GetAbsOrigin()

GetAbsOrigin() returns the absolute (i.e. world-relative) origin of an entity as a vector. Its sister function SetAbsOrigin() changes it.

The origin is actually stored in local space (i.e parent-relative), meaning that the Abs functions perform extra calculations to get their return value. Use the more direct GetLocalOrigin() and SetLocalOrigin() if possible; they work directly with the stored value (m_vecOrigin).

Note:In fact, GetAbsOrigin() seems to recalculate its return value only if it was changed since the last time it was used, otherwise it just returns cached value stored in a member variable.

Alternatives

A different approach for finding the position of an entity can be useful, particularly when dealing with a brush entity which might not have a valid origin. Bear in mind that these alternatives are all significantly more expensive than the 'vanilla' functions listed above.

Vector WorldSpaceCenter()

Returns the centre of an entity's bounding box.

void CollisionProp()->CalcNearestPoint(&vecWorldPt,*pVecNearestWorldPt)

Finds the nearest point on the entity's bounding box to vecWorldPt. Modifies pVecNearestWorldPt with the result instead of returning anything.

Cells

Starting with Left 4 Dead, CBaseEntity transmits its origin in "cells". This splits the entity's local coordinate space up into cells of 32 units X/Y/Z, then uses that reduced range to compress the networked value to a mere 5 bits per ordinate. The index of the current cell also has to be networked, but it changes relatively infrequently.

This system leads to a huge reduction in network traffic at the price of imprecision. The sensitivity of prediction requires the local player to be sent an uncompressed origin.

Note: In Alien Swarm, CBasePlayer assumes that an uncompressed origin will be sent...but does not actually send one. This leads to the client-side entity being stuck at world origin. Either network the origin yourself, or for a quick fix override ShouldRegenerateOriginFromCellBits() and return true.