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Fundamentals. Rawesak Tanawongsuwan [email protected]. 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. - PowerPoint PPT Presentation
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Fundamentals
Rawesak [email protected]
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.