The problem being addressed by ICM is that different devices and media have different ranges of hue, saturation and intensity/brightness. They also may have different neutral and white points and may create colors either by adding colors to black or by subtracting them from white. Without proper translation, a color image moved from one device to another is likely to be altered in disconcerting ways.
ICM works by correcting color inputs to a "neutral" representation that attempts to represent true color intensity, saturation and hue. It uses a Red-Green-Blue representation that is easily converted to the Cyan-Magenta-Yellow representation used by subtractive devices. Image outputs are translated to the colors appropriate for the output device. Profiles describing how to convert the device's native color to or from neutral are either built into the device or provided to the OS as .ICM files. Strings of conversions ("links") are supported that, in principal, allow a complete set of conversions e.g. scanned image to screen to printer to be specified without tinkering with settings for each individual image.
There are several different methods of conversion supported. One approach is to map color components so that the range of each is preserved even though the absolute values may change. This is known as "picture intent" and tends to look 'natural' even though the actual color fidelity may be poor. It is generally recommended for photographs and similar material.
Another approach is to try to map the colors accurately and to alter colors that can not be represented correctly to something that can. This is called "proof intent" or "colorimetricly accurate" conversion. The images may look odd even though most of the colors are more accurately reproduced than in other conversions. It often used for proofing material that will eventually be output to some other device/medium.
A third approach is to adjust hue and brightness while trying to represent saturation as accurately as possible. This preserves contrast differences and usually yields acceptable colors even if they may not be exactly the colors in the original. is called Graphic Intent and is generally used for presentation graphics.
There is lots of material on the wonders of ICM. There is remarkably little on the nuts and bolts of configuring and using it.
Return To Index Copyright 1994-2002 by Donald Kenney.