Transcript 9 jrowanSpring2013VideoTVFilmV2.pptx

Digital Media
Lecture 9: Video, TV & Film
Georgia Gwinnett College
School of Science and Technology
Dr. Jim Rowan
In the next several lectures…
Film & TV & Video & Animation
Issues that arise from conversion
Analog vs Digital
Test Pattern
TV Broadcast…
Digital replaces Analog
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Why Digital Broadcast?
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reduced spectrum use
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greater capacity
multiple programs on one freq
better quality picture
HDTV
can use compression
allows multiple HD signals on one freq.
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allows user interaction
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TV Broadcast…
Difference with poor reception
Analog vs Digital
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Analog…
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as signal gets weaker
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image gets less distinct
“ghosts (white shadows) appear”
gracefully degrades
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TV Broadcast…
Difference with poor reception
Analog vs Digital
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Digital…
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with digital, you either have signal
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or you don’t have signal so…
lose signal
everything goes black
audio stops
ungraceful degrading
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Two ways to make Moving Pictures:
Video & Animation
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In this class:
– Video
• shot with a camera
• captures images from the world
• then play them back
– Animation
• create frames individually
• using inkscape and blender
• play them back
Two ways to make Moving Pictures:
Video & Animation
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In this class:
– Video
• shot with a camera
• captures images from the world
• then play them back
– Animation
• create frames individually
• using inkscape and blender
• play them back
Video (and Film)
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Works because of persistence of vision
– human perception causes still images
played in rapid succession to fuse into
motion
– http://en.wikipedia.org/wiki/Frame_rate
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Fusion frequency
– ~ 40 frames per second
– depends on the brightness of the image
relative to the viewing environment
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Less than that
– first flickering
– then individual images appear losing the
illusion of motion
Film: How it works
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Plays at 24 frames per second
– Show the image
– Block the light to make it dark
– Move to the new image
– Allow the light through to show the
new image
– Without “blacking out” the change
from one image to the next the
image would be blurred
Film Trivia 1
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Films are longer than one reel long
How does the person who runs the
cameras know when to change?
There are two projectors, one running,
one waiting
A black hole in the film appears ~5 sec
before the switch is made
Another black hole in the film appears
and the projector operator switches
Film Trivia 2
Watch the credits…
 Foley artist?
 24 frame manager?
 Telecine?
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Video & TV
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Two versions
– Interlaced
• Rising from a TV legacy
– Progressive scan
• Rising from a computer legacy
Interlaced
– Captured (and displayed) as “fields”
– First the odd numbered lines are
captured (or displayed)
– Then the even numbered lines are
captured (or displayed)
–…
Interlaced
–…
– This reduced the bandwidth needed
to transmit images that moved for
early TV
• The glowing phosphor of the CRT stayed
glowing for a while after the electron
beam was turned off
• Allowing the other field to be drawn and
complete the TV image
Interlaced fields
Raster scan
Interlaced scan
Interlaced problem:
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Rapid
motion
resulted in
the “comb
effect”
Progressive scan
Progressive scan
•Each line on the screen is painted one after the
other from top to bottom
•Electronics are faster now so interlacing is not
required
•If captured progressively, then the playback is
straight forward
•If captured as interlaced fields, playing them
back progressively is problematic
•disadvantage of progressive scan is that it
requires higher bandwidth than interlaced video
that has the same frame size
Interlace problem:
the left-column
images are
progressive scan
the center-column
images are interlaced
the right-column
images use line
doublers
bottom images are
anti-aliased
http://en.wikipedia.org/wiki/File:Indian_Head_interlace.gif
Video… it’s big
How do you deal with it?
Playback degradation (graceful
degradation)
 Compression
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Video… it’s big
Handling with Playback
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Transport or playback not fast enough
to keep up with the story?
– something’s got to give
– there’s too much data to either transport or
display
Some players just freeze the image and
halt the audio
– this kills the ability to tell the story
 Some players (like quicktime) make
attempts to “degrade gracefully”
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Video: Graceful degradation
Graceful degrading allows the story to
continue
 Some players drop frames
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– first showing as a “slide show” while continuing to
play the audio
– then holding the last image while continuing to play
the audio stream
– this effectively loses the illusion of motion but
continues the “story” as an audio stream
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Video: Graceful degradation
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 Some play lower resolution images
while remaining synched to the audio
stream
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– this continues the illusion of motion (at a lower
resolution) and continues the “story” with the audio
stream
Video: The “progress bar”
Quicktime example
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Click to play a quicktime video
Quicktime window opens
It is in “play” mode (the pause icon is
showing
Doesn’t begin to play, instead a gray
colored bar starts filling the progress
bar
At some point it begins to play
Why? It’s predicting how long the
download will take
The progress bar
Video is big so: reduce its size
using compression
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On the capture side
– Digitization & compression can be
carried out by hardware to be fast
– Can be done in the camera
(hardware)
– Can be done in the computer
(software)
hardware vs software
compression
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Hardware compression... user has no
control over it... it is hardwired
– It can be in the camera
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Software compression... is
computationally expensive... it’s a slow
process
– Provides for the most flexibility since it can
be changed
– Can use different software coder-decoders
(codec), picking and choosing what fits your
needs better
Compression in the camera:
hardware compression
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Our cameras?
– Mini DV format
– Compress each captured image into a jpeg
image
• This is called intra-frame compression
– Present a digital stream of bits to the
computer over a firewire connection
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With compression you get artifacts
with software compression…
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Analog is presented to the computer through a
video capture card
Compression is done (usually) in the video
capture card
Allows for a really small camera because the
work (the compression and the analog to
digital conversion) is done elsewhere
More about
Analog vs Digital
An analog signal to the computer is
susceptible to noise corruption
 Digital signal is not
 What’s the big deal?
 Consider compressing a video of a wall
painted a solid color
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– Analog noise will cause small fluctuations
from pixel to pixel
– RLE can’t compress it because each pixel is
a bit different
Comparing cameras
iSight to MiniDV
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iSight (or a webcam) is built into the
Macs in this room
– Presents an analog signal to the computer
– Subject to analog noise
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The cameras we can check out from the
library are Mini DV format and record
on tape
– Presents a digital signal to the computer
Our video cameras
compress using
jpeg
analog signal
computer
webCam
the scene
digital
signal
miniDV
compression
!!!NOISE!!!
video capture card
compression
iMovie
We’ve seen…
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Converting TV to Video is
problematic
– Interlacing
• comb effect
Next:
 Converting Film to Video is
problematic
– Matching 24 frames to 30 frames
• Telecine problem
Film to Video
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Problematic (interleaved)
– video is 30 frames per second
– film is 24 frames per second
How do you make 30 frames from
24?
 One way: The 3-2 pull down…
 AKA Telecine
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Film to interlaced video:
Refer to Supplemental text:
Moving Images: Film
Moving Images: TV
Moving Images: Video
Next
Lecture 10:
Video & compression techniques
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