lengthen or shorten vector

[mike cardeiro]

this may seem trivial, but vector mathematics is not my strong suit.

if I have a vector between point A and point B, how do I shorten it (or lengthen it), but maintian a center point between A and B, like this

  A------------B  old vector

  A  --------  B new vector or

--A------------B-- another new vector

[Dan Ebberts]

Vectors don't really have a position, just length and direction. What are you trying to do exactly?

Dan

[mike cardeiro]

Hey Dan,

I have this expression that disperses layers in a comp between 2 nulls; so the first layer will get position of null 1, the last layer gets position of null 2 and all layers in between get evenly dispersed along a line between them.

what i am trying to figure out is a way to lengthen or shorten the line between the nulls.  the position expression on each of the layers looks like this (the 2 nulls are the last 2 layers in the comp)

n = thisComp.numLayers;
v1 = thisComp.layer(n-1).position;
v2 = thisComp.layer(n).position;
if (index == 1){
  v1
}else if (index == n-2){
  v2
}else{
  v1 + (v2 - v1)*((index-1)/(n-3))
}

Mike

[Dan Ebberts]

This should do it:

offset = 25; // positive makes line longer

n = thisComp.numLayers;
v1 = thisComp.layer(n-1).position;
v2 = thisComp.layer(n).position;
v = normalize(v2 - v1)*offset;
v2 += v;
v1 -= v;
if (index == 1){
  v1
}else if (index == n-2){
  v2
}else{
  v1 + (v2 - v1)*((index-1)/(n-3))
}

Dan

[mike cardeiro]

Awesome! thanks Dan, this is exactly what I needed.

Mike