Image resolution

The resolution of images in a PDF document is expressed in dpi, "dots per inch", and determines the amount of information they contain.

The term dpi commonly refers both to:

• The number of pixels per inch in the digital image, which determines its quality. Less pixels per inch (a low dpi value) result in larger pixels, and therefore a poorer image.
• The number of physical dots a printer can place per inch, which determines the printer's quality.

For example, when printing an image with a resolution of 300 dpi on a high-quality printer that has a resolution of 1200 dpi:

• The image contains 300 pixels per inch in both the horizontal and vertical dimensions
• The printer can place up 1200 dots per inch both horizontally and vertically
• As a result, each pixel in the image is repro­duced on paper with 4 (horizontal) x 4 (vertical) dots = 16 dots.