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Computer Systems

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

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  1. Computer Systems Nat 5 Computing Science Data Representation Lesson 4b: Storing Graphics EXTENSION

  2. 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

  3. Nat 5 Vector 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.

  4. Nat 5 Vector 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…

  5. National 5 Storing 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

  6. National 5 Colour 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]

  7. National 5 True 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

  8. National 5 Increasing Bit Depth

  9. National 5 Storage 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

  10. National 5 Storage 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

  11. 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

  12. 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

  13. Tasks National 4 National 5 Worksheet 4A and 4B • Worksheet 4

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