Find angle of a point from center of circle

Question

If I have an image 720, 720 that looks like this..

enter image description here

How do I work out the angle of the touched x,y given that the center x and y are 360, 360 A lot of calculations I see for this assume the origin is 0,0 (which is top left) so I get incorrect results. I am assuming 0 is always to the top and not rotated.

What exactly did you find so far? If your problem is only about the coordinates of the center, we will be happy to help you! — Jean Logeart, Jan 23 '12 at 07:53
Traditionally, 0 degrees is to the right, 90 is up, 180 is left, and 270 is down. (I may misunderstand your phrasing.) — , Jun 25 '15 at 02:59

score 19 · Answer 1 · edited Jul 13 '17 at 12:32

19

Here is the general formula:

angle = atan2(mouseY - cirleCenterY, mouseX - circleCenterX);

edited Jul 13 '17 at 12:32

Bill

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answered Jan 19 '13 at 19:03

Sileria

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score 13 · Accepted Answer · edited Oct 27 '13 at 00:02

13

May be clearer this way:

(Math.toDegrees( Math.atan2(fromLeft - 360.0, 360.0 - fromTop) ) + 360.0) % 360.0

Adding a 360 degree turn and applying the modulo operator gives you the positive angle, which atan2 does not.

edited Oct 27 '13 at 00:02

Aran Mulholland

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answered Jan 23 '12 at 08:15

minopret

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This seems the closest to any solution suggested here. I misunderstood in my previous comment, if I substitute x touch coordinate for fromLeft and y for fromTop I get a positive angle back at least. Clicking at around 3 o'clock does give around 90 degrees as expected but if I click at 6 0'clock I get close to 360 degrees which wasn't expected, 9 o'clock reports the correct 270 degrees. It looks like this is pretty close but still some problem with 12 and 6 o'clock in that calculation – Kyros Jan 23 '12 at 16:57
If I click at 12 0'clock I get 180 and 6 o'clock I get 0 so these are somehow reversed. – Kyros Jan 23 '12 at 17:13
Corrected from "fromTop - 360.0" to "360.0 - fromTop". Tested at JavaWIDE. – minopret Jan 24 '12 at 01:40
1

Did you test this snippet? `atan2` is defined (counterintuitively) as `atan2(y, x)` but you're passing `(x, y)`... – Bill Jul 13 '17 at 12:33
@Bill Thanks. I agree now that it's worthwhile to state clearly the coordinate axes that are used in this question. They are not oriented as usual for x-y coordinates in mathematics. They are oriented as usual for Java2D, Postscript, and so on. That is why I named my parameters `fromLeft` and `fromTop`. In regard to testing: Yes, I think you'll agree now that the point 720 from left, 360 from top, is at 90 degrees; the point 360 from left, 720 from top, is at 180 degrees. – minopret Oct 18 '17 at 07:49

score 0 · Answer 3 · answered Jan 23 '12 at 07:51

0

java.lang.Math.atan2(y-360,x-360);

answered Jan 23 '12 at 07:51

Maurice Perry

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score 0 · Answer 4 · answered Jan 23 '12 at 08:16

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the screen coordinates don't go the way of the trigonometric ones.

use java.lang.Math.atan2(-(y-360),x-360);

answered Jan 23 '12 at 08:16

vlad-ardelean

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Find angle of a point from center of circle

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