The rule used to move colours that do not fit when converting from a larger gamut to a smaller one.
ICC defines four: perceptual (compress everything to preserve relationships), relative colorimetric (keep in-gamut colours exact, clip the rest, map white to white), saturation (keep vividness for graphics) and absolute colorimetric (keep exact values including paper white, for proofing).
No intent is 'correct'. Photographs usually suit perceptual; logos and flat colour usually suit relative colorimetric.
A saturated sRGB blue converted to CMYK loses more under relative colorimetric than a gradient does under perceptual.
It decides whether out-of-gamut colours are clipped flat or everything shifts slightly to make room.
An intent is a choice, and its effect is measured by comparing colours before and after conversion, as a ΔE for each. The two colorimetric intents are defined exactly: in-gamut colours keep their values, relative to the destination's white in one case and absolutely in the other. The perceptual and saturation intents are not defined by the ICC and are left to whoever builds the profile, so two profiles for the same press can compress colours differently. Black point compensation, which scales the darkest source tone to the darkest printable one, is a separate switch that the ICC recommends alongside relative colorimetric.
| Colour | CIELAB chroma | Largest printable chroma | ΔE00 moved by clipping |
|---|---|---|---|
| Sage Green #9caf88 | 21.5 | 80.4 | 0.0 |
| Terracotta #e2725b | 54.8 | 64.5 | 0.0 |
| Orange #ffa500 | 83.8 | 81.9 | 0.4 |
| Blue #0000ff | 131.2 | 53.1 | 15.2 |
Sage green needs a chroma of 21.5 and the press can reach 80.4 at that lightness and hue, so a colorimetric conversion leaves it exactly where it is. Blue needs 131.2 against 53.1 available and is moved 15.2 ΔE00 to the gamut edge. A perceptual conversion would spread that loss, shifting the sage a little so that the blue keeps its distance from its neighbours.
How this was calculated: Colourwise's cusp-triangle approximation of the printable gamut, built from the solid and overprint patches of the published characterisation data. A full ICC profile would give somewhat different boundaries.
A photograph with a saturated sky or flower is converted with relative colorimetric.
Every tone beyond the gamut collapses onto its edge and the petals or the sky lose their modelling. Perceptual keeps the gradation at the cost of slightly less saturated colour everywhere.
A brand colour that is already printable is converted with perceptual.
The intent shifts it anyway, to make room for colours the file does not contain, and the result misses a colour the press could have hit exactly.