In our case, we could not use a single additive pose, as we have multiple grips, firearms and the grips can be placed at different locations on a rail.
Preparation:
- an animation curve to control when to blend in/out of the left hand override pose.
- virtual bones to be use as parent-child - VB temp_root & VB temp_child1.
- a generic grip pose for each grip, based on a base rifle pose - in our case, the AR-15.
- predetermined offset from ik_hand_gun.
Animation blueprint:
- use an evaluate node to play the generic grip pose.
- copy ik_hand_gun location & rotation to VB temp_root
- copy ik_hand_gun location to VB temp_child1
- copy ik_hand_l to rotation to VB temp_child1
- offset VB temp_child1 by a predetermined offset.
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feed this into a layered blend by pose that is splitting off clavicle_l, ik_hand_l & VB temp_root.
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copy ik_hand_gun location & rotation to VB temp_root
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copy VB temp_child1 location & rotation to ik_hand_l
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feed this into a blend node with the alpha controlled by the animation curve made earlier.
You may need to add additional nodes/logic to handle a pole vector for your left elbow, depending on your requirements. We do, as we have low ready and high ready positions to take into account.
Last thing you need to do is to remember to use the animation curve in any animation that animates the left arm to blend out/in of the grip pose:
Hope that points in you in the right direction.