Point transformations child/parent with matrix

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Hello everybody,

I have to make a small rig for a skeleton, to see if it's correct by simple cinematic movement.

I need help on matrix & positions: This is about a knee flexion.

At the frame 1 I need to play with point positions: I can move the knee anywhere I want.
But my ankle would not move because the position is already knowed.

So I made a little combo with nulls to tune the knee and I wish to have a relation between the knee but not as child/parent because I want to have free control on the knee without moving the ankle.

On the knee we have 2 curve with a carve OP generating an axis line with up/normal/tangent and then I've got a central point with a correct orientation from the axis.

I can generate a line between this central point to the ankle so we can see a schematic schin.

in that situation my ankle is in the center of the world : {0,0,0 }
and when I connect it to my knee, it's going to {0.32,0.15,-1.85}

the movement is really good but I want to start with the initial position at 0,0,0 and then rotate around the knee

What did I miss to keep the inital pos then apply the matrix transformation?

I hope it's not to confused.

Thanks for your help!!
Greetings
Fred

Attachments:
2019-12-16_14h31_30.png (85.6 KB)
initial_point_position.png (162.4 KB)
with xform enabled.png (179.8 KB)
2019-12-16_14h31_24.png (200.4 KB)

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I've found another way, if someone's looking for a same action:

Finally i've done it similar by objects then I did put a parent blend constraint, with an update offset when its ready to move.
and it's working like a charm.
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