UDIM naming convention: tile numbers, file names and Unreal import

How UDIM tile numbers work

UDIM stands for U DIMension. Instead of fitting the whole UV map into the single 0 to 1 square, you spread UV islands over several squares (tiles), and each tile gets its own image. That lets the parts that matter get a big texture and the rest a smaller one.

The numbering, as the Blender manual puts it: the main tile is 1001, the next tile 1002 goes to its right, the array is ten tiles wide (1001 to 1010 on the first row), and the next row starts above the main tile, so 1011 is directly above 1001.

• From UV to tile: UDIM = 1001 + floor(U) + 10 × floor(V). A UV point at (2.3, 1.6) is in tile 1001 + 2 + 10 = 1013.
• From tile to UV: subtract 1001, then the last digit is the U column and the rest is the V row. Tile 1024: 1024 − 1001 = 23, so U column 3 (U 3 to 4) and V row 2 (V 2 to 3).
• Why ten columns: U only runs from 0 to 10 in the UDIM scheme. RenderMan’s docs state that UDIM is only defined for U values from 0.0 to 10.0 and positive V values, and warn against negative UVs in any tile workflow.

Autodesk Maya writes the same rule as 1000 + (u + 1 + v × 10), which is the same number.

UDIM tile table: 1001 to 1020

The first two rows of tiles, with the matching names in the two u/v styles (explained in the next section). U and V are the UV ranges each tile covers.

Quick check: every tile in the first column (1001, 1011, 1021, …) starts at U = 0, and every tile on the bottom row (1001 to 1010) starts at V = 0.

UDIM file name patterns by app

There are two families of tile names. UDIM numbers (1001, 1002, …), labelled the Mari style in Maya and RenderMan, are what Unreal, Blender, Houdini and USD read. u/v names write the column and row into the file name instead, and they come in a 0-based and a 1-based flavour.

Mudbox vs ZBrush: 0-based or 1-based?

Both write the tile’s column and row into the name, but they count from different numbers. In the Mudbox style the first tile (U 0 to 1, V 0 to 1) is u1_v1; in the ZBrush style the same tile is u0_v0. Maya’s UV Tiling Mode list names them exactly that way: “0-based (Zbrush)” and “1-based (Mudbox)”, and RenderMan’s atlas styles use the same examples. Blender’s <UVTILE> token is the 1-based (Mudbox) form.

To convert: Mudbox u = ZBrush u + 1, and UDIM = 1000 + Mudbox u + 10 × (Mudbox v − 1). So u3_v2 in Mudbox style is UDIM 1013.

A name pattern to copy

A clear, engine-friendly pattern is <prefix>_<asset>_<map>.<UDIM>.<ext>. Epic’s recommended asset naming is [AssetTypePrefix]_[AssetName]_[Descriptor]_[Variant] with T_ as the texture prefix, so a rock’s colour tiles become:

• T_Rock_BaseColor.1001.png
• T_Rock_BaseColor.1002.png
• T_Rock_BaseColor.1011.png

Use a dot before the four digits. Blender’s Detect UDIMs looks for a four-digit number after a dot just before the extension, and Unreal’s documented pattern is BaseName.####.

How to import UDIM textures into Unreal Engine

In Unreal, UDIM tiles are imported as a Streaming Virtual Texture (SVT). Epic lists UDIM support under SVT, not under the other method, Runtime Virtual Texturing.

1. Turn on virtual textures for the project: Project Settings > Engine > Rendering > Virtual Textures > Enable virtual texture support. The editor must be restarted after this change.
2. Name the tiles BaseName.####.ext with exactly the same BaseName, for example T_Rock_BaseColor.1001.png and T_Rock_BaseColor.1002.png.
3. Put all tiles in one folder and import one of them. Unreal scans the source folder for other images with the same BaseName and a different tile number, and imports the whole group as one Virtual Texture asset.
4. Sample it with a virtual sampler in the material. Epic’s docs say virtual textures use a Virtual sampler type in the Texture Sample node, such as Virtual Color instead of Color.

Tiles do not have to be the same size. Epic’s example is a 2 × 2 set with two 2048 px and two 128 px tiles, which the engine samples like one large texture; that is how UDIM lets a hero area get more pixels. Epic also notes that UDIM textures are flipped vertically during import.

Converting an ordinary texture by hand (ticking Virtual Texture Streaming in the Texture Editor) breaks the materials that already use it until you switch their samplers to a Virtual type. Right-click > Convert to Virtual Texture updates them for you.

UDIM in Blender, Houdini and RenderMan

Blender

• Load a set: Image > Open, pick any one tile and keep Detect UDIMs on. Blender looks in the same folder for files with a four-digit number before the extension and loads them as one tiled image.
• Make tiles: in the Image or UV Editor, Sidebar > Image > UDIM Tiles, add tiles starting at 1001 and use Fill so each one gets an image. A tile that is not filled is not saved with the image.
• Save: put <UDIM> in the file name (monster-basecolor.<UDIM>.png) and Blender writes one file per tile.

Houdini and USD

Write <UDIM> where the number goes, as in kaiju<UDIM>.exr. SideFX notes that most modern tools, including USD’s path resolver, expand it. Houdini’s file dialog can mistake a UDIM set for a frame sequence; choose <UDIM> rather than $F from its menu.

RenderMan

PxrTexture reads Mari UDIM, Mudbox and ZBrush tile sets. With an Atlas Style set, the file name carries a placeholder (_MAPID_ in RenderMan 25) that is replaced by the tile number or u/v name.

Why a UDIM import fails, and the fix

UDIM and the AI 3D Texturing Studio

Our AI 3D Texturing Studio paints one texture in the 0 to 1 UV square; it does not write UDIM tile sets. If a model’s UVs are spread over several tiles (more than 2% of the UV points outside 0 to 1), the studio treats them as not usable for painting and makes a fresh single-tile UV atlas, then exports one texture inside the GLB.

That suits quick props, prints and previews. For a hero asset you plan to finish in Unreal with UDIMs, keep the UDIM layout in your DCC app and use the studio for look development on a 0 to 1 copy.

Questions