Upcoming Deadlines

Ninth Homework (Stop-motion Character Animation)Due by 8am on Tuesday, April 10th 20 points (10 points if late)

Tenth Homework (Outline for Second Term Paper)Due by 8am on Tuesday, April 17th 20 points (10 points if late)

For full schedule, visit course website:ArtPhysics123.pbworks.com

Pick up a clicker, find the right channel, and enter Student ID

Extra Credit Opportunity

Attend any event at 55th San Francisco International Film Festival, April 19–May 3, in San Francisco.

Present proof of you attendance (receipt, photos, etc.) for 10 points of extra credit.

Homework Assignment #9

In this assignment you will create a simple stop-motion animation of a moving character.

For your character, use a doll or stuffed animal that's easily posed.

You may even use an inanimate object, such as a table lamp, as long as you can animate it so that it looks like a character.

Homework Assignment #9

http://www.youtube.com/watch?v=EiEVdTQGGTM

Homework Assignment #9Your animation should be composed of at least a dozen

different images but you can use the same photo in two or more frames to get timing right.

Try to make the motion of the character as realistic as possible so that your audience will momentarily forget that it's not actually alive.

In homework assignment #6 you created a stop-motion animation of an inanimate falling object; if you're not happy with how you produced your earlier animation then re-read that assignment for alternative ways of creating stop-motion.

Homework Assignment #9You may work together with one or more classmates to

create a single animation for the team.

Clearly indicate the work done by each person, for example if different persons animate different characters or different scenes.

More is expected from a team effort; an animation created by a three person team should present three times as much work as a solo effort.

Homework Assignment #9Post your animation clip to your blog in an entry entitled

"Stop-Motion Character Animation.“

Assignment is due by 8am on Tuesday, April 10th

20 points (if late, 10 points)

The top three clips in the class, as selected by another celebrity judge, will receive a bonus of 20 extra points.

Special Guest Animation JudgeThe top three stop-motion character animations (next Wednesday’s homework) will be picked by Jason Spencer Galsworthy of Dreamworks Animation (and formerly with Aardman Animation).

Winner in Fall 2010

http://theodorejreid.blogspot.com/2010/11/character-animation.html

Survey QuestionCompared with the other homework assignments, the reverse video reference

assignment was:

A) A lot more interesting

B) A bit more interesting

C) About average

D) A bit less interesting

E) A lot less interesting

Review QuestionIn the passing position the pelvis drops slightly on the non-weight bearing side. This motion is called:

A) Pelvic rotationB) Pelvic listC) Knee flexionD) Hip hula-hulaE) The Twist

Pelvic List

Pelvic list keeps the center of gravity from rising as much when the body passes over the weight-bearing leg, keeping the center of gravity on a flatter path of action.

CG

Stride

Passing Position

Passing Position

Passing Position

Stride

CG

Walking is more efficient with pelvic list.

Path of Action ofCG without List

With Pelvic List

Walki

ng Forward

B) Pelvic list

Review Question

In normal walking, the side-to-side and up-down motion shown here is for the:

A)LegB)ArmC)Center of gravityD)Heel of the footE)Hand

Slow Fast

Side-to-Side

Up & Down

Walking

Forward

Figure 8Loop

8-Loop & U-Loop

C) Center of Gravity

The center of gravity shifts up & down and also side-to-side. CG makes a Figure-8 loop when walking slow

Makes a U-shape loop when walking fast.

Slow Fast

Side-to-Side

Up & Down

Walking

Forward

Figure 8Loop

Joints & Levers

Articulated FiguresIn computer animation characters are articulated figures, like marionette puppets.

The challenge for animators is to move all the elements (arms, legs, hands, etc.) believably from frame to frame.

Joints

Revolute JointPrismatic Joint

(Slider)

Two kinds of joints: Revolute (rotation) and Prismatic (extension/contraction).

Nearly all joints in animals are revolute joints

Axis of RotationRotation occurs around a line called the axis of rotation.

The axel is the axis of rotation for a set of wheels.

Revolute JointsRevolute joints may have a single axis of rotation, like hinge joints and pivot joints, or a variable axis, such as the ball and socket joint.

Elbow

Shoulder

Forearm

Forward KineticsWith forward kinetics (FK) the animator has to specify the axis of rotation and the angle of rotation for each joint on a moving limb.

Lift the arm by a shoulder rotation

Move forearm by an elbow rotation

Forward KineticsForward kinetics (FK) is a tedious process for the animator and it is difficult to maintain certain constraints, such as the planted foot in a walk.

Inverse KineticsWith inverse kinetics (IK) the animator positions the end effector, such as the hand, and the computer calculates the required joint rotations.

Lift the arm by positioning the hand

Inverse KineticsRotations calculated by the computer using IK are not always natural poses for a character. Furthermore, the timing may not be correct.

Ball & Socket

Ball & Socket

Hinge

x

x

Raising hand into “High Five” pose

AwkwardIK Pose

Elbow

Wrist

Shoulder

Understanding Rotation

Just like any other type of motion, rotation is governed by Newton’s laws:

• Law of Inertia• Law of Acceleration• Action-Reaction Principle

Let’s see how inertia, force, reaction, etc. appear in the context of rotational motion.

Sir Isaac Newton

Mass is a measure of inertia for linear motion.

Rotational inertia is similar concept for rotation.

Inertia

M mGold brick Normal brick

Difficult to move Easy to move

x xWood Bat Plastic Pee-wee Bat

Difficult to Rotate Easy to Rotate

Rotational Inertia

Rotational inertia depends on:

• Total mass of the object

• How the mass is distributed

The farther the object’s mass is from the axis of rotation, the larger the rotational inertia.

