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Computer Systems

Computer Systems. Nat 4.5 Computing Science Data Representation Lesson 4: Representing and Storing Graphics EXTENSION. Nat 4/5. REVISION. How does a computer system store a black and white image? What storage space is required for a B+W image 300*700 pixels?

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Computer Systems

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  1. Computer Systems Nat 4.5 Computing Science Data Representation Lesson 4: Representing and Storing Graphics EXTENSION

  2. Nat 4/5 REVISION • How does a computer system store a black and white image? • What storage space is required for a B+W image 300*700 pixels? • What storage space is needed for a 3*5 inch image with a resolution of 600 dpi? • Explain the difference in the storing of a vector and bitmap graphic.

  3. Nat 4/5 ANSWERS Revision: • Store as a series of bits, 1’s for black and 0’s for white. • 300*700/8/1024=25.63 = 26Kb • (3*600)*(5*600)/8/1024= 659.17 =660Kb • Bitmap stored as pixels and Vector stored by it’s attributes.

  4. Nat 4/5 Lesson Aims • Pupils at National 5 levelwill be able to: • Describe how a computer system stores a colourbitmapimage. • Calculate the storage requirements of a colour bitmapimage • Describe the advantages bitmap graphics have over vector graphics • Describe the advantages vector graphics have over bitmap graphics

  5. National 5 Colour Bitmaps • The colour bitmap method is exactly the same as for black and white with one difference. • Each pixel is not black and white but can represent a variety if colours.

  6. National 5 Colour Bitmaps • Each pixel has a binary value representing the colour. The amount of colours is known as the bit depth. • So an image with 8 bit colour depth could have 256 Colours • True Colour is defined as an image with 24bit colour depth. 16,777,216 colours!

  7. National 5 True Colour • True Colour is defined as an image with 24bit bit depth. • This means that 16,777,216 colourscan be represented. • 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) * 24bits = 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 Alternate Storage Space Example • Sometimes you will be given the size and the resolution of the image. • One way in which this can be measured is dpi • Dots per inch is the amount of pixels in an inch. • A 16bit colour image is 4 inches by 6 inches with a resolution of 300dpi. • Step 1: ((Length x dpi) x (Breadth x dpi)) * bit depth • ((4*300) * (6*300))* 16 = 2,160,000 pixels • Step 2: number of pixels * bit depth • 2,160,000 * 16 = 34,560,000 bits • Step 2: Convert into appropriate units • 34,560,000 /8 = 34,560,000 bytes • 34,560,000 bytes /1024/1024 = 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. National 5 What Vector and Bitmap graphics are used for Vector Bitmap • Plans • Logos • Promotional posters • Large scale banners • Photo editing • Life like pictures • Computer drawing • Special effects, for example, blurring and texture

  14. National 5 Summary • In colour bitmaps the amount of colours in the picture is represented by a binary number • The amount of colour is known as the bit depth • You calculate the storage requirements by multiplying the amount of pixels by the bit depth • Or by using the size of the graphic multiplying it by the dpi and then by the bit depth • Vector and Bitmap graphics are used for different task

  15. Two methods of calculating storage space required • A graphic 300 pixels by 600 pixels and a bit depth of 16 then 300*600*16 = 2880000 bits / 8 = 360000 bytes / 1024 = 351.5625 = 352 Kb • A 16 bit 5*7colour image with dpi of 600 • (5*600)*(7*600)*16= 201600000 bits/8 = 25200000/1024 = 24609.375/1024 = 24.0325927734375 = 24.1 Mb

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