creating orient and rotating points around vector

I think Toadstorm posted this somewhere on odforce... but it's a useful little vex snippet

 vector fwd = v@N;

vector up = {0,1,0};
// if you don't have a defined up vector but want to compute one the way houdini does it natively,
// compute the rotation that rotates +Z to N and apply this to +Y...
// matrix3 d = dihedral({0,0,1}, fwd);
// up = normalize(d * up);matrix3 m = maketransform(fwd, up);
// now you can rotate this around whatever axis you want.
vector axis = normalize(chv("axis")); // or use an existing vector attribute!
float angle = @noiseVal*radians(ch("angle")); // or use a random float attribute!
rotate(m, angle, axis);// now convert this rotated matrix to a quaternion and bind to p@orient. copy to points and/or DOPs will understand this.
@orient = quaternion(m);

rotating point normals, with vops

To rotate a point's normals (or any vector for that matter) you need to multiply it with a matrix and an angle (usually in radians) and around a specified axis.

A much neater/easier-to-grasp method is to use a VOP.
Give your points some initial Normal values, eg point them in X.

Make a point VOP.
Connect the N output into a multiply. We want to multiply this N vector and get a vector output.
The output of the multiply should go to the N output.

Make a DegToRad node (skip if you speak radians) and connect that to the angle input of a Rotate node. Promote the DegToRad's angle input so you can tweak it at object level.

In the Rotate node, set the vector that you wish to spin the normal vector around. It might be Y. 0,1,0.
Or you could be doing something  cool like calculating the vector that an edge lies on and spinning something around that.

Connect the output of the Rotate to the second input of the Multiply.

Now jump back out of the VOP and you should be able to see your normals rotating when you change the angle input.

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Of course this is just a couple, maybe one, of lines in VEX, but who really has time to memorise that? heh.

recalculating velocity

Nicked from Walter..

Velocity is a vector..
Velocity=speed x direction.
I was using this because some of my POPs had too much speed and were going through my collision objects.

If you store the normalised version of your velocity, this is the direction. Then you can multiply it by a new speed.. Perhaps you want to make it 85% of it's velocity until it reaches your max target?

So...in VEX:


vector direction=normalize(@v);

float speed=length(@v);

if(speed>chf("maxSpeed")){

speed*=chf("dampMultiplier");
}

using object's rotation as a vector value.

A simple method would be to set the world aligned object's vector value to pure X,Y or Z (eg <<1,0,0>> for all X)before it ever gets transformed. As long as inherit attributes is active, the vector will also be transformed/rotated etc)
Useful for setting vectors to point in the direction of the object.