OK, sorry if this is already somewhere.... I couldn't find it when I did a search.

I'm working on a 2D project where I have images moving around the screen. They have a 'heading' vector that is sort of generated randomly. I'm trying to calculate an angle of rotation based on the dot product between the heading vector and a 'straight' vector of (1.0f, 0.0f) ... I'm using this because the original image for rotation is facing to the left. (its actually a left arrow --> )

It seems like its sort of working, but not quite right. Sometimes they aren't oriented correctly, and when I put in a negative value for the heading like (0.0f, -1.0f) it rotates as if the heading were (0.0f, 1.0f)..... meaning the sign was lost somewhere in the dot product calculation I believe.

I'll try to paste the pertinent code below ......

/*
* OnStarting
*
* Called when the game engine's OnStarting method gets called before
* any frames are drawn.
*/
public void OnStarting(GraphicsDevice device)
{

Random rand = new Random();

// initialize attributes

this.Drawer = new SpriteBatch(device);
this.Texture = Texture2D.FromFile(device, texturePath);

this.Heading = new Vector2(0.0f, 1.0f);

// normalize the heading
this.Heading.Normalize();
Vector2 straight = new Vector2(1.0f, 0.0f); // default orientation -->


// get a vector for rotation --> dot product between perfectly right and the current heading
this.rotation = (Heading.X*straight.X) + (Heading.Y*straight.Y); // dot product formula
this.rotation = Math.Acos(this.rotation);

}

/*
* Update
*
* Called every frame to update
*/
public void Update(float dt)
{
this.Position += this.Heading;
}


/*
* Draw
*
* Called every frame to render
*/
public void Draw()
{
// I think this is relative to the texture (0, 0), but not sure
Rectangle src = new Rectangle(0, 0, this.Texture.Width, this.Texture.Height);

this.Drawer.Begin(SpriteBlendMode.AlphaBlend);

// this.Drawer.Draw(this.Texture, this.Position, Color.White);

this.Drawer.Draw( this.Texture,
this.Position,
src,
Color.White,
(float)this.rotation,
new Vector2(this.Texture.Width/2, this.Texture.Height/2),
1.0f,
SpriteEffects.None,
1.0f);

this.Drawer.End();
}

Any help would be greatly appreciated. I'll keep slugging through math books =)