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 refers to the total amount of pixels horizontally in a display, such as:

  • Computer monitors - 1440p means 1440 pixels horizontally compared to 4K which is 3840 pixels (roughly 4 thousand)

  • TVs - HD (high definition) televisions have a screen resolution of 1080p, 1080 pixels horizontally compared to newer 4K televisions with 3840 pixels

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

• Another consideration of screen resolution is the physical size of the of the display

• The number of pixels per square inch (PPI) is known as pixel density

• Pixel density can mean images to need to be scaled up or down to fit, sometimes causing loss in 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 can also refer to the number of colours that can be represented in an image

• It is calculated using the formula:

  • Colour depth = 2ⁿ (where n = number of 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