Digital image basics

Digital Image Basics

Digital images
Digital images are electronic photos taken of a scene or scanned from documents. These images are composed of pixels and each pixel is assigned a tonal value (black, white, shades of gray, or color).
Bit depth
Bit depth is determined by the number of bits used to define each pixel. The greater the bit depth, the larger the number of tones (grayscale or color) that can be represented. 1 bit can represent two tones, typically black and white. A 2-bit image has four possible combinations: 00, 01, 10, 11, which represent black, dark gray, light gray, and white. The number of tones that can be represented for a 2-bit image is 2= 4. In other words, an 8-bit image has 28 different tones that can be assigned to each pixel.
Black and white images
Black and white mages are made up of pixels each of which hold gray levels which span the full range from black to white. Assuming 256 gray levels, each black and white pixel can be stored in a single byte (8 bits) of memory.

28 =256

Color images

Color images are made up of pixels each of which holds three numbers corresponding to the red, green, and blue (RGB) levels of the image at a particular location. Assuming 256 levels for each RGB primary level, each  pixel can be stored in three bytes (24 bits) of memory. This corresponds to roughly 16.7 million different possible colors.

28of Red x 28 of Green x 28 of Blue = 224=16,777,216

Note that for images of the same pixel size , a black and white version will use three times less memory than a color version.

Binary or Bilevel images

Binary images use only a single bit to represent each pixel. Since a bit can only exist in two states—on or off, every pixel in a binary image must be one of two colors, usually black or white. This inability to represent intermediate shades of gray limits their usefulness in dealing with photographic images.

Resolution is the ability to distinguish fine detail. The higher the resolution, the more information the image contains. We use dpi (dots per inch) to refer to the quality of an image. A 300dpi image could be a good quality image in that you can see fine detail. However, pixels have no set size as they expand or contract to fill the space available.
Pixel dimensions
Pixel dimensions are the horizontal and vertical measurements of an image expressed in pixels. For example, an 8” x 10” photo that is scanned at 300dpi has the pixel dimension of 2400 x 3000 ( 8 x 300 = 2400, 10 x 300 = 3000).
File size

File size is determined by the number of pixels so it is important to know how many pixels the image contains. An image with higher resolution will have a bigger file size. A color image will also have a bigger file size than a grayscale image.

1 byte = 8 bits

1 kilobyte (KB) = 1024 bytes

1megabyte (MB) = 1024KB

1Gigabyte (GB) = 1024 MB

Resizing images

Use photo editing tools, like Photoshop, to change document size, pixel dimensions, or as a calculator for file size.

If you are using a Mac, you can open images in the built-in software Preview and resize your image in Tools➝ Adjust Size.



For more information:

Image pixels

Resolution and file size