UE4中PhysX BroadPhase（碰撞检测的粗略阶段）

PhysX的BroadPhase（碰撞检测的粗略阶段），具体是用AABB（轴向包围盒）来做碰撞检测

具体算法有两种：SAP（Sweep And Prune）和MBP（Multi Pruning Box，多个剪枝盒）

 

SAP（Sweep And Prune）

当场景中有大量的物体（大世界有百万级别）时，即使它们已按AABB的三个轴向xyz做了排序，一个动态物体在移动过程与它们进行AABB判断也是一个很耗时操作

 

TG_EndPhysics的卡顿与BpSAP.UpdateWork直接相关

 

 

MBP（Multi Pruning Box，多个剪枝盒）

为了解决与物体判断AABB的次数，可以使用MBP来将整个场景划分成多个块，这样动态物体只用与当前所在块中的物体进行AABB判断，大大减少BroadPhase（碰撞检测的粗略阶段）的耗时

 

全局Default Broadphase Settings

注1：勾上Use MBPOn Client，并设置MBPBounds和MBPNum Subdivs的数值

注2：MBPBounds的Min、Max缺省为0，表示区域为整个场景

注3：MBPNum Subdivs为2，即将MBPBounds划分成2x2，一共4个块

 

该配置会保存在DefaultEngine.ini中

[/Script/Engine.PhysicsSettings]

DefaultBroadphaseSettings=(bUseMBPOnClient=True,bUseMBPOnServer=False,bUseMBPOuterBounds=False,MBPBounds=(Min=(X=0.000000,Y=0.000000,Z=0.000000),Max=(X=0.000000,Y=0.000000,Z=0.000000),IsValid=0),MBPOuterBounds=(Min=(X=0.000000,Y=0.000000,Z=0.000000),Max=(X=0.000000,Y=0.000000,Z=0.000000),IsValid=0),MBPNumSubdivs=2)

 

在地图中可以覆写全局Default Broadphase Settings

 

具体代码详见：UnrealEngine\Engine\Source\Runtime\Engine\Private\PhysicsEngine\PhysScene_PhysX.cpp

void FPhysScene_PhysX::InitPhysScene(const AWorldSettings* Settings)

 

相关CVar变量

p.ForceMbpClient	Forces all created scenes to use MBP on client builds
p.ForceMbpServer	Forces all created scenes to use MBP on server builds
p.ForceNoKKPairs	Disables kinematic-kinematic pairs. This is required when using APEX destruction to correctly generate chunk pairs - when not using destruction this speeds up the broadphase by early rejecting KK pairs.
p.ForceNoKSPairs	Disables kinematic-static pairs. This makes converting from static to dynamic a little slower - but provides better broadphase performance because we early reject those pairs.
p.OverrideMbpNumSubdivisionsClient	Override for number of subdivisions to perform when building MBP regions on a client, note regions are only generated when a scene is created - this will not update the scene if it's already running (0 = No override, 1>16 - Override number)
p.OverrideMbpNumSubdivisionsServer	Override for number of subdivisions to perform when building MBP regions on a server, note regions are only generated when a scene is created - this will not update the scene if it's already running (0 = No override, 1>16 - Override number)

 

参考

Dive Into PhysX Broad Phase In UE4