I’ve been working with a legacy skeleton for a while where left foot, right foot, left hand, and right hand IK bones are all simply children of the root bone.
I’m trying to understand why the mannequin IK bones are set up the way they are. There is the foot IK root bone and the hand IK root bone. Then there’s the gun IK bone. And left and right hands are children of the gun IK bone.
What is the purpose of the IK root bones as opposed to having all IK bones children of the main root bone? And what is the purpose of the gun IK bone? Does that make something easier somehow?
Out of the box they don’t do anything and are used as an IK target for coded or BP features like 2-bone-IK.
Setup
In your authoring app the IK is constrained, not linked, to it’s parent bone (Left hand IK to left hand bone) and the IK chain maintains its positon relative to the main character root.
In Unreal 4 you can then set up a 2-bone-IK and set the target to use the IK bones that you can use to pull the joints (lets say the left arm) to the corrected position referenced by the IK target.
Typical use.
Unreal 4 interpolates everything so the relative position of a joint is generally not the same position as authored and more so when blending between animation clips that causes floating issues as to the need to maintain contact points.
For example Aim offsets has problems maintaining the correct contact point with the forward stock of the rifle as it interpolates to the pitch and yaw changes. Using the left hand IK you can constraint the IK to the weapon and using 2-bone-IK pull the joints in the left hand chain to the relative position of the forward stock and locks it into place.
The weapon IK is not really an IK but an auxiliary effector and more useful for the animator to do a lazy reload of say an M4 (for example). In say MotionBuilder one can constraint the weapon_IK to the left hand and **** the weapon with the right hand instead of doing a tactical reload. (magazine dropped and reload and cocked with the left hand)
Overall though it seems they are there for legacy purposes of a past requirement of say moving character assets from UDK or a previous engine where they were required
Thing is, I’ve been able to do all of those things my way without the extra IK root bones which is why I’m confused why they exist.
I’d be interested in examples of why I need the IK Hand Root and IK Foot Root bones as opposed to having those just be children of Root itself. I’ve never seen IK Hand Root and IK Foot Root move away from Root.
Same with that Gun bone…
Here’s currently my setup just to clarify some more:
Root
(Main Skeleton itself)
Pelvis
Spine 01
etc...
Left Leg
etc...
Left Hand IK Target
Right Hand IK Target
Left Foot IK Target
Right Foot IK Target
(Knee Pole Vector Targets, 2 Bone IK has the option of specifying these)
Left Knee IK Target
Right Knee IK Target
(Elbow Pole Vector Targets)
Left Elbow IK Target
Right Elbow IK Target
This is the setup used in Unreal 4 Mannequin that I’m trying to understand
Root
(Main Skeleton itself)
Pelvis
Spine 01
etc...
Left Leg
etc...
Hand IK Root (Not sure what this extra hand IK root is for)
Gun (Located at the right hand)
Left Hand IK Target
Right Hand IK Target
Foot IK Root (Not sure what this extra foot IK root is for)
Left Foot IK Target
Right Foot IK Target
(Also in UT4 I saw there were a few extras, not sure if these are even used, maybe for legacy UT3 anims because this is how it was in UDK)
Hand IK Root Alt
Left Hand IK Target
Right Hand IK Target
Foot IK Root Alt
Left Foot IK Target
Right Foot IK Target
With my setup, I plan on having either the left hand or the right hand reach out to interact with arbitrary objects in the world. I currently have it working beautifully for holding different sized guns by making the left hand move to the correct spot on the gun with IK. I had to modify existing Skeletal controls a bit which I can describe how if anyone’s interested but it works in the end.
Also in Maya my control rig is set up so I control knee and elbow directions using pole vector targets. And I can constrain the bones in the skeleton to those controls as well so what I do in Maya matches exactly in game.