Child parent rendering (flash) in XNA

Question

i am currently in the process of implementing a render engine that works like flash's. Where a sprite is drawn inside another sprite and reflects all transformation (rotation, scale, etc) on the parent.

I have my transformation setup like so.

    public Matrix transform
    {
        get
        {
            Matrix m_transform =
                Matrix.CreateTranslation(new Vector3(x, y, z)) *
                Matrix.CreateRotationZ(rotation) *
                Matrix.CreateScale(scale)
                ;
            return m_transform;
        }

    }

But i cannot use spritebatch to render the sprite with a matrix. Is there a way to properly do this ?

Thanks alot ;)

Answer 1

Use the following helper method which decomposes any Matrix into a set of position, rotation and scale that can be used directly with a SpriteBatch. It's basically a wrapper around the existing Matrix.Decompose method but making it SpriteBatch friendly:

public static void DecomposeMatrix(ref Matrix matrix, out Vector2 position, out float rotation, out Vector2 scale)
{
    Vector3 position3, scale3;
    Quaternion rotationQ;
    matrix.Decompose(out scale3, out rotationQ, out position3);
    Vector2 direction = Vector2.Transform(Vector2.UnitX, rotationQ);
    rotation = (float) Math.Atan2(direction.Y, direction.X);
    position = new Vector2(position3.X, position3.Y);
    scale = new Vector2(scale3.X, scale3.Y);
}

It should be pretty straightforward, but here's an example of how to use the method above:

Matrix transform = /* The transform of your sprite */

Vector2 position, scale;
float rotation;
DecomposeMatrix(ref transform, out position, out rotation, out scale);

spriteBatch.Draw(texture, position, null, Color.White, rotation, Vector2.Zero, scale, SpriteEffects.None, 0.0f);

Notice how the origin parameter is left as Vector2.Zero in the example. This is because I usually already embed the origin transformation into the matrix, so the resulting position already includes it.

For more information on the subject of implementing a sprite hierarchy in XNA, I will simply leave two links to related answers of mine:

• Is there a simple way to group two or more sprites, so all of them will be dependent on each other?
• How to rotate “stacked sprites” in XNA? (Scenario 2)

Answer 2

Shawn Hargreaves write this http://blogs.msdn.com/b/shawnhar/archive/2011/01/12/spritebatch-billboards-in-a-3d-world.aspx article on his blog about it. Create BasicEffect instance, set transforms and draw:

spriteBatch.Begin(0, null, null, null, null, basicEffect);
//draw something
spriteBatch.End();