Chapter 3- Media Representation and Formats.ppt

Chapter 3
Media Representation
and Media Formats

 Media is represented in various forms like text,
images, audio, video, animation and graphics.
 All these kinds of media need to be represented in
digital form so they can be viewed, exchanged, edited
and transmitted in a standard manner.
 This chapter will discuss:
 Different media representation in digital form and
 Commonly used format for these medias
Introduction

 Images by themselves are used in various forms for a
variety of applications. These might be:
 photographs, gray or color
 used with text in documents
 fax is another image representation used in
communication
 Images are combined to create an interesting
application using different methods
 Images are also basic element of video
Digital images

 Images are digitally represented as pixels
 Images is defined by
 Width
 Height and
 Pixel depth (number of bits per pixels)
 Pixel depth is same for all pixels of a given image
 Number of bits per pixels used in an image is depends
on the color representation grey or color
Digital representation of images

 For instance
 in a grey scale image the pixel depth is 8bits per pixel
 In color image each R, G, B channel may be presented by 8
bits. So the depth for color image would be 8+8+8= 24bits
 An additional channel alpha is used to composite the pixels
of the foreground with the background image, producing
the final image on the far right.
 In this case the pixel depth would be 32 bits
 So the number of channels typically ranges from 1 to 4 and
accordingly pixel depth changes.

 Image size can be calculated as:
 Resolution * depth = image size
 (width * height) * depth = image size
 For example image size of a grey scale image having
resolution of 640*480 pixels using 4 bits per pixels is:
 Image size = resolution * depth
 Image size = 640*480* 4
 Image size = 1228800 bits
For further examples refer the book and note book.

 Color images or other continuous images are printed using
halftone technology to save the printing cost.
 Aspect ratio:
 Ration of picture width to its height is known as aspect ration.
 Plays an important role in standards.
 Commonly used aspect ratios are:
 3:2 for developing and printing photographs
 4:3 for television images
 16:9 for high definition images
 47:20 for cinema videos

 Digital image formats depends on the software and
hardware used for the image.
 Some commonly used formats are:
 Bmp
 Pcx
 Gif
 Jpg, jpeg
 Png
 Psd,
 etc

 Throughout twentieth century motion pictures were
stored on films
 Then medium changed to tapes and provide public
direct access to movies
 Digital videos further altered the field by making
available precedent level of:
 Visual quality
 Distribution
 Interaction
Digital video

 Video, whether analog or digital, is represented by a
sequence of discrete images shown in quick succession.
 Each image in the video is called a frame, which is
represented as a matrix of pixels defined by a width,
height, and pixel depth. The pixel depth is represented in a
standardized color space such as RGB.
 These image attributes remain constant for all the images
in the length of the video.
 Thus, video has the same properties such as width, height,
and aspect ratio.
Representation of digital video

 In addition, two important properties govern video
representation: frame rate and scanning format.
 The rate at which the images are shown is the frame
rate.
 Film is displayed at 24 frames per second.
 Television standards use 30 frames per second (NTSC)
or 25 frames per second (PAL).
 If the frame rate is too slow, the human eye perceives
an unevenness of motion called flicker.

 NTSC (developed by the National Television Systems
Committee)
 PAL (Phase Alternating Line), and
 SECAM (Système Electronique Couleur Avec Mémoire)
 The first commercial television program was broadcast in
monochrome in the United States in 1941, with standards
developed by NTSC.
 NTSC’s standards for color television were published in
1954
 They have evolved since then to cover VCRs, and have also
influenced digital cable, HDTV, and digital video.

 Although digital video is thought of as a three-dimensional
signal in space and time,
 The analog video signal used in broadcast is scanned as a
one-dimensional signal in time, where the spatiotemporal
information is ordered as a function of time according to a
predefined scanning convention.
 The standardization process implemented in the broadcast
of analog video for television mandated a few
requirements, which were necessary for making television
transmission viable:
Analog video and television

 YUV color space conversion and interlaced scanning.
 These requirements, although not necessary for
digital video representation, still need to be
supported in the digital world because of the well-
entrenched standards for analog television displays
 Analog displays will gradually transition into digital
display devices, but for now, both need to be
supported.
Analog video and television

 Video frames are represented using a color format
which is generally RGB.
 RGB color space is used by CRT display devices such
as TV.
 For transmission purpose RGB is transformed into
YUV signal.
 This is done to reduce the bandwidth and is based on
experiments with the human visual systems.
Conversion to YUV

 Y is called luminance (brightness) and UV is called
chrominance (colors)
 Human eyes are sensitive to the brightness.
 During subsampling Y is left untouched while for
every 4 pixels an average information is stored as
chrominance.
 For a 16*16 pixels frame , four 8*8 pixels blocks are
created for Y component while one 8*8 pixels block is
created for U & V component.
Conversion to YUV

 The separation is intended to reduce the transmission
bandwidth.
 Following figures explains the conversion and
subsampling process.
Conversion to YUV

 For synchronization of transmission purposes, the
line-by-line analog raster signal has to be rendered on
your television in a corresponding manner, as the
data is received.
 This synchronization is carried out by the cycles in the
power outlet (60 Hz for NTSC, 50 Hz for PAL).
 Every 1/60th of a second, the electron gun is reset by
the vertical sync to draw the beginning of the next
frame.
Analog video scanning

 However, to meet the synchronization needs, each frame
is broken down into two fields—an odd field and an even
field.
 The odd field consists of the odd-numbered scan lines and
the even field consists of the even-numbered scan lines, as
shown in figure.
 The electron gun at the back of the TV tube first draws the
odd lines of the on-screen image, and
 Then during a second pass, it draws the even-numbered
lines.
Analog video scanning

1. Composite Video
 Composite video is also called baseband video or RCA
video. It is the analog waveform that conveys the image
data in the conventional NTSC television signal.
 Composite video contains both chrominance (color) and
luminance (brightness) information, along with
synchronization and blanking pulses, all together in a single
signal.
 This is done to reduce bandwidth and achieve real-time
transmission.
Types of video signals

 However, in composite video, interference
between the chrominance and luminance
information is inevitable and tends to worsen
when the signal is weak.
 Some DVD players and videocassette
recorders (VCRs) accommodate composite
video inputs/outputs for the purpose of
connecting to standard NTSC televisions,
which only accept composite video.

