Collision mesh

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A static collision mesh (red wireframe)
A jointed collision mesh (yellow wireframes)

A collision mesh is a cheap 3D mesh used by VPhysics. It is sometimes called a collision model, after the C++ object that loads it into the engine, but as far as creating models is concerned, it is a vertex mesh more or less like any other.

Collision meshes are invisible, and should be extremely low-detail compared to the model's visible components (see the images to the right for two examples). The lack of precision will not be noticed, except in that it doesn't grind the simulation to a halt!

Caveats

Collision meshes consist of convex elements only
Any Concave elements will be made into convex elements via the "shrink wrapping" method.
To create a concave mesh build separate convex objects, intersecting each other if necessary, and compile with $concave. Every convex piece adds to the simulation cost so be sparing.
All collision elements must be sealed
There can be no holes anywhere. If studiomdl says "Model has 2-dimensional geometry" you've made a mistake (or at least need to thicken your mesh).
All collision elements must be fully smooth and share the same texture
Source uses the vertex normals of a mesh to determine whether or not the mesh is a sealed element. If a model shares more than one texture it will also cause the "Model has 2-dimensional geometry" error.
If you are making a collisionmodel for a ragdoll or any model with bones, each piece must be fully weighted to a single bone.
Collision pieces must have a weight of "1" to only one bone. If a piece shares weights across multiple bones, it will generate an error on compile.

Smoothing

This section covers how to make a smoothed collision mesh in various modelling packages. Remember that you also need to eliminate any holes in each convex shape.

Blender

  1. In (Icon) Object Mode, run the "Shade Smooth" operator on your mesh.
  2. Remove the (Icon) Edge Split modifier if you've added it for some odd reason.

Maya

  1. Use the Combine command to combine every part into a single editable mesh.
  2. Do NOT append edges or merge verticies after using the Combine command.
  3. Set selection mode to Edges, then select all edges in your collision mesh, then click on the Normals menu and click Soften Edge.
  4. Set up a very basic UVW map, such as an Automatic unwrap.

XSI

A collision mesh created in XSI 6.01. The reference mesh has been moved aside for visibility.
  1. Select every part of the collision mesh and use Model -> Create -> Polymesh -> Merge to combine them into one object.
  2. Use Render -> Get-> Property-> Geometry Approximation. In the Polygon Mesh tab, uncheck Discontinuity: Automatic. All hard edges on the model will disappear.
  3. Apply a texture and a projection to the collision mesh. Anything will do.

3DS Max

  1. Create individual pieces of collision out of simple shapes, or break apart your reference mesh and cap all of the holes as well as well all verts.
  2. Use the Attach function to combine every part into a single editable mesh. (optional)
  3. Do NOT weld vertices together after using the Attach function. All objects within the mesh should be individually selectable using the "Select Element" option within the Edit Mesh modifier.
  4. In Edit Poly, or Edit Mesh, go into either element mode or polygon mode and select all the faces in the mesh. Set them to "smoothing group" "1" in the modifier panel.
  5. Apply a single texture to the collision mesh with at least a basic UWV map. It does not matter what texture as long as the entire collisionmodel uses only 1 material. If you do not specify a texture some exporters will not properly export the face.

SketchUp

  1. Build a collision mesh out of one or more groups of faces. Each group must form a solid convex shape.
  2. Smooth each edge in the group by selecting all the faces inside and then right clicking and selecting "Smooth/Unsmooth edges". Set the slider to the 180 degrees to smooth all edges.
  3. Apply the same material to each face of the collision model. Note you must apply the material to each face, not the group as a whole.
  4. Combine multiple groups of faces into a single component.
  5. Set the axis of the component to the same as your reference model.

Lightwave

  1. Build a collision mesh out of primitives such as boxes and cylinders. As noted above, make sure each primitive is entirely solid and a separate object.
  2. Hit Q to add the mesh to a new surface, and give it a name.
  3. In the Surface Editor (F5), select the new surface, and check the smoothing checkbox. Give it a smoothing threshold of 180.

Cinema 4D

  1. Build a collision mesh out of primitives. Each group must form a solid convex shape.
  2. Create a new Connect Object and drag every primitive into it.
  3. Remove all tags from the primitives (Phong, Textures..) except the UVW tag.
  4. Apply your model's material to the Connect Object and export as SMD.

Milkshape 3D

  1. Build your collision mesh. Each group must form a solid convex shape.
  2. Clear all smoothing groups and assign all your meshes into smoothing gourp 1.
  3. Export your smd.

See also