WEBVTT

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Inverse kinematics, or IK, is one of the most common bone constraints, as it helps make

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moving limbs and other joints very intuitive and easy to animate.

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But what is IK?

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Well, inverse kinematics is the opposite, or inverse, of forward kinematics, or FK.

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So let's figure out what FK is first.

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To do this, I'm going to demonstrate using a simple arm rig.

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You can find this base arm rig in the same character mesh file available in the description

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down below in the collection called arm base rig.

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Now let's talk about what exactly FK is.

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FK is simply how bone parent chains work by default.

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In other words, moving the parent moves the child.

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The reason why it's called forward kinematics is because the transformation of the bones

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only influences bones that are further forward down the chain of bones.

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Which way is forward?

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Well, if you pretend the tails of the bones are like arrows, then forward down the chain

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is simply the direction the bones are pointing.

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You can also see that moving the child right now will not affect the parent at all.

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Great, now we know what forward kinematics is.

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So what is inverse kinematics?

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Well, we know it's the opposite, but what does that mean?

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Well, instead of influencing bones forward down the chain, inverse kinematics allows

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for the transformation of a bone near the end of a chain to influence bones that are

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further up the chain of bones.

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Basically it's when a child bone has influence over the parent bone in some way.

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To demonstrate this, I want to quickly show you the auto IK option.

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Let's open the right hand side menu by pressing N or clicking this arrow here.

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Then go to tool and you'll see the auto IK option.

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This is a simple checkbox Blender has that doesn't permanently change your armature at

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all but rather helps when you want to pose bones very quickly.

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By enabling this, you can very easily move bones at the end of chains to influence the

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transformation of the entire chain itself.

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This is especially handy when animating hands or feet.

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In real life, when we raise our hands, we don't think, oh, I'll move my upper arm and

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then extend my forearm quickly so that it allows my hand to travel upwards.

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No, we tell our hands to move upwards and our upper arm and forearm act accordingly

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to accomplish that movement.

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It's this kind of natural thinking that IK allows for animators to do with their rigs.

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With IK, by simply telling the hand bone to move upwards, the upper arm and forearm bones

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will automatically determine where they have to be to accomplish that movement.

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Likewise, it's also very helpful for making sure feet stay glued to the ground when needed.

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Great, now that we understand what IK is, let's figure out how to apply it to our rigs.

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The auto IK feature is nice for quick posing, but it doesn't give us full control over the

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IK and it doesn't work for animation.

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So instead, what we want to do is use the inverse kinematics bone constraint to set

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it up properly.

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The first step is simple.

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Let's go into pose mode, select our forearm bone and add the inverse kinematics bone constraint

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from the bone constraints tab indicated by this wrapped blue bone icon in the properties

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editor.

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From there, you'll notice that we have a few input fields to fill.

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The target object will be the armature itself, even though what we really want is to target

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a specific bone, but that's within that armature, so let's select armature from the dropdown.

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Then we can select the bone we want in the bone input field.

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This works in a similar way to the track 2 and stretch 2 constraints, where our target

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bone is where we want our active bone to point to.

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So in this case, we'll want to choose the hand bone.

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However, when we select our hand bone now and try to move it, it doesn't actually seem

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to do anything.

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In fact, it's not translating at all as it's still connected to the forearm bone.

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Let's simply go into edit mode and disconnect our hand bone from the forearm bone, but keep

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it parented.

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Now as you can see, we can translate our hand bone and it influences the forearm bone,

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but it also influences the entire rig.

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This is not what we want, we only want the arm to bend, so let's go into our inverse

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kinematics bone constraint and go to our chain length option.

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The default is 0, and this simply goes up the entire bone chain the active bone is parented

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to.

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So in our case, it goes all the way down to the bottom of the spine.

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This is indicated by the yellow dotted line here.

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By changing the chain length value, our yellow dotted line will show how far up the chain

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our inverse kinematics will take effect.

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You might see a bit better in wireframe mode.

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However, the only bones we want influenced by the inverse kinematics constraint are the

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forearm and upper arm bones.

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So we want a chain length of 2.

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Now when we move our hand bone, it correctly influences only the forearm and upper arm.

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But I don't think we want it to be freaking out like this.

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It doesn't seem like it's very animatable or stable, so what's happening here?

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Well quite simply, remember in the parenting video when I said you can't form a loop in

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the parents?

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Well we sort of just did here.

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If you think about it, the forearm is pointing to the hand, but the hand is parented to the

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forearm.

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This means that when the hand moves, the forearm will try to point to the hand.

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But if the forearm moves to point to the hand, the hand will have to move because it's parented

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to the forearm, thus moving the hand further, which will move the forearm further to point

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to the hand, which will move the hand further because it's parented, which will move the

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forearm to point to the hand, etc. etc.

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This is called a cyclic dependency, and we don't want that.

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So let's go ahead and unparent the hand entirely so that we can move our hand independently

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from our forearm.

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Nice, as you can see, moving our hand now bends the elbow in a very natural way.

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The only issue we have now is because the hand is no longer connected to the forearm,

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we can now stretch the hand out away from the forearm, which isn't ideal.

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So the most common workaround for this is to simply go back into edit mode, re-parent

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the hand to the forearm, then duplicate the hand bone and unparent the duplicate.

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Now we have an independent controller bone that we can point to instead.

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Let's call this bone hand underscore ik.

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Going back into pose mode, we can select our forearm, go into the bone constraints tab,

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and change the bone target from hand to hand ik.

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Now as we move our hand ik bone, the forearm and upper arm deform nicely, and we can move

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this hand ik controller out without stretching the hand itself.

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However, now to rotate the hand bone itself, we would have to switch between animating the

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hand ik and the original hand bone.

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That's a bit tedious for our animators, so let's add a simple copy rotation constraint

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for the original hand deformation bone to follow the hand ik bone for easy animation.

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I'm also going to make the hand ik bone a bit bigger in edit mode, so it's easier to differentiate

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from the original hand bone, although giving it a nice custom bone shape is an option as

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well.

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Great, now we have a nice basic ik rig.

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Except, you may notice that sometimes when we move the hand bone around, the elbow sort

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of bends the wrong way.

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This is not ideal, and some of you may remember that there is still one more input field in

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our inverse kinematics constraint.

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Let's go back into it and see what it is.

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Yes, the pole target input field.

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This will be helpful for solving the issue of joints bending the wrong way during inverse

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kinematics.

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A pole target is simply a reference object or bone that the elbow joint or inverse kinematics

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joint will try to point to as the bones bend.

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For this, we can simply create a new bone by duplicating the hand ik bone and moving

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it behind the elbow.

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I'm going to rotate it so that it points away, but it doesn't matter because only its location

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is actually referenced.

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Let's rename this to elbow underscore target.

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Now we can go back into our inverse kinematics constraint settings and simply input the armature

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again for the object field.

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For the bone input, we'll want to select elbow underscore target.

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And now, as you can see, when we move our hand ik bone, the elbow never bends in the

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wrong direction anymore.

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Except that's because the elbow doesn't bend at all.

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This is because, quite simply, our bones were created too straight and it doesn't

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know which direction to start bending.

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So let's very quickly go into edit mode and just drag our elbow joint slightly backwards.

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Now as you can see, our elbows bend just fine.

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Except it's still in the wrong direction.

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But don't worry, that's because we forgot to set one more value in the inverse kinematics

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constraint.

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Let's go ahead and keep our elbow bent and then select our forearm bone again.

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We can go into the inverse kinematics constraint and click and drag this pull angle value

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until it points to the pull target correctly.

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And that's it!

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We now have a working inverse kinematics rig in Blender.