Computer Animation Ying Zhu Georgia State University Character Animation with Armature

Skeletal animation Skeletal animation is the standard way to animate characters A character is represented in two parts A polygon mesh model of the character (called skin) A set of bones (called skeleton, or armature) used for animation

Armature “An armature is the name of the kinematic chains used in computer animation to simulate the motions of virtual human or animal characters.” – Wikipedia In character animation, directly animating the polygon mesh is too time consuming. Instead, you attach the polygon mesh to an armature. Then you animate the armature, which in turn animates the polygon mesh model.

Benefits and drawbacks Benefits of skeletal animation Reduce the number of moving parts that an animator needs to control Can build hierarchical skeletal structures Can apply inverse kinematic techniques to make an animator’s job easier Weaknesses Cannot simulate realistic muscle or skin motion

Rigging and skinning Rigging: the process of building a hierarchical set of bones for animation Skinning: the process of attaching polygon meshes appropriately to the bones

How to add armature? In Object mode, press Space bar and add an armature How to extend a bone? Select a bone, enter Edit mode. Select tip of the bone, press E or ctrl + LMB to extrude bones Each extruded bone will be the child of the previously selected bone This way you can build a hierarchical skeleton model

Different representations Select armature, in Editing panel, under “Armature” Octahedron: useful for positioning and extending Stick: slim, useful for weight painting Envelope

Armature parameters Select armature, in Editing panel, under “Armature” X-Ray Display bones in front of the polygon mesh Usually you want to turn this on X-Axis Mirror Use Shift + E to symmetrically extrude bones Names: draw bone names

Armature parameters In Edit Mode, in Editing panel, under “Armature Bones” tab BO: edit name of the bone In this panel, you can also edit the parent/child relationship between bone You can also press/unpress the “Con” button to change connections between bones Useful for inverse kinematics (IK)

Naming convention Naming convention for symmetrical bones Include word “left” (or l), “right” (or r) in the bone names Separate the word “left” and “right” with other part of the bone name with a separator (dot ‘.’, dash ‘-’, underscore ‘_’, or space) For example: arm.l, Left.hand, Foot-r, Righthand, etc. Be consistent This is required for convenient tools like X-Mirror weight painting

Armature Posing Select armature, enter Pose mode RMB to select bones, then you can rotate, move, or scale bones

Skinning Two methods: (preferred) Apply an “armature” modifier to the polygon mesh Make polygon mesh a child of armature

Skinning through modifier Give your armature a meaningful name By default it’s called “Armature” Select the polygon mesh, add an “Armature” modifier Type the Armature name in “Ob:” field Usually un-press “Envelopes” button and keep “Vert.Groups” select to use Weight Paint (Use Envelopes only for simple skinning)

Skinning through parenting Select the mesh first, select the armature Make sure you select all the bones you want to associate with the mesh Press Ctrl + P Make the polygon mesh a child of the armature In subsequent menu “Create vertex groups?” select “Create from bone heat” if you want to use Weight Paint Select “Create from envelopes” for simple skinning After this step, you can do weight painting

Weight paint Weight paint is a technique for attaching polygons to each individual bone Armature must be in “Pose” mode Otherwise cannot select bones in the “Weight paint” mode Select the polygon mesh, enter “Weight Paint” mode

Weight Paint In weight paint mode, the color of the polygons indicate the influence (weight) of a bone on these polygons Blue means zero influence (0) Red means maximum influence (1.0) Other color (e.g. green, yellow, or orange) means somewhere between 0.0 and 1.0.

Weight Paint Select a bone to see which polygons are influenced by this bone Initially the polygons are probably blue Use paint brush to paint on the polygons in order to change the influence (weight) of the selected bone on the polygons Only takes effect if you paint on vertices, not on faces You’ll see the color of the polygons change

Weight Paint parameters In Editing panel, under “Paint” tab Weight: select the current influence level Opacity: press on the brush Size: size of the brush Paint method: mix, add, sub, mul(tiply), etc. All faces: turn on or off the brush circle Spray: continuously apply paint pressure

Weight Paint parameters In Editing panel, under “Paint” tab X-Mirror: paint symmetrically if the bone names are symmetrical (this is why you should follow the naming convention) Clear: clear all previous paint for the selected bone so you can start over (like a “undo all” button)

Posing in Weight Paint mode There is usually no need to switch betwene Weight Paint and Pose mode You can manipulate the bones in Weight Paint mode When you move the bones, you may see some polygons are mistakenly attached (or not attached). Just paint over them

Edit mode Note that a polygon mesh is not animated by the bones in Edit mode If you select the polygon mesh and enter Edit mode, the mesh will go back to “rest” position If you select bones and enter Edit mode, the bones will go back to “rest” position

Weight paint and vertex group What weight paint essentially does is to create a vertex group for each bone. You can see the vertex group associated with each bone in Edit mode Select polygon mesh, enter Edit mode Under “Link and Materials” tab, see “Vertex Groups” field Select a bone by name, press “Select” button to see the vertex group for that bone

Weight paint and vertex group You can add or remove polygons to/from each vertex group Give more control to animators than just weight paint

Armature and Sub-surf modifiers Usually the Armature modifier should be above the Sub-surf modifier in the modifier stack This means the Armature will animate the base mesh first, and then Sub-surf will happen later

Inverse Kinematics Inverse Kinematics (often called IK) is an important technique for character animation When we place a hand or foot to a certain position, we want the rest of the arm/leg to follow the hand/foot. IK means that when a child bone is moved, the parent bones are automatically moved to follow it. On the other hand, forward kinematics means that only child bones will follow parent bones

How to add IK to armature? Select a bone (usually the tip on a bone chain) Enter Pose mode Add a “IK Solver” constraint The “IK Solver” constraint is only available in the Pose mode

IK parameters Can change the “ChainLen” number to add/remove bones from the IK chain By default, “ChainLen” is 0, which means all the bones that are connected with the selected bone are in the IK chain You can also add a “Target” object so that the IK chain will track to the target object E.g. catching a ball You can make the IK chain Rot(ate) with the target You can adjust the influence of IK on the bone chain

What is motion capture? Motion capture involves measuring an object's position and orientation in physical space, then recording that information in a computer-usable form. Objects of interest include human and non-human bodies, facial expressions, camera or light positions, and other elements in a scene. In most instances, a live subject, most likely human (but possibly and animal or puppet), is used as the source of data which is transformed into another form.

Motion capture (MoCap) MoCap is used extensively in films But game industry accounts for 85-90% of the total MoCap usage Almost all games use MoCap to drive character animation

Importing motion capture data You can import motion capture data into Blender As of 2009, Blender can import two MoCap file formats: BVH and C3D Go to File  Import. In case you don’t see the importing scripts, reinstall Blender.

Importing motion capture data A MoCap file will be imported as an armature You can then attach polygon mesh to it and do weight painting, etc. But you can change the armature pose at any frame

MoCap data Lots of MoCap data online (many of them free) http://mocap.cs.cmu.edu/ http://accad.osu.edu/research/mocap/mocap_data.htm http://sites.google.com/a/cgspeed.com/cgspeed/motion-capture/cmu-bvh-conversion http://www.mocapdata.com/