> For the complete documentation index, see [llms.txt](https://docs.gta.clothing/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.gta.clothing/game-mechanics/mesh-physics-yld.md).

# Mesh Physics (YLD)

## Introduction

Most clothing only needs normal skeleton weights. Components with loose sections, such as long coats, skirts or suitable hair, can use a separate **Mesh Physics (YLD)** file when those sections should continue moving dynamically.

{% hint style="info" %}
Want to author the simulation in Blender? Follow [Creating Cloth Physics (YLD)](/cloth-modding/cloth-physics-yld.md). This page explains the file relationship and how Durty Cloth Tool handles it.
{% endhint %}

## What are YLD Files?

**YLD files** contain character-cloth simulation data associated with a component YDD. The visible model remains in the YDD, while the matching YLD provides the low-density simulation mesh and related physics data used by that component.

## Purpose of YLD Files

The **Mesh Physics (YLD)** file can provide:

1. **Dynamic Cloth Simulation**: Loose areas can react to character movement instead of following skeleton weights rigidly.
2. **Simulation Binding and Collision**: The YLD works with the visible cloth region, pinned attachment area and configured collision bounds.

Durty Cloth Tool currently builds character YLD companions for component drawables. Do not treat this as the workflow for fragment/prop cloth stored in a YFT.

## Components of a YLD File

A **YLD file** contains the data needed by the character-cloth simulation. Durty Cloth Tool can attach, import, export, preview and build a matching YLD with a component, but it does not provide controls for editing individual physics fields. Author or edit those files with a current Sollumz release.

{% hint style="warning" %}
Do not copy numeric values from an unrelated item or assume that one setting is a universal preset. Start from a comparable working component, change one variable at a time and test the result in-game.
{% endhint %}

## How Mesh Physics (YLD) Files Work

The **YLD** is tied directly to the **YDD** that provides the visible component geometry, materials, LODs and normal rig. The YLD does not replace correct skeleton weights, materials or the visible model.

**Example**:\
A long coat may use a YDD for its visible shape and a matching YLD for the simulated lower section. Exact binding, collision and movement depend on both files and must be tested together.

Without a matching YLD, DCT does not generate character-cloth simulation automatically. The item continues to use its normal visible mesh and skeleton weights.

## Related File Naming and Structure

For automatic DCT resolution, a binary YLD should sit beside the corresponding YDD, share the same base name and end with `.yld`. See [Auto Cloth Data Resolving](/basics/good-to-know/auto-cloth-data-resolving.md).

**Example file structure**:

```
jbib_000_u.ydd
jbib_diff_000_a_uni.ytd
jbib_000_u.yld
```

In this example, `jbib_000_u.ydd` and `jbib_000_u.yld` form the model/physics pair. Matching names alone do not prove that their meshes and simulation data are compatible.

{% hint style="info" %}
When using Sollumz XML, import the `.ydd.xml` first, then select the matching `.yld.xml` manually in the component's **Mesh physics file (.yld)** field. DCT converts the XML and manages the final binary file.
{% endhint %}

## Durty Cloth Tool Workflow

Once a YLD is attached to a component, DCT:

* Keeps it with the drawable in the project and item packages.
* Generates the final component filename and cloth metadata during the build.
* Includes the matching YLD in supported component resource output.
* Makes the attached physics available to the clothing preview.

Use the [Cloth Analysis Report](/basics/essential-knowledge/cloth-analysis-report.md) to catch a missing linked file and the [3D Preview](/basics/essential-knowledge/3d-preview-clothes.md) with animations to catch obvious motion problems.

{% hint style="warning" %}
DCT does not validate the YLD hierarchy, simulation topology, pins, visible-mesh binding, bone weights, collision capsules or physics values. Run Sollumz Cloth Diagnostics and complete an in-game test before release.
{% endhint %}

## Common Use Cases for YLD Files

**1. Capes, Skirts and Long Coats**\
The loose lower section can move independently while the fixed section remains normally rigged.

**2. Hair**\
Suitable long hair components can use a YLD for selected loose sections.

**3. Loose Sections Within Components**\
Tails, tassels or scarf-like areas that are part of a supported component can use the same character-cloth approach when the visible mesh, material and proxy are prepared correctly.

## Summary

**Mesh Physics (YLD)** files are an optional companion for component areas that need simulated movement. Durty Cloth Tool handles the project association, generated naming, metadata and resource output, while Sollumz handles the simulation authoring and diagnostics. Build with [Build Options](/basics/essential-knowledge/build-options.md), then follow [Testing Addon Clothes](/getting-started/testing-addon-clothes.md) for final verification.