Storing Graphics - Shawlands Academy
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Transcript Storing Graphics - Shawlands Academy
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. American Standard Code for Information
Interchange
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
Nat 4/5
Lets look at a bitmap picture
The whole image is made up of
individual pixels.
Pixel stands for picture element
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
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
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:
1. Amount of pixels wide x Amount of pixels high
2.
This answer is in bits
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
If you already know the amount of pixels in the
image then use this method
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
(4*300) * (6*300) = 2,160,000 bits
Step
1: (Length x dpi) x (Breadth x dpi)
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
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…
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.