Today’s Agenda

Welcome!

Go over Quiz 1 and terminology review

Split up and discuss Chapter 1-3 (Present your chapters)

Break

Lecture: Compression Basics

Lecture: Batch Templates (review Compressor)

Project #2

Take Breaks as needed

Take Quiz #2

For Next Class: Read chapter 4-6
Chapter Review
Presentations
 What was the main point of the chapter?
 What are the key concepts?
 What are the key terminology from the chapter?
 How does it relate to compression?
 What are some questions you still from the
chapter?
Two types of Compression:
Lossless and Lossy Compression
 Lossless: eliminate just redundant bits • Modest
reductions (for images, 2-3X on average) •
Compression reproduces exact original dataset
 Lossy: eliminate least important bits (too) • Major
reductions (up to 100X or more) • Decompression
reproduces only a similar copy
Compression Techniques
Compression techniques used for digital video
can be categorized into three main groups:
General purpose compression techniques for
any kind of data.
Intraframe compression techniques work on
images.
Interframe compression techniques work on
image sequences rather than individual
images.
Comparison
Lossless Compression
Lossy Compression
High Quality
Discards redundant data
Larger file sizes
Small file sizes
Complete control of parameters
Trade off between size and quality
Two types: Spatial (intraframe)
versus temporal (interframe)
Things to consider when compressing…
 Type of media: are you using QT, MPEG1…?
 Frames per second: 24, 25, vs. 29.97
 Audio quality: Noise reduction
 De-interlacing: you need to do this otherwise you
video will have artifacts (unclear outlines)
 Audio and video data rates: this will determine the file
size and your format for distribution
 Frame size/resolution: pay attention to the size
because the smaller the size, the smaller the file.
 Color fidelity: the accuracy of the colors in the picture
which depends on bit depth.
Some Notes For
Understanding
 Formats versus Codecs
 Formats are “container” files. They describe the type
of file that your project is in (e.g. MOV, AVI, WMV).
Formats are the “wrapping” for data streams. They
describe how streams are stored in the file, NOT the
contents of the streams.
 Codecs describe how a file will be compressed and
decompressed.
 Erroneous: “The MOV file has good video
compression” – formats have nothing to do with how
a video is compressed, but rather, how the streams
are stored.
 Correct: “The data streams in this MOV file can be
compressed with an H.264 codec, which will allow for
a better picture.”
Common Formats
(container Files)
 Quicktime (.mov)
 MPEG-2 (.VOB)
 AVI
 MPEG-4 (.MP4)
 DivX (.DIVX)
 Audio Interchange Format
 Windows Media (.WMV,
.WMA, .ASF, .ASX)
 MPEG-1 (.MPG, .MPEG,
.MPE)
File (.AIFF, .AIF) and
Windows WAVE audio
(.WAV)
 Matroska (.mkv)
 Flash Video (.flv)
Common Video Codecs
 Cinepak
 Sorenson Video
 Motion-JPEG
 WMV
 MPEG-1
 DivX
 MPEG-2
 XviD
 MPEG-4
 H.264
 Intel Indeo Video
Common Audio Codecs
 PCM (Pulse-code
modulation)
 A-law and Mu-law
 IMA/ADPCM
 QDesign Music and
Qualcomm PureVoice
 MPEG-1 audio (MP1, MP2,
MP3)
 AC3
 AAC
 WMA
 Ogg Vorbis
Codecs (MPEG 1)
 Mpeg 1 (moving pictures expert group)
 Mainly used for CD’s – produces quality to a VHS
tape.
 Mpeg 1 only supports progressive scan videos.
Codecs (Mpeg 2)
 Is one of the most popular codecs in use today.
