Transcript JrowanITEC2110-01&04VideoPart1.ppt

Digital Media
Dr. Jim Rowan
ITEC 2110
Video
Roll call
Barton, Paul H.
Bois, Lauren C.
Bonds, Allison E.
Duncan, Jarred T.
Lawson, Joseph I.
Mulongo, Julio B.
Pennison, Heather L.
Reilly, Daniel J.
Sanchez-Casas, Jon F.
Simson, Davis
Sinnock, Grant A.
Swaim, Mark S.
Tran, Dung Q.
Vyas, Anand A.
Woldeyohannes,
Tesfamichael
Roll call
Jones, Crystal L.
Marsh, Kerreen A.
Thompson, Daniel G.
Tran, Christopher V.
Video
• Works because of persistence of vision
• Fusion frequency
– ~ 40 frames per second
– depends on the brightness of the image relative to
the viewing environment
• Less than that
– flickering
– individual images appear losing the illusion of
motion
Video
• Video vs Animation...
– Video - capture of frames and then
playback
– Animation - create frames individually and
then playback
Video
• Computationally demanding
– Capture must be fast enough to capture sufficient
frames to produce the illusion of motion
– Transport (if across the web) must be fast enough
to carry those captured frames at a rate fast
enough to produce the illusion of motion
– Playback must be fast enough to play those
captured frames at a rate fast enough to produce
the illusion of motion
Video
• If the transport and playback is not fast
enough, frames will be dropped
• Video players make compromises
differently
– Some drop frames holding the last image
• effectively losing the illusion of motion but
continuing the story as a slide show
– Some play lower resolution images
– Some continue to play audio
Video gets big fast
• NTSC
– North America and Japan
– 640 x 480
– 24 frames per second
• Using 3 byte color (24 bits, one byte per
color) each frame ~ 1megabyte
• One second of video (uncompresssed) is 26
Megabytes
• One minute is 1.6 gigabytes
Video gets big fast
• PAL is even bigger
– Western Europe and Australia
– 768x576
– 25 frames per second
• Using 3 byte color (24 bits, one byte per
color) each frame ~ 2 megabytes
• One second of video (uncompresssed) is 26
Megabytes
• One minute is 1.85 gigabytes
More on size...
• Uncompressed this exceeds most home
computer interface standards
• Strains the internal speed of the home
computer
• Strains the storage capability of home
computer
• WAY exceeds what can be carried by
the net
What to do?
• Apply compression
• On the capture side
– digitization & compression needs to be
carried out by the hardware to be fast
enough
– Can be done in the camera (VTR)
– Can be done in the computer (iSight cam)
Compression in the VTR
• Within the camera
– 3 (at least) different formats internally
• Differing error correction and compatibitibility
– Recording on different media
• CD
• tape
• Mini DV or DV format
– Connected to computer using firewire
– All 3 formats present the same stream of bits to
the computer
• Artefacts that interfere with processing and
recompression are created
Compression in the computer
• 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 is done elsewhere
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 solid color
– Analog noise will cause small fluctuations from
pixel to pixel
– RLE can’t compress it because each pixel is a bit
different
Consider compressing
this using RLE
analog signal
!!!NOISE!!!
computer
iSight
Camera
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
hardware vs software
compression
• Hardware conversion... user has no control
over it... it is hardwired
– It is 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
– Can use different software coder-decoders
(codec), picking and choosing what fits your needs
better
Next?
• Streaming video!
Questions?