above 1001.

The number can be calculated: 1001 + u + 10 × v, where u and v are the column and row indices starting from 0. The tile in the third column and the second row (u = 2, v = 1) is therefore 1013.

When to switch to UDIMs

• An object too large for a single texture at the level of detail you want. Several tiles cost less than a giant image. The guide texel density in Blender explains how to set that density.
• Areas of unequal importance: a character's face deserves more pixels than the soles of its shoes.
• Textures already delivered as UDIMs, numbered 1001, 1002 and onwards, by a texture painting application or another member of the team.

For a prop or a set seen from a distance, a single texture remains simpler: each tile adds a file to manage.

Spreading UVs across several tiles

1. Unwrap the mesh as for any UV map.
2. Decide how many tiles to use and which part of the model goes in each.
3. In the UV Editor, move each group of islands into its tile and scale it as on a standard UV map. A move of exactly one unit (G, X, 1) brings the islands into the neighbouring tile.
4. To arrange them automatically, UV > Pack Islands offers the Pack To option: Closest UDIM packs the islands into the tile closest to the centre of the selection, Active UDIM into the tile of the active image or, with no image, into the tile under the 2D cursor.

Each island must fit within a single tile: an island straddling two tiles picks up two different images.

Creating tiles in the Image Editor

Create a tiled image with Image > New (Alt+N) by ticking the Tiled option. The tiles are then managed in the Image Editor or the UV Editor, sidebar, Image tab, UDIM Tiles panel:

• Add Tile adds tiles: starting number (Number), number of tiles (Count), optional label (Label) and the Fill option to fill them with a generated image.
• Remove Tile deletes the selected tile, along with its content if it has not been saved.
• Fill Tile fills an existing tile with a generated image.

A point the manual flags: a tile that has not been filled is not saved with the image. Create the tiles with Fill, or fill them before saving.

Naming and loading UDIM textures

Blender identifies each tile in the file name with a token in angle brackets, replaced when saving as well as when loading:

• <UDIM>: the four-digit number. An image saved as character-basecolor.<UDIM>.png gives character-basecolor.1001.png, character-basecolor.1002.png, and so on.
• <UVTILE>: the u1_v1 notation, where u and v start at 1. Tile 1001 becomes u1_v1.

To load textures produced elsewhere: in the Image Editor, Image > Open (Alt+O), select one of the files in the series and leave the Detect UDIMs option ticked. Blender looks for the other tiles in the same folder and loads them as a single image. Detection relies on a four-digit number placed just before the extension. In the material, a single Image Texture node is enough for all the tiles.

The manual notes one limitation: you cannot assign an existing file to a tile that has already been created. To replace the content of a tile, save the image, then replace the file on disk while keeping its name, or edit it in another application.

Painting on UDIMs

Blender paints directly on the tiles, with 2D painting in the Image Editor as well as 3D painting in the 3D Viewport. Painted images are saved separately from the .blend file, with Image > Save: remember to do it after each session.

UDIMs, video memory and rendering

UDIMs do not reduce memory by themselves: each loaded tile takes up its own space in memory at render time, and ten 4K tiles weigh ten times as much as one 4K image. The saving comes from choosing the resolution tile by tile. When the scene exceeds the memory of the graphics card, the render slows down sharply or stops: the article fixing the Out of Memory error in Blender (in French) details the settings that free some up.

For a render on another machine, all the tiles must go with the scene. A forgotten tile does not necessarily block the render: it is simply missing from the image.

Common mistakes

• A tile disappears after saving: it had not been filled. Use Fill or Fill Tile before saving.
• Blender only loads one tile: Detect UDIMs was not ticked, or the files do not carry the four-digit number just before the extension.
• Part of the model shows the wrong texture: a UV island spills over into the neighbouring tile.
• Sharpness changes from one area to another: a 1K tile sits next to a 4K tile on the same visible surface. Even out the level of detail between neighbouring tiles.
• Paint changes are lost: the image was not saved, only the .blend file was.