Introduction to Computer Animation using Computer Graphics Techniques

Module 5: Computer Animations
Dr. Ramesh S. Wadawadagi
Department of ISE, NCET, Bengaluru

Computer Animation
What is Animation?
Making objects change over time
according to scripted actions
What is Simulation?
Predict how objects change over
time according to physical laws


Computer animation is the process used for
generating animated images (moving images) using computer
graphics.
• Animators are artists who specialize in the creation of
animation.
• From Latin animātiō, "the act of bringing to life";
from animō ("to animate" or "give life to") and -ātiō ("the act
of").
• 'Animating' is moving something which can't move itself.
• Animation adds to graphics the dimension of time which
vastly increases the amount of information which can be
transmitted.
Introduction


Computer animation generally refers to any time
sequence of visual changes in a picture.

In addition to changing object positions using
translations or rotations, a computer-generated
animation could display time variations in object size,
color, transparency, or surface texture.

Another consideration in computer-generated animation
is realism. Many applications require realistic displays.

Some applications may be displayed with exaggerated
shapes and unrealistic motions and transformations.
Introduction

Real-time animation:

In a real-time computer-animation, each stage of the
sequence is viewed as it is created.

Thus the animation must be generated at a rate that is
compatible with the constraints of the refresh rate.
Frame-by-frame animation:

For a frame-by-frame animation, each frame of the
motion is separately generated and stored.

Later, the frames can be recorded on film, or they can
be displayed consecutively on a video monitor in “real-
time playback” mode.
Two basic methods for constructing a
motion sequence are:

Raster Methods for Computer Animation

We can create simple animation sequences in our programs
using real-time methods.

We can produce an animation sequence on a raster-scan
system one frame at a time, so that each completed frame
could be saved in a file for later viewing.

The animation can then be viewed by cycling through the
completed frame sequence, or the frames could be transferred
to film.

If we want to generate an animation in real time, however, we
need to produce the motion frames quickly enough so that a
continuous motion sequence is displayed.

Because the screen display is generated from successively
modified pixel values in the refresh buffer, we can take
advantage of some of the characteristics of the raster screen-
refresh process to produce motion sequences quickly.

Double Buffering

One method for producing a real-time animation with a
raster system is to employ two refresh buffers.

We create a frame for the animation in one of the buffers.

Then, while the screen is being refreshed from that buffer,
we construct the next frame in the other buffer.

When that frame is complete, we switch the roles of the
two buffers so that the refresh routines use the second
buffer during the process of creating the next frame in the
first buffer.

When a call is made to switch two refresh buffers, the
interchange could be performed at various times.

The most straight forward implementation is to switch the
two buffers at the end of the current refresh cycle, during
the vertical retrace of the electron beam.

Introduction to Computer Animation using Computer Graphics Techniques

Generating Animations Using Raster
Operations
1) Block Transfer method

We can also generate real-time raster animations for
limited applications using block transfers of a
rectangular array of pixel values.

A simple method for translating an object from one
location to another in the xy plane is to transfer the group
of pixel values that define the shape of the object to the
new location.

Then no viewing or visible-surface algorithms need be
invoked.

2) Color Table method

We can also animate objects along 2D motion paths using
color table transformations.

Here we predefine the object at successive positions along the
motion path and set the successive blocks of pixel values to
color-table entries.

The pixels at the first position of the object are set to a
foreground color, and the pixels at the other object positions
are set to the background color.

Then the animation is accomplished by changing the color-
table values so that the object color at successive positions
along the animation path becomes the foreground color as the
preceding position is set to the background color.

Design of Animation Sequences
A basic approach is to design such animation
sequences using the following Development
Stages:
1) Storyboard layout
2) Object definitions
3) Key-frame specifications
4) Generation of in-between frames

Storyboard

The storyboard is an outline of the action.

It defines the motion sequences as a set of basic events
that are to take place.

Based on the type of animation to be produced, the
storyboard could consist of a set of rough sketches.

Along storyboard a brief description of the movements,
or a list of the basic ideas for the action need to be listed.

Originally, the set of motion sketches are attached to a
large board that was used to present an overall view of the
animation project.

Hence, the name “storyboard.”

Object definition

An object definition is given for each participant
in the action.

Objects can be defined in terms of basic shapes,
such as polygons or spline surfaces.

