Rendering Intents (Gamut Mapping)
The rendering intent specifies how out-of-gamut colors of the device-independent Lab color space are to be converted into the printer gamut. This conversion is necessary because the Lab color space holds much more colors than the printer gamut.
A common task for the CMM (color management module) is to convert the image (input) into Lab and then convert it into CMYK (printer):
To the right we see visual representations of the RGB, CMYK and LAB Color spaces. LAB is the largest of the 3 and covers all colors that RGB and CMYK do, as such it is ideal as the "in between" color space.
However, as we can see in the illustration to the right, RGB and CMYK do not completely match up, certain colors in RGB can not be matched exactly in CMYK. This is called a gamut mismatch.
When such a gamut mismatch occurs, the CMM uses the rendering intent to decide how the out-of-gamut colors are handled in the conversion. Since the colors can't be matched fully, there is usually a tradeoff between different effects on color accuracy.
The listing below explains the most common rendering intents available in ErgoSoft RIP Software.
Colorimetric Rendering Intents
The absolute colorimetric and relative colorimetric rendering intents do not compress the Lab color space to the printer gamut. Colors that are within the printer gamut reproduced are reproduced exactly. Colors that are outside the printable gamut are projected into it. White color in the image will be realized by printing colored pixels in order to compensate the deviation of the media color from ideal Lab white. This media color compensation can lead to light colors being printed with unexpected combinations of ink and may look wrong to the eye.
The relative colorimetric rendering intent adjusts the reference white point (Lab white) to the white of the media. This means that white color in the image will not be printed and remain white in the output; all other light colors are printed - even when the original color tone is actually lighter than the output media. The deviation of the media color from ideal Lab white is not compensated, which can lead to some color shifting in light tones.
Perceptual Rendering Intent and Saturation
The perceptual rendering intent compresses the larger color spaces so it fits into the smaller one, while retaining the effective distance between colors (They are adjusted in relation to the black and white points of each color space). Each colors coordinates in the color space are moved to a new position, which makes exact reproduction of the colors coming from the transitional LAB color space impossible, but preserves the general impression of the colors in the image. Color gradients for example will remain gradients, but their start and endpoint colors will likely not exactly match the original ones. The most common use for the perceptual rendering intent is to reproduce images, photos etc. where the overall impression of the image is more important than matching individual colors.
Saturation is a modified version of the perceptual rendering intent that places greater importance on the saturation of colors. Using the saturation rendering intent, color gradients will be printed with higher color intensities, but become less smooth in their color transitions. This is useful for vector graphics like diagrams and charts that deal with full color areas rather than smooth transitions.
Relative Colorimetric with Black Point Compensation
This rendering intent combines the relative colorimetric rendering intent with the perceptual rendering intent. The reference white point is adjusted to that of the medium, which affects all colors in the image even if they are in gamut. Out of gamut colors are handled similar to the perceptual rendering intent, in that they are moved to the outer edges of the color space, i.e. as close as possible to their original out of gamut color value.
This results in more saturation but less texture in dark colors compared to the perceptual rendering intent and more texture in dark colors compared to the relative colorimetric rendering intent.