Computer Systems Nat 4.5 Computing Science Data Representation Lesson 4: Representing and Storing Graphics EXTENSION
Dec 13, 2015
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
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