 Is used in satellite TV broadcast, digital television
set boxes, and DVD’s.
 It provides widescreen support for DVD’s and
good quality.
 Used for digital standard definition cable (3-15
Mbit/s) and high definition television (at 15-30
Mbit/s).
Codec (mpeg 4 – 2 and 10)
 Comes in two types: Mpeg 4 (part 2) and Mpeg
4 (part 10).
 Mpeg 4 Part 2 – earlier codec typically offered
improvement in the compression ratio over
Mpeg 2 and was mainly used for internet and
broadcast distribution.
 Mpeg 4 Part 2 – was too complex and many
professionals waited for the newer version to
come out, so this codec never really was
popularized.
Codec (mpeg 4 part 10)
 Also known as H.264
 Offers compression that results in a significant data
rate savings
 Efficient transmission of data
 Presets for a wide variety of applications
 Has been adapted for many hardwares:
 Sony Playstation
 Apple’s iPod
 Mac OS
 HD DVD and Blu-ray
Codec (Sorenson 3)
 Considered one of the best video compression
codecs for action based content as apposed to
CG or computer graphics based content.
 The Sorenson 3 codec is used by Apple’s QT and
is also very popular for creating movie trailers for
the internet.
Codec (wmv)
 Stands for Windows Media Video and is created
by Microsoft.
 More for streaming video media technology.
 Based on the Mpeg 4 format.
 Most popular method of streaming or
progressive download of video over the internet.
 The newest addition is WMV HD which is the HD
version of WMV.
Codec (flash)
 Web programers have the choice of using 2
codecs to display movies on the web in flash
format.
 On2 VP6 codec and Sorenson Spark codec.
 On2 VP6 – uses elaborate compression scheme,
but requires processing power to play its frames –
quality if outstanding. Better for Flash movies.
Codec (Divx)
 DivX is a video codec created by DivXNetworks
which is now known as DivX, Inc.
 Specializes in compressing long video footage
into small file sizes and the video quality if
maintained.
 Based on lossy Mpeg 4 Part 2 compression and
available for both Mac and PC’s.
Some Notes
 Apple TV – A device created by Apple to stream
content from different sources (YouTube, MobileMe,
Flickr, iTunes, Netflix. Works similarly to an iPod.
 QVGA – Quarter VGA (240x320 pixels)
 VGA (480x640 pixels), in 4:3 aspect ratio. This is the basic
resolution for computers, but below SD standards.
 DV in SD can either be in 4:3 or 16:9 aspect ratio.
 3GPP – Generator Partnership Project
(telecommunications – cell phones)
 Phone networks: 1) GSM, 2) GRPS, 3) CDMA, 4) EDGE, 5)
UMTS – all for mobile devices
Interlaced
versus
progressive
Interlaced versus Progressive
 Interlaced Scanning
 Progressive scanning
 NTSC- 525-line, 60 fields/30 frames-per-second at
60Hz system for transmission and display of video
images.
 PAL - 625 line, 50 field/25 frames a second, 50HZ
system.
Interlaced
 Presents the odd lines first
and then the even lines.
 Developed as a result of
limitations in television
technology.
 Causes blur during motion
sequences – because of
the slight delay between
the fields.
 Better for broadcasting
compatability.
Progressive
• Lines scanned in
sequential order
• Less jitter during playback
• Better for moving images
• All computer monitors are
progressive – so you must
de-interlace your video
before you can display.
• Easier editing and
compression
Aspect Ratio

