What this resolution estimate means
A small detail spans an angle at the viewer’s eye. Under the selected model, feature spacing in inches is viewing distance × tan(detail angle). Taking its reciprocal gives an image sampling density in pixels per inch. The default is one arcminute, an explicit modeling assumption rather than a universal production rule.
At one meter and one arcminute, the result is about 87.3 PPI. A 24 × 36-inch image then requires about 2096 × 3144 pixels at the unrounded model threshold. The next listed density of 100 PPI provides a convenient round setting, not a guarantee of sharpness for every person or image.
Why text and line art can need more
Reading letters requires recognizing shapes with multiple strokes and gaps. Fine logos, small type, diagonal edges and close inspection introduce requirements beyond distinguishing a single angular feature. Use a higher target, vector artwork where appropriate and the printer’s requirements. The modeled minimum is especially unsuitable as a blanket promise for safety signage or critical lettering.
PPI, DPI and the original image
PPI describes image pixels over physical size. Printer DPI describes hardware dots and should not be substituted directly into an image-resolution model. Upsampling or editing a density tag does not restore original detail. Check the source pixels at final size and inspect a physical proof under the intended viewing conditions.
Continue the file preparation
After choosing image resolution, set the physical trim, bleed and safe area. Ink coverage is a separate step: a high pixel count says nothing about original CMYK separations, total ink, overprint or output-profile compatibility.