PPI (Pixels Per Inch) is the metric for pixel density in the input or digital file, indicating the quality and level of detail reproducible in graphic design software. Designers use this unit to define image clarity before entering the Prepress process.

The rule of doubling the LPI (Lines Per Inch) (or using a 1.5 to 2 factor) for PPI is a crucial rule of thumb. This ratio ensures that there is sufficient pixel data for every halftone dot, which is necessary to maintain the quality of tonal transitions and fine details in Lithography printing. This prevents quality loss or image pixelation in the output.