Rendering Intent is a term to describe how colors both inside and outside the gamut of the printer are 'mapped' into the colors available on the printer (as described by the printer's ICC profile). The range of colors that a printer or monitor can show or that a scanner or digital camera can capture varies from device to device. It also varies with the whiteness of the paper, in the case of a printer.
The effect of these rendering intents can be subtle or nearly invisible on