View source for Zh/Cloth Simulation

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'''布料物理模拟'''提供了一种在模型上模拟布料变形的方法。{{src2|4}} 的物理引擎 {{L|Rubikon}} 支持布料模拟。
{{Warning|此页面大部分内容都是基于老版本起源2引擎的，新的内容（包括ModelDoc）暂未被添加！}}
{{note|该系统基本上没有记录，官方资料也不多。}}
{{todo|根据 HLA 的新功能进行更新！}}

== 支持的引擎 ==
=== {{source|4}} ===
{{dota2|4}} 的原始 {{source|4}} 版本包含一个传统的布料模拟系统。{{src2|4}} 中的布料系统似乎向后兼容该系统。

=== {{source2|4}} ===

布料模拟在{{dota2|4}}中主要用于角色服装。在 {{L|SteamVR Home}}（官方拳击场环境中被击败的拳击机器人模型）中也有使用。
<s>当前SDK中的模型编辑器对制作布料的支持有限。尚未发布的 ModelDoc 引擎内工具似乎将取代它，但其功能尚不清楚。阀门工作流程似乎使用 Maya 或 Modo 为网格绘制每个顶点的属性。DMX 模型格式可用于将这些属性导入引擎工具。</s> <br>
待补充新内容。
{{todo|There may be per-mesh or per-bone attributes as well.}}

The cloth system appears to have received significant upgrades after the initial Dota 2 implementation, including collision with world meshes. Old versions of the Source 2 tools include help files for the model editor that document many of the cloth features. These help files were removed with an update that changed the tool help file format. Converted versions of the help files can be obtained [https://github.com/Rectus/source2_toolhelp_converted here].

{{todo|Jigglebones in Source 2 appear to be implemented with cloth simulation.}}

{{todo|Keyvalues in the .vmdl file suggest other softbody systems exist.}}

{{bug|In SteamVR Home, cloth will stop simulating after a half second unless an animation is playing on the model.}}

== Features ==
Known features include:
* Can drive meshes either directly, through a proxy mesh or through manually set up nodes.
* Supports collision with other objects: Spheres and capsules, convex hulls, and even arbitrary meshes are supported.
* {{todo|Self collision within the cloth does ''not'' seem possible.}}
* Supports wind from {{L|env_wind}}
* Pressure forces
* Volumetric solver
* Cloth can be driven from animations
* Bend and stretch springs {{todo|Shear springs may exist.}}

== How it works ==
A cloth piece consists of a set of cloth nodes connected together with quads and rods. Quads connect four nodes together and act as cloth surfaces, while rods connect two nodes and act as stiff rods or ropes. 

The cloth nodes can drive bones in the model skeleton, which in turn drive the render mesh. Apart from manually setting up nodes and bones, the model compiler can generate cloth procedurally from attributes painted on the vertices of a mesh. The compiler will then generate a cloth node and a bone on each vertex with cloth enabled.

The attributes can be painted directly onto the render mesh, but also onto a separate cloth proxy mesh. The cloth will them be generated from the vertices of the proxy mesh instead. The proxy mesh can automatically skin and drive the render mesh, or both meshes can be skinned to the same skeleton and the joints back-driven from the cloth. This can be used if the render mesh doesn't have good topology to support cloth, or to make a simpler more performant cloth setup.

{{todo|It appears possible to attach physics colliders to bones driven by cloth. Can it be used to have cloth affect other objects and/or receive feedback through the collider?}}

{{todo|Cloth parameters can be modified with animation events. The command syntax is undocumented.}}

== External tools ==

{{note|Many of the features are still undocumented. No official best-practices are available.}}

Cloth attributes can be painted on with an external modeling tool. A plugin is required to export a mesh with the attributes. 

Valves plugins have not been released, but an experimental fork of the {{L|Blender Source Tools}} addon capable of exporting {{L|DMX/Source_2_Vertex_attributes|DMX vertex attributes}} is available [https://github.com/Rectus/BlenderSourceTools here].

== Ways of adding cloth ==

To edit cloth properties, a collision file is needed in the model. In the model editor, right click '''Cloth (Compiled)''' or ('''NULL Collision''' if no cloth was generated) in the outliner, and choose '''New Collision'''. An item for cloth settings will now be available, as well as several more right-click options.

{{warning|Many of the commands and options may crash the tools. Make sure to save often.}}

=== Paint onto render mesh ===

Paint the <code>cloth_enable</code> attribute onto vertices to be simulated.

The by default a hidden setting in the model forces back-solving to be used. To enable cloth processing on the render mesh, open the .vmdl file of the model in a text editor, navigate to the <code>m_clothProxyMeshList</code> category and set <code>m_bDriveMeshesWithBacksolvedJointsOnly = false</code>

==== Pros ====
* Relatively simple

==== Cons ====
* Expensive on high poly meshes
* Requires planned mesh topology


=== Auto-match render mesh to proxy mesh ===

Author a proxy mesh and paint the <code>cloth_enable</code> attribute onto it.

To show the cloth proxy mesh category in the model editor, edit the .vmdl file navigate to the <code>m_clothProxyMeshList</code> category and add a set of curly brackets inside the square brackets of the <code>m_clothProxyMeshList</code> key inside. It should look like: <code>m_clothProxyMeshList = [ {} ]</code>

To make the auto matching work, add the proxy mesh to the proxy list, and uncheck all the back-solve options. Note that both meshes need to be skinned to the same skeleton to be matched.

==== Pros ====
* Auto-matches render mesh to cloth

==== Cons ====
* May not match well in all cases

=== Back-solve mutual bones on proxy mesh to drive render mesh ===

Enable either of the back-solving options for the proxy mesh.

==== Pros ====
* Gives good control on how the render mesh deforms

==== Cons ====
* Only deforms mesh from existing bones


=== Add cloth piece on predefined bones ===

Create a grid of bones in an external modeling tool. It is recommended to keep a good naming conversion.

Right click the collision properties and choose 'Add Cloth Piece'. Use the UI to select bones set up the cloth.

==== Pros ====
* Might be a good way to add ropes

==== Cons ====
* Hard to use UI
* Many features missing or undocumented
* Bones have to be in a grid pattern and oriented right
* Tools will crash when misconfigured


=== Manually add nodes on predefined bones ===

Use the cloth properties to add nodes manually. It is recommended to turn off AutoComp while doing this.

==== Pros ====
* Exact control over topology

==== Cons ====
* Extremely hard to use UI
* Many features missing or undocumented
* Tools will crash when misconfigured


=== Author it in ModelDoc? ===

Wait for Valve to release the tool (this may take a long time).

==== Pros ====
* Added more recently than other methods

==== Cons ====
* May not be a complete way to author cloth
* Unavailable for mortals

== Using models with cloth ==
=== SteamVR Home ===
Only the {{L|prop_dynamic}} entity class seems to work properly with cloth, although it can be parented to other entities. {{todo|The entity may have to be parented to a bone of the parent entity in order not to crash.}}

{{bug|The prop needs to have an animation playing at all times for the cloth not to sleep and reset. The animation does not need to move any of the existing bones though, so any animation file with multiple frames can be added and set to loop for models that do not already have animations.}}

{{ACategory|Modeling}}
{{ACategory|Source 2}}
{{ACategory|Physics}}