Computer Systems

Nat 4.5 Computing ScienceData RepresentationLesson 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.

REVISION

Nat 4/5

Revision:1. Store as a series of bits, 1’s for black and 0’s

for white.

2. 300*700/8/1024=25.63 = 26Kb

3. (3*600)*(5*600)/8/1024= 659.17 =660Kb

4. Bitmap stored as pixels and Vector stored by it’s attributes.

ANSWERS

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 BitmapsThe 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 BitmapsEach 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 ColourTrue 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. 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 bitsStep 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

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 = 4.119 Mb

National 5

AdvantagesCan be manipulated at

pixel levelCan 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

AdvantagesDo not lose quality

when scaledRequire less storage

spaceObjects are easily

moved/manipulatedResolution

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

VectorPlansLogosPromotional postersLarge scale banners

Bitmap

What Vector and Bitmap graphics are used for

National 5

Photo editingLife like picturesComputer drawingSpecial 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 = 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