For 2 days I have been going insane over something very simple. All I am trying to do is to transform a parent matrix, and transform its child-matrices inversely, so that the child matrices keep their global transform. First I tried simply applying the localised ("internalised") inverse transform-change. It's a per-frame operation and there was always a gradual shifting away from the global position of child-object/matrices. No matter how much I experimented with the orders of rotation/translation, I couldn't get it right. The rotation part looks good and stable, but the translation always shifts away.

So I finally gave up and wanted to try something less prone to easily overlooked "inaccuracies": every frame I compute the last global position (since rotation is working, I only need the translation, I guess) by transforming the child-position with last frame's parent-transform. Then I inverse-transform this global position with the updated (this frame's) parent-transform, to re-localize the global position to the new transform. This is about as simple as it gets, and should be completely independent from any "localizing velocity issues".

... and still my child matrices are drifting away from their global positions. I just don't get it. Am I missing something?

edit: when the parent is not moving/rotating at all, parent-transformation is constant over time, and I only say...

- vector globalPos = parentTrans.transform(localPos);

- vector newLocalPos = parentTrans.tranformInv(globalPos);

... I would expect to see the child positions move with the parent, like when leaving the local positions unchanged. But even now, I get a drift away from the parent-origin, increasing with distance, for every child-position. How do I localize a position to a transform, if not by inverse-transforming???

Leaving the positions alone produces no drift, applying the 2 lines above does. Wth??? This can't be rounding errors. The shifting is much too systematic and smooth for that.

**Edited by Max Power, 04 April 2014 - 05:56 AM.**