GameBlender v2.10 - Physics FAQ

  1. What's a dynamic object?
  2. What are linV and angV?
  3. How do DoFh and RotFh work now?
  4. How do Damp and Rot Damp work now?
  5. How do I get rid of gravity?
  6. How do I tweak the bouncing of dynamic objects?
  7. What's the difference between damping and friction?
  8. What is Anisotropic Friction?
  9. Who do I contact for suggestions, comments, or bugs regarding this FAQ, or the GameBlender physics?

What's a dynamic object?

An object is dynamic if it has mass. Gravity, forces, and impulses (collision bounce) only make sense for objects that have mass. Also, only dynamic objects experience drag, or velocity damping (a crude mimic of air/water resistance). Note that for dynamic objects using dLoc and dRot may not have the desired result. Since the velocity of a dynamic object is controlled by the forces and impulses, any explicit change of position or orientation of an object may not correspond with the velocity. For dynamic objects it's better to use the linV and angV for explicitly defining the motion.
 

What are linV and angV?

These are linear and angular velocity. The linear velocity is a vector representing the speed of the objects center. The angular velocity is a vector representing the rotation speed of an object. The direction of the angular velocity represents its rotation axis, and the length represents its counter-clockwise rotation speed. Velocities can be set for both dynamic and non-dynamic (kinematic) objects. Note that for dynamic objects velocities are also affected by forces, collisions, gravity, etc.
 

How do DoFh and RotFh work?

DoFh and RotFh determine whether the dynamic object should react to the 'springiness' of the material (Fh Dist, Fh Damp, and Fh Force). When DoFh is enabled, a spring-damper system will keep the object at some distance hovering above the surface, as if it were lying on a mattress. The spring works in the direction of the global z-axis countering the effect of gravity. When RotFh is enabled, the object's local z-axis will be aligned with the normal to the surface beneath the object. Fh Dist is the resting distance of the linear spring, Fh Damp is the damping factor for both the linear and angular spring, and Fh Force is the spring constant for both the linear and the angular spring.
 

How do Damp and Rot Damp work?

Damp is the damping factor for the linear velocity, and Rot Damp is the damping factor for the angular velocity. The factor expresses the decrease in velocity over one second. Currently, the damping is regardless of the mass of the objects, so light objects drop as fast as heavy objects. Time for a little questionnaire...
  1. Are on average your damping values close to

    2.         A. 0.05
            B. 0.5
            C. 0.95
  2. Would you like to have mass-sensitive damping, i.e., heavy objects drop faster?

    3.         A. Yeah, that'll be really useful.
            B. Nah, leave it as it is.
            C. I have no idea what you're talking about.
Please, send you're answers to gino@blender.nl
 

How do I get rid of gravity?

Set the amount of gravity in the world settings to zero.
 

How do I tweak the bouncing of dynamic objects?

Dynamic objects can bounce for two reasons. Either you have Do Fh enabled and have too little damping, or you are using a Restitut value in the dynamic material properties that is too high. (Note that if you haven't defined a material, the default restitution will be 1.0., which is the maximum value)  For the first case, the amount of bounce can be decreased by increasing the damping. For the latter case, make sure you have defined a material for at least one of the colliding objects, and set its Restitut value to a smaller value. The Restitut value determines the elasticity of the material. A value of zero denotes that the relative velocity between the colliding objects will be fully absorbed. A value of one denotes that the total momentum will be preserved after the collision.
 

What's the difference between damping and friction?

Damping is a decrease of velocity in % per second. Damping is useful to achieve a maximum speed. The larger the speed the greater the absolute decrease of speed due to drag. The maximum speed is attained when the acceleration due to forces equals the deceleration due to drag. Damping is also useful for damping out unwanted oscillations due to springs.

Friction is a force tangent to the contact surface. The friction force has a maximum that is linear to the normal, i.e., the force that presses the objects against each other, (the weight of the object).  The Friction value denotes the Coulomb friction coeffcient, i.e. the ratio of the maximum friction force and the normal force.  A larger Friction value will allow for a larger maximum friction. For a sliding object the friction force will always be the maximum friction force. For a stationary object the friction force will cancel out any tangent force that is less than the maximum friction. If the tangent force is larger than the maximum friction than the object will start sliding.
 

What is Anisotropic Friction?

For some objects you need to have different friction in different directions. For instance a skate board will experience relatively little friction when moving it  forward and backward, but a lot of friction when moving it side to side. This is called anisotropic friction. Anisotropic friction is enabled by selecting the Anisotropic button in the Real-time window.  After selecting this button, three sliders will appear in which the relative coefficient for each of the local axes can be set. A relative coefficient of zero denotes that along the corresponding axis no friction is experienced. A relative coefficient of one denotes that the full friction applies along the corresponding axis.  

Who do I contact for suggestions, comments, or bug reports regarding this FAQ, or the GameBlender physics?

Please, send everything you wish to share to gino@blender.nl