Tuesday, October 25, 2011

Gamut mapping

The question came up during the conference as to differences in the size of the source and destination spaces. Here's an answer from Adobe's Jim King: "When the gamut of the source and destination colorspaces differ a "gamut mapping" needs to be performed. Those colors that cannot be represented in the destination need to be altered to colors that can be represented.

If one is working with a company logo or any other object where it is important to preserve the colors colorimetrically, then one will want to colorimetrically match source colors to matching colors in the destination. For those colors that have no match some "closest possible" match should be found. This type of gamut mapping is called "colorimetric".

Experience has shown that colorimetric gamut mapping of pictures can be improved upon and so a "perceptual" method of gamut mapping is preferred for pictures. Usually these mappings make changes even to the colors that can be matched. The human visual system is more sensitive to relative color differences than it is to absolute differences so changing the mapping to preserve those differences yields better looking results.

When producing a business graphic or other schematic material, it may be more important to have the best solid saturated colors that a device can produce than to have an accurate color that the device produces in a poorly rendered way. So for these kinds of objects one uses a "saturated" gamut mapping.

There is a correspondence between gamut mapping methods and what has come to be called "rendering intents". More about rendering intents a little later.

The ICC has agreed to a basic set of four of rendering intents shown here. For the perceptual and saturated intents considerable latitude is allowed in the interpretation and implementation since no one has demonstrated a single best method to do the gamut mapping."


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