[1] Information Representation

• How lossless compression reduce file size (3)

  · Uses RLE · Identifies group of repeated characters · Replaces them with index
• Why use compression:

  · Reduce file size · Faster to transmit · Available to email
• Lossy methods:

  · Reduce bit depth · Reduce number of colours · Reduce image resolution
• Difference between lossy + lossless

  · lossless allows original file to be recreated exactly · lossless based on replacement, lossy based on discarding · lossy makes decisions what important to discard · lossy reduce file size more
• Sampling

  Amplitude of sound wave taken at different points in time

  Recording of sound at a regular interval and storing the value
• Sampling rate


  · number of times that the amplitude measured per unit of time · the number of samples taken per second · higher sampling rate = more accurate digital representation
• Sampling resolution

  · number of bits used to represent each sample · higher sampling resolution = smaller quantization error
• Increase sample resolution (3)

  · more bits per sample · larger file size · greater accuracy
• Increase sample rate

  · more samples per unit of time · increase file size · increased accuracy
• Analogue to Digital Converter (ADC)

  · converts analogue sound into digital signals which can be digitally stored (binary)
• Analogue vs digital description (3)

  · analogue cannot be read by computer · digital cannot be output · need for ADC/DAC
• Bit Rate

  Sampling Rate * Sampling Resolution number of bits required to store 1 sec of sound.
• Sound file size formula

  Bit Rate * Length of SoundFile
• Pixel

  Smallest identifiable component of image colour represented by binary stored in bits
• Colour depth

  Number of bits used to represent each colour
• Image resolution

  Number pixels wide by number pixels high
• Screen resolution

  Monitor specification. Larger number = more pixels
• File header + contents

  · Stores data about image · file size, format, number of bits per pixel
• Increase colour depth

  · more bits per pixel · increase file size · increase quality
• Bitmap:

  · Made up of pixels · Each pixel has one colour stored as binary · When enlarged, pixels get bigger + pixelates · Large File size (Data stored about all pixels) · Compressed with significant reduction
• Vector:

  · Constructed through CAD/drawing packages · Set of instructions to draw shape · Enlarged - recalculated so no pixelation · Smaller file size – only contain instructions
• Difference between bitmap + vector:

  · Bitmap pixels. Vector set of drawing instructions · Enlarged: bitmap pixelated, vector not · Bitmap larger file size. Need to store data about each pixel · Bitmap compressed with greater reduction in file size · Bitmap more suitable for photograph. Vector for shape
• Benefits of vector instead of bitmap:

  · Resize no pixelation – recalculated · Smaller file size · Stores commands instead of pixels
• How is bitmap image encoded (1)

  solid colour assigned to each pixel
• bitmap File size:

  Number pixels * colour depth

  n bits has 2^n colours per pixel
• Drawing object + properties

  Individual shape in vector graphic
• Property + example

  Data about shape e.g. colour of triangle
• Drawing List

  List of shapes in vector graphic
• role of Editing software for pics

  · Resize · Crop · Blur · Red eye reduction
• Byte

  8 bits
• Nibble

  4 bits
• Kilobyte

  1000 bytes 1*10^3
• Kibibytes

  1024 bytes (2^10)
• Megabytes

  1,000,000 bytes 1*10^6
• Mebibyte

  1024 * 1024 bytes 2^20
• Gigabyte

  1,000,000,000 bytes 1*10^9
• Gibibyte

  1024 * 1024 * 1024 bytes 2^30
• Terabyte

  1,000,000,000,000 bytes 1*10^12
• Tibibyte

  1024 * 1024 * 1024 * 1024 bytes 2^40
• Binary prefixes

  Byte, Kibibyte, Mebibyte, Gibibyte, Tebibyte