Width: Height

4:3 – Standard (actual ratio is 1.33:1)
 33% wider than it is tall

16:9 – HDTV (broader viewer field) (actual ratio is 1:78:1)
 78% wider than it is tall

Film does not adhere to any set standards for aspect ratios but there
are 4 commonly used aspect ratios for film:
 1.37:1 – this is close to the 4:3 SD that we see on TV (pre-1950’s
movies)
 1.66:1 – many Disney cartoons and European movies
 1.85:1 – most American movies today (also called 1.78:1 which is the
same as HDTV)
 2.35:1 – most epic movie directors (Star Wars, Lord of the Rings)
Letterboxing
 Occurs when you watch a 16:9 content on a 4:3
television screen.
 Two horizontal black bands appear along the
top and bottom of the screen.
 Reduces the height of the video, regardless of
whether the desired aspect ratio is 16:9, 1.85:1, or
2.35:1.
 The difference is that you get wider black bars
on top.
Pillarboxing
 Watching 4:3 content onto a 16:9 widescreen
television.
 2 vertical black bands appear on the sides of
the screen.
 Windowboxing:
 Using both letterboxing and pillarboxing.
Example of scaling
Example of Scaling
Frame rates
 Film = 24 frames per second
 PAL = 25 frames per second
 NTSC = 30 frames per second
 True frame rates:
 NTSC = 29.97 frames per second
 FILM = 23.98 frames per second
Conversions
 29.97fps/30fps = .999 (or 99%)
 .1% slower than 30fps
 Some extra time is required to pass on color
information from the input to the output and this
requires the actual frame rate to be slightly lower
than 30 fps.
 Why do we need telecine?
 Because if you were to show a 1 hour film on NTSC, it
would end in 48 minutes.
Telecine
 Is the process where studios add additional
frames to the original film in order to increase the
frame rate while converting film to video so you
watch the film in the correct speed.
 Also the device that is used to perform process.
 One method of doing this is 3:2 pulldown.
 This process involves taking the first frame of the
film, convert it into 3 fields.
 The the second frames of the film and this time it
converts only two fields and so on until the end of
the film.
Compression
Chapters 1-3
Chapter 1Seeing and Hearing
 Compression is the art of converting media into
a more compact form while sacrificing the least
amount of quality possible.
 How the human brain perceives images and
sounds.
 Seeing: Light, luminance (how bright objects
appear), color, white, space, motion
 Hearing: sounds, ear functions, psychoacoustics
Chapter 1
 What is light?
 Light is composed of particles (photons) at various
frequencies – the higher the frequency, the shorter
the wavelength.
 Visible light is between 380-750 nanometers – red
has the lowest frequency and violet has the
highest frequency (a rainbow).
 Our eyes see light reflected – when light hits the
retina of our eyes we see an image (a camera).
The retina of our eyes turns the light into impulses
(like camera film or the CCD in a camera).
Chapter 1
 The Retina:
 Made of Cones and rods
 Rods are sensitive to low light and fast motion, but
they detect only luminance (brightness), not
chrominance (color).
 Cones detect detail and chrominance and come
in three different varieties: sensitivity to blue, red or
green. But they don’t work well in low light.
 We are most sensitive to green, less to red and
least to blue (why we use green screen).
Chapter 1
 Main concepts of Luminance (Y)
 Most colors can be seen as a mixture of green, red,
and blue.
 We perceive luminance better than color.
 Our brain processes brightness differently than color-
our perception of brightness is based mainly on how
much green we see in something.
 Y’=.587 Green + .299 Red + .114 Blue
What is white?
 White varies a lot on the context –
 Our brain automatically calibrates our white
perception – this makes white balancing difficult.
 Outdoors and bright – 6500 K
 Indoor incandescent light bulbs -3000K
Persistence of Vision
 The many times a second something needs to
move for us to perceive smooth motion in order
to not see it as unrelated images.
 The sense of motion is achieved by playing back
images that change at least 16 times per second
(16FPS) – anything below that looks like a slide
show.
Sound
 Variation in pressure – changes in air pressure
determine vibrations (sound).
 The speed of air pressure change determines
loudness (amplitude).
 How fast the speed changes determines
frequency.
 Harmonic overtones (harmonics) and
enharmonic overtones (percussion, explosions,
door slams) are harder to compress.
Basic Concepts
 We can see brightness better than color.
 Color perception is blurry and slow.
 We can see detail in luminance than in color.
 We can perceive motion than things standing
still.
 The less harmonics in your audio, the better it will
compress.
Chapter 2
 Sampling: the process of breaking up an image into
discrete pieces – the smaller the square the more
samples there are. Each square is a picture element or
pixel.
 Most web codecs use pixels in which height and width
of the pixels are equal.
