How is the scengraph traversed?

Engines, Libraries and Tools Xith3D Forums
#1

A new question which has nothing to do with gluUnProject.

Suppose we have the following pseudocode

addChild(Translation)
Translation.addchild(Rotation)
Rotation.addChild(ourShape)

This should create a branch which looks like:
Root --> Translation --> Rotation --> outShape

Supposed our object starts at (0,0,0)

I thought the above would first translate the object (lets say to (0,10,0))

Then it would do a rotation around the origin (0,0,0)

However, when I actually code this up it does the rotation around the center of the object. I’m a little confused by that. I thought if you want to rotate the object around it’s own center you need to do the rotation first then the translation.

Can someone point out where I’m wrong in my thinking?

On another note. Shouldn’t these two blocks of code do the exact same thing? BLock 1 works for me and Block 2 does not.

Block 1: (Position is a vector3f)


      translate.setTranslation(position);
      translateGroup.setTransform(translate);

Block 2:

  (position is the same as above)
            Matrix4f transMatrix = new Matrix4f();
            transMatrix.setTranslation(position);
            translate.set(transMatrix);
            translateGroup.setTransform(translate);

Thanks. ~shochu

#2

You’ve got it back to front.

Your tree should look like this:

Root -> Rotate -> Translate -> ourShape.

Because the transforms are applied starting at the outer nodes (in this case the shape) and folded inwards.

The order in which the above transforms are applied is “Translate” then “Rotate”

Root -> Translate -> Rotate -> ourShape as you saw rotates the shape in it’s place (which is also useful).

When in doubt - try reversing the order as the chances are you just got it back to front :slight_smile:

With your second question - read the javadoc and/or source.

You will see that “set” (unlike “setTranslation”) resets all other components (rotation and scale) to the identity matrix. “setTranslation” only effects the translational components. This is a very important “feature” of Transform3D’s methods - which ones reset and which ones don’t.

I am guessing that’s the problem - if you post the complete method I can look at it closer (although it’s just a moot point isn’t it? It’s much cleaner to use setTranslation which is why it’s provided).

Hope that helps,

Will.

#3

THanks =) That helped. I was assuming that the transformation were being applied from the root down to the leaf but I guess it’s from the lead up to the root. :slight_smile: