Representing Images (AQA GCSE Computer Science)

Pixels

What is a bitmap?

• A bitmap image is made up of squares called pixels, meaning picture elements

• A pixel is a single point in a image

• Each pixel is stored as a binary code

• Binary codes are unique to the colour in each pixel

• A typical example of a bitmap image is a photograph

• The more colours and more detail in the image, the higher the quality of the image and the more binary that needs to be stored

Image Size & Colour Depth

What is image size?

• image size is the total amount of pixels that make up a bitmap image

• The image size is calculated by multiplying the height and width of the image (in pixels)

• In general, the higher the image size the more detail in the image (higher quality)

What is colour Depth?

• Colour depth is the number of bits stored per pixel in a bitmap image

• The colour depth is dependent on the number of colours needed in the image

• In general, the higher the colour depth the more detail in the image (higher quality)

• In a black & white image the colour depth would be 1, meaning 1 bit is enough to create a unique binary code for each colour in the image (1=white, 0=black)

• In an image with a colour depth of 2, you would have 00, 01, 10 & 11 available binary codes, so 4 colours

• As colour depth increases, so does the amount of colours available in an image

• The amount of colours can be calculated as 2ⁿ (n = colour depth) 

What is the impact of image size and colour depth?

• As the image size and/or colour depth increases, the bigger the size of the file becomes on secondary storage

• The higher the image size, the more pixels are in the image, the more bits are stored

• The higher the colour depth, the more bits per pixel are stored

• Striking a balance between quality and file size is always a consideration

Calculate Bitmap File Size

How do you calculate the size of a bitmap image?

• Calculating the size of a bitmap image is carried out with the following formula:

  • Image size x colour depth OR

  • Image width x image height x colour depth

Example

OR

Converting Between Binary Data & Bitmaps

How do you convert binary data into a bitmap image?

• To convert binary data into a bitmap image:

  • Image metadata is read

  • Using this information binary data can be mapped to individual pixels

  • A bitmap image is created

What is metadata?

• Metadata is data about data

• Metadata is additional information stored with the image, it provides context and information

• Examples of metadata that are stored are:

  • Image size

  • Colour depth

  • Author - Who created the image?

  • Date/Time - When and what time was the image created/taken?

  • Location - Where was the image taken?

Example

• A bitmap image with binary data:

• And metadata of:

  • Width: 9px

  • Height: 9px

  • Colour depth: 1 bit

• 1 bit is a monochrome image (B&W), typically 1 = black and 0 = white

• Every 9 pixels a new line is created

• The resulting image would be:

How do you convert a bitmap image into binary data?

• To convert a bitmap image into binary data:

  • Look at how many unique colours are used in the image and decide on the minimum number of bits needed to represent them (colour depth)

  • For each pixel, write down the binary code for the colour

  • Join together to create a binary bit pattern

Example

• A bitmap image

• 4 unique colours = 2 bit colour depth (white, red, green, black)

• white = 00, red = 10, green = 01, black = 11

• The binary digits can be joined together to make a binary bit pattern