Image representation

Published by Patrick Mutisya 10 months and 11 days ago

Image Representation


An image is a visual representation of objects, scenes, or data. On a computer, images are stored as a series of pixels, which are small colored dots or squares.


Key Terminology

  • Pixels - A pixel (short for "picture element") is the smallest unit of a digital image. Each pixel has a specific color, and when combined with other pixels, it forms an image.
  • Resolution: The number of pixels in the image (width height).
  • Color depth: The number of bits used to represent each pixel's color.
  • Compression: Reducing the file size of an image.
  • Aspect Ratio: The ratio of the width to the height of an image (e.g., 16:9).

Types of Image Representation


Images can be represented in two main ways on computers:


1.      Bitmap (Raster) Images

  • They are made up of a grid of pixels.
  • Some file formats include: BMP, PNG, GIF, JPEG.
  • Each pixel has a specific color stored in binary.
  • The higher the resolution, the more detail the image has but also the more storage it requires.
  • It is the best choice for photos and detailed images.
  • It loses quality when scaled up or down (becomes pixelated).


2.      Vector Images

  • Made up of mathematical paths and shapes like lines, curves, and polygons.
  • Some file formats include SVG, EPS, PDF.
  • The computer uses mathematical equations to draw the image, so no pixels are involved.
  • They Can be resized without losing quality (scalable).
  • They are better for logos or illustrations.
  • They are not suitable for detailed images like photographs.

 

How Pixels Store Colors

  • Computers use binary to store information, so colors must be represented using numbers.
  • Common methods for storing colors:

 

A. Monochrome (1-bit)

  • The image is black and white.
  • Each pixel is represented by a single bit: 0 for black, 1 for white.

 

B. Grayscale (8-bit)

  • An image contains shades of gray, from black to white.
  • Each pixel is represented by 8 bits (1 byte), allowing 256 shades of gray (from 0 to 255).

 

C. True Color (24-bit)

  • Each pixel contains a color made up of three components: Red, Green, and Blue (RGB).
  • Each component is represented by 8 bits, so the full pixel is represented by 24 bits (3 bytes).
  •  This allows for over 16 million possible colors (256 values for each of the 3 components: 256 256 256 = 16,777,216).

 

Image File Size calculation

  • The size of an image file depends on Resolution and Color depth
  • Formula for file size:

File Size (in bits) = Width Height Color Depth (in bits per pixel)

 

Image Compression


Images are often compressed to reduce file size. There are two types:


A. Lossy Compression

  • Some data is lost during compression to reduce file size.
  • An example file format is JPEG.
  • They have Smaller file sizes.
  • Image quality is reduced.

B. Lossless Compression

  • No data is lost; the image can be restored exactly as it was.
  • An example file format is PNG, GIF.
  • There is no loss in image quality.
  • They have larger file sizes compared to lossy compression.