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Computer Systems

Computer Systems. Nat 4/5 Data Representation Lesson 4: Storing Graphics. REVISION. What does ASCII mean? Describe how text is stored by a computer system What is a control character?. ANSWERS. Int 2 Revision : A merican S tandard C ode for I nformation I nterchange

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Computer Systems

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  1. Computer Systems Nat 4/5 Data Representation Lesson 4: Storing Graphics

  2. REVISION • What does ASCII mean? • Describe how text is stored by a computer system • What is a control character?

  3. ANSWERS • Int 2 Revision: • American Standard Code for Information Interchange • Each value has a unique ASCII value and this is stored in binary by the computer • A key that has an effect on screen

  4. Lesson Aims • Pupils at National 5 levelwill be able to: • Describe how a computer system stores a black and white bitmap image • Calculate the storage requirements of a black and white bitmap image. • Explain the difference between a vector and bitmap image • Explain the terms pixel, dpi and resolution

  5. Nat 4/5 Lets look at a bitmap picture The whole image is made up of individual pixels. Pixel stands for picture element

  6. Nat 4/5 Bitmap Images • A bitmap image stores each individual pixel. • First you need to understand black and white images. These are constructed as follows: 0 is stored for a white pixel, 1 is stored for a black pixel Each black and white pixel takes up 1 bit of storage

  7. Nat 4/5 Resolution • Resolution is the term given to the amount of pixels that the image is made of • Also used for the amount of pixels displayed on the screen • Higher resolution = clearer and more detailed picture Pixel Pattern using 8x8 grid Pixel Pattern using 16x16 grid

  8. Nat 4/5 Calculating Storage Requirements • You are often required to calculate the storage requirements of a black and white bitmap image • The steps are as follows: • Amount of pixels wide x Amount of pixels high • This answer is in bits • You then convert the number into appropriate units (Kilobytes/Megabytes) • Divide until its in an appropriate unit.

  9. Nat 4/5 Storage Space Example • A black and white image is 800 pixels by 900 pixels. Calculate the storage requirements and express the answer in appropriate units. • Step 1: Length x Breadth • 800 * 900 = 720,000 bits • Step 2: Convert into appropriate units • 720,000/8 = 90,000 bytes • 90,000 bytes /1024 = 87.89 Kb If you already know the amount of pixels in the image then use this method

  10. Nat 4/5 Alternate Storage Space Example • Sometimes you will be given the size of the image 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 black and white 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 bits • Step 2: Convert into appropriate units • 2,160,000/8 = 270,000 bytes • 270,000 bytes /1024 = 263.67 Kb If you are not given the amount of pixels in the image then use this method

  11. Nat 4/5 Vector Graphics • The image below is an example of a vector image. • For example attributes for the eyes(ellipses) may be • Centre X/Y-co-ordinates • Fill Colour • Border Style • Border Colour etc…

  12. Nat 4/5 Summary • Bitmap images store every single pixel • 1bit for each pixel • 0 for white pixel, 1 for black pixel • Resolution is the amount of pixels that make up an image • Higher resolution means you have a more detailed image.

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