Representing Images

Title: Representing Images

Do Now

Previous term

List two differences between RAM and ROM

Current term

All time

What is a pixel

What is abstraction?

Do Now Answers

Previous term

RAM is volatile, ROM is non - volatile.
RAM can be read from and written to. ROM is read-only.

Current term

All time

A single dot of colour in a digital bitmap image or on a computer screen.

The process of removing unnecessary detail from a problem so that the important details can be focused on.

Computer Science Exam dates

Paper 1

15th May 2024

Paper 2

21st May 2024

Lesson Objectives

• Understand what a pixel is

• Describe the following for bitmaps: image size & colour depth. Know that the size of a bitmap image is measured in pixels (width x height).

• Calculate bitmap image file sizes based on the number of pixels and colour depth

Computers work in binary. All data must be converted into binary in order for a computer to process it. Images are no exception.

​One type of digital image is a bitmapped image. These are made up of pixels, a small square of solid colour. In many images, the pixels are too small for the human eye to see, so the colours appear to merge together.

Consider a simple black and white image. If black is represented as 0, and white as 1, then a simple black and white picture can be created using binary.

To create the picture, a grid can be set out and the squares, known as pixels, coloured (0 - black and 1 - white):

Representing images

Many images need to use colours. To add colour, more bits are required for each pixel. The number of bits determines the range of colours. This is known as an image's colour depth.

For example, using a colour depth of two, ie two bits per pixel, would allow four possible colours, such as:

• 00 - black

• 01 - dark grey

• 10 - light grey

• 11 - white

Each extra bit doubles the range of colours that are available:

• one bit per pixel (0 or 1) - two possible colours

• two bits per pixel (00 to 11) - four possible colours

• three bits per pixel (000 to 111) - eight possible colours

• four bits per pixel (0000 to 1111) - 16 possible colours

• 16 bits per pixel (0000 0000 0000 0000 to 1111 1111 1111 1111) - over 65,000 possible colours

The more colours an image requires, the more bits per pixel are needed. Therefore, the more the colour depth, the larger the image file will be.

​

Colour depth

Multiple Choice

What are images made up of?

• Pixels
• Pixies
• Pictures

Multiple Choice

The higher the colour depth - the 

• Larger the file size
• The more colours available
• Better the image quality
• All of the above

Image quality is affected by the resolution of the image.

Image resolution is the number of pixels in a specific area of an image. It is expressed as dots per inch (dpi), or pixels per inch (ppi), eg:

• 72dpi = 72 dots per inch

• 200 ppi = 200 pixels per inch

In a low-resolution image, the pixels are larger and therefore, fewer are needed to fill the space. This results in images that look blocky or pixelated. An image with a high resolution has more pixels, so it looks a lot better when it is enlarged or stretched. The higher the resolution of an image, the larger its file size will be.

​Image resolution

Multiple Choice

What is meant by the resolution of an image?

• The number of pixels in an image
• The number of bits each pixel can use to store colour data

Multiple Choice

Which is the higher resolution?

• 16 x 16
• 1080 x 720
• 8 x 8

Files contain extra data called metadata. Metadata includes data about the file itself, such as:

• file type

• date created

• author

An image file also includes metadata about the image data itself, such as:

• the height and width of the image - this defines how many rows and columns the pixels are to be arranged in

• the resolution

• the colour depth

Without this metadata, the image data would not be correctly interpreted, meaning the image could not be correctly displayed.

Metadata

The file size of a bitmap image is calculated using the number of pixels in the image, and its colour depth. The formula is:

Width × Height × Colour depth

To work out the file size in bytes instead of bits, simply divide by 8.

Example 1:

An image has pixels in a grid that is 3 columns wide and 4 rows high. The colour depth is 2 bits:

W(3) × H(4) × CD(2) = 24 bits, or 3 bytes.

Example 2:

An image has pixels in a gride that is 4 columns wide and 8 rows high. The colour depth is 3 bits:

W(4) × H(8) × CD(3) = 96 bits, or 12 bytes.

File Size

Summary of the Calculation

1. Work out how many pixels are in the image

2. Multiply the number of pixels by the colour depth

3. If the question requires, convert your answer (which is in bits) into bytes by dividing it by 8.

4. If the question requires it to be written in kilobytes or megabytes, divide your answer (in bytes) by 1000 (for kb) or 1000000 (for mb)

Multiple Choice

What is meant by the colour depth of an image?

• The number of pixels in an image
• The number of bits each pixel can use to store colour data

Multiple Choice

An image with a low colour depth will take less time to upload or download

• True
• False

Multiple Choice

How many different combinations of binary numbers can you make with a 4 bit image?

• 16

• 256

• 4

Multiple Choice

What language do pixels store colour data in?

• English
• Data
• Binary