In addition, a description is often given of the
movements that are to be performed by each
character or object in the story.

Key frame specifications

A key frame is a detailed drawing of the scene at a certain
time in the animation sequence.

Within each key frame, each object (or character) is
positioned according to the time for that frame.

Some key frames are chosen at extreme positions in the
action; others are spaced so that the time interval between key
frames is not too great.

More key frames are specified for intricate motions than for
simple, slowly varying motions.

Development of the key frames is generally the responsibility
of the senior animators, and often a separate animator is
assigned to each character in the animation.

Generation of in-between frames

In-between frames are the intermediate frames between the
key frames.

The total number of frames, and hence the total number of in-
betweens, needed for an animation is determined by the
display media that is to be used.

Film requires 24 frames per second, and graphics terminals
are refreshed at the rate of 60 or more frames per second.

Typically, time intervals for the motion are set up so that
there are from three to five in-betweens for each pair of key
frames.

Depending on the speed specified for the motion, some key
frames could be duplicated.


As an example, a 1-minute film sequence with no
duplication requires a total of 1,440 frames.

If five in-betweens are required for each pair of key
frames, then 288 key frames would need to be developed.

There are several other tasks that may be required,
depending on the application.

These additional tasks include motion verification,
editing, and the production and synchronization of a
soundtrack.

Traditional Animation Techniques

Film animators use a variety of methods for
depicting and emphasizing motion
sequences. These include:

Object deformations

Spacing between animation frames

Motion anticipation and Follow-through

Action focusing.

Traditional Animation Techniques

One of the most important techniques for
simulating acceleration effects, particularly for
nonrigid objects, is squash and stretch.

Figure 4 shows how this technique is used to
emphasize the acceleration and deceleration of a
bouncing ball.

As the ball accelerates, it begins to stretch.

When the ball hits the floor and stops, it is first
compressed (squashed) and then stretched again as
it accelerates and bounces upwards.


Another technique used by film animators is
Timing, which refers to the spacing between
motion frames.

A slower moving object is represented with more
closely spaced frames, and a faster moving object
is displayed with fewer frames over the path of the
motion.

This effect is illustrated in Figure 5, where the
position changes between frames increase as a
bouncing ball moves faster.


Object movements can also be emphasized by
creating preliminary actions that indicate an
anticipation of a coming motion.

For example, a cartoon character might lean
forward and rotate its body before starting to run;
or a character might perform a “windup” before
throwing a ball.

Similarly, follow-through actions can be used to
emphasize a previous motion.


After throwing a ball, a character can continue the
arm swing back to its body; or a hat can fly off a
character that is stopped abruptly.

An action also can be emphasized with staging,
which refers to any method for focusing on an
important part of a scene, such as a character
hiding something.

General Computer-Animation Functions:

Many software packages have been developed:

(i) General animation design

(ii) Performing specialized animation tasks

Typical animation functions include:

(i) Managing object motions

(ii) Generating views of objects

(iii) Producing camera motions

(iv) Generation of in-between frames

General Computer-Animation Functions:

Some animation packages, such as Wavefront for
example, provide special functions for both the
overall animation design and the processing of
individual objects.


Others are special-purpose packages for
particular features of an animation, such as a
system for generating in-between frames.

A set of routines is often provided in a general
animation package for:

Storing and Managing the object database.

Object shapes and associated parameters are
stored and updated in the database.

Other object functions include those for generating
the Object motion and those for rendering the
object surfaces.

Computer-Animation Languages

We can develop routines to design and control
animation sequences within a General-purpose
programming language, such as:

C, C++, Java, Lisp, Python, or Fortran

Several specialized animation languages have been
developed, such as.

Unity, Rust, OpenGL, Swift, Kotlin, Processing
etc.

Computer-Animation Languages

These languages typically include:

Graphics editor

Key-frame generator

In-between generator

Standard graphics routines

The Graphics editor allows:

An animator to design and modify object shapes
using spline surfaces

Apply constructive solid-geometry methods

Other representation schemes


An important task in an animation specification is
Scene Description. It includes:

Positioning of objects and light sources

Defining the photometric parameters

Setting the camera parameters.

Another standard function is Action
Specification.

It involves the layout of motion paths for the
objects and camera.

We need the usual Graphics Routines:

Viewing and Perspective transformations

Geometric transformations to generate object
movements as a function of accelerations

Kinematic path specifications

Visible-surface identification

Surface-rendering operations.


