Storing Images

• What are bitmap images

  • Most images you use are bitmap images (e.g. photos) and they are made out of pixels arrange on a grid 
  • Pixels are tiny , single-colour squares/dots

  
• The colour of pixels

  • Each pixels is assigned a binary code/value which represents the colour of that pixel 
  • The number of colours available in an image is related to the number of bits the code has - you can make a greater range of shades and colours by increasing the number of bits for each pixel

  
• 1-bit images and 2-bit images

  • Black-and-white images only use 2 colours, so they only need 1-bit for each pixel 
  • White is represented by 0 , and black is represented by 1 
  • 2-bit images can be made up of 4 colours , and each pixel can be one of 4 binary values , 00 , 01 , 10 and 11

  
• What are vector images

  • Vector images are images drawn by the computer following precise mathematical instructions to create lines and objects 
  • Vector images are usually smaller in file size compared to bitmap images because each pixel in a bitmap image is stored as an individual binary value 

  
• What is colour depth

  • Colour depth is the number of bits used for each pixel 
  • Given the colour depth, you can work out how many colours can be made using this formula - total number of colours = 2ⁿ (Where n = number of bits per pixel) 
  • Most devices use a 24-bit colour depth, with 8 bits for the levels of red, green and blue in each pixel 
  • This colour depth can produce more colours than the human eye can see (estimated to be 10 million)

  
• What is the total number of colours in a 1-bit image

  2 colours
• What is the total number of colours in a 2-bit image?

  16 colours
• What is resolution

  • The image resolution is the number of pixels in the image (It’s sometimes given as width x height) 
  • The higher the resolution , the more pixels the image is made of, so the better the quality of the image 

  
• What is the number of colours in a 24-bit image

  16,777,216 colours
• How to work out file size in bits

  • To work out how many bits an image will take up , use the formula :
  • File size (in bits) = width x height x colour depth
• Calculate the file size, in MB of an 8-bit image that is 2000 pixels wide and 1000 pixels high?

  • Find the image resolution of image (width x height) - 2000 x 1000 = 2,000,000 
  • Find the colour depth of the image - Image has 8 bits, so the colour depth of 8 
  • Multiply the image resolution by the colour depth, to get file size in bits, - 16,000,000 
  • Divide 16,000,000 bits by 8 to convert it into bytes (8 bits in 1 byte), - 2,000,000 bytes 
  • Divide 2,000,000 bytes by 1000 twice to convert to MB (B > KB > MB) to get a final answer of 2 MB
• Calculate the resolution of the image below (Hint: count the pixels to find width and height)

  48 bits

  
• Calculate the file size in KB of this 1 bit image

  • By counting, you can see the width = 8 and the height = 6  - Resolution = 48 
  • The colour depth is 1 bit (The image is 1-bit and only has 2 colours) 
  • File size in bits = resolution x colour depth = 48 
  • File size in bytes = 48 divided by 8 = 6 bytes
  • File size in KB = 6 divided by 1000 = 0.006 KB