How to get cardinal mouse direction from mouse coordinates

Question

is it possible to get the mouse direction (Left, Right, Up, Down) based on mouse last position and current position? I have written the code to calculate the angle between two vectors but I am not sure if it is right.

Can someone please point me to the right direction?

    public enum Direction
    {
        Left = 0,
        Right = 1,
        Down = 2,
        Up = 3
    }

    private int lastX;
    private int lastY;
    private Direction direction;

    private void Form1_MouseDown(object sender, MouseEventArgs e)
    {
        lastX = e.X;
        lastY = e.Y;
    }
    private void Form1_MouseMove(object sender, MouseEventArgs e)
    {
        double angle = GetAngleBetweenVectors(lastX, lastY, e.X, e.Y);
        System.Diagnostics.Debug.WriteLine(angle.ToString());
        //The angle returns a range of values from -value 0 +value
        //How to get the direction from the angle?
        //if (angle > ??)
        //    direction = Direction.Left;
    }

    private double GetAngleBetweenVectors(double Ax, double Ay, double Bx, double By)
    {
        double theta = Math.Atan2(Ay, Ax) - Math.Atan2(By, Bx);
        return Math.Round(theta * 180 / Math.PI);
    }

I would go so far as to say that the angle method is incorrect, period. The angle difference between two vectors doesn't tell you anything about the actual direction (on the cartesian plane) that the point has moved. — Ron Warholic, Nov 06 '09 at 18:32
Are you trying to accommodate for mouse jitter? - For example if the user has consistently moved 100 px to the right but midway jittered back and forth a few pixels, you don't want to accidentally sample that jitter and assume they're going the other way because the overall picture doesn't lend itself to that conclusion — John K, Nov 06 '09 at 18:38
No, I am writting a game which an image on the form should follow the direction of the mouse — Ioannis, Nov 06 '09 at 21:02

score 13 · Accepted Answer · answered Nov 06 '09 at 18:29

Computing the angle seems overly complex. Why not just do something like:

int dx = e.X - lastX;
int dy = e.Y - lastY;
if(Math.Abs(dx) > Math.Abs(dy))
  direction = (dx > 0) ? Direction.Right : Direction.Left;
else
  direction = (dy > 0) ? Direction.Down : Direction.Up;

score 6 · Answer 2 · answered Nov 06 '09 at 18:30

I don't think you need to calculate the angle. Given two points P1 and P2, you can check to see if P2.x > P1.x and you know if it went left or right. Then look at P2.y > P1.y and you know if it went up or down.

Then look at the greater of the absolute values of the delta between them, i.e. abs(P2.x - P1.x) and abs(P2.y - P1.y) and whichever is greater tells you if it was "more horizontal" or "more vertical" and then you can decide if something that went UP-LEFT was UP or LEFT.

score 1 · Answer 3 · answered Nov 06 '09 at 18:31

1

0,0 is the top left corner. If current x > last x, you're going right. If current y > last y, you're going down. No need to calculate the angle if you're just interested in up\down, left\right.

answered Nov 06 '09 at 18:31

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11
1

score 0 · Answer 4 · answered Nov 06 '09 at 18:30

Roughly speaking, if the magnitude (absolute value) of the horizontal move (difference in X coordinates) between the last position and the current position is greater than the magnitude (absolute value) of the vertical move (difference in Y coordinates) between the last position and current position, then the movement is left or right; otherwise, it is up or down. Then all you have to do is check the sign of the movement direction to tell you if the movement is up or down or left or right.

You shouldn't need to be concerned about angles.

How to get cardinal mouse direction from mouse coordinates

4 Answers4

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