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 Post subject: Collision Flags & Groups
PostPosted: Tue Sep 05, 2017 10:50 am 
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Joined: Fri Aug 04, 2017 10:38 am
Posts: 2
I'm a little confused in CollisionFlags & CollisionFilterGroups
I'm using BulletUnity (which is build upon BulletSharp - Bullet wrapper for C#).

Here every RigidBody has 3 properties:
Code:
   BulletSharp.CollisionFlags collisionFlags
   BulletSharp.CollisionFilterGroups collisionGroupsIBelongTo
   BulletSharp.CollisionFilterGroups collisionMask

Each one could contain multiple flags on
(the only exclusion I met is TerrainCollisionObject, which has same props, but each of them could contain only 1 flag, but I suppose, it's not proper realization).

I tried to figure out, what every of these 3 props means.
If I've understand correct:
  • `collisionGroups` & `collisionMask` are used in broadPhase to weed out pairs, which should not collide (logically - e.g. arrow with grass)
    `collisionGroups` describes our object, while `collisionMask` describes "which other objects our one should collide" (logically)
  • I suppose, `collisionFlags` describes our object for narrow phase, but I couldn't find code, where it is used for this purpose.

So, my questions are:
  1. Am I right in my understanding of what `collisionGroups` & `collisionMask` are & how they used?
  2. What is `collisionFlags` & how it poperty is used basically & in connection with other 2 props?
    For example, why we need "static" and "kinematic" flags among CollisionFlags enum, if we already have same-named options among CollisionFilterGroups enum, which we could set in `collisionGroups` for our object?

Thank for your help!

PS:
here are corresponding enums (to perhaps save your time to find them in bullet code):

Code:
   enum CollisionFlags
   {
      CF_STATIC_OBJECT= 1,
      CF_KINEMATIC_OBJECT= 2,
      CF_NO_CONTACT_RESPONSE = 4,
      CF_CUSTOM_MATERIAL_CALLBACK = 8,//this allows per-triangle material (friction/restitution)
      CF_CHARACTER_OBJECT = 16,
      CF_DISABLE_VISUALIZE_OBJECT = 32, //disable debug drawing
      CF_DISABLE_SPU_COLLISION_PROCESSING = 64,//disable parallel/SPU processing
      CF_HAS_CONTACT_STIFFNESS_DAMPING = 128,
      CF_HAS_CUSTOM_DEBUG_RENDERING_COLOR = 256,
      CF_HAS_FRICTION_ANCHOR = 512,
      CF_HAS_COLLISION_SOUND_TRIGGER = 1024
   };

   enum CollisionFilterGroups
   {
      DefaultFilter = 1,
      StaticFilter = 2,
      KinematicFilter = 4,
      DebrisFilter = 8,
      SensorTrigger = 16,
      CharacterFilter = 32,
      AllFilter = -1 //all bits sets: DefaultFilter | StaticFilter | KinematicFilter | DebrisFilter | SensorTrigger
   };


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PostPosted: Tue Sep 05, 2017 11:57 am 
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Joined: Tue Sep 30, 2014 6:03 pm
Posts: 363
Location: San Francisco
Your understanding of collisionGroups & collisionMask is correct.

The collisionFlags bits are used as boolean indicators to enable/disable various per-object behaviors. The "collision" part of the name is a bit of a misnomer: objectFlags might have been more correct.


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