Video and Animation
Prof. S. Shakya
1
Motion
Both video and animation give us a sense of
motion
They exploit some properties of human eye’s
ability of viewing pictures
Motion video is the element of multimedia
that can hold the interest of viewers in a
presentation
2
Visual Representation
The visual effect of motion is due to a biological
phenomenon known as persistence of vision
An object seen by the human eye remains mapped on the eye’s
retina for a brief time after viewing (approximately 25 ms)
Another
A th phenomenon
h contributing
t ib ti to
t the
th vision
i i off motion
ti is
i
known as phi phenomenon
When two light sources are close by and they are
illuminated in quick succession,
succession what we see is not two
lights but a single light moving between the two points
Due to the above two phenomena of our vision system, a
discrete sequence of individual pictures can be perceived as
a continuous sequence
3
Visual Representation
Temporal aspect of Illumination—To represent
visual reality, two conditions must be met the rate of
repetition of the images must be high enough to guarantee
smooth motion from frame to frame
the rate must be high enough so that the persistence of
vision extends over the interval between flashes
The frequency at which the flicking light source must be
repeated before it appears continuous is known as the
f i
fusion frequency
f
• This depends on the brightness of the light source
• The brighter the light source the higher the fusion frequency
It is known that we perceive a continuous motion to happen
at any frame rate faster than 15 frames per second
PAL television system
y has a frame rate of 25 frames/s
4
Visual Representation
Another problem known as flicker occurs
due to a periodic fluctuation of brightness
p
perception
p
A technique known as interleaving
improves the view by
-dividing a frame into two fields, each contains
t ea
the alternative
te at e sca
scan lines,
es, a
and
d
- displaying the field in twice of the frames rate
5
Video resolution
The smallest detail that can be reproduced in the image is a
pixel
i l
Practically, some of the scene inevitably fall between
scanning lines, so that two lines are required for such
picture elements
Only about 70% of the vertical detail is presented by
the scanning lines
Aspect ratio is the ratio of the picture width to height.
It is 4:3 for conventional TV
The picture width, horizontal resolution and the total detail
content of the image can be calculated
6
Video resolution
Conventional video systems have relative low resolution
- compare to computer screens: typical resolution of
640 x 480, even up to 1024 X 768
One consequence
q of this low resolution is that video played
p y
on computer screen are usually in a small window
On the other hand, even with this low resolution, the amount
of data in video is huge
Consider PAL TV at 25 frames per second, if we sample at
352 x 288 with 16 bits per pixel, the raw video size is
352 x288 x16 x 25 = 40.55Mbit/s = 5Mbytes/s
-Compare this with a typical Ethernet bandwidth of 10Mbit/s
-or a double speed CD-ROM drive of 300Kbyte/s
Therefore we need to compress the video data
Therefore,
7
Digitalising Video
We need to capture or digitize video for playing back on
comp ters or integrating into m
computers multimedia
ltimedia applications
We need to take a lot of samples
At 25 frames per second, each frame requires 1/25 = 40ms
There 625 scan lines in each frame
frame, giving each scan line is
40ms/625 = 64µs
At a horizontal resolution of 425 pixel, the time for sampling
each ppixel is 64µs/425
µ = 0.15µs
µ , i.e.,, sampling
p g rate is at least
7Mhz
This requires very fast hardware
Hardware required to capture video:
• Video sources: TV, VCR, LaserDisc player, Camcorder
• Video capture card
• Storage space: large hard disk
8
Video capture cards
There are many different video capture cards on the market
The common features in these cards are:
Can accept composite video or S-VHS in NTSC or PAL;
high-end capture cards can accept digital video (DV)
Video input mixer and ADC— to select/combine video
sources, to convert analog video signal to digital samples
Video frame buffer— temporary storage for video frame
Video processor— to filter or enhance the video frame, e.g.,
reduce noise, adjust brightness, contrast and colour
Compressor/encoder—
p to compress
p and encode the digital
g
video into a required format
Interface to the system PCI bus
9
Video capture cards
10
Video formats
AVI (Audio Video Interleaved) format was defined by
Microsoft for its Video for Windows systems
It supports video playback at up to 30 frames per second on
a small window (typical size 300X200 with 8 or 16 bit colour)
It is a software-only system
It supports a number of compression algorithms
QuickTime was originally developed by Apple for storing
audio and video in Macintosh systems
It supports video playback at up to 30 frames per second on
a small window (typical size 300X200 with 8 or 16 bit colour)
It is a software-only system
It supports a number of compression algorithms
11
Animation
To animate something is, literally, to bring it to life
An animation covers all changes that have a
visual effect
Visual effect can be of two major kinds:
motion dynamic— time varying positions
update dynamic— time varying shape, colour,
texture or even lighting,
texture, lighting camera position,
position etc
etc.
