Calculating the angle between the line defined by two points

Question

I m currently developing a simple 2D game for Android   I have a stationary object that s situated in the center of the screen and I m trying to get that object to rotate and point to the area on the screen that the user touches   I have the constant coordinates that represent the center of the screen and I can get the coordinates of the point that the user taps on   I m using the formula outlined in this forum   How to get angle between two points    It says as follows  If you want the the angle between the line defined by these two points and the horizontal axis   double angle   atan2 y2 - y1  x2 - x1    180   PI     I implemented this  but I think the fact the I m working in screen coordinates is causing a miscalculation  since the Y-coordinate is reversed   I m not sure if this is the right way to go about it  any other thoughts or suggestions are appreciated

User · Answer

in android i did this using kotlin   private fun angleBetweenPoints a  PointF  b  PointF   Double           val deltaY   abs b y - a y          val deltaX   abs b x - a x          return Math toDegrees atan2 deltaY toDouble    deltaX toDouble

User · Answer

A few answers here have tried to explain the  screen  issue where top left is 0 0 and bottom right is  positive  screen width  screen height  Most grids have the Y axis as positive above X not below   The following method will work with screen values instead of  grid  values  The only difference to the excepted answer is the Y values are inverted          Work out the angle from the x horizontal winding anti-clockwise     in screen space          The value returned from the following should be 315       lt pre gt     x y -------------           1 1                                           2 2     lt  pre gt      param p1     param p2     return - a double from 0 to 360     public static double angleOf PointF p1  PointF p2           NOTE  Remember that most math has the Y axis as positive above the X         However  for screens we have Y as positive below  For this reason          the Y values are inverted to get the expected results      final double deltaY    p1 y - p2 y       final double deltaX    p2 x - p1 x       final double result   Math toDegrees Math atan2 deltaY  deltaX         return  result  lt  0     360d   result    result

User · Answer

Assumptions   x is the horizontal axis  and increases when moving from left to right  y is the vertical axis  and increases from bottom to top   touch x  touch y  is the point selected by the user   center x  center y  is the point at the center of the screen   theta is measured counter-clockwise from the  x axis   Then   delta x   touch x - center x delta y   touch y - center y theta radians   atan2 delta y  delta x    Edit  you mentioned in a comment that y increases from top to bottom  In that case    delta y   center y - touch y   But it would be more correct to describe this as expressing  touch x  touch y  in polar coordinates relative to  center x  center y    As ChrisF mentioned  the idea of taking an  angle between two points  is not well defined

User · Answer

with pygame   dy   p1 y - p2 y dX   p2 x - p1 x  rads   atan2 dy dx  degs   degrees rads  if degs  lt  0      degs   90   it work for me

User · Answer

the origin is at the top-left of the screen and the Y-Coordinate increases going down  while the X-Coordinate increases to the right like normal  I guess my question becomes  do I have to convert the screen coordinates to Cartesian coordinates before applying the above formula    If you were calculating the angle using Cartesian coordinates  and both points were in quadrant 1  where x 0 and y 0   the situation would be identical to screen pixel coordinates  except for the upside-down-Y thing   If you negate Y to get it right-side up  it becomes quadrant 4       Converting screen pixel coordinates to Cartesian doesnt really change the angle

User · Answer

Had a need for similar functionality myself  so after much hair pulling I came up with the function below         Fetches angle relative to screen centre point    where 3 O Clock is 0 and 12 O Clock is 270 degrees         param screenPoint     return angle in degress from 0-360      public double getAngle Point screenPoint        double dx   screenPoint getX   - mCentreX         Minus to correct for coord re-mapping     double dy   - screenPoint getY   - mCentreY        double inRads   Math atan2 dy  dx           We need to map to coord system when 0 degree is at 3 O clock  270 at 12 O clock     if  inRads  lt  0          inRads   Math abs inRads       else         inRads   2   Math PI - inRads       return Math toDegrees inRads

[java] Calculating the angle between the line defined by two points

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