xatlas

A cleaned up version of thekla_atlas.

Mesh charting, parameterization and atlas packing. Suitable for generating unique texture coordinates for baking lightmaps.

Screenshots

Example - Cesium Milk Truck

| Viewer | Random packing | Brute force packing | |---|---|---| | | | |

Example - Godot Third Person Shooter demo

Graphite/Geogram

Changes from thekla_atlas

• Smaller code size - from about 18 KLOC to 10 KLOC
• Easier to integrate and build - a single source/header file pair instead of around 120 files and 10 directories.
• Atlas resolution option for outputting multiple atlases.
• Flexible data description API for input meshes.
• Better tolerance of bad input geometry. Zero length edges and zero area faces are ignored.
• Support for packing multiple atlases/parameterizations into a single atlas.

How to use

Generate an atlas (simple API)

1. Create an empty atlas with xatlas::Create.
2. Add one or more meshes with xatlas::AddMesh.
3. Call xatlas::Generate. Meshes are segmented into charts, which are parameterized and packed into an atlas.

The xatlas::Atlas instance created in the first step now contains the result: each input mesh added by xatlas::AddMesh has a corresponding new mesh with a UV channel. New meshes have more vertices (the UV channel adds seams), but the same number of indices.

Cleanup with xatlas::Destroy.

Example code here.

Generate an atlas (tools/editor integration API)

Instead of calling xatlas::Generate, the following functions can be called in sequence:

1. xatlas::ComputeCharts: meshes are segmented into charts.
2. xatlas::ParameterizeCharts: charts are flattened into 2D parameterizations.
3. xatlas::PackCharts: charts are packed into one or more atlases.

All of these functions take a progress callback. Return false to cancel.

You can call any of these functions multiple times, followed by the proceeding functions, to re-generate the atlas. E.g. calling xatlas::PackCharts multiple times to tweak options like unit to texel scale and resolution.

See the viewer for example code.

Pack multiple atlases into a single atlas

1. Create an empty atlas with xatlas::Create.
2. Add one or more meshes with xatlas::AddUvMesh.
3. Call xatlas::PackCharts.

Example code here.

TODO

• Segmentation: improve chart merging by using similar metrics to chart growing
• Segmentation/Parameterization: detect geometry with zero Gaussian curvature (e.g. a cylinder) and unwrap as a single chart
• Viewer: better lightmap baking
• Viewer: chart picking in scene and texture views

Technical information / related publications

Ignacio Castaño's blog post on thekla_atlas

P. Sander, J. Snyder, S. Gortler, and H. Hoppe. Texture Mapping Progressive Meshes

K. Hormann, B. Lévy, and A. Sheffer. Mesh Parameterization: Theory and Practice

P. Sander, Z. Wood, S. Gortler, J. Snyder, and H. Hoppe. Multi-Chart Geometry Images

D. Julius, V. Kraevoy, and A. Sheffer. D-Charts: Quasi-Developable Mesh Segmentation

B. Lévy, S. Petitjean, N. Ray, and J. Maillot. Least Squares Conformal Maps for Automatic Texture Atlas Generation

O. Sorkine, D. Cohen-Or, R. Goldenthal, and D. Lischinski. Bounded-distortion Piecewise Mesh Parameterization

Y. O’Donnell. Precomputed Global Illumination in Frostbite

Used by

ArmorPaint

Bakery - GPU Lightmapper

Godot Engine

Graphite/Geogram

Filament

Lightmaps - An OpenGL sample demonstrating path traced lightmap baking on the CPU with Embree

toy / two

Wicked Engine

Related projects

Microsoft's UVAtlas - isochart texture atlasing.

simpleuv - Automatic UV Unwrapping Library for Dust3D.

Ministry of Flat - Commercial automated UV unwrapper.

Lightmapper - Hemicube based lightmap baking. The example model texture coordinates were generated by thekla_atlas.

aobaker - Ambient occlusion baking. Uses thekla_atlas.

seamoptimizer - A C/C++ single-file library that minimizes the hard transition errors of disjoint edges in lightmaps.

Models used

Gazebo model by Teh_Bucket