Calc Facing

**kingdeking** · 08-11-2012

Hey,

I have problems calculating how my player needs to rotate in order to face a mob.

What I do now is this:

Code:

facing = (Pos.x * Object.x + Pos.y * Object.y + Pos.z * Object.z ) / ( sqrt(  pow(Pos.x, 2) + pow(Pos.y, 2) + pow(Pos.z, 2) ) * sqrt(  pow(Object.x, 2) + pow(Object.y, 2) + pow(Object.z, 2) ) );

temp = acos(facing);

This basically gives me the angle between two vectors. The two vectors are the player and the mob. But the value temp is not 0, nor 2Pi, when I face a mob.

**miceiken** · 08-11-2012

Dude. you make a new thread for everything, and everything I see from you has already been answered atleast 5 times before. For god's sake please search before making a new thread.

**kingdeking** · 08-11-2012

I dont understand sry

**Bananenbrot** · 08-11-2012

Well... clear your linear algebra up. What you are utilizing is the dot product between two vectors. By incidence, it defined as

Code:

a . b = |a| * |b| * cos(<the angle between a and b>)  
<=> (a . b) / (|a| * |b|) = cos(<the angle between a and b>)
<=> arccos((a . b) / (|a| * |b|)) = <the angle between a and b>

So you are missing arccos for the angle to be between 0 and 2 Pi. Actually.

But what you are trying to compute is not the facing, but the angle between your position and the object's position relative to the origin [1] (and even considering the z coordinate, which will give you inappropriate results if you think about it).
You would need to compute the angle between the direction you are facing and the offset of the object's position relative to your own.
i.e.:

Code:

float ownFacing = LocalPlayer.GetFacing(); // the angle like that saved somewhere in the movement struct 
C2Vector direction(-sin(ownFacing), cos(ownFacing)); // facing of 0 is facing north iirc
C2Vector objectOffset = static_cast<C2Vector>(object.GetPosition() - LocalPlayer.GetPosition());
float dot = direction.Dot(objectOffset);
float denominator = direction.GetLength() * objectOffset.GetLength();
return arccos(dot/denominator);

Other examples in this forum are using Atan of the triangle described by objectOffset and substracting the own facing value. But this should be the "normal" vector computational way for computing facing delta.

Edit: note that there might be an error in the way I compute the angle (WoW's 'special' coordinate system, the facing north assumption, etc.), but It should generally provide the right results.