Key-frame systems were originally designed as a
separate set of animation routines for generating
the in-betweens from the user-specified key
frames.

Parameterized systems allow object motion
characteristics to be specified as part of the object
definitions.

The adjustable parameters control such object
characteristics such as:

Degrees of freedom

Motion limitations

Allowable shape changes


Scripting Systems allow object
specifications and animation sequences to be
defined with a user-input script.

From the script, a library of various objects
and motions can be constructed.

Morphing

Transformation of object shapes from one
form to another form is termed as
morphing, which is a shortened form of
“metamorphosing.”

The morphing can be modeled by
transitioning polygon shapes through
generating in-between frames from one key
frame to the next key-frame.


Assume two key frames k and k+1, each
with a different number of line segments
specifying an object transformation.

We can adjust the object specification in the
any frame so that the number of polygon
edges is the same for the two frames.

This preprocessing step is illustrated in
Figure 8.


A straight-line segment in key frame k is
transformed into two line segments in key
frame k + 1.

Step 1: Because key frame k + 1 has an extra
vertex, we add a vertex between vertices 1
and 2 in key frame k to balance the number
of vertices in the two key frames.

Step 2: Using linear interpolation to
generate the in-betweens, we transition the
added vertex in key frame k into vertex 3’
along the straight-line path shown in Fig. 9.

Example of a triangle linearly expanding into a
quadrilateral

General preprocessing rules for equalizing
key frames:

Decide on number of edges or the number of
vertices to be added to a candidate key frame.

We first consider equalizing the edge count,
where parameters Lk and Lk+1 denote the
number of line segments in two consecutive
frames.

The maximum and minimum number of lines
to be equalized can be determined as:

Simulating Accelerations: (Motion path
estimation)

Curve-fitting techniques are often used to
specify the animation paths between key
frames.

Given the vertex positions at different key
frames, we can fit the positions with linear
or nonlinear paths.

Figure 11 illustrates a nonlinear fit of key-
frame positions.

To simulate accelerations, we can adjust the
time spacing for the in-between frames.


If the motion is to occur at constant speed
(zero acceleration), we use equal-interval
time spacing for the in-between frames.

For instance, with n in-between frames and
t1 and t2 times of key-frames (Figure 12), the
time interval between the key frames is
divided into n+1 equal subintervals, yielding
an in-between spacing of:


Then, this time value tBj is used to calculate
coordinate positions, color, and other physical
parameters for that frame of the motion.

However, speed changes are usually necessary
at some point in an animation film or cartoon,
particularly at the beginning and end of a
motion sequence.

The startup and slowdown portions of an
animation path are often modeled with spline
or trigonometric functions.

Parabolic and cubic time functions have been
applied to acceleration modeling.


Animation packages commonly render
trigonometric functions for simulating
accelerations.

To model increasing speed, we want the time
spacing between frames to increase so that
greater changes in position occur as the object
moves faster.

We can obtain an increasing size for the time
interval with the function:

Motion Specifications:
Defining how an animation is to take place by
providing the transformation parameters such as:

Motion path parameters

External forces that act on objects

Details on how objects interact to produce
motion

Three Types of Motion Specifications:

Direct Motion Specification

Goal-Directed Systems

Kinematics and Dynamics

Direct Motion Specification

Here, we use explicitly set the values for the
rotation angles and translation vectors.

Then the geometric transformation
matrices are applied to transform coordinate
positions.

We could use some approximating equation
involving these parameters to specify certain
kinds of motions.

Example bouncing ball, for instance, with a
damped, rectified, sine curve.

where A is the initial amplitude (height of the
ball above the ground), ω is the angular
frequency, θ0 is the phase angle, and k is the
damping constant.

Goal-Directed Systems

Specify the motions that describe the actions
in terms of the final results.

An animation is specified in terms of the
final state of the movements.

Values for the motion parameters are
determined from the goals of the animation,
hence, goal directed.

For example, we could specify that we want
an object to “walk” or to “run” to a
particular destination.

Kinematics and Dynamics

Specify the animation by giving motion
parameters (position, velocity, and
acceleration) without reference to causes or
goals of the motion.

For example, if a velocity is specified as (3,
0, −4) km per sec,

Then this vector gives the direction for the
straight-line motion path and the speed is
calculated as 5 km per sec.

