Bitmap Images (Cambridge (CIE) A Level Computer Science): Revision Note

Bitmap encoding

What is a bitmap?

• A bitmap image is made up of squares called pixels

• A pixel is the smallest element of a bitmap 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 vs screen resolution

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

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

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

• Screen resolution is the number of pixels a screen can display horizontally and vertically, given as width x height, e.g. 1920 x 1080, such as:

  • Computer monitors - a 1440p monitor is 2560 x 1440 pixels, compared to a 4K monitor at 3840 x 2160 pixels

  • TVs - HD (high definition) televisions are 1920 x 1080 pixels (1080p, where 1080 is the number of pixel rows), compared to newer 4K televisions at 3840 x 2160 pixels

  • YouTube - the quality button allows a user to change the video playback resolution from 144p (144 pixel rows) up to 4K

• If an image's resolution does not match the screen resolution, the image is scaled up or down to fit the screen, which can reduce its quality

Colour/bit 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

• Colour depth determines the number of colours that can be represented in an image

• The number of colours is calculated using the formula:

  • Number of colours = 2ⁿ (where n = colour depth in bits)

Calculating the size of a bitmap file

How do you calculate the size of a bitmap image?

• Estimating the size of a bitmap image can be carried out with the following formula:

  • Resolution x colour/bit depth

Example

• When bitmap images are saved, a file header is created

• This contains:

  • File type (.bmp or .jpg)

  • File size

  • Image resolution

  • Colour depth

  • Any type of compression if used

Image quality & file size

What is the relationship between image quality and file size?

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

• The higher the resolution, 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

• Compression can be used on high quality images to help reduce the file size