====== Version 1.6 ======

This time we are working with Matrixes to position and rotate sprites. Lots of code from the previous examples has been removed to keep the code base clear and to the point.

Download the source code, compile and run. It should look something like this: 

{{:wiki:rotating_skull.png?direct&600|}}

===== Source code =====

[[https://github.com/tornseglare/ShootThemAll/releases/tag/v1.6]]

===== Our code =====

    public class PositionScaleRotation
    {
        public Vector2 position;
        public float scale;
        public float rotation;
    }

Let's start with the simple class containing position, scale and rotation for an object. The new thing here is the rotation, keep in mind rotation is in radians. (2 * pi == one full 360-degree turn, pi == 180 degrees, pi / 2 == 90 degrees, you get the idea.)

    static readonly double fullCircle = Math.PI * 2.0f; // equals 360 degrees, but expressed in radians.

The Update() constantly increases the variable angle used for the rotation. 3600.0d decides the speed of the rotation and is just a number I found that looked good. Increase the value to slow down the rotation and vice versa.

    protected override void Update(GameTime gameTime)
    {
        base.Update(gameTime);
        
        angle += (float)(fullCircle / 3600.0d * gameTime.ElapsedGameTime.TotalMilliseconds);
    }

The Draw() function contains all the fun with matrixes. Let's start with creating a matrix:

    Matrix positionMatrix = Matrix.CreateTranslation(150.0f, 150.0f, 0.0f);

Here we create a translation-maxtrix, which translates (moves) 150 pixels in both the X and Y-axis. We leave the Z-axis at zero. Since monogame is actually a 3D framework we work with 3 dimensions, but ignore the Z-axis in all our examples to simulate a 2-dimensional environment.

    Vector3 stick = new();
    stick.X = 80;
    stick.Y = 0;
    stick.Z = 0;
    
    Matrix rotationMatrix = Matrix.CreateTranslation(stick);

If you take a look at the first image in this article you will see a skullisch thing with a circular arrow. The centre of this rotation is at 150, 150 pixels, which is defined by the ''positionMatrix''.

The new variable ''stick'' is a Vector3, containing X,Y,Z and we simply set the X-axis to 80. This will be the radius of the circle. 

Next, we create the ''rotationMatrix'', which is just another translation-maxtrix. We could have given the X,Y, and Z in the same way as for the ''positionMatrix'', but I wanted to name the ''stick'' so we can think of it as a stick, sticking out from the centre and rotating.

    rotationMatrix *= Matrix.CreateRotationZ(angle);

This time we are creating a z-rotation-matrix. This is rotating around the Z-axis. We send the ''angle'' along, which is ever-increased in the Update() function. (The CreateRotationZ() mod the given angle with 2pi, so we don't need to worry about that in our code in the Update() function.)

Next, we multiply the ''rotationMatrix'' with the z-rotation-matrix. I guess I should have named it stickMatrix, not rotationMatrix, sorry about that. Anyway, the effect of this multiplication is that the stick gets rotated!

    positionMatrix += rotationMatrix;

Next, we add the rotated stick to our 150,150 -position. 

    Vector3 v3position = Vector3.Transform(Vector3.One, positionMatrix);

To get a Vector3-object we use the Vector3.Transform() function. We give it the Vector3.One which is just a Vector3 object with X,Y and Z set to 1. I guess it multiplies the two and returns the result. 

    Vector2 position = new();
    position.X = v3position.X;
    position.Y = v3position.Y;

Since the spriteBatch.Draw() wants a Vector2, let's just transfer the X and Y to a Vector2 named ''position''.

    spriteBatch.Draw(Art.Skull, position, Color.White);

Finally, we draw the skull at our carefully crafted position. 

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