Forward Velocity

I need to know only the forward velocity of an object (X on local axis). According to the Wiki, this can be attained by multiplying llGetVel() by llGetRot(), however this doesn't seem to work. When I do this and pull the .x component out of the resulting vector, it doesn't seem like I end up with forward velocity. Accelerating in different directions causes this number to vary between negative and positive in a rather useless fashion. Has anyone had any success with this functionality before?

Hello!

The best way to get the forward velocity in meters / second would be:

llVecMag(llGetVel() * llGetRot());

For more information about llVecMag() you can go to
http://secondlife.com/badgeo/wakka.php?wakka=llVecMag

This would be an application of the dot product, wouldn't it? Somebody catch me if I'm completely wrong, here.

Code snippet:

// Gets the 'unit vector' showing which way the x-axis (forward) is pointing

vector forward = llRot2Fwd( llGetRot() ) ;

// Gets the velocity of the object in region coordinates

vector velocity = llGetVel() ;

// This gets the x-axis component of the object's velocity.
// ( velocity * forward ) gives the 'dot product' of these two vectors, ie. a float
// representing the magnitude of the projection of the velocity vector on the
// forward vector.  Multiplying this by the forward vector then gives the velocity
// vector of its motion along its own x-axis (forward)

vector forward_velocity = forward * ( velocity * forward ) ;

Thanks! I'll give that a try after work tonight.

Hehe, let me know if it works! I'm curious, too.

For the interested -- I was trying to do this the other day, and it seems the math here is slightly flawed (or I just read it wrong, over and over).

It actually comes out more along the lines of:

vector myVel = llGetVel();
rotation myRot = llGetRot();
vector myFwd = llRot2Fwd(myRot);
vector localizedVel = myFwd * ((myVel * myRot) * myFwd);

I'm no good with this sort of math, so i'm not sure if this is the right or most efficient way to do this -- however, it seems to work, where the one before (without adjusting the velocity with the rotation) seems to return it in a global sense.

If someone can more efficiently do this math, or provide an explanation of why it needs this, i'd appreciate it.

Here's what I use:

vector curVel = llGetVel() / llGetRot();