Velocity is a measure of speed and direction. In Source, it is a
Vector that an entity will try to move the length of once per second: a velocity of
(0,0,-200) means that you are falling at 200 units per second.
Absolute, local and base
There are actually three different velocity vectors:
- Local velocity: generated by the entity
- Base velocity: applied to the entity from other sources
- Absolute velocity: the sum of the local and base velocities; the actual value used to move
Having separate local and base velocities allows for the proper handling of inertia. If an NPC is standing on a fast-moving train, the motion of the carriage is stored in its base velocity while the motion the NPC creates itself when walking up and down is stored in its local velocity. There is no danger of either value overwriting the other, or of the NPC thinking it's walking at 150MPH.
When an entity moves, the code looks a little like this:
SetLocalVelocity( vecSomeValue ); SetAbsVelocity( GetLocalVelocity() + GetBaseVelocity() ); // frametime is in seconds, e.g. 0.033 SetAbsOrigin( GetAbsOrigin() + (GetAbsVelocity() * gpGlobals->frametime) );
- Objects don't so much travel through space as teleport a very small distance each frame. Note: Beware an object moving so fast that it doesn't pass through intervening space, which can happen unexpectedly if the server's framerate dips too far. To do: How can this be best avoided?
gpGlobals->frametimeis used to regulate the entity's speed regardless of how long each frame takes to calculate. (See Vector for more detail on this operation.)