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Computer Systems. Nat 5 Computing Science Data Representation Lesson 4b: Storing Graphics EXTENSION. Lesson Aims. Pupils at National 5 level will be able to: Describe how a vector image is stored Calculate the storage requirements of a bitmap image with colours

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computer systems

Computer Systems

Nat 5 Computing Science

Data Representation

Lesson 4b:

Storing Graphics EXTENSION

lesson aims
Lesson Aims
  • Pupils at National 5 levelwill be able to:
    • Describe how a vector image is stored
    • Calculate the storage requirements of a bitmap image with colours
    • Explain the difference between a vector and bitmap image
    • Explain the advantages and disadvantages of bitmap images compared to vector images
vector graphics
Nat 5Vector Graphics
  • This image is an example of a vector image.
  • A vector image is made up of lots of overlapping shapes
  • Each of the shapes can be edited separately from the other shapes.
vector graphics1
Nat 5Vector Graphics
  • A vector image is stored using

a different method than bitmapped images

  • The attributes (or qualities) of each shape are stored.
    • EG, the eyes(ellipses) may be stored as:
      • Centre X/Y-co-ordinates
      • Fill Colour
      • Border Style
      • Border Colour etc…
storing colour bitmaps
National 5Storing Colour Bitmaps
  • Storing a colour bitmap is exactly the same as for black and white with one difference - Each pixel is not just black and white but can represent a variety of colours.
  • This is done by allocating more than one bit for each pixel:
    • 2 bits/pixel = 4 colours
    • 4 bits/pixel = 16 colours
    • 6 bits/pixel = 64 colours
colour bitmaps
National 5Colour Bitmaps
  • The amount of bits allocated to each pixel is known as the bit depth.
    • So an image with 8 bit colour depth could have 256 colours [28 = 256]
    • True Colour is defined as an image with 24 bit colour depth. It has 16,777,216 colours! [224 = 16,777,216]
true colour
National 5True Colour
  • True Colour is defined as an image with 24 bit bit depth.
  • The colour code for each pixel is constructed of a single 8 bit number for each of the main 3 additive colours.
    • Red, GreenandBlue

RGB Colour Codes

storage space example
National 5Storage Space Example

A true colour image is 800 pixels by 900 pixels.

Calculate the storage requirements and express the answer in appropriate units

  • Step 1: (Length x Breadth) * bit depth
    • (800 * 900) * 24 bits = 17,280,000 bits
  • Step 2: Convert into appropriate units
    • 17,280,000/8 = 2,160,000 bytes
    • 2,160,000 bytes /1024 = 2,109.375 KB
    • 2,109.375 Kb/1024 = 2.06 MB
storage space example 2
National 5Storage Space: Example 2
  • A 16 bit colour image is 4 inches by 6 inches with a resolution of 300dpi.
  • Step 1: (Length x dpi) x (Breadth x dpi)
    • (4*300) * (6*300)= 2,160,000 pixels
  • Step 2: number of pixels * bit depth
    • 2,160,000 * 16 = 34,560,000 bits
  • Step 3: Convert into appropriate units
    • 34,560,000 /8 = 4,320,000 bytes
    • 4,320,000 bytes /1024 = 4,218.75KB
    • 4,218.75KB /1024 = 4.119 MB
bitmap graphics pros and cons

National 5

Bitmap Graphics – Pros and Cons

Advantages

Disadvantages

Requires large storage space

Image becomes jagged when scaled

  • Can be manipulated at pixel level
  • Can create a wide array of graphic effects
  • Can represent photo-realistic images
vector graphics pros and cons

National 5

Vector Graphics – Pros and Cons

Advantages

Disadvantages

Cannot be edited at pixel level

Cannot show photo realistic scenes

Will usually require particular applications to open

  • Do not lose quality when scaled
  • Require less storage space
  • Objects are easily moved/manipulated
  • Resolution independent
tasks
Tasks

National 4

National 5

Worksheet 4A and 4B

  • Worksheet 4