Colour management on Canvas Prints

The diagram below, taken from Jonathan Sachs’ excellent tutorial on Color Management for computers and is a simplified illustration of a color-managed workflow.Programs that utilize color management are described as being ICM-aware, where ICM stands for Image Color Management. ICM standards are maintained by the International Color Consortium.
In a color-managed workflow, the color response of each device, each image file, and each image in the computer’s active memory is characterized by a file called an ICC profile. ICC profiles have the extension “.icm” and are stored in specific locations on Windows/Mac computers.

ICC Profiles consist primarily of tables that relate numeric data, for example, RGB (222,34,12), to colors expressed in a device-independent CIE color space called a Profile Connection Space (PCS)– either CIE-XYZ or CIELAB. The colors may be the objects sensed by a scanner or produced by a printer or monitor. They can also refer to one of the numerous color spaces. Monitor profiles have the same format as color space profiles. Profiles may contain additional data, such as a preferred rendering intent and gamma, Monitor profiles often contain instructions for loading video card lookup tables, i.e., for calibrating the monitor.

The heart of color management is the translation or gamut mapping between devices with different color gamuts and files with different color spaces. Mapping functions are shown in the yellow boxes in the illustration, above. They are performed by a Color Matching Module or Method (CMM), also called a Color Engine, using data in the profiles. The CMM combines the input and output profiles, both of which are referenced to a PCS, to perform a direct conversion between the devices or color spaces. It interpolates data in printer profile tables, which would be prohibitively large if all possible color values were included.

Gamut mapping is performed with one of the four rendering intents (gamut mapping algorithms) recognized by the ICC standard and by Windows ICM 2.0. The rendering intent determines how colors are handled that are present in the source but out of gamut in the destination. Since there are several nomenclatures for gamut mapping, I use a color code to distinguish the sources: ICC, Windows ICM 2.0, Picture Window Pro. I’ll generally stick with the ICC nomenclature.

1. Perceptual, also called Picture or Maintain Full Gamut. This is PW Pro’s default, and is generally recommended for photographic images. The color gamut is expanded or compressed when moving between color spaces to maintain consistent overall appearance. Low saturation colors are changed very little. More saturated colors within the gamuts of both spaces may be altered to differentiate them from saturated colors outside the smaller gamut space. In the diagram on the right, the left and right of the color space blocks represents saturated colors; the middle represents neutral gray. Perceptual rendering applies the same gamut compression to all images, even when the image contains no significant out-of-gamut colors. Bruce Fraser points out that for an image with unsaturated colors, e.g., with pastels, Relative Colorimetric rendering may produce a slightly more accurate result. Perceptual gamut mapping is mostly reversible; it is most accurate in 48-bit color. None of the other rendering intents is reversible.

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2. Relative Colorimetric, also called Proof or Preserve Identical Color and White Point. Reproduces in-gamut colors exactly and clips out-of-gamut colors to the nearest reproducible hue. Not reversible. See diagram. Bruce Fraser says, “Look at the relative gamuts of your source and destination: The same image may need different rendering intents for different output process. For example, an image might benefit from perceptual rendering when printed to an inkjet printer, but when the same image is going out to the much larger gamut of a film recorder, relative colorimetric rendering might work much better. If an image doesn’t contain any important strongly saturated colors, you’ll probably get a better result using relative colorimetric rendering than you would using perceptual.”

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3. Absolute Colorimetric, also called Match or Preserve Identical Colors. Reproduces in-gamut colors exactly and clips out-of-gamut colors to the nearest reproducible hue, sacrificing saturation and possibly lightness. On tinted papers, whites may be darkened to keep the hue identical to the original. For example, cyan may be added to the white of a cream-colored paper, effectively darkening the image. Rarely of interest to photographers.
4. Saturation, also called Graphic or Preserve Saturation. Maps the saturated primary colors in the source to saturated primary colors in the destination, neglecting differences in hue, saturation, or lightness. For block graphics; rarely of interest to photographers.

~ by canvasprints on March 20, 2008.