Demo: Inertia Sticks

Two metal pipes of the same mass

Rotate

Leadweights

Easyto

Rotate

Hardto

Rotate

Axis ofRotation

Tightrope Walkers

The Great Blondin Is First to Walk Across Niagara Falls (June 30, 1859)

Tightrope walkers carry a long pole to increase their rotational inertia, which slows their rotation when they’re off balance.

Demo: Ramp RacingThe two disks and rods have the same mass but

one has connecting rods near the center while the other has rods near the rim.

When rolled down a ramp, which wins the race?

A

B

A)Wheel AB)Wheel BC)Virtually a tie

Demo: Ramp Racing

A) Wheel AThe wheel with the rods near the center

wins the race because it has a smaller rotational inertia.

Winner

Quiz Question

A short rod and a long rod swing like pendulums.The two rods weight the same. Which rod has the larger rotational inertia?

A) Short rod

B) Long rod

C) Rods have equalrotational inertias

Quiz Question

The farther the mass is from the axis of rotation, the larger the rotational inertia.

B) Long rod

Fast

Slow

Demo: Long Legs

Long legs have greater rotational inertia than short legs so long legged animals have a slow walking stride.

Drag of Articulated Limbs

Fast Swing

Slow Swing

Slow

Fast

Drag (inertia) is seen with

articulated limbs

Demo: Drop the Stick

A pair of meter sticks stand upright against a wall; one has a hunk of clay on the end.

Which stick will swing down and hit the floor first?

The one without the hunk of clay.

Why?Clay increases rotational

inertia, which slows the rotation.

Axis of Rotation

Demo: Drop the Stick

Axis of Rotation

X CG

X CG

Another way to understand why the weighted stick takes longer to fall is that it’s center of gravity is higher.

The higher it is, the longer it takes for an object to fall to the ground.

Longer stick tips over more slowly

Tripping and Falling

Axis of Rotation

X CG

X CG

If small child trips, he hits the ground more quickly than an adult.

Can view this two ways:*Child has small rotational

inertia.*Child’s center of

gravity is initially closer to the ground.

Demo: Balancing a Pool Cue

Try to balance a cue stick in the palm of your hand.

When the stick starts to fall you have to correct the balance quickly.

How can you slow the rotation of the stick to make this easier to do?

Demo: Balancing a Pool CueMuch easier to balance when heavy end up.

X

X

Center of Gravity

Center of Gravity

Stick rotates quickly when the heavy end is down.

When a force causes a rotation, we identify this as a torque.

Torque depends on• Magnitude of Force• Lever Arm

(Torque) = (Force) x (Lever Arm)

Torque

Lever ArmLever arm is the perpendicular distance from axis of rotation to the direction of the force.

Opening or Closing a Door

For a door the hinge is the axis of rotation.

For maximum lever arm (and maximum torque), push perpendicular to the door at the edge opposite from the hinges.

DoorHinge

Pirouettes

A pirouette is typically done en dehors (moving leg starts at the front or the side and moves towards the back).

A ballet turn done on one leg, starting with one or both legs in plié and rising onto demi-pointe (usually for men) or pointe (usually for women).

http://www.youtube.com/watch?v=694S8oNXRZM

Torque for a Pirouette

The farther the distance between the feet, the greater the lever arm so the greater the torque for creating the rotation.

Push onFloor

ReactionForce

LeverArm

Feet apart(Easy)

Feet together(Harder)

Fouetté (Whipped) Turns

Done properly, the dancer returns to the same spot at the end of the turn.

A turn executed with a quick thrust of the moving leg as it passes in front of or behind the supporting leg.

http://www.youtube.com/watch?v=TV-N0QWyeac

Torque for Fouetté Turns

Push onFloor

ReactionForce

LeverArm

Lower heel to the floor

Push off while swinging right leg

Lift heel and return to point

The torque first creates a rotation of the arm & leg, then whole body rotates together

Fantasia (1940)The Dance of the Hours from Disney’s Fantasia is the most famous ballet scene in animation.

http://www.youtube.com/watch?v=0pbQdtkbCcQ

Enter the Dragon (1973)

http://www.youtube.com/watch?v=usdcpWXPaDY

The wrong way and the right way to do a spin kick

Action/Reaction for TorquesFor every action force there is an equal and opposite

reaction force.

Similarly, the action and reaction torques, caused by these forces, are equal.

Accelerations from these torques dependon objects’ rotational inertia.

If the action torque creates a clockwise rotation, the reaction creates a counter-clockwise rotation (and vice versa).

Action/Reaction for Torques

Action

ReactionWeightlifter exerts an action torque on the weights, rotating them counter-clockwise.

The weights exert a clockwise reaction torque, pulling the weightlifter forward.

Action / reaction torques are equal and opposite but resulting accelerations are unequal.

Action/Reaction in Fouetté Turns

1

2

3

4

5

6

The torque from pushing off gives angular momentum to the right arm and leg, which rotate freely from #1 to #4.

Then the right arm and leg are made to rotate back in the opposite direction.

By action/reaction, the torso recoils and rotates in the original direction.

1

2

3

4

5

6

Anchors Aweigh (1945)The musical Anchors Aweigh has a famous scene in which Gene Kelley dances with Jerry the Mouse.

The scene climaxes with a series of fouetté turns.

http://www.youtube.com/watch?v=xRMGRpDCW6k

Next LectureCreating Scale

Next Homework Stop-motion Character Animation

Due Tuesday, April 10th (Week from today)

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