Representing Images (AQA GCSE Computer Science)

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

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) 

Colour Depth	Amount of Colours
1 bit	2 (B&W)
2 bit	4
4 bit	16
8 bit	256
24 bit	16,777,216 (True Colour)

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

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

Image Files

(Image size) x (Colour Depth)

Size of bitmap image =      Image size 500 x 500 Image size = width x height Colour Depth 24 bits 24 bits = 3 bytes (500x500) x 24 (500x500) x 3 = = 6,000,000 bits 750,000 bytes = 750 KB

OR

Image Files

(Image width) x (Image height) x (Colour Depth)

Size of bitmap image =      Image width 500   Image height 500   Colour Depth 24 bits 24 bits = 3 bytes (500 x 500 x 24) (500 x 500 x 3) = = 6,000,000 bits 750,000 bytes = 750 KB

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:

Binary data

111111111110111011100010001100000001100000001110000011111000111111101111111111111

• 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:

Binary data
1	1	1	1	1	1	1	1	1
1	1	0	1	1	1	0	1	1
1	0	0	0	1	0	0	0	1
1	0	0	0	0	0	0	0	1
1	0	0	0	0	0	0	0	1
1	1	0	0	0	0	0	1	1
1	1	1	0	0	0	1	1	1
1	1	1	1	0	1	1	1	1
1	1	1	1	1	1	1	1	1

Bitmap image

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