Question

I have a question concerning model rotation in XNA. The question is – what should I do (which values should be changed and how) to rotate green model in that way (red arrows):

http://img843.imageshack.us/i/question1.jpg/

Code used to draw :

DrawModel(elementD, new Vector3(-1, 0.5f, 0.55f), 0,-90,0);

private void DrawModel(Model model, Vector3 position, float rotXInDeg, float rotYInDeg, float rotZInDeg)
    {
        float rotX = (float)(rotXInDeg * Math.PI / 180);
        float rotY = (float)(rotYInDeg * Math.PI / 180);
        float rotZ = (float)(rotZInDeg * Math.PI / 180);

        Matrix worldMatrix = Matrix.CreateScale(0.5f, 0.5f, 0.5f) * Matrix.CreateRotationY(rotY) *Matrix.CreateRotationX(rotX)*Matrix.CreateRotationZ(rotZ)* Matrix.CreateTranslation(position);
        Matrix[] xwingTransforms = new Matrix[model.Bones.Count];
        model.CopyAbsoluteBoneTransformsTo(xwingTransforms);
        foreach (ModelMesh mesh in model.Meshes)
        {
            foreach (BasicEffect effect in mesh.Effects)
            {
                effect.EnableDefaultLighting();

                effect.View = cam.viewMatrix;
                effect.Projection = cam.projectionMatrix;
                effect.World = (xwingTransforms[mesh.ParentBone.Index] * worldMatrix);
            }
            mesh.Draw();
        }

    }

---

So I tried to apply your solution by changing my code slightly:

Matrix worldMatrix = Matrix.CreateScale(0.5f, 0.5f, 0.5f) * Matrix.CreateRotationY(rotY) * Matrix.CreateRotationX(rotX)  * Matrix.CreateTranslation(position)
                * Matrix.CreateRotationX((float)(45 * Math.PI / 180)) * Matrix.CreateRotationZ(rotZ) * Matrix.CreateRotationX((float)(-45 * Math.PI / 180)); 

By changing rotZ parameter indeed I was able to rotate the model. However the effect is not what I wanted to achieve http://img225.imageshack.us/i/questionau.jpg/, it changed its position. Is it because of a faulty model or some other mistake? I want the “cylinder” to remain in its position. Do you know how can I do this?

Answer 1

Rotation with rotation matrices are cummulative. So you can probably calculate your rotation matrix by rotating the model 45 degrees “down”, then apply your rotation around your wanted axis, then rotating the model 45 degrees “up” again. The product of these three matrices should give you your desired matrix.