Computing
Computer Science LearnITWithMrC ⛯ Year 7 Year 8 Year 9 GCSE
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Lesson Video

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Lesson Tasks

  • Watch the Lesson video
    Make notes if needed.
  • Open your Learning Journal
    Complete Task 1 in your Learning Journal
  • Complete the learning activities
    Make sure you complete the book tasks in your Unit Booklet
  • Complete End of Task Assessment
    Update your learning objectives

What do I need to Learn?

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I need to learn how to describe how the size of a bitmap image is measured in pixels (width x height)
I need to learn how to describe how a bitmap represents an image using pixels and colour depth.
I need to learn how to explain how the number of pixels and colour depth can affect the file size of a bitmap image.
I need to learn how to calculate bitmap image file sizes based on the number of pixels and colour depth.
Convert binary data into a bitmap image.
I need to learn how to convert a black and white image into binary data.

Key Terms

pixel resolution pixels per unit dots per inch (dpi) colour depth bit depth metadata width and height colour map

Task 1 - Getting organised Click to see more


Task: Learning Journal

Open your Learning Journal by clicking on the image below

Learning Journal

Good notes will help you organise and process data and information

Task 2 - Adding colour Click to see more

Colour depth

The system described so far is fine for black and white images, but most images need to use colours as well. Instead of using just 0 and 1, using four possible numbers will allow an image to use four colours. In binary this can be represented using two bits per pixel:

  • 00 – white
  • 01 – blue
  • 10 – green
  • 11 – red

    • While this is still not a very large range of colours, adding another binary digit will double the number of colours that are available:

  • 1 bit per pixel (0 or 1): two possible colours
  • 2 bits per pixel (00 to 11): four possible colours
  • 3 bits per pixel (000 to 111): eight possible colours
  • 4 bits per pixel (0000 – 1111): 16 possible colours
    • …
  • 16 bits per pixel (0000 0000 0000 0000 – 1111 1111 1111 1111): over 65 000 possible colours

    • What is colour depth and how does it effect the size of an image file?

    Colour
    depth in bits
    (n)    		 Number
    of possible
    colours
    (2n)    		 Example
    binary code(s) used to
    store the colour information
    about each pixel
    1 2 0, 1
    2 4 00, 01, 10, 11
    3 8 000, 001, 111 etc.
    4 14 0000, 0001, 1110 etc.
    8 256 10010101, 11101101 etc.
    24 16,777,216 010110101110001101001101

    Colour depth describes the number of bits of memory that are used to store the colour information about each pixel in a bitmap image.


    1 bit colour depth image - allowing 2 colours

    With 1 bit colour depth the number of bits used to store the information about each pixel is 1. This allows 2 colours, represented by 0 or 1.

    3 bit colour depth image - allowing 8 colours

    With 3 bit colour depth the number of bits used to store the information about each pixel is 3. This allows 8 colours, represented by the binary codes 000, 001, 010, 011, 100, 101, 110 or 111.

    SUMMARY: The greater the colour depth of a bitmap image, the greater the file size because more memory is used to store the colour data about each pixel.

    Task: Colour Depth

    Open your student workbook at page 38 Read through the notes then complete the following tasks in your book.
    1. Task 20
    2. Task 21

  • When you are finished screenshot your answer into your Learning Journal

    • Task 3 - Calculating file size Click to see more

    The key formula for working out an image file size is:
    File size = width * height * bit depth + metadata

    Metadata

    Why does metadata need to be included in an image file?

    A typical example of the metadata stored in a digital image
    A typical example of the metadata stored in a digital image

    Metadata is needed in a bitmap image file because the software that displays an image needs to know:

    • The height and width of the image – so each line of the image starts in the correct place.
    • The resolution – so the image displays at the correct size.
    • The colour depth – so the correct number of bits are used to represent the colour of each pixel.

    Colour depth

    Below you can see the effect of colour depth on both the quality of the image and the file size.

    What is resolution?

    Resolution is a measure of how much detail there is in an image.
    In a bitmap image the resolution depends on the pixel density, the number of pixels per unit (not the total number of pixels in the image). The resolution depth of a screen image is usually measured in pixels per inch (PPI). The resolution depth of a printed image is usually measured in dots per inch (DPI).
    The effect of zooming in on a high and a low resolution bitmap image

    The effect of zooming in on a high and a low resolution bitmap image

    A high resolution image can be magnified and still stay sharp. A low resolution image will appear pixelated (the individual pixels will be clearly visible) if it is magnified to the same size as a high resolution image.

    SUMMARY: A bitmap image with a high resolution will have a greater the file size than the equivalent low resolution image because more memory is used to store the colour data of the extra pixels.

    Because vector images are only stored as a set of mathematical instructions, halving the size of a vector image would effectively make no difference to the file size. However, halving the dimensions of a bitmap image would effectively make the file size ¼ of the original.

    File sizes

    Task: Calculating Image Size

    Open your student workbook at page 42 Read through the notes then complete the following task in your book.
    1. Task 22

  • When you are finished screenshot your answer into your Learning Journal

    • Task 4 - Converting Binary into Bitmap Click to see more

    Decryption 

    Binary grid


    Characters used to be designed on an 8 x 8 grid, and created using eight numbers from 0-255.  Each number was converted to binaryand the resulting pattern of 0s and 1s was used to make a pattern of black and white dots on a single row.

    128
    64
    32
    16
    8
    4
    2
    1

    Design a character by entering numbers from 0-255 in the boxes to the left of the grid.  Each number is converted to binary and used to create the pattern of dots.

    Task:

    Open your student workbook at page 43 Read through the notes then complete the following tasks in your book.
    1. Task 23
    2. Task 24

  • When you are finished screenshot your answer into your Learning Journal

    • Task 5 - Representing images - End of Chapter recap Click to see more

    Task:

    Open your student workbook at page 46 Complete the end of chapter recap task below.
    1. Task 25

  • When you are finished screenshot your answer into your Learning Journal

    • Task 6 - Lesson Review/Homework Click to see more

    Complete any unfinished tasks

    Summing it all up

    Lets look at the learning outcomes and decide which one best describes our current level of understanding :

    Tick the one you feel is closest to your level

    Learning Outcomes

    • I have a basic understanding of how with a little help from my teacher
    • I can show my teacher that without their help.
    • independently and I can also explain it to others and can complete any extension tasks I am given.

    🠜 Now update your learning objectivesClick on the Assessment image

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    Lesson Notes

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    Revision

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    Activity

    Bitmap Images