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SYLLABUS

consideration for the compression of files for the transfer and display of data  11ATAR


PURPOSE

Understanding the features of image / audio files and formats can help you make appropriate choices to ensure that your use of image / audio is as successful as possible

  • What is your target? Know whether your image / audio file is going to be displayed
  • Which file format is best for a particular media?mp3, mov
  • How important is the quality of the image when transferred, copied, or displayed?
  • How important is the speed at which the image transfers or loads?
  • How important is the file size?

DETAILS

LOSSY COMPRESSION

  • Saves space by discarding pixels not needed for viewing the file.
  • When opened the pixels are restored using colours from the available set
  • Data is lost every time a lossy file is edited and saved

LOSSLESS COMPRESSION

  • In an image, it saves space by packing up and grouping common colours
  • no data lost when the file is opened and colours restored
  • e.g. 1. in one part of a picture it has many pixels the same colour. it is easy to compact that part of the picture to 239 pixels of green instead of repeating the pixel 239 times. This is how run-length encoding works (RLE)
  • e.g. 2. in an audio file there are parts the human ear cannot hear, so those parts can be removed making the file much smaller in size, but still lossless

FILE SIZES

  • the more pixels captured, the larger the file size
  • the more pixels captured, the larger the possible print size
  • less pixels cannot be made more, without losing quality
  • more pixels can be made less
  • how you are going to use an image = file size
  • minimum file size for 300 dpi at 100mmx150mm is 2 megapixels for quality prints
  • minimum file size for 300 dpi at 200mmx250mm is 5 megapixels for quality prints

 

 
 
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