Technical Specifications 3 of 7
Resolution
PNG fundamentally contains pixels. It may also contain a pHYs chunk describing pixels per unit along the horizontal and vertical axes. When its unit is the metre, software can derive an intended physical resolution. When the unit is unknown, pHYs describes only pixel aspect ratio. If pHYs is absent, physical pixel size is unspecified.
Resolution metadata does not increase image detail. The production-relevant value is effective resolution: the number of actual pixels divided by the intended physical output size.
When evaluating resolution:
- Use actual pixel dimensions, not only the PPI value shown by an editor
- Calculate at the final production size
- Follow the maker's resolution range for the selected method and product
- Inspect fine text, edges, and tonal detail at actual pixels
- Do not assume that resampling restores missing source detail
Effective resolution examples
| Pixel dimension | Output dimension | Effective resolution |
|---|---|---|
| 3000 px | 10 in | 300 pixels per inch |
| 3000 px | 20 in | 150 pixels per inch |
| 1200 px | 4 in | 300 pixels per inch |
| 1200 px | 12 in | 100 pixels per inch |