Question

I am trying to make a simple game in XNA.

I have a player which has spritesheet next to it.
The spritesheet is a sort of weapon, with a tip.

How can I get this sprite to rotate with the tip facing the mouse position?

        float y2 = m_Mouse.Y;
        float y1 = m_WeaponOrigin.Y;
        float x2 = m_Mouse.X;
        float x1 = m_WeaponOrigin.X;

        // Get angle from mouse position.
        m_Radians = (float) Math.Atan2((y2 - y1), (x2 - x1));

Drawing with: 
activeSpriteBatch.Draw(m_WeaponImage, m_WeaponPos, r, Color.White, m_Radians, m_WeaponOrigin, 1.0f, SpriteEffects.None, 0.100f);

while this makes it rotate, it does not follow the mouse correctly, and it behaves weirdly.

Any hints on how to make this work?

Another issue I have is to define a point and which is the muzzle, and update it based on the angle as well, so that shots will fire correctly from that point towards the mouse.

Thanks

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Screenshots:

Thanks again, turned out to be a fun game.

Answer 1

Basically, use Math.Atan2.

Vector2 mousePosition = new Vector2(mouseState.X, mouseState.Y);
Vector2 dPos = _arrow.Position - mousePosition;

_arrow.Rotation = (float)Math.Atan2(dPos.Y, dPos.X);

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Proof of concept (I used the plus texture for the cursor – it doesn’t show on the sceenshot unfortunately):

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“What is _arrow?”

In that example _arrow is of type Sprite, which might come in handy in some situations, and will sure make your code look a bit cleaner:

public class Sprite
{
    public Texture2D Texture { get; private set; }

    public Vector2 Position { get; set; }
    public float Rotation { get; set; }
    public float Scale { get; set; }

    public Vector2 Origin { get; set; }
    public Color Color { get; set; }

    public Sprite(Texture2D texture)
    {
        this.Texture = texture;
    }

    public void Draw(SpriteBatch spriteBatch, GameTime gameTime)
    {
        spriteBatch.Draw(this.Texture, 
                         this.Position, 
                         null, 
                         this.Color, 
                         this.Rotation, 
                         this.Origin, 
                         this.Scale, 
                         SpriteEffects.None, 
                         0f);
    }
}

Declare:

Sprite _arrow;

Initiate:

Texture2D arrowTexture = this.Content.Load<Texture2D>("ArrowUp");
_arrow = new Sprite(arrowTexture)
        {
            Position = new Vector2(100, 100),
            Color = Color.White,
            Rotation = 0f,
            Scale = 1f,
            Origin = new Vector2(arrowTexture.Bounds.Center.X, arrowTexture.Bounds.Center.Y)
        };

Draw:

_spriteBatch.Begin();
_arrow.Draw(_spriteBatch, gameTime);
_spriteBatch.End();