JPEG artifacts in a print file: the damage that resizing cannot undo
A JPEG discards detail in order to be small, and the parts it discards do not come back. What arrives instead is structure the artwork never had: square blocks in flat areas, and a faint echo along every hard edge. On a screen these hide inside the image. Enlarged onto fabric they become texture, and texture in a flat colour field is what a customer notices first.
| Compression block size | 8 × 8 pixels — fixed by the format, regardless of image size |
|---|---|
| An 8 px block printed at 300 DPI | 0.68 mm across |
| The same block at 150 DPI | 1.35 mm across — now readable as a square |
| Where blocks show first | large flat areas: a solid background, a broad letterform |
| Where ringing shows first | high-contrast edges: black text on white, logo outlines |
Why re-saving makes it worse and re-sizing does not help
Each save quantises what the previous save already quantised, so the damage compounds while the file gets no smaller in any useful sense. Resizing then interpolates between damaged pixels: the blocks become soft-edged blocks rather than absent ones, and the ringing spreads over more millimetres than it started with.
What is recoverable and what is not
Damage that is uniform can sometimes be reduced without touching the artwork. Damage that overlaps real detail cannot be separated from it, because at that point the artifact and the design occupy the same pixels.
- Reducible: blocking in a flat field with no detail in it
- Reducible: mild ringing along an edge that is otherwise sharp
- Not reducible: fine detail that was compressed away entirely
- Not reducible: text small enough that its strokes and the ringing are the same width
The version to ask for
Any format that does not re-encode on save answers this: PNG for raster artwork, the source document if there is one, the vector if the design began as one. What matters is that the file has been through the compressor once at most, not that it is large.
FAQ
- Can artifacts be told apart from the design?
- Sometimes. Blocking follows a fixed 8 × 8 grid that the artwork has no reason to align with, and that regularity is what makes it measurable.
- Does a high-quality JPEG avoid the problem?
- It reduces it rather than removing it. The format still works in blocks, and enlargement still turns those blocks into millimetres.
- Is a screenshot of a JPEG better or worse?
- Worse in general: the screenshot fixes the artifacts into new pixels at screen resolution, so the damage is now baked in at a lower resolution than the original.