DPI metadata sets an intended physical density
JPEG and PNG can store resolution metadata that layout and print software may use to choose a default physical size. The number is metadata; it does not describe extra hidden pixels.
Changing DPI should not secretly resample the image
If a 3000 × 2000 image is rewritten from 72 DPI to 300 DPI without resampling, it is still exactly 3000 × 2000 pixels. Only the suggested physical density changes.
- 3000 pixels ÷ 300 PPI = 10 inches.
- 3000 pixels ÷ 150 PPI = 20 inches.
- The first print has more source pixels per inch because it is physically smaller.
PNG and JPEG store density differently
PNG commonly uses a pHYs chunk expressed as pixels per meter. JPEG commonly exposes JFIF density fields and may also contain EXIF resolution metadata. Different applications can prioritize metadata differently, so important print jobs should still be checked in the destination application.
Use effective PPI to judge print quality
A printer asking for a 300-DPI file usually needs enough pixels for the requested physical size. Rewriting a metadata value cannot make a small or blurry image suitable for a large print.
Keep the original and verify the rewritten copy
OpenFileTools preserves the encoded pixel data and writes a new download. Keep the original, then inspect the new file's metadata and final physical size in the print/layout workflow that requested the DPI value.