VAT (Vertex Animation Texture) on Unity HDRP examples

This is a repository that contains examples of the use of VAT (Vertex Animation Texture) on Unity High Definition Render Pipeline (HDRP).

In this document, "VAT" refers explicitly to the texture encoding method used in Houdini and SideFX Labs.

Several types of VAT: Soft, Rigid, Fluid and Sprite

System requirements

• Unity 2019.4
• HDRP 7.4

How to use VAT with Shader Graph

At first, export VAT files from Houdini. It consists of a geometry file (.fbx), texture files (.exr) and a realtime data file (.json). Then import .fbx and .exr files into Unity. The texture files must be imported with the following settings:

• sRGB (Color Texture): Off
• Non-Power of 2: None
• Generate Mip Maps: Off
• Format: "Automatic" is recommended. You can select a lower BPP format with sacrificing quality.
• Compression: "None" is recommended. You can try other options, but usually, they don't work with non-power of two textures.

There are three types of shader graphs under the "Shader Graph" group: Soft, Rigid, and Fluid. You can use corresponding VAT methods with these shader graphs.

These shader graphs have some VAT dependent properties.

The "_numOfFrames", "_posMax" and "_posMin" properties must be set based on the realtime data. Open the exported .json file with a text editor and copy-paste these values to the corresponding properties.

You can use the packed-normal encoding (the "Pack normals into Position Alpha" option in the VAT exporter) with enabling the "Use Packed Normals" option in the material settings. Note that this may significantly increase the quantization errors in normal vectors.

To animate the mesh, you have to control the "Current Frame" property manually. Using a timeline would be the handiest way to do it.

The structure of the shader graph is quite simple. You can easily extend it to add features, like adding a albedo map or support of different surface types.

For example, an extended shader graph named "Soft Lerp" is used in the Soft example, that interpolates positions/normals between consecutive frames to achieve smooth animation.

How to use VAT with Visual Effect Graph

You can use a Sprite VAT to move particles in a Visual Effect graph; Other methods are not supported at the moment.

This project contains the following three subgraph operators that are designed to be used with VAT.

VAT Particle Count -- Calculates the number of particles contained in VAT. This is convenient to determine the number of particles to emit in the Spawn context.

VAT Particle UV -- Calculates the texture coordinates for each particle. This can be used to retrieve data from a position map.

VAT Convert Position -- Converts position data into a object space position vector.

For detailed usage of these operators, see Sprite example in this project.

Frequently Asked Questions

Is it possible to support Universal RP?

Yes. Although these examples are created with HDRP, you can use the same approach on Universal RP. These shader graphs and VFX graphs can be converted by changing a few options. Check the URP branch for details.