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Ninth Homework (Stop-motion Character Animation)
Due Tuesday, October 30th (Next week)
20 points (10 points if late)
Tenth Homework (Outline of Second Term Paper)
Due Tuesday, November 6th (In 2 weeks)
10 points (5 points if late)
For full schedule, visit course website:
www.Animation123.com
Extra Credit Opportunities
Extra credit opportunities:
•
•
•
•
•
Cartoon Art Museum
Mill Valley Film Festival
Walt Disney Family Museum
Exploratorium
The Tech Museum (Mythbusters)
Visit course website for more info.
Homework Assignment #9
Create a stop-motion animation with a believable
animated character.
Post your animation clip to your blog in an entry
entitled “Stop-Motion Character Animation.”
As before, describe how you created it.
Assignment is due by 8am on
Tuesday, October 30th
20 points (if late, 10 points)
The top three clips in the class, as selected by another
celebrity judge, receive a bonus of 20 extra points.
Survey Question
Compared 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 Question
Although it’s not a very
comfortable way to walk,
in terms of energy
efficiency the most
efficient way to walk is to
not bend the knees and
to not rotate the hips.
A) True or B) False
Simplified Walking Model
Contact
Pose
B) False
This style of walking CG
is not efficient since
the center of gravity
has to be lifted by a
significant distance
with each step.
CG
Contact
Pose
Passing
Position
Passing
Position
Passing
Position
The body has to do work to raise the CG and
much of that energy (30-40%) is lost to friction.
Review Question
In the passing position the pelvis drops
slightly on the non-weight bearing side.
This motion is called:
A) Pelvic rotation
B) Pelvic list
C) Knee flexion
D) Hip hula-hula
E) The Twist
Pelvic List for the Passing Leg
Contact
Pose
B) Pelvic list
Contact
Pose
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.
Passing
Position
Passing
Position
Passing
Position
CG without pelvic list
CG with pelvic list
Review Question
In normal walking,
the side-to-side and
up-down motion
shown here is for the:
A)
B)
C)
D)
E)
Leg
Arm
Center of gravity
Heel of the foot
Hand
Side-to-Side
Up & Down
Figure 8
Loop
Slow
Fast
Swaying
Slow
Fast
Center of gravity moves
both up-and-down and
side-to-side, making an
-shaped loop when
walking slow and a U-shape
loop when walking fast.
Slow
Fast
85 steps
per minute
120 steps
per minute
Walks
Part II (cont.)
Overlapping Actions in Walks
Psychology experiments have
shown that we can recognize a
human walk (including gender)
even when shown only a handful
of isolated points in motion.
This recognition comes from the
timing; if you pause the video the
perception goes away.
Shoulder Rotation
The shoulders rotate opposite from the hips,
swinging over the planted leg.
Arm Swing
The arm swings back and forth, also like a pendulum,
roughly 180o out of phase with the leg.
The arm and leg are roughly the same length so they
swing back and forth with about the same period.
Who Framed Roger Rabbit? (1988)
Notice the complementary, counter-rotating
motion of Jessica’s upper and lower body.
Video Analysis of Arm & Leg
Hand and Ankle
Hand and ankle on opposite sides follow
similar triangular or half-teardrop pattern.
Richard Williams’ ASK
Williams shows a similar half-teardrop path
of action in the motion of the ankle.
Rotation Balance
Moving your legs
(and hips) as you
walk requires a torque
(rotational force)
to turn them.
It takes less effort if you
balance the rotation of the
lower body with
an opposite rotation of
your upper body.
Katie Corna
Demo: The Twist
Try dancing The Twist in the normal way, moving
the hips opposite from the shoulders.
Then try to dance it the wrong way, moving hips
and shoulders together, back and forth.
http://www.youtube.com/watch?v=8FTTFo6mcug
Joints & Levers
Articulated Figures
In 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
Two kinds of joints: Revolute (rotation) and
Prismatic (extension/contraction).
Prismatic Joint
(Slider)
Revolute Joint
Nearly all joints in animals
are revolute joints
Revolute Joints
Forearm
Revolute 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.
Shoulder
Elbow
Forward Kinetics
With 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 Kinetics
Forward 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 Kinetics
With 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 Kinetics
Rotations calculated by the computer using IK are
not always natural poses for a character.
Furthermore, the timing may not be correct.
