# Question on Node rotations

Hi, I am writing a Collada viewer, and was wondering about rotation elements that are children of nodes.

Using the Seymour_triangulate.dae file as an example, it contains the following snippet:

<node id=“l_hip” name=“l_hip” sid=“l_hip” type=“JOINT”>
<translate sid=“translate”>0.450181 0.049889 0.340342 </translate>
<rotate sid=“jointOrientZ”>0 0 1 -81.9269 </rotate>
<rotate sid=“jointOrientY”>0 1 0 -3.10948 </rotate>
<rotate sid=“jointOrientX”>1 0 0 0 </rotate>
<rotate sid=“rotateZ”>0 0 1 42.183 </rotate>
<rotate sid=“rotateY”>0 1 0 -5.78704 </rotate>
<rotate sid=“rotateAxisX”>1 0 0 -150 </rotate>

I have two questions:

1. I’m assuming the rotations need to be done in order, but why are there two different rotations about each of the x, y and z axes, and what space are these axes defined in?

2. Do I need to do the translation before the rotations?

Nevermind part 2, the specification specifically mentions the rotations are without any translation.

<translate> and <rotate> cumulates in the order they appear.
Just like a matrix stack in a rendering API.

Actually I find this a very confusing statement in the spec.

Basically the order matters, and you should use the ‘getContents’ to iterate over the proper order of transforms (<lookat>,<matrix>,<rotate>,<scale>,<scew>,<translate>) and build the resulting matrix, using post-multiplication (as if each individual transform element first was converted to a matrix).

According to the Collada 1.4.1 spec:

Rotations change the orientation of objects in a coordinated system without any translation.
Translations change the position of objects in a coordinate system without any rotation

I wonder what this ‘without translation’ part in the <rotate> and ‘without rotation’ in the <translate> part is refering too?
Those transforms act in a local coordinate system, and they concatenate in order, so they include all translate’s, rotate’s and others (scale etc) previous applied in all the transforms in this node’s transform stack. In a node-hierarchy, they also include all transforms (rotates,translates etc) from its parent… That’s why ‘without translation’ and ‘without rotation’ is confusing: the local coordinate system does include translation/rotation.

Erwin

It’s just saying that the respective element does not contain information that causes a transformation other then what is expected.

Yes, in other words they define the local coordinate system.