In a colour-managed workflow, images will need to be converted from one profile to another for various reasons. During such a conversion, colours in the source profile may not be available in the destination profile. Colours that exist in both profiles can be converted quite easily. The out-of-gamut colours need to be dealt with more carefully. The rendering intent determines how the colour conversion is performed.

We need to gain an understanding of how the rendering intents behave. Four rendering intents are available: saturation, relative colourimetric, absolute colourimetric, and perceptual.

Saturation

With the saturation rendering intent, as the name implies, the most important property of each colour is the saturation. This is the purity of a colour, and it is what we think of as having vibrant colours.

The saturation rendering intent does not change colours that are within the gamut of the destination profile. For out-of-gamut colours, the colour is changed to maintain the saturation of the colour without regard for the hue. In other words, a highly saturated green that doesn't exist in the destination profile could be converted to a highly saturated red hue if that is the closest match in terms of saturation. This could be an obvious problem for photographic images. Although the results will not be as garish as changing a colour from green to red, the important point is that the actual colour appearance can potentially change quite dramatically, causing problems for photographic images.

Because of the way the saturation profile works, it is not recommended for use with photographic images. It is designed for situations that require saturated colours where you don't care about the hue, such as when using charts in a PowerPoint presentation.

Absolute Colourimetric

The absolute colourimetric rendering intent maintains the appearance of colours that are within the gamut of the destination space, but changes the values of colours that are out of gamut to the closest reproducible hue. Saturation and lightness may be sacrificed in order to obtain the closest matching hue. Absolute colourimetric includes white in the conversion, so that the "colour" of white in the source profile is reproduced in the destination profile. That means that white in your image may not match paper white, which can cause the image to look like it has a colour cast. This is because the human visual system is incredibly capable of adapting to different light sources, always translating white to appear white. With absolute colourimetric, the eye may adapt to paper white, resulting in a colour cast if the white of the image doesn't match paper white. Black is not adjusted with the Absolute Colourimetric rendering intent.

This issue may cause you to avoid using absolute colourimetric as a rendering intent, and in general I would say that is a good decision. However, there are some situations that make it useful. For example, if you are using your printer as a proofing device of a different output method, the absolute colourimetric rendering intent can allow you to proof what white will look like in the final print, producing a more accurate proof.

Relative Colourimetric

Relative colourimetric is virtually identical to absolute colourimetric, in that it keeps colours that are within the gamut of the destination space unchanged, but it changes the value of colours that are outside that gamut to the closest matching hue. The difference with relative colourimetric is that it maps white in the source profile to white in the destination profile, so you can achieve an accurate white that is as pure as possible based on the destination profile.

Perceptual

The perceptual rendering intent attempts to retain the perception of colours within the image by maintaining the relationships between those colours. When there are colours outside the gamut of the destination profile, the colours are effectively compressed to fit within that profile. All colours are shifted so that the relationships between colours are maintained, even if the actual colours shift slightly. This can cause a loss of saturation for many colours, which can be a problem for some images. It may be necessary when there are significant colours outside the gamut of the destination profile.

The perceptual rendering intent is best suited for photographic images that contain a significant number of colours that are outside the gamut of the destination profile, because the relationships between all colours in the image will be maintained, producing the best visual result. We'll talk more about how to determine if there are significant colours that are outside the destination colour gamut later.

The Right Rendering Intent

There is no single best rendering intent. As a general rule for photographic images, I would only use the relative colourimetric or perceptual rendering intents. I tend to favour the perceptual intent for many images printed to inkjet printers, but for colour space or profile-to-profile conversions, I generally prefer relative colourimetric. There is no right answer for every situation but understand that you may want to use a different rendering intent for different images even with the same output process.