2. S-Video
 S-Video (Super-Video, sometimes referred to as Y/C Video) is a video
signal transmission in which the luminance signal and the
chrominance signal are transmitted separately to achieve superior
picture clarity.
 The chrominance signal (C) is formed by combining the two
chrominance signals U and V into one signal along with their
respective synchronization data
 This is unlike the traditional composite video where all three
channels are combined together into one signal.
 Separating the Y and C channels and sending them separately
reduces problems caused by interference between the luminance
and chrominance signals and yields a superior visual quality.

3. Component Video
 Component video strives to go a step further than S-
Video by keeping all three Y, U, V (or equivalent)
components separate.
 Consequently, the bandwidth required to broadcast
component video is more than the composite or S-
Video and, correspondingly, so is the visual quality.
 The separation of these components prevents
artifacts due to inter-signal interference.

 A class of digital television called HDTV supports a
higher resolution display format along with surround
sound. The visual formats used in HDTV are as
follows:
 720p—1280 720 pixels progressive
 1080i—1920 1080 pixels interlaced
 1080p—1920 1080 pixels progressive
High definition television

 Sound is a form of energy transmitted in form of
pressure waves
 Captured by electromechanical devices such as
microphone
 The first sound capturing/reproducing equipment
appeared in the late 1800s in the form of a
phonograph created by Thomas Alva Edison and a
gramophone by Emile Berliner.
Digital Audio

 Digitizing an analog audio signal requires sampling
and quantization. The process of conversion to digital
sound is known as pulse code modulation (PCM).
 The analog sound is sensed at evenly spaced time
intervals, producing digital audio samples. The
number of samples per time unit (sampling rate) must
be specified during the digitization process.
 All samples are represented by the same number of
quantization bits.
Digital representation of audio

 The sampling rate and the quantization bits per sample are
the main properties of the PCM signal and need to be
carefully chosen so that it is possible to reconstruct the
analog equivalent.
 In addition to sampling rate and quantization, another
characteristic commonly used to describe audio signals is
the number of channels, which may be:
 one (mono),
 two (stereo),
 or multichannel (surround sound)
Digital representation of audio

 Surround sound aims to create a three-dimensional sound
experience.
 It refers to the use of multiple audio tracks to engulf the
listening audience in many sources of sound, making them
feel as if they are in the middle of the action or concert.
 Surround sound usage is standard in movie theaters,
where the surround sound movie soundtrack and the
loudspeaker equipment allow the audience to hear sounds
coming from all around them
Surrounded Sound

 Spatial audio, though similar in intent to create a surround
sound like experience, attempts to create directional
effects using fewer channels, typically two stereo
channels.
 This can be classified as virtual surround sound processes
(for example, Sound Retrieval System [SRS] and other
proprietary algorithms) that make use of only two left and
right speakers and psychoacoustics effects to emulate true
surround sound.
Spatial sound

 Computer graphics has evolved into an advanced field having
applications in high end markets such as visual effects movies,
interactive and multiplayer games etc.
 Most of today’s graphics-related applications tend to be three
dimensional, interactive
 Available on a variety of platforms from computers, to game
consoles, to handheld devices.
 From a multimedia point of view, 2D computer graphics has
been a predominant force since the early 1990s to create
interactive games.
 These were distributed in the form of interactive multimedia CD-
ROMs for a variety of entertainment and educational purposes.
Graphics

 Graphics objects can be represented as vectors or rasters.
 Vector graphics are geometric entities saved in a vector format
having attributes such as color.
 For example, a shape can be represented as a sequence of points to be
connected in a specific order and filled with a color.
 The advantage of vector representations is that they provide infinite
resolution
 However, vector graphics need to be converted to raster images to be
displayed.
 Raster images are represented as a grid of pixels, each pixel having
x, y coordinates and a value that corresponds to a color.
 Sample operations performed on raster images include painting,
compositing, and filtering effects.
Graphics

 In vector graphics, objects are represented as smooth
or discrete primitives.
 Smooth 2D primitives include parametrically defined
entities, such as circles, ellipses, and other control
point–defined curves such as splines.
 Discrete primitives, such as triangles and polygons
have now become a choice for representing vector
graphics because all hardware graphics cards have
been designed to render such primitives.
2D vector graphics representation

 Animations are created by showing different images
over time.
 In 2D graphics, these different images are raster
representations of a vector object(s) that at each
image frame has undergone a slight change
 Operations performed are translations, rotations,
scaling, shearing, and so forth
Animation using 2D graphics

 Translation involves moving the object from one
location to another.
 Rotation is defined by an angle and an axis. The
object rotates about an axis by an angle in either the
clockwise or anticlockwise direction.
 Scaling is defined by a scale amount and a point about
which scaling occurs.
 The point about which scaling occurs is normally
referred to as the scale pivot.
Animation using 2D graphics

Chapter 3- Media Representation and Formats.ppt

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Chapter 3- Media Representation and Formats.ppt