image representation

• basics of images

  n number of bits = 2^n combinations and therefore colours

  
  

  computers need the image in a format it understands, a pattern of 1's and 0's

  
  

  the data type is digital (not the same as analogue which is continuous and in theory has an infinite degree of accuracy)
• colours

  there is an infinite range of possible colours but humans cannot detect all of them.

  
  

  Images use a range of these colours depending on allocation of bits per pixel.

  
  

  more bits per pixel = more colour combinations

  
  

  each pixel is given a hex colour value
• capturing images

  ->digital cameras break up what we see into a grid of pixels

  ->light sensors measure the intensity of colour in each pixel

  ->depending on the bit depth; a larger or smaller range of these measurements is converted into a binary code (using an analogue to digital converter)

  
  

  the number of pixels recorded (based on resolution) affects the size of the file

  
  

  images comprised of pixels are called bitmaps

  eg BMP, JPG, GIF, PNG, TIF
• image resolution

  density of pixels over an area

  resolution defines the number of pixels used to make up a bitmap (therefore the accuracy of the colours used as more pixels = smaller sections measured and therefore more detail)

  72 dpi = screen resolution

  300 dpi = print quality

  (1,000,000 = 1 megapixel)
• image size

  The resolution of an image does not equal its actual size as the size of the pixel itself can be changed.

  increasing the size of an image will enlarge each pixel and cause a deterioration in bitmap quality
• colour depth

  colour depth is the same thing as bit depth

  in a bitmap each pixel has a finite number of colours it can be, based on the bit depth (2^n)

  an increase in bit depth = an increase in colours = an increase in quality

  (it is the same but opposite for a decrease)
• colour codes

  colours are given RGB values denoted in HEX.

  This is to make it easier to use and remember by people as the computer still stores the information in binary.

  32 bit = 8 bits for RED, 8 bits for GREEN, 8 bits for BLUE.

  (the last 8 are used to represent the opacity of the pixels colour)
• file size of images

  file size = number of pixels (resolution) * colour depth

  eg a 2 megapixel camera with 24 colour depth will have an image with file size:

  2,000,000 * 24 = 48,000,000 /8 = 6,000,000 bytes (6 MB)

  quality of the image is traded against file size as an increase in quality (higher resolution or colour depth) will increase the file size.
• meta data

  meta data is information about the data being stored

  It is contained in the same file and adds to file size (we usually ignore it in calculations)

  
  

  examples:

  date taken, width * height, colour depth, location