# Getting the % that a point is on a line

Alright, so I got two points in 3d space, so they have a x,y, and z. Now if the line's y - which I get like so:

Vector3 v = new Vector3();
v = a.subtract(b, v);
v.normalizeLocal();

float cosine = (float) v.dot(v);
float angle = (float) Math.toDegrees(Math.acos( cosine ));


Now I have a third point (the c point), which is inside the line of a and b. now I need to know how to get point a and b to 0 and 1, than I need to find out where c is on the line. my goal is to have something like c.y = b.y - (a.y * c.x and c.z's point on the line (which will be between 0 and 1))

So how do I do this?

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Your first order of business is to construct a parametric equation $\mathbf h(t)=(a+bt\quad c+dt\quad e+ft)$ such that $\mathbf h(0)$ corresponds to one endpoint, and $\mathbf h(1)$ corresponds to the other endpoint. (Hint: direction cosines). –  Ｊ. Ｍ. Dec 7 '10 at 18:26

I'm not entirely sure that this answers what you're asking--if not, please comment so that I can revise it.

If you have two points $A$ and $B$, the set of points $P=(1-t)\cdot A+t\cdot B$, where $t$ is a real number, is the line through $A$ and $B$, parameterized with $A=P|_{t=0}$ and $B=P|_{t=1}$.

If you know for sure that $C$ is on the line through $A$ and $B$, then set $C=(1-t)\cdot A+t\cdot B$ and solve for $t$ in any one of the components (if $C$ is in fact on the line, then the value of $t$ will be the same, regardless of which component you use; if $C$ is not on the line, then the value of $t$ will not be the same for each component).

note: edited to fix equation $P=$ in the second paragraph, and correspondingly in the third paragraph.

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So the dots are dot products? what are they? also what does P|t=0 mean? –  CyanPrime Dec 8 '10 at 0:10
@CyanPrime: The dots are scalar multiplication: $t\cdot(x,y,z)=(tx,ty,tz)$. By $P|_{t=0}$ I meant evaluate $P$ when $t=0$. –  Isaac Dec 8 '10 at 0:21
Alright, so c = (t * a.x, t * a.y, t * a.z) + (1-t * b.x, 1-t * b.y, 1-t * b.z) And t can be any number I want? –  CyanPrime Dec 8 '10 at 1:11
@CyanPrime: For $0<t<1$, the point will be between $A$ and $B$; for $t<0$ or $t>1$, the point will still be on the line, but not between $A$ and $B$. –  Isaac Dec 8 '10 at 1:15
Does having a different number as t change the outcome? If so, what number should I use to make it between 1 and 0? –  CyanPrime Dec 8 '10 at 1:16