Detect a collision with the edge of a circle

Question

I am using a script to generate random particles within a circle based on it's radius. What I would like to do next is detect when the particle collides with the circles edge.

I'm guessing I need to use a for loop to store the coordinates of the circles circumference in an array but I'm unsure what math is needed to do this.

Here's what I've got down from the answer below. It doesn't seem to be working though:

Variable par is a particles moving with the circle, emitters contains x,y, positions of the centre of the circle while the prop height contains the radius.

var fromC = Math.sqrt( (par.y-(emitters[i].y ) )^2 + (par.x- (emitters[i].x))^2); 

if(fromC >= emitters[i].height){
    par.vx *= -1;
    par.vy *= -1;
}

Thanks in advance.

If you know the center of the circle and the radius you could just check the distance from the center. — atomman, Jan 27 '13 at 14:39
@ATOzTOA Updated with variable info. It's definitely firing, just not when the particles hit the circles edge. — Infra Stank, Jan 27 '13 at 15:24
possible duplicate of [circle-circle collision](http://stackoverflow.com/questions/1736734/circle-circle-collision) — Bergi, Jan 27 '13 at 15:38
Also have a look at all the other [circle collision-detection questions](http://stackoverflow.com/search?tab=votes&q=%5bcollision-detection%5d%20circle) — Bergi, Jan 27 '13 at 15:40

ATOzTOA · Accepted Answer · 2013-01-27T17:28:33.487

1

The issue is with your square operation, ^ is NOT power operator in javascript.

Use this:

var fromC = Math.sqrt( Math.pow((par.y - emitters[i].y), 2) + Math.pow((par.x - emitters[i].x), 2) ); 

if(fromC >= emitters[i].height){
    par.vx *= -1;
    par.vy *= -1;
}

edited Jan 27 '13 at 17:28

answered Jan 27 '13 at 15:26

ATOzTOA

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No joy, seems to be throwing lots of NANs. – Infra Stank Jan 27 '13 at 15:30
`NaN`s? How is that possible with a positive root argument? Are you absolutely sure all parameters are correctly initialized? Try `console.log()`ging the `par` and `emitters[i]` objects. – Mattias Buelens Jan 27 '13 at 15:39
Here's what I'm getting going fromC em.y em.x em.height par.y par.x : NaN 306.5 338.5 190.85 433.2453792351848 333.1283476399258 . – Infra Stank Jan 27 '13 at 15:53
Can you print fromC without the Math.floor? – ATOzTOA Jan 27 '13 at 16:01
Without Math.floor : NaN 306.5 338.5 260.6 31.176133123692125 580.35272803111 – Infra Stank Jan 27 '13 at 16:09
Not quite sure what you mean? – Infra Stank Jan 27 '13 at 16:16
Are you printing the values using the code in your question or the one in my answer? – ATOzTOA Jan 27 '13 at 16:17
Got you, still getting NaNs with the original: NaN 306.5 338.5 201.9 617.4620188029949 110.25046234992328 – Infra Stank Jan 27 '13 at 16:22

score 0 · Answer 2 · answered Jan 27 '13 at 14:41

0

Just calculate the distance between the point and the center of the circle ( square root( (y2-y1)^2 + (x2-x1)^2) and compare with the radius

answered Jan 27 '13 at 14:41

Charles Bretana

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Thanks, I've added some code to my question based on the answer. It isn't working though, could you give it a look. – Infra Stank Jan 27 '13 at 15:10

Detect a collision with the edge of a circle

2 Answers2