Rotating a vertex using transformation matrix

Question

I'm trying to rotate a vertex using the lwjgl Matrix4f class. I tried this code:

    float rotY = (float) Math.toRadians(180);

    Matrix4f transMat = new Matrix4f();
    transMat.rotate(rotY, new Vector3f(0.0f, 1.0f, 0.0f));
    transMat.translate(new Vector3f(1.0f, 0.0f, 0.0f));

    Vector4f vecPosMod = new Vector4f(1.0f, 0.0f, 0.0f, 1.0f);
    Matrix4f.transform(transMat, vecPosMod, vecPosMod);

It should rotate a Vector3f(1.0f, 0.0f, 0.0f) by 180 degrees but unfortunately after all calculations vecPosMod is (-2.0, 0.0, 1.7484555E-7, 1.0). I want it to be (-1.0, 0.0, 0.0, 1.0). How?

score 1 · Accepted Answer · answered Jul 20 '12 at 04:27

You are applying a translation along the X axis to your matrix after the rotation. This is translating the position one unit along the X axis after the rotation is performed resulting in -2.0 instead of -1.0.

Try this:

  float rotY = (float) Math.toRadians(180);

  Matrix4f transMat = new Matrix4f();
  transMat.rotate(rotY, new Vector3f(0.0f, 1.0f, 0.0f));

  Vector4f vecPosMod = new Vector4f(1.0f, 0.0f, 0.0f, 1.0f);
  Matrix4f.transform(transMat, vecPosMod, vecPosMod);

It gives the following result:

(-1.0 0.0 8.742278E-8 1.0)

The 8.742278E-8 is probably as close to 0.0 as you can get after applying a rotational transform with floats in java.

Rotating a vertex using transformation matrix

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