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=COLLADA Animation Import: How It Works=

==Abbreviations used==
tm = transformation (rotation, scale, translation, etc.), tms = transformations

==The Problem==

DAE has a very specific way of describing object tm. For example (excerpt from [[#References|[1]]]):

<pre>
<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 1.000000 -81.926788</rotate>
<rotate sid="jointOrientY">0 1.000000 0 -3.109476</rotate>
<rotate sid="jointOrientX">1.000000 0 0 0</rotate>
<rotate sid="rotateY">0 1.000000 0 0</rotate>
<rotate sid="rotateAxisX">1.000000 0 0 -150.000000</rotate>
<node id="l_knee" name="l_knee" sid="l_knee" type="JOINT">
<translate sid="translate">1.833014 -0.024761 -0.002519</translate>
<rotate sid="jointOrientZ">0 0 1.000000 2.742172</rotate>
<rotate sid="jointOrientY">0 1.000000 0 -8.695618</rotate>
<rotate sid="jointOrientX">1.000000 0 0 -120.102058</rotate>
<rotate sid="rotateZ">0 0 1.000000 0</rotate>
<rotate sid="rotateAxisX">1.000000 0 0 -167.476486</rotate>
...
</pre>

This shows 2 nodes, "l_knee" is a child node of "l_hip" (it's tms are in parent space).
Both have a series of tms (there can be any number them) that have to be applied sequentially to produce the final tm.

Each tm can be driven by anim curve.

==How This Fits Into Blender==

To correctly translate DAE tms to a Blender object's rotation, scale, location, the following is done:

Separately for rotation, scale and location:
* find all keys at which to sample
* at each key: evalute the whole transform, write it

This is simple. Now the difficult part.

==How Bone Animation Is Translated==

For bone animation, DAE joint tm cannot be used "as is" for a Blender bone. Because:
* in Blender, Y axis is the bone length direction. In DAE, there's no such convention (there're no bones actually, only arbitrarily-oriented nodes)
* in Blender, bone animation is stored as relative to bone rest pose, in DAE it's plain object-space

So bone animation has to be converted somehow.

The following formula is used to translate a bone animation key:

<pre>
K = iR * M * iR_dae * R
</pre>

Where R and iR are Blender bone rest matrix and it's inverse, iR_dae is an inverse of DAE joint rest matrix, M is an evaluated joint matrix.
All matrices are world-space.

(M * iR_dae) gives a world-space change of a joint from it's rest pose.
Post-multiplying this change by R gives a world-space Blender bone matrix.
Pre-multiplying the result by iR gives a matrix that changes a Blender bone rest pose the same way it changed a DAE joint.

It took me 2 weeks to determine this formula. Yes, I should read more maths books :)

==References==

[1] Seymour_anim2.dae from [http://collada.org/owl http://collada.org/owl]

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