Image resolution: check pixels at printed size

Native and effective resolution

An image has pixel width and height. The PDF then places it in an area expressed in points or physical units. Effective resolution comes from that relationship: the same image may be sufficient at 40 mm and insufficient at 200 mm.

Preflight must account for transformation matrices, crops and repeated use. One image can therefore have several effective resolutions in the same document.

Choose a threshold for the production condition

Close-viewed work on coated stock often needs more detail than a poster seen from a distance. Grayscale, color and one-bit images may also require different thresholds.

The right threshold belongs in the production profile. It should reflect process, screening, viewing distance and commercial tolerance instead of applying one universal number to every file.

What the report should show

A useful alert identifies the page and occurrence, reports the measured effective value and repeats the expected value. A generic “low-resolution image” message forces the operator to investigate again.

• Affected page and occurrence.
• Pixel dimensions and placement size.
• Horizontal and vertical effective resolution.
• Profile threshold and measured difference.
• Blocking, warning or REVIEW status for the job.

Why upsampling is not a safe fix

Artificially adding pixels can smooth or sharpen an image, but it does not faithfully restore missing detail. Conservative preflight automation should expose the risk rather than silently create an image that is larger without containing more information.

A reliable correction normally means replacing the asset with a suitable version, reducing printed size or explicitly accepting the risk for that product.

Use resolution in a READY / REVIEW workflow

A blocking threshold can route insufficient occurrences to REVIEW with evidence. Images above threshold pass without intervention; near-limit cases may remain warnings when the profile allows it.

Store the decision with the profile version so a later threshold change cannot reinterpret an older job retroactively.