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Digital Data
• Bits are units of data that can only have one of two values.
• A byte is eight bits.• Groups of bits can be interpreted as numbers
to base 2, but can also be treated as characters, colours, etc.
Bytes to remember
• 1 byte = 8 bits• 2 bytes = 16 bits• 3 bytes = 24 bits• 4 bytes = 32 bits• .• .• .
Analogue and Digital Representation
Digitization
• Converting a signal from analogue to digital form– Analogue signal can vary continuously, digital is
restricted to discrete values• Two-stage process– Sampling – measure the value at discrete intervals– Quantization – restrict the value to a fixed set of
quantization levels
Sampling and Quantization
Sampling and Quantization
• The sampling rate is the number of samples in a fixed amount of time or space.
• The quantization levels are the set of values to which a signal is quantized.
Analogue vs Digital
Benefits of digital signals
• High fidelity• Noise tolerance
Frequency Domain
• Any periodic waveform can be decomposed into a collection of frequency components– Each one is a pure sine wave
• The collection of frequencies and their amplitudes represent the waveform in the frequency domain– Compute the frequency domain representation
(frequency spectrum) using the Fourier Transform
– Higher frequency components are associated with abrupt transitions
Spatial and temporal signals
• Spatial and temporal signals are made up of pure sine wave components at different frequencies.
• The Fourier Transform operation can be used to compute a signal’s representation in the frequency domain.
• Higher-frequency components are associated with abrupt transitions.
Sampling Theorem
• If the highest frequency component of a signal is at fh the signal can be properly reconstructed if it has been sampled at a frequency > 2fh– Nyquist rate
• Undersamping leads to aliasing– Sound distortion, image 'jaggies' or Moiré
patterns, jerky or retrograde motion
Aliasing
Anti-aliased
Aliased
Aliasing
Moiré Patterns
Too Few Quantization Levels
• Reducing memory requirements by using fewer bits for each value means fewer quantization levels are available
• Cannot distinguish between values that fall between levels
• Images: banding and posterization• Sound: coarse hiss, loss of quiet passages,
general fuzziness (quantization noise)
Image banding effects
Posterization
Digital Representation of Media
• There are established ways of representing images, video, animation, sound and text in bits.
• Media data may be represented as a textual description in a suitable language, or as binary data with a specific structural form.
Image
• Images are displayed as arrays of pixels and represented using an internal model. Generating the pixels from the model is called rendering.
Bitmaps vs Vector Graphics
• Images may be modelled as bitmaps or vector graphics.
• A bitmap is an array of logical pixels (stored colour values) that can be mapped directly to the physical pixels on the display.
• In vector graphics, the image is stored as a mathematical description of a collection of individual lines, curves and shapes making up the image, which requires computation to render it.
Good Things for Vector Graphics
• Often smaller than bitmaps• Resolution-independent• Scalable without loss of quality – only suitable for certain sorts of synthetic image,
not photographs
Combining Vectors & Bitmaps
• Rasterize vectors– Lose all their vector properties
• Trace bitmaps– Difficult and can only produce an approximation
(parameterized)
Layers
• Organizational device used in both sorts of graphics, especially useful in bitmaps– Permits separation and manipulation of different
parts of a bitmapped image• Digital version of clear sheets of acetate,
stacked on top of each other– Areas without coloured pixels/graphic objects are
transparent so lower layers show through• Compositing – combine layers using different
blending modes (digital collage)
Creatively Blending Layers
http://www.carlvolk.com/photoshop15.htm
Creatively Blending Layers
http://www.carlvolk.com/photoshop15.htm
Graphics/Image Data Types
RGB Components
Moving pictures, Videos, Animation
• Moving pictures can be created as live-action or animation.
• Live-action must be stored as video.• Animation may be represented in other more
flexible or efficient ways.• Video frames require a lot of storage so video
is invariably compressed for delivery.
Sound
• Sound can be represented as a sequence of samples after digitization.
• CD audio is sampled at 44.1 kHz, higher sampling rates are sometimes used.
• Audio delivered over the Internet is compressed, often using the MP3 codec.
Text
• A character set is a mapping from characters to character codes.
• Unicode is a character set capable of representing text in all known languages.
• A font is a set of character shapes, called glyphs.
Layout and Typography
• Many aspects of layout must be controlled when text is displayed.
Interactivity
• Interactivity is produced by executing a program in response to user input.
• In multimedia, programs are often written in a scripting language, such as JavaScript or ActionScript.
Compression
• Compression must often be applied to media data.
• Compression may be lossless or lossy.– Lossless – image can be reconstructed exactly
from compressed version– Lossy – some information discarded, image can
only be reconstructed approximately
Compression
• Different compression algorithms are applicable to different types of media data. Their effectiveness depends on the characteristics of the data itself.
