Math in setLocalTranslation

Nike · February 23, 2015, 4:16am

I’m looking for a mathematical discussion of setLocalTranslation. I’ve come up absolutely empty - even in a wide-open Google search. And nothing in the JME Wiki at all beyond people having difficulties.

Please point me to a real definition, not a textual arm waving. I have a math background and the math part is easy. The definitions are hard.

Nike

Momoko_Fan · February 23, 2015, 4:59am

Not sure what you mean? There’s no math in setting local translation, it just modifies an internal vector.
Perhaps you mean how the world transform is derived? In that case, you can find it in Transform.combineWithParent():

github.com

jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/math/Transform.java#L193


 * vector3f is created to hold the value. The value, once stored, is
 * returned.
 *
 * @param scale The store location for this transform's scale.
 * @return The value of this transform's scale.
 */
public Vector3f getScale(Vector3f scale) {
    if (scale == null) {
        scale = new Vector3f();
    }
    scale.set(this.scale);
    return scale;
}
/**
 * Sets this transform to the interpolation between the first transform and
 * the second by delta amount.
 *
 * @param t1 The beginning transform.
 * @param t2 The ending transform.
 * @param delta An amount between 0 and 1 representing how far to

Nike · February 23, 2015, 6:00am

Momoko_Fan

Looking at the source code references, and the Spatial.java source as well, and piecing it together…

The Spatial class has ‘LocalTransform’ and ‘WorldTransform’ members. These are instances of the ‘Transform’ class.

The ‘Transform’ class has a displacement vector member, a Quaternion rotation member and a scale member.

setLocalTranslation sets the vector value of the translation vector inside the localTransform member inside the Spatial class.

The spatial.java file gives:
public void setLocalTranslation(float x, float y, float z) {
this.localTransform.setTranslation(x, y, z);
setTransformRefresh();
}

Okay, so now I must guess that the local translation is applied to position-rotate-scale the spatial with respect to the parent node. That’s different than I had imagined.

Thanks
Nike

pspeed · February 23, 2015, 7:35am

That’s precisely what a scene graph is, though.

This might help bring you up to speed on what a scene graph is and explain a bit about relative transforms in the process:
http://wiki.jmonkeyengine.org/doku.php/jme3:scenegraph_for_dummies