Transcript 9 ITEC2110VideoPart1Spring2011.ppt

Digital Media
Dr. Jim Rowan
ITEC 2110
Video
Lecture URLs
http://en.wikipedia.org/wiki/Interlace
http://en.wikipedia.org/wiki/Progressive_scan
http://en.wikipedia.org/wiki/Telecine
http://en.wikipedia.org/wiki/Digital_terrestrial_television
http://en.wikipedia.org/wiki/ATSC_Standards
http://en.wikipedia.org/wiki/NTSC
http://en.wikipedia.org/wiki/PAL
http://en.wikipedia.org/wiki/SECAM
In the next several lectures…
Film & TV & Video & Animation
Issues that arise from conversion
Analog vs Digital
http://www.bealecorner.com/gl1/res/gl1resint.jpg
Test Pattern
TV Broadcast…
Digital replaces Analog
Why Digital Broadcast?
reduced spectrum use
greater capacity
multiple programs on one freq
better quality picture
HDTV
can use compression
allows multiple HD signals on one freq.
allows user interaction
TV Broadcast…
Standards
Analog (old school)
NTSC (north America)
PAL (western europe)
SECAM (eastern europe)
Digital
ATSC (see map in wikiPedia)
TV Broadcast…
Difference with poor reception
Analog…
as signal gets weaker
image gets less distinct
“ghosts (white shadows) appear”
gracefully degrades
Digital…
with digital, you either have signal
or you don’t have signal so…
lose signal
everything goes black
audio stops
ungraceful degrading
Two ways to make Moving Pictures:
Video & Animation
• 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
• 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)
• 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
• Fusion frequency
– ~ 40 frames per second
– depends on the brightness of the image relative to
the viewing environment
• Less than that
– first flickering
– then individual images appear losing the illusion of
motion
Film how it works
• 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
• 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?
Video & TV
• Two versions
– Interlaced
• Rising from a TV legacy
– Progressive scan
• Rising from a computer legacy
Interlaced
http://en.wikipedia.org/wiki/Interlace
– Captured (and displayed) as “fields”
– First the odd numbered lines are captured (or
displayed)
– Then the even numbered lines are captured (or
displayed)
– 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
http://www.infocellar.com/television/files/interlace1.gif
Interleaved scan
http://www.geniusdv.com/weblog/archives/2007/08/14/interlaced_vs_progressive.gif
Interlaced problem:
• Rapid motion
resulted in the
“comb effect”
http://en.wikipedia.org/wiki/File:Interlaced_video_frame_%28car_wheel%29.jpg
Interlace problem:
the center-column
images are interlaced
the left-column
images are
progressive scan
the right-column
images use line doublers
bottom images are anti-aliased
Progressive scan
http://www.geniusdv.com/weblog/archives/2007/08/14/interlaced_vs_progressive.gif
Progressive scan
http://en.wikipedia.org/wiki/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
Video… it’s big
How do you deal with it?
• Playback degradation (graceful degradation)
• Compression
Video… it’s big
Handling with Playback
• 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”
Video: Graceful degradation
• Graceful degrading allows the story to continue
• Some players drop frames
– 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
• Some play lower resolution images while remaining
synched to the audio stream
– this continues the illusion of motion (at a lower resolution) and continues
the “story” with the audio stream
Video: The “progress bar”
Quicktime example
• 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
Video is big so: reduce its size
using compression
• 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
• Hardware compression... user has no control
over it... it is hardwired
– It can be in the camera
– It can be in the video card
• Software conversion... 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
• 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
• With compression you get artifacts
with software compression…
• 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
– 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
• iSight (or a webcam) is built into the
Macs in this room
– Presents an analog signal to the computer
– Subject to analog noise
• 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
!!!NOISE!!!
computer
webCam
video capture card
compression
digital
signal
miniDV
the scene
compression
iMovie
Our video cameras
compress using jpeg
analog signal
!!!NOISE!!!
computer
webCam
video capture card
compression
digital
signal
miniDV
the scene
iMovie
compression
640 x 480 = 307,200
307,200 can be represented
by < 24 bits, call it 3 bytes
RLE:
307,200 (3bytes) +
RGB (3 bytes) ~ 6 bytes
640 x 480 = 307,200 bytes
Noise makes each pixel
a little different
RLE:
307,200 bytes x RGB (3bytes)
= 921600 bytes
We’ve seen…
• 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
• 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…
Film to interlaced video:
http://en.wikipedia.org/wiki/Telecine