Question

I’m new to XNA and would like to develop a light-weight 2D engine over it, with the entities organized into parent-child hierarchy. I think of matrix when drawing children, because their position, rotation and scale are depend on their parent.

If I use SpriteBatch.Begin(), my rectangles can be drawn on the screen, but when I change them into:

        this.DrawingMatrix = Matrix.Identity;
        this.SpriteBatch.Begin(SpriteSortMode.Deferred, BlendState.AlphaBlend, SamplerState.LinearClamp, DepthStencilState.None, RasterizerState.CullClockwise, null, this.DrawingMatrix);

nothing is drawn anymore. I even tried new Matrix() or Matrix.CreateTranslation(0, 0, 0) for DrawingMatrix.

My first question is: why doesn’t it work? I’m not working with any camera or viewport.

Secondly, before drawing an entity, I call the PreDraw to transform the matrix (I will then reset to original state at PostDraw):

    protected virtual void PreDraw(Engine pEngine)
    {
        pEngine.DrawingMatrix *=
            Matrix.CreateTranslation(this.X, this.Y, 0) *
            Matrix.CreateScale(this.ScaleX, this.ScaleY, 1) *
            Matrix.CreateRotationZ(this.Rotation);
    }

Please clarify the correction of above code. And I need to scale not at the origin, but at ScaleCenterX and ScaleCenterY, how can I achieve this?

ADDED: Here is an example of my engine’s draw process:

1. Call these code:

       this.DrawingMatrix = Matrix.CreateTranslation(0, 0, 0);
       this.SpriteBatch.Begin(SpriteSortMode.Deferred, BlendState.AlphaBlend, SamplerState.LinearClamp, DepthStencilState.None, RasterizerState.CullClockwise, null, this.DrawingMatrix);
   

2. Call PreDraw(), with is:

   protected virtual void PreDraw(Engine pEngine)
   {
       pEngine.DrawingMatrix *=
           Matrix.CreateTranslation(this.X, this.Y, 0) *
           Matrix.CreateScale(this.ScaleX, this.ScaleY, 1) *
           Matrix.CreateRotationZ(this.Rotation);
   }
   

3. Call Draw(), for example, in my Rect class:

   protected override void Draw(Engine pEngine)
   {
       pEngine.SpriteBatch.Draw(pEngine.RectangleTexture, new Rectangle(0, 0, (int)this.Width, (int)this.Height), new Rectangle(0, 0, 1, 1), this.Color);
   }
   

If I replace above Begin code with this.SpriteBatch.Begin(), the rectangle is drawn correctly, so I guess it is because of the matrix.

Answer 1

First issue is a simple bug: The default for SpriteBatch is CullCounterClockwise, but you have specified CullClockwise causing all your sprites to get back-face-culled. You can pass null if you just want to use the default render states – you don’t need to specify them explicitly.

(You would need to change the cull mode if you used a negative scale.)

To answer your second question: You need to translate “back” to place the scaling origin (your ScaleCenterX and ScaleCenterY) at the world origin (0,0). Transformations always happen around (0,0). So normally the order is: translate sprite origin back to the world origin, scale, rotate, translate to place sprite origin at desired world position.

Also, I hope that your PostDraw is not applying the reverse transformations (you made it sound like it does). That is very likely to cause precision problems. You should save and restore the matrix instead.