Ball &
Socket
Shoulder
x
Awkward
IK Pose
Wrist
Hinge
Elbow
Ball &
Socket
x
Raising hand into “High Five” pose
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
Sir Isaac Newton
Let’s see how inertia, force, reaction, etc. appear
in the context of rotational motion.
Inertia
Mass is a measure of inertia for linear motion.
Gold brick
Normal brick
M
m
Difficult to move
Easy to move
Rotational inertia is similar concept for rotation.
Wood Bat
x
Plastic Pee-wee Bat
x
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
Easy
to
Rotate
Rotate
Hard
to
Rotate
Lead
weights
Axis of
Rotation
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?
A) Stick A (No clay on the end)
B) Stick B (Clay on the end)
C) Hit ground at the same time
A
B
Drop the Stick
A) Stick A (No clay on the end)
Stick B has higher rotational
inertia so it rotates slowly.
Can also think of it as the CG
starting higher for Stick B.
Longer stick
tips over more
slowly
X
X
CG
CG
Tripping and Falling
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.
X
CG
X
CG
Axis of Rotation
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 Cue
Stick rotates
quickly when
the heavy end
is down.
Center of
Gravity
X
Much easier to
balance when
heavy end up.
Center of
Gravity X
Torque
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)
Lever Arm
Lever 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.
Hinge
Door
Mechanical Lever
Lever converts an effort force into a load
force by ratio of distances from fulcrum.
Push down
with a small
effort force
over a large
distance
Fulcrum
Lift a large
load weight
over a small
distance
Axis of rotation goes through the fulcrum point.
First Class Levers
First Class Lever: Fulcrum is located between the
input force (effort) and output force (load).
Load
Arm
Effort Arm
Fulcrum
Lift a heavy load using small effort by having a
long effort arm and/or short load arm.
Efficiency vs. Speed
Efficient but slow
Load
Effort
Fulcrum
Effort
Load
Fulcrum
Inefficient but fast
Examples of First Class Levers
Scissors
Crowbar
Effort
Load
Load
Fulcrum
Effort
Crowbar and scissors
are efficient (big load
force) but slow (long
effort distance).
Examples of First Class Levers
A trebuchet
is inefficient
but fast.
Effort
Load
Fulcrum
Second Class Levers
Second Class Lever: Load is located in between
the effort force and the fulcrum.
Load
Arm
Effort Arm
Fulcrum
Lift a heavy load using small effort by having a
long effort arm and/or short load arm.
Examples of Second Class Levers
Wheel Barrow
Load
Effort
Hole Punch
Fulcrum
Second class levers are
always efficient and slow.
Third Class Levers
Third Class Lever: Effort force is located in
between the load and the fulcrum.
Load
Arm
Effort Arm
Third class levers are always inefficient but fast
(since load arm is longer than effort arm).
Examples of Third Class Levers
Broom
Jaw
Fulcrum
Effort
Effort
Load
Load
Human Arm as a Lever
Biceps muscle exerts effort force close to your
elbow (fulcrum) to raise your forearm (load).
This is what type
of lever?
A) First class
B) Second class
C)Third class
Human Arm as a Lever
Biceps is a third class lever
Load
so a large effort force acts
over a small distance to
move a small load over a
large distance.
Effort
Triceps is also a third class
lever, which pulls the arm in
the opposite direction.
Because muscles can only
contract, they’re almost always
found in pairs, like biceps/triceps.
Human Foot as a Lever
To lift the body on the toes, the
gastrocnemius (one of the
strongest muscles in the body)
contracts, lifting the heel upward.
This is an
example of a
second class
lever.
Raising heel,
as in walking.
Effort
Load
Fulcrum
Weight Shift
Weight shift forward shortens the load arm,
reducing the required effort to lift the heel.
Effort
Line of
Gravity
Line of
Gravity
Effort
Load
Fulcrum
Load
Fulcrum
Human Neck as a Lever
The neck muscles are the
body’s only first class lever
This lever is not efficient
since the effort arm is
shorter than the load arm.
In general, the levers
in animals’ bodies
sacrifice efficiency for
speed and to keep a
compact body form.
Load
Fulcrum
Effort
Enter the Dragon (1973)
The wrong way and the right way to do a spin kick
http://www.youtube.com/watch?v=usdcpWXPaDY
Next Lecture
Creating Scale
Next Homework
Stop-motion Character Animation
Due Tuesday, Oct. 30th (Week from today)