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javamouserectanglesdrawing2d

How to rotate object to always face mouse?

I'm trying to make a game and I need the player(rectangle) to always be looking at the mouse, I have found some pages on this but I can't seem to understand the math. Main:

 g2d.rotate(calculateRotation, x,y);
 g2d.fill(player);
 g2d.rotate(-calculateRotation, x,y);

Mouse Listener:

 int mx = e.getX();
 int mY = e.getY();

float rotation = Math.atan((mouseX-playerX)/(mouseY-playerY)); //<--- I don't know

would it be something like this?

Solution

  • You should use linear algebra- instead of using sines and cosines, you use vectors.

    If you have P1=(x1,y1) (where the player is) and P2=(x2,y2) (where the mouse pointer is), then you have the vector V=(x2-x1,y2-y1)=(v1,v2), which has length v=|V|=sqrt(v1^2+v2^2). Then you have the versor (which is a vector of length=1) M=(v1/v,v2/v)=(m1,m2).

    Then instead of computing an angle, you can rotate points by mapping (x,y)->(x* m1-y* m2, x* m2+y*m1).

    See also https://en.wikipedia.org/wiki/Rotation_matrix#In_two_dimensions

    (and remember to take care in the case V=0)

    Note: using atan is OK, but you will need to check the signs of x and y.. If they are both negative you'll find the wrong angle; and if one is positive and the other is negative, you still don't know if your arrow points NW or SE.