Question

Hey all, i’ve been working on this small project in XNA the goal is pretty basic
i have a 3D model which pops up somewhere on the screen, the object is suppose to rotate on it’s y axis,

The problem is:
what happens instead is that the models rotate around themselves and also start to orbit the middle point between themselves and the 0,0,0 point in the screen, so it looks like the middle model is rotating normally while the other 2 act in the afore mentioned way (included img’s of the rotating models below).

“some” more details

My architecture is like this, i have 3 Models (they take Game,Position,Size @ their ctor)

private void initDradles()
    {
        m_Dradles[0] = new DradlePositionColor(this, new Vector3(15, 0, 0), 8);
        m_Dradles[1] = new DradlePositionColor(this, new Vector3(0, 0, 0), 8);
        m_Dradles[2] = new DradlePositionColor(this, new Vector3(-15, 0, 0), 8);
        this.Components.Add(m_Dradles[0]);
        this.Components.Add(m_Dradles[1]);
        this.Components.Add(m_Dradles[2]);
    }

these models first appear on the screen in the manner seen below on image 1

they all share the same BasicEffect shader but with a different world matrix

The basic effect creation and assignment to the models

protected override void Initialize()
    {
        float k_NearPlaneDistance = 0.5f;
        float k_FarPlaneDistance = 1000.0f;
        float k_ViewAngle = MathHelper.PiOver4;

        // we are storing the field-of-view data in a matrix:
        m_ProjectionFieldOfView = Matrix.CreatePerspectiveFieldOfView(
            k_ViewAngle,
            GraphicsDevice.Viewport.AspectRatio,
            k_NearPlaneDistance,
            k_FarPlaneDistance);

        // we want to shoot the center of the world:
        Vector3 targetPosition = new Vector3(0, 0, 0);//Vector3.Zero;
        // we are standing 100 units in front of our target:
        Vector3 pointOfViewPosition = new Vector3(0, 0, 130);
        // we are not standing on our head:
        Vector3 pointOfViewUpDirection = new Vector3(0, 1, 0);

        // we are storing the point-of-view data in a matrix:
        m_PointOfView = Matrix.CreateLookAt(
            pointOfViewPosition, targetPosition, pointOfViewUpDirection);

        base.Initialize();
    }

    protected override void LoadContent()
    {
        // we are working with the out-of-the box shader that comes with XNA:
        m_BasicEffect = new BasicEffect(this.GraphicsDevice, null);
        m_BasicEffect.View = m_PointOfView;
        m_BasicEffect.Projection = m_ProjectionFieldOfView;
        m_BasicEffect.VertexColorEnabled = true;

        // we are working with colored vertices
        GraphicsDevice.VertexDeclaration = new VertexDeclaration(
            GraphicsDevice, VertexPositionColor.VertexElements);

        // we did not use certain clockwise ordering in our vertex buffer
        // and we don't want antthing to be culled away..
        this.GraphicsDevice.RenderState.CullMode = CullMode.None;

        foreach (var dradle in m_Dradles)
        {
            dradle.BasicEffect = m_BasicEffect;
        }

        base.LoadContent();
    }

In each model theres this code in the update method:

private void BuildWorldMatrix()
    {
        m_WorldMatrix = Matrix.Identity;
        m_WorldMatrix *= Matrix.CreateScale(m_Scales);
        m_WorldMatrix *= Matrix.CreateRotationX(m_Rotations.X);
        m_WorldMatrix *= Matrix.CreateRotationY(m_Rotations.Y);
        m_WorldMatrix *= Matrix.CreateRotationZ(m_Rotations.Z);
        m_WorldMatrix *= Matrix.CreateTranslation(m_Position);
            ///*I*/ Matrix.Identity *
            ///*S*/ Matrix.CreateScale(m_Scales) *
            ///*R*/ Matrix.CreateRotationX(m_Rotations.X) *
            //        Matrix.CreateRotationY(m_Rotations.Y) *
            //        Matrix.CreateRotationZ(m_Rotations.Z) *
            ///* No Orbit */
            ///*T*/ Matrix.CreateTranslation(m_Position);
    }

    public override void Update(GameTime gameTime)
    {
        m_Rotations.Y += (float)gameTime.ElapsedGameTime.TotalSeconds;
        BuildWorldMatrix();

        base.Update(gameTime);
    }

and this code in the draw method

public override void Draw(GameTime gameTime)
    {
        m_BasicEffect.World = m_WorldMatrix;
        m_BasicEffect.Begin();
        foreach (EffectPass pass in m_BasicEffect.CurrentTechnique.Passes)
        {
            pass.Begin();
            Render();
            pass.End();
        }
        m_BasicEffect.End();

        base.Draw(gameTime);
    }

    public override void Render()
    {
        renderVerticesBox(m_BodyVertices);
        renderVerticesBox(m_HandleVertices);
        Game.GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(
            PrimitiveType.TriangleFan, m_PyramidVertices, 0, 4);
    }

    private void renderVerticesBox(VertexPositionColor[] m_Vertices)
    {
        Game.GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(
            PrimitiveType.TriangleStrip, m_Vertices, 0, 8);

        Game.GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(
            PrimitiveType.TriangleStrip, m_Vertices, 10, 2);

        Game.GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(
            PrimitiveType.TriangleStrip, m_Vertices, 14, 2);
    }

image when rotating
alt text http://www.freeimagehosting.net/uploads/b0aa83d0d6.png

image when starting

alt text http://www.freeimagehosting.net/uploads/ee9e5c379c.png

Answer 1

I think you need to move your line

m_WorldMatrix *= Matrix.CreateTranslation(m_Position);

up a few lines, do this before applying the rotation matrices.