Vector math, finding the angle

Question

I am trying to learn XNA by writing a small 2D game, it's a Top-Down perspective and Im trying to have double movement, moving along the axis using Left-Right and Up-Down keys, as well as looking right at the mouse cursor, so that the player can run and aim at the same time.

I have one vector for the player position (m_PlayerPos), and one vector for the mouse position (m_MousePos), and im trying to get the correct angle towards the mouse position.

Im using the formula method:

public static float Angle(Vector2 from, Vector2 to)
{
    return (float)Math.Atan2(from.X - to.X, from.Y - to.Y);
}

This works, but for some reason the method only works half-way, along the x-axis. When the mouse is to the exact left of right of the player, the player looks right at the mouse.

But if I move to the top of the player, it looks down, and if the mouse is below the player, the player looks up. So I need to inverse the Y axis, but Im not sure how.

Thanks in advance for any feedback.

Answer 1

Use to.Y - from.Y.

Answer 2

Multiply it with (0.0, -1.0) (or just multiply the Y component by -1.0). This will mirror the vector along the horizontal axis and should achieve the result you want.

Answer 3

In screen space the origin is in the top-left corner with the Y axis pointing downward whereas in eucledean space the Y axis points upwards. That's why you observe the Y axis being "flipped".