 But DV uses non-square pixels which are rectangular.
 Sampling Time – at least 15fps to see motion, 24fps for
video and audio to appear in sync, and 50 fps for fast
motion to be clear.
 Sampling Sound – sampling rate is the frequency at
which loudness changes are sampled.
Chapter 2
 Sampling rate of audio:
 CD 44.1kHz
 Consumer audio no more than 48kHz.
 96 kHz and 192kHz are exclusively used for
authoring not for delivery.
Chapter 2
 Quantization: the process of assigning discrete numeric values
to the theoretically infinite possible values of each sample.
 1 byte = 8 bits
 2 bytes = 16 bits
 Most video codecs use 8 bits per channel.
 RGB – 256 levels of brightness between black and white
 Y’CbCr – 219 levels of brightness between black and white.
Color Sampling
 4:4:4 – RGB; most common form for Y’CbCr
 4:2:2 – most commonly used in professional video
 4:2:0 – the idea color space for compressing
progressive scan video and used in all codecs,
for broadcast, DVD, Blu-ray, or web. Also used
for PAL DV, HDV, and AVCHD
 4:1:1 – Color space for NTSC DV25 (DV, DVC,
DVCPRO, and DVCAM). You should not encode
to 4:1:1, only for acquisition. It causes blocking
or loss of detail if you encode this way.
Basic Concepts
 Resolution is a made up concept – there is no
resolution in nature.
 Video has two spatial dimensions: height and width
 Audio has only one: loudness (or the air pressure at
any given moment).
 Both video and audio are sampled – but it is easier for
audio to be compressed in a high quality format
(uncompressed) because data rates are lower.
 8 bits – the default depth for compresssion for both
RGB and Y’CbCr.
Chapter 3
 Spatial and temporal compression
 The more redundancy in the content, the more it
can be compressed
 The Shannon limit – there is a limit to how small
you can compress a file.
 Blocking and ringing
Two types of Compression:
Lossless and Lossy Compression
 Lossless: eliminate just redundant bits • Modest
reductions (for images, 2-3X on average) •
Decompression reproduces exact original
dataset
 Lossy: eliminate least important bits (too) • Major
reductions (up to 100X or more) • Decompression
reproduces only a similar copy
Comparison
Lossless Compression
Lossy Compression
High Quality
Discards redundant data
Larger file sizes
Small file sizes
Complete control of parameters
Trade off between size and quality
Two types: Spatial (intraframe)
versus temporal (interframe)
1) Spatial Compression (Lossy)
 Also called Intraframe compression
 Looks at similarities among the pixels of the
images within a video frame, such as patterns,
graphics, etc.
 As you increase the spatial compression, you
increase the data rate and file size.
 A codec that uses spatial compression will use a
quality slider control.
2) Temporal compression
•
Also called interframe compression
•
Compression over time
•
Stores only changed information between frames.
•
Looks for ways to store data that has not changed from one frame to another.
•
The codec can construct the image based on the information deconstructed from the frames.
•
It keeps at least 1 uncompressed frame to be able to re-construct the frame – also known as the
Keyframe.
•
You get a smaller file size and better than spatial compression.
•
However, only effective for videos that have little movement.
•
When there is movement, result in high keyframe creation.
Keyframe
 A keyframe is a
video frame
which is
unprocessed by
the video
codec.
 Use data to
reference
frames by the
codec during
decompression.
 The first frame is
always the
keyframe.
Lossy Compression
Spatial (Intraframe Compression)
Temporal (Interframe compression)
Similarities among pixels “within”
the frame.
Similarities “across” time.
Keeps 1 keyframe uncompressed
and repeats this process during
scene changes.
Creates a keyframe and stores only
changed data over time. It
preserves data in the frame.
Not very effective for motion
because that would mean
creating a lot of keyframes for
each change.
How decompression works
 How the codec reconstructs video frames
Keyframe
 A keyframe is a
video frame
which is
unprocessed by
the video
codec.
 Use data to
reference
frames by the
codec during
decompression.
 The first frame is
always the
keyframe.
Project #2: On Your Own
 Available Footage: Poker Footage 1 and Poker
Footage 2
 Transcode:
 Poker Footage 1 and 2
(Codec: Apple Devices) – all videos should have Text
Overlay with a description of the specs.
 Poker Footage 1 and 2 with Timecode, lower left,
fade in/out.
 Poker Footage 1 and 2 with Watermark, fade in/out.
 Poker Footage 1 and 2 with Color Correction
 Poker Footage 1 and 2 with Letterbox, Scale,
Panavision 2.35:1, Fade in/out
Review Questions
 What windows make up the Compressor interface?
 Why is it important to set your destination? How do
you create a new destination for your target?
 What are the steps for creating an output file?
 What types of presets are available in Compressor?
 In what window do you adjust filters? How do you do
this?
 What are the differences between file Formats and
Codecs?