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I have two points (A, B) and the length of d. How can I find point C?


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2 Answers 2

up vote 26 down vote accepted

Compute a vector V from A to B, and normalize it.

V = (B - A) / |B - A|

Since the vector is normalized, it will have a length of one, and it will indicate the direction of B relative to A. If you then scale the vector by d you will have the displacement from A to C

V' = d * V

which you can simply add to A to yield C:

C = V' + A
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works perfect!!!! thanks!!! :) –  eempee Oct 10 '11 at 18:25
btw: could anyone recommend me a book where I can learn about stuff like that? :) –  eempee Oct 10 '11 at 20:40
3D Math primer is a good one: amazon.com/Primer-Graphics-Development-Wordware-Library/dp/…;. –  Ricky AH Oct 11 '11 at 8:03
Didn't read this one, but I've been told that is also pretty good amazon.com/Mathematics-Programming-Computer-Graphics-Second/dp/… –  Ricky AH Oct 11 '11 at 8:05
See this question: gamedev.stackexchange.com/questions/1210/… –  Josh Petrie Oct 11 '11 at 15:06

In case your d is a ratio instead of a length, e.g 0.5 puts it on equal distance from A and B, you can do the following and avoid a normalization:

C = A * (1 - ratio) + B * ratio
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