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Colour Management - How It Works - What are Rendering Intents?

When using ICC Colour Management software applications,  such as Adobe Photoshop, Fujifilm ColourKit and many proofing and profiling applications, etc., you will frequently come accross the term "Rendering Intent".

But what does it mean?  This article will hopefully give you a better idea of what Rendering Intents are, and more importantly, which one to use.

All ICC Profiles contain at least one, and usually four, Rendering Intents. These define how the reproduction process should cope with colours and tones which are either outside of the devices colour gamut, or near the edge of the colour gamut, to give the desired colour "rendering".

When reproducing colour using a medium with a smaller or different shaped colour gamut than the original photograph, print or scene, it is inevitable that some colours or tones may be "lost" or degraded. These are said to be "out-of-gamut". This applies to the tonal range or "density" as well as to colours. An extreme case would be a colour transparency, which is designed to be viewed by transmitted light, which could have a maximum density range as high as D4.0, being reproduced in a newspaper with it's dull "grey" paper, where the density range may be lower than D1.2.

Clearly compromises have to be made. The Rendering Intents do this, but in quite different ways.
The Four ICC Rendering Intents are: 

Colorimetric; There are two of these: Absolute and Relative. To reproduce an image colorimetrically means as accurately as possible for colour. The down side is that colours which are out of gamut may be "mapped" to be the same, thereby losing detail by being "clipped". Dark shadow areas may well fill in. These Intents are designed for use when the original has a smaller gamut than the reproduction; i.e. when proofing. When proofing we typically have a larger colour gamut in the (usually inkjet) proofer than with the printing press we are simulating. So out of gamut colours are seldom a problem.

Absolute Colorimetric:  This will simulate the paper colour by adding a "tint" of that colour. It is therefore to be used with great caution!  It is normally only used in digital proofing. We would not advise using this for scanned images as they will probably reproduce too "flat" with heavy highlights.

Relative Colorimetric:  This will attempt to map the white point of the image "relative" to that of the original. This is usually ideal for proofing. It is sometimes used with certain reflection copy scanned images, and digital camera images. In which case use PhotoShop's Black Point Compensation or Fuji's appropriate Range Mode.

Perceptual:  Also sometimes known as "Photographic". This will "pull" out of gamut colours into gamut. Colours at  or near the edge of the gamut will also be pulled in to give a distinction between them. This will generally give a pleasing result. Different ICC-profiling software vendors may achieve different results with the Perceptual intent. In particular, profiles generated with older software versions may exhibit a lack of saturation. Perceptual should always be used for colour transparencies, and usually also gives the best results for colour prints.

Saturation:  This will pull saturated colours out to the edge of the gamut, thereby increasing saturation, or colour "strength". It is intended to be used for vector graphics; i.e. "logos", lineart, etc. Never use it for photographs or proofing!

Photoshop's Black Point Compensation:
This is found under "Colour Settings" (where you should set your defaults) or under "Image / Mode / Convert to Profile" (where uou can select it when converting individual Images from RGB to CMYK for example). It is used to automatically map the black point of an input profile to that of an output profile, when they are different. Typically the input colour space (RGB scanner or digi-camera)) will have a darker black point than that of the output . If Black Point Compensation is not used, the shadow areas could be filled-in, losing detail, in the output CMYK image. This feature is especially useful if using the Relative Intent. It should normally be left ON. 

Fujifilm ColourKit Range Modes:
These are available in ColourKit Editor and Image Processor software, and are used to modify the Absolute or Relative Rendering Intent, as appropriate, while ranging the image.  Primarily for digital camera or reflection copy images.
Absolute - Best Facsimile: 
This was designed to be used with reflection originals, giving a more "image friendly appeareance than the standard ICC mode".  However we do not like this and much prefer to use Perceptual or Relative.
Relative -  Map Image White to paper White: 
This compensates for any colour cast in the white point of the image and the paper before mapping them together.
Relative - Black and White Mapping:
The white and black points of the image are mapped into range if necessary, and any colour casts are reduced. If you are using Relative rendering intent, for reflection scans or digital camera images, we reccomend this mode.