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Library 101: Digital Image Basics

Guide to best practices for cataloging images for research projects.

Definitions

Pixel

The term "pixel" is short for "Picture Element." Pixels are the small dots or squares which comprise a digital image. For example, a resolution of 640 x 480 is comprised of a matrix of 640 by 480 pixels or 307,200 pixels in total.

PPI vs. DPI

Pixels are the dots on your monitor, and dots are, the dots on paper. PPI refers to the number of pixels per inch in a digital image. DPI refers to the number of dots per inch that your printer will print on paper.

Resolution “Image Quality”

The more pixels in a digital image, the better the image quality. Image resolution is described by either the pixel width and height of the image, the number of pixels per inch or the total number of pixels in an image. Different resolutions for a quality image are required for computer screens, PowerPoint presentations and printing on paper.

Megapixels

Cameras use the term megapixels, the total number of pixels per image to define the resolution for image quality.

Bit-Depth (Tonal Gradations in a Digital Image)

Bit depth is the number of bits used to define or comprise each PIXEL. The more bits the more colour gradations and more complex image. Digital images are produced in black and white (bitonal), grayscale, or colour.

A  bitonal image consists of pixels which have 1 bit each. Two tones are depicted, usually black and white.

A grayscale image is comprised of pixels with multiple bits of information, ranging from 2 to 8 bits or more.

A color image is typically represented by a bit depth ranging from 8 to 24 or higher.  For example, the following bits illustrate how many colour values or tones are represented: 

1 bit (21) = 2 tones

2 bits (22) = 4 tones

3 bits (23) = 8 tones

4 bits (24) = 16 tones

8 bits (28) = 256 tones

16 bits (216) = 65,536 tones

24 bits (224) = 16.7 million tones