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

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

1.What does ASCII mean? 2.Describe how text is stored by a computer system 3.What is a control character?

 Int 2 Revision : 1. A merican S tandard C ode for I nformation I nterchange 2.Each value has a unique ASCII value and this is stored in binary by the computer 3.A key that has an effect on screen

Lesson Aims  Pupils at National 5 level will 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

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

Bitmap Images  A bitmap image stores each individual pixel.  First you need to understand black and white images. These are constructed as follows: Nat 4/5 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 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

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 Nat 4/5 Pixel Pattern using 8x8 gridPixel Pattern using 16x16 grid

Calculating Storage Requirements  You are often required to calculate the storage requirements of a black and white bitmap image  The steps are as follows: 1. Amount of pixels wide x Amount of pixels high  This answer is in bits 2. You then convert the number into appropriate units (Kilobytes/Megabytes)  Divide until its in an appropriate unit. 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 Nat 4/5 If you already know the amount of pixels in the image then use this method

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 Nat 4/5 If you are not given the amount of pixels in the image then use this method

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… 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. Nat 4/5

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