2020-11-10_08:30:07
A Static Mesh can have collision shapes. Because doing collision detection on the high-polygon mesh is often not necessary and too expensive. Collision shapes are what are sent to the physics engine. A collision shape on the Static Mesh is required for enabling Simulate Physics on a Static Mesh Component.
Some functionality in the engine (which?) require that the collision shape is the Root Component in the Actor.
Simple collisions are a collection of primitive or convex shapes that approximate the shape of the Static Mesh.
Collision shapes are added in the Static Mesh Editor, in Top Menu Bar > Collision. A number of collision primitives can be added. Each shape is added, not replacing an existing one. The collision shapes are listed under Details Panel > Collision > Primitives. There is one array per shape type: sphere, box, capsule, convex, and tapered capsule. The Details Panel doesn't support adding or deleting collision primities. To delete a collision primitives select it in the viewport and press Delete. A selected collision shape can be translated and rotated with the usual W, E, S controls. By adding and positioning multiple shapes we can produce a combined shape that roughly matches the shape of our StaticMesh. Another type of shape is K-DOP. This is a family of convex hull shapes. Adding one to the StaticMesh adds it to the Convex Elements list in Details Panel > Collision > Primitives.
The collision primitives are held by a class named UBodySetup.
UStaticMesh has a UBodySetup member named BodySetup.
UBodySetup contains a bunch of other stuff as well.
UBodySet has a FKAggregateGeom member named AggGeom.
FKAggregateGeom contains the same lists as we saw in the Collisions category of the Static Mesh's Details Panel.
A complex collision is a secondary Static Mesh that is used for collision detection. Use this when a collection of primitives is insufficient.
It is set in Details Panel > Collision > Complex Collision Mesh. It is possible to use the mesh itself as the complex collision mesh. A Static Mesh can have multiple LODs. It is never not a good idea to swap the collision mesh in the way a rendering mesh is swapped. Therefore we set a single LOD to use for collisions in Details Panel > General Settings > LOD For Collision.
The UStaticMesh class has the two properties UStaticMesh* ComplexCollisionMesh and int32 LODForCollision.
The complex collision can be created in a mesh creation software, such as 3D Studio Max, together with the visual mesh.
The collision mesh must be named UCX_<FULL_NAME_OF_MESH>_/d+/.
For example, a model that contains the two meshes named Container and Lid can have collision meshes named
- UCX_Container_00
- UCX_Container_01
- UCX_Lid_00
When importing a mesh that has collision shapes included, disable Auto Generate Collision in the FBX Import Options.
The collision will show up as a Simplified Collision, using a Convex. At least in the tutorial at Building Better Pipelines - Static Meshes @ learn.unrealengine.com. Don't know what will happen if the collision mesh isn't convex.
Convex decomposition is the process of approximating a mesh using a collection convex shapes. Multiple simple convex shapes are often more performant than a single complicated highly detailed mesh. A collection of convex shapes can often approximate a arbitrary shape quite well.
The Static Mesh Editor has a Convex Decomposition Panel. If not, enable it in Top Menu Bar > Window > Convex Decomposition.
[[2020-07-04_15:24:50]] Collisions
[[2020-11-10_08:30:07]] Collision presets