Finding CGPoint by adding into distance of two CGPoints

Question

I have two CGPoints A and B. Distance between A and B can be found with ccpDistance(A,B).

With this information, I need to find programmatically CGPoint(x,y) of X by adding small distance into distance of A-B. A-B may can make a connecting line in horizontal, vertical, diagonal or any orientation. Purpose is to extend the imaginary line and find a point at the end of it.

Is it possible in Box2D for iOS? If Yes, then how?

          X            Y   X
         /               .  \ 
        /                  . \
       B                     .B
      /                        \
     /                          \
    /                            \
   /                              \
  A                                A

Lars Blumberg · Answer 1 · 2012-08-28T17:07:21.003

I suggest the following method which doesn't make use of any triangular function. I suppose it's faster than Lukman's suggestion but I didn't measure times.

static inline CGPoint ccpLineExtend(const CGPoint from, const CGPoint to, const CGFloat factor)
{
    return CGPointMake(from.x + factor * (to.x - from.x), from.y + factor * (to.y - from.y));
}

factor = 1.0 means that the resulting point is equal to point to. factor = 2.0 means that the resulting point is as twice as far from from as to is.

score 1 · Accepted Answer · answered Mar 23 '11 at 03:29

1

Use the following function (put it in one of your .h files):

static inline CGPoint
ccpLineExtend(const CGPoint v1, const CGPoint v2, const CGFloat extension)
{
    double angle = atan2(v2.y - v1.y, v2.x - v1.x);
    return ccp(v2.x + extension * cos(angle), v2.y + extension * sin(angle));
}

answered Mar 23 '11 at 03:29

Lukman

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Thankyou so much. It works like charm for the first figure. It is not considering reverse angle like in figure 2. It is giving Y point at dotted distance. I so much hope you can provide solution for this one too. – WaJiyaz Mar 23 '11 at 13:39
@WJK: Are you sure you calculated it right? Because a simple test of `ccpLineExtend(ccp(3, 0), ccp(0, 4), 5.0)` gives the correct `-3.0, 8.0` .. the trigonometric formula can't be wrong. Oh yeah, if you want to use it in Box2D, don't forget to divide both x and y components by PTM_RATIO – Lukman Mar 23 '11 at 15:43
You are right. It's straight trigonometry. It is only happening to me for angle between 90-135 degrees. May be because I have load many calculations, so some value might be impacting it. Thanks. – WaJiyaz Mar 23 '11 at 17:47
some other value was affecting lineExtend, it's solved now. Thanks again :) – WaJiyaz Mar 23 '11 at 20:34

score 0 · Answer 3 · answered May 07 '19 at 17:41

@Lars's answer was best and simplest for me. Here is the Swift 4 version:

let newPoint = CGPoint(x: origin.x + factor * (end.x - origin.x), y: origin.y + factor * (end.y - origin.y))

If you have the current distance, you can add and subtract from it or change it altogether by calculating:

let factor = newDistance/currentDistance

In my case, I had a distance calculator from CGPoint to CGPoint using an extension:

extension CGPoint {
    func distance(to point: CGPoint) -> CGFloat {
        let x = self.x - point.x
        let y = self.y - point.y
        return sqrt(pow(x, 2) + pow(y, 2))
    }
}

Finding CGPoint by adding into distance of two CGPoints

3 Answers3