Assessment
Assessment
Learning objective
Onderdelen in deze les
Assessment
Learning objective
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Images
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Describe bitmap image representation
Light reflecting off the object is captured through the lens of the device.
The colour of light detected at each part of the image is stored as single points known as pixels, (picture elements).
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Describe bitmap image representation
This ‘map of pixels’ is referred to as a
bitmap image.
As with any data in a computer, this must be represented as a sequence of bits.
Each pixel is represented by a binary sequence.
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Fill in the blanks
The colour of light detected at each part of the image is stored as single points known as pixels
A bitmap image is a grid of pixels
Each of these pixels in the image need to be represented by a binary sequence
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The number of bits per pixel used in a bitmap image
is referred to as its colour depth.
A large colour depth allows an image to display
more colours.
However, this results in an image requiring a larger number of bits for its representation.
Describe bitmap image representation
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Describe bitmap image representation
The shortest possible binary sequence for a pixel is 1 bit.
Each state can be used to represent a different colour.
1 = black
0 = white
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As the binary sequence for a pixel is increased, more colours can be represented.
Describe bitmap image representation
11 = green
10 = red
01 = black
00 = white
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The number of sequences, and therefore the number of colours, can be worked out as 2n where n is the number of bits.
Describe bitmap image representation
11 = green
10 = red
01 = black
00 = white
2n = 2 x 2 = 4
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How many colours can be represented by a sequence
of 4 bits?
2n
3
6
8
16
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Describe image resolution
Image resolution is the size of an image measured
in pixels.
It is measured by the number of pixels across the width and height of the image.
width
height
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Describe image resolution
Image resolution can be expressed as its pixel dimensions.
Image resolution = width x height
180 pixels
300 pixels
Image resolution = 180 x 300
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Describe image resolution
Image resolution can also be expressed by the total number of pixels in the image.
Image resolution = width x height
180 pixels
300 pixels
Image resolution = 180 x 300
Image resolution = 54,000 pixels
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Which statement is correct?
Resolution is the number of colours an image can represent.
Resolution is the total number of pixels in an image.
Resolution is the number of bits used for each pixel.
Resolution is the file size of an image.
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These properties are used to find the file size.
Calculate bitmap file size
colour depth
1111111100000000
16 bits used to represent the colour of each pixel
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The image has 240 000 pixels of 16 bits.
Calculate bitmap file size
1111111100000000
file size = resolution ✕ colour depth
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The image has 240 000 pixels of 16 bits.
Calculate bitmap file size
1111111100000000
file size = 240 000 ✕ 16
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Calculate bitmap file size
1111111100000000
file size = 3 840 000 bits
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Calculate bitmap file size
1111111100000000
file size (bytes) = 3 840 000 ÷ 8
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Calculate bitmap file size
1111111100000000
file size (bytes) = 480 000
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Text Representation
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The Problem
Computers only store: 0 and 1
But we need them to store: A B C ? ! £ 😊
Big Question:
How can a computer store text if it only understands binary?
If you had to invent a system for storing letters using numbers, what would you do?
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Encoding messages
Imagine you are designing a computer system.
You need to give each character a number:
If we encode the message
CAB!
it becomes
3125
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Character sets
This is exactly the basic idea behind character sets. The computer does not “understand” the letter. It stores a code that represents the letter.
Why would it be a problem if every computer used a different code for A?
Files and messages would not display correctly on different computers.
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But computers need binary
Our made-up code used denary numbers:
A = 1
B = 2
C = 3
But computers store numbers in binary.
So a real computer might store:
A = 01000001
B = 01000010
C = 01000011
The letter itself is not stored. The computer stores a binary number, and the character set tells the computer which character that number represents.
How many keys on a standard QWERTY Keyboard
104
87
105
91
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Representing text characters
Shaping the learning
104 keys on a keyboard
95 distinct printable characters
A computer understands only 1s and 0s, each key is given a unique 8 bit binary number. When a key is pressed the computer is sent that binary code.
Printable characters include letters, numbers, punctuation, symbols and even spaces
one key does not equal 1 character
one key can create many characters
upper case + lower case, numbers + symbols
Character sets
Shaping the learning
A character set is a set of letters, symbols and digits that can be represented by a computer
There are 2 major types we use
ASCII (standard and extended)
Unicode (16 bit and 32 bit)
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ASCII
Shaping the learning
Extended ASCII (American Standard Code for Information Interchange) has become the standard code, used worldwide.
It uses 8 bits to store all possible combinations with 256 possible options.
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The problem with ASCII
Shaping the learning
ASCII was designed mainly for English characters.
It can represent basic letters, numbers and symbols.
But the world uses many more characters:
é ñ ü أ Ж 你 £ € 😊
ASCII does not have enough space for every language and symbol.
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ASCII
Could Standard ASCII store this message?
Hello! 😊 ¿Cómo estás?
Which characters could ASCII probably store?
Which characters might need Unicode and why?
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Unicode
Shaping the learning
We invented UNICODE to solve this problem.
UNICODE uses 32 bits (2 sets of 16 bits) to represent every character in various languages around the world.
Unicode has space for up to 149,878 characters
https://symbl.cc/en/unicode-table/
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Calculate and compare text file sizes
Consider the text “ASCII and Unicode encode text.”
There are 30 characters to represent including the spaces and the full stop.
If ASCII encoding is used, each character requires 7 bits for
its representation.
Therefore, the total
file size to store this text would be 7 ✕ 30 = 210 bits
210 bits / 8 = 26.25 bytes
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In bytes, what is the file size of an ASCII text file with
the text “password” in it?
7 bytes
56 bytes
8 bytes
120 bytes
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Sound representation
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Describe sound
Sound is a pressure wave that causes air particles to vibrate.
Izzy
A microphone detects these vibrations and creates a changing electric signal that matches the vibrations.
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Describe sound
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Describe sound
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A higher amplitude makes a sound …
Higher pitched
Lower pitched
Quieter
Louder
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Drag and Drop
1
2
3
4
5
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Describe sound
Representing sound
Create a binary sequence from a sound wave
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Create a binary sequence from a sound wave
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Create a binary sequence from a sound wave
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Create a binary sequence from a sound wave
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Fill in the blanks
When digitising sound, a sample is taken of the sound wave.
A measurement of the wave is taken from a reference point to be turned into a binary sequence.
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Create a binary sequence from a sound wave
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Create a binary sequence from a sound wave
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What is measured in hertz when digitising sound?
Amplitude
Sample rate
Resolution
Frequency
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The number of bits used in digitisation is reffered to as...
Audio depth
Resolution
Sample rate
Frequency
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Calculate sound file size
Sample rates are often expressed in kilohertz (kHz)
so need converting.
Duration is often expressed in minutes so will also need to be converted.
1 kilohertz = 1000 Hz
1 minute = 60 seconds
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A 4-minute sound recording has been made using
a sample rate of 10 kHz and a sample resolution of 6 bits.
Calculate sound file size - We do
file size
=
sample resolution
sample rate
✕
duration
✕
The file size is calculated as:
file size
=
file size
=
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Calculate sound file size
file size
=
sample resolution
sample rate
✕
duration
✕
file size
=
file size
=
A 2-minute sound recording has been made using
a sample rate of 20 kHz and a sample resolution of 4 bits.
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