Character Animation
Articulated Figure Animation:

A basic technique for animating people,
animals, insects, and other creatures is to
model them as articulated figures.

These are hierarchical structures composed
of a set of rigid links that are connected at
rotary joints (Figure 17).


We model animate objects as moving stick
figures, or simplified skeletons.

These can later be wrapped with surfaces
representing skin, hair, fur, feathers, clothes,
or other outer coverings.

The connecting points, or hinges, for an
articulated figure are placed at the shoulders,
hips, knees, and other skeletal joints, which
travel along specified motion paths as the
body moves.


For example, when a motion is specified for
an object, the shoulder automatically moves
in a certain way and, as the shoulder moves,
the arms move.

Different types of movement, such as
walking, running, or jumping, are defined
and associated with particular motions for
the joints and connecting links.


For example, a series of walking leg motions,
might be defined as in Figure 18.

Both kinematic-motion descriptions and inverse
kinematics are used in figure animations.

Motion Capture (MoCap):

Digitally recording the movement of a live
actor.

Convert the movement of an animated
character using the information captured.

The animated character will perform the
same series of movements as the live actor.

The classic motion capture technique
involves placing a set of markers at strategic
positions on the actor’s body, such as the
arms, legs, hands, feet, and joints.


It is possible to place the markers directly on
the actor, but more commonly they are
affixed to a special skintight body suit worn
by the actor.

The actor is them filmed performing the
scene.

Image processing techniques are then used to
identify the positions of the markers in each
frame of the film, and their positions are
translated to coordinates.


These coordinates are used to determine the
positioning of the body of the animated
character.

The movement of each marker from frame to
frame in the film is tracked and used to
control the corresponding movement of the
animated character.

Periodic Motions:

When we construct an animation with
repeated motion patterns, such as a rotating
object.

We need to be sure that the motion is
sampled frequently enough to represent the
movements correctly.

In other words, the motion must be
synchronized with the frame-generation rate
so that we display enough frames per cycle to
show the true motion.

Periodic Motions:

Figure 19 illustrates one complete cycle in
the rotation of a wagon wheel with one red
spoke that makes 18 clockwise revolutions
per second.

If this motion is recorded on film at the
standard motion-picture projection rate of 24
frames per second, then the first five frames
depicting this motion would be as shown in
Figure 20.

Periodic Motions:

In a computer-generated animation, we can
control the sampling rate in a periodic
motion by adjusting the motion parameters.

For example, we can set the angular
increment for the motion of a rotating object
so that multiple frames are generated in each
revolution.

Thus, a 3◦
increment for a rotation angle
produces 120 motion steps during one
revolution, and a 4◦
increment generates 90
steps.

Periodic Motions:

For faster motion, larger rotational steps
could be used, so long as the number of
samples per cycle is not too small and the
motion is clearly displayed.

When complex objects are to be animated,
we also must take into account the effect that
the frame construction time might have on
the refresh rate.

OpenGL Animation Procedures:

Raster operations and color-index
assignment functions are available in the
core library, and routines for changing color-
table values are provided in GLUT.

Other raster-animation operations are
available only as GLUT routines because
they depend on the window system in use.

●
Double-buffering operations, if available, are
activated using the following GLUT
command:
glutInitDisplayMode (GLUT_DOUBLE);
●
This provides two buffers, called the front
buffer and the back buffer, that we can use
alternately to refresh the screen display.
●
While one buffer is acting as the refresh
buffer for the current display window, the
next frame of an animation can be
constructed in the other buffer.

●
We specify when the roles of the two buffers
are to be interchanged using:
GlutSwapBuffers ( );
●
To determine whether double-buffer
operations are available on a system, we can
issue the following query:
glGetBooleanv(GL_DOUBLEBUFFER, status);
●
A value of GL TRUE is returned to array
parameter status if both front and back
buffers are available on a system. Otherwise,
the returned value is GL FALSE.

●
For a continuous animation, we can also use
glutIdleFunc (animationFcn);
●
Where parameter animationFcn can be
assigned the name of a procedure that is to
perform the operations for incrementing the
animation parameters.
●
This procedure is continuously executed
whenever there are no display-window events
that must be processed.
●
To disable the glutIdleFunc(), we set its
argument to the value NULL or the value 0.

Introduction to Computer Animation using Computer Graphics Techniques

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