By now, you're familiar with Object mode and Edit mode. We're going to use another mode that's specific to armatures:
Pose mode.
With the armature selected, press Ctrl TAB . This essentially substitutes Object Mode for Pose Mode. In other words, you can now press TAB and switch between Pose mode and Edit Mode. If you need to get Object mode back, press Ctrl TAB again, and you can then switch between Object and Edit mode with TAB . You know you are in Pose mode when you select a bone and it is outlined in light blue.
For armatures, Edit mode is used to construct the armature. Object mode is to move the entire armature as a whole. Pose mode is used for, well, posing. In Pose mode, you can grab, rotate, and scale each bone individually.
Bone names, upper body
upper_arm.l selected in Edit mode.
upper_arm.l selected in Pose mode.
Armature selected in Object mode (individual bones cannot be selected
in Object mode).
Try selecting the upper arm in Pose mode and rotating it in Front view. Notice how all bones "downstream" of it rotate as well.
Now select the lower arm and rotate it in Front view. The upper arm stayed in place, but the lower arm and everything
"downstream" rotated. This is the essence of parenting.
That is, the upper arm is the parent of the lower arm. The lower arm is in turn the parent of the hand bone. Another way to say that is that the hand is the child of the lower arm. These parent-child relationships were automatically created when we extruded the bones. The extruded bone becomes the child of whatever it was extruded from. That's the reason we started from the lower spine and extruded upward, as well as starting at the shoulder and extruding toward the fingers.
Clear the rotation of all bones by using A twice to select all, then
Alt R to clear rotation. The bones are now reset to their original rotations. You'll end up using this command a lot, along with the related command Alt G , which clears location.
Turn on Auto IK in the Armature panel, under the Edit buttons.
upper_arm.l rotated in Front view.
lower_arm.l rotated.
Clearing the rotation of all bones in the armature.
Turn on Auto IK in the Armature panel.
Select the tip of the arm and move it with G . Note that it moves much differently now!
A little explanation: Forward Kinematics, or FK, is the way of moving bones that we first used. That is, rotate the upper arm, and its children (and children's children!) follow along. The opposite of FK is Inverse Kinematics (IK), where we move a child and the parents follow along. In reality, there is some fancy math going on in the background that tries to point the chain of bones toward the target. What's the target? For Auto-IK, it's whatever bone you have selected. In this case, the target is the finger2.l bone. What's the chain? It's the lineage of bones going all the way back to the
great-great-great-(etc)-grandparent.
In our armature, when we moved the finger bone, all the bones in the chain tried to point to wherever we moved it. An orange line showed up, connecting the finger2.l bone to the spine1 bone. The orange line points to the root of the chain:
spine1 is the highest parent of finger2.l, and the chain is everything between spine1 and finger2.l.
It would be nice if the spine didn't move so much when we moved the arm. We'll fix this by essentially breaking the IK chain at the shoulder so only the arm moves and the spine stays still.
Go into Edit mode of the armature with TAB . Note that even though you may have just moved some bones around in Pose mode, upon entering Edit Mode everything goes back to the way it was. In Edit Mode, you're viewing the bones as they are in Rest position, and once you go back out to Pose mode, your posed armature will return.
Select the upper_arm.l bone in Edit mode.
finger2.l moved with Auto IK.
Select upper_arm.l in Edit mode.
In the Armature Bones panel under the Edit buttons, deselect the Con button. In this panel, the child of:
menu indicates that this bone, upper_arm.l, is the child of shoulder.l. We want to keep that relationship, but we'd like to allow upper_arm.l to be disconnected from shoulder.l . . . and therefore break the IK chain.
Con stands for Connected. By deselecting this button, we disconnected the upper_arm.l bone from the shoulder.l bone.
To test this new setting, switch to Pose mode ( TAB ).
Reset the armature by pressing A twice to select all bones, Alt G to clear locations, and Alt R to clear rotations.
Now, move the finger2.l bone again. Much different! The orange line now points to the root of the chain, which is the upper_arm.l bone.
While we were able to extrude bones symmetrically, we have to make changes to the settings separately. To disconnect the upper_arm.r bone from shoulder.r in the same way,:
Switch to the armature's Edit mode Select upper_arm.r.
Deselect the Con button.
Test the armature.