Dots per inch (dpi) measures how many image dots fit along one inch. It gives a physical way to talk about resolution: pack dots farther apart and their individual marks can become visible; pack them tightly enough and the eye sees a smooth image instead. section 1.2
Dpi is measured in one direction, but a printed area has dots in two directions. A 1,200 dpi printer places 1,200 × 1,200 = 1,440,000 dots in one square inch. On a US Letter page, that works out to 1,200 × 1,200 × 8.5 × 11 = 134,640,000 black-or-white dots. section 1.2
Why the same image can need different dpi
A printer may use 600 or 1,200 dpi, while a computer or tablet screen may use 100 to 300 dpi and show shades of gray or color. What counts as enough resolution also depends on viewing distance: a page seen at 12 inches may need 600 dpi, while a distant billboard may need only 20 or 50 dpi. section 1.2 section 1.3
Figure 1.3 Three different resolutions
Compare the same capital A at 60, 30, and 15 dpi to see how reducing dot density makes its edges increasingly blocky. section 1.2
More dpi is not the only route to a smoother-looking mark. Screens can use gray or colored pixels, and anti-aliasing uses shades of gray to make edges appear smoother at resolutions that would otherwise look rough. section 1.2 section 1.3