The visual effects is the result of exploiting the
properties of human vision system as described
above (in the section about video)
A computer animation is an animation performed
by a computer using graphical tools to provide
visual effects
12
Input process
The first step in producing computer animation is input
process
Key frames have to be created and input into the computer
Key frames are the frames in which the objects being
animated are at extreme or characteristic positions
They can be drawn using traditional artistic tools, such as
pen and brush, and then digitised
The digital images may need to be cleaned up
They can also be created using drawing or painting tools
directly
In composition stage, the foreground and background
figures are combined to generate the individual frames
13
Inbetween process
The animation of movement from one
position to another needs a composition
of frames with intermediate p
positions in
between the key frames
The process of inbetweening is performed
in computer animation through
interpolation
p
The system is given the starting and ending positions
It calculates the positions in between
14
Inbetween process
The easiest interpolation is linear interpolation
It has many limitations: the object does no move smoothly, look
unreal
Spline interpolation can make object move more smoothly
I b
Inbetweening
i also
l involves
i l interpolating
i l i the
h shapes
h off
objects
Some animation involves changing the colour of objects
This is
Thi i usually
ll done
d using
i colour
l look-up
l k table
bl (CLUT)
By cycling through the colours in the CLUT, the objects’ colours will
change
Morphing is a popular effect in which one image transforms
into another
15
Controlling animation
Full explicit control —the animator provides a description of
everything that occurs in the animation
either by specifying simple changes, such as scaling,
transformation
or by providing key frames
Procedural control —using a program to calculate the position,
angle, etc. of the objects
In physical systems, the position of one object may influence the
motion of another
Constraint-based systems —movement of objects that are in
contact with each other is constraint by physical laws
An animation can be specified by these constraints
Tracking live action —
People or animals act out the parts of the characters in the
animation
The animator trace out the characters
16
Controlling animation
Kinematics refers to the position and velocity of points
The ball is at the origin at time t = 0. It moves with a constant
acceleration in the direction (1,1,5) thereafter.
The final result of an animation is the sum of all the steps. If
it does not fit
fit, the animator has to try again
again. This is known
as forward kinematics.
Inverse kinematics (IK) is concerned with moving a
skeleton from one pose to another.
another
The animator specifies the required position of the end effecter, the IK
algorithm will calculate the joint position, angle, etc.
Dynamics
y takes into account the p
physical
y laws that g
govern
the masses and forces acting on the objects
The ball is at the origin at time t = 0 second. It has a mass of 200 grams. The
force of gravity acts on it.
17
Displaying animation
The rules governing the showing of video apply to
animation as well
ell
The frame rate should be at least 10, preferably 15 to 20, to
give a reasonably smooth effect
There are basically three common ways to display
animation
Generate a digital video clip
Manyy Animation tools will export
p an animation in common digitalg
video format, e.g., QuickTime
Create a package including runtime system of the animation
tool
F example,
For l DiDirector
t can createt a projector
j t including
i l di allll casts.
t
The projector can then be distributed and play the animation.
Show the animation in the animation tool
18
Animation tools
Macromedia Director and Flash
It is one of the most popular interactive animation
tool for generating interactive multimedia
applications
MetaCreations Poser
It understands human motion and inverse kinematics, e.g., move an
arm the shoulders will follow.
Discreet 3D Studio Max
Very
y popular
p p for creating
g 3D animations
Animation language—VRML (Virtual Reality
Modeling Language)
19
Video formats
MPEG (Motion Picture Expect Group) is a working group under
ISO
There are several versions of mpeg standard.
The most commonly used now is mpeg-1
It requires hardware support for encoding and decoding (on
slow systems)
The maximum data rate is 1.5Megabit/sec
The next generation mpeg-2 is now getting popular
Mpeg-2 improves mpeg-1 by increasing the maximum data
rate to 15Mbit/sec
It can interleave audio and video
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