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

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

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

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

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. 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! National 5

True Colour  True Colour is defined as an image with 24bit bit depth.  This means that 16,777,216 colours can be represented.  The colour code for each pixel is constructed of a single 8 bit number for each of the main 3 additive colours.  Redgreenblue  Red, green and blue National 5 RGB Colour Codes

Increasing Bit Depth Higher colour Depth = More Colours File Size Increases National 5

Storage Space Example .  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, Kb  2, Kb/1024 = 2.06 Mb National 5 A true colour image is 800 pixels by 900 pixels. Calculate the storage requirements and express the answer in appropriate units

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 = Mb National 5

Advantages Can be manipulated at pixel level Can create a wide array of graphic effects Can represent photo- realistic images Disadvantages  Requires large storage space  Image becomes jagged when scaled Bitmap Graphics – Pros and Cons National 5

Advantages Do not lose quality when scaled Require less storage space Objects are easily moved/manipulated Resolution independent Disadvantages  Cannot be edited at pixel level  Cannot show photo realistic scenes  Will usually require particular applications to open Vector Graphics – Pros and Cons National 5

Vector Plans Logos Promotional posters Large scale banners Bitmap What Vector and Bitmap graphics are used for National 5 Photo editing Life like pictures Computer drawing Special effects, for example, blurring and texture

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 National 5

Two methods of calculating storage space required  A graphic 300 pixels by 600 pixels and a bit depth of 16 then 300*600*16 = bits / 8 = bytes / 1024 = = 352 Kb  A 16 bit 5*7colour image with dpi of 600  (5*600)*(7*600)*16= bits/8 = /1024 = /1024 = = 24.1 Mb