Calculating the angle between the line defined by two points

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 thepoint selected by the user. (center_x, center_y) is the point at the center of thescreen. theta is measured counter-clockwise from the +x axis. Then:

delta_x = touch_x - center_xdelta_y = touch_y - center_ytheta_radians = atan2(delta_y, delta_x)

Edit: you mentioned in a comment that y increases from top to bottom. In thatcase,

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.

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 < 0)        inRads = Math.abs(inRads);    else        inRads = 2 * Math.PI - inRads;    return Math.toDegrees(inRads);}

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.  * <pre> * x,y ------------- *     |  1,1 *     |    \ *     |     \ *     |     2,2 * </pre> * @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 < 0) ? (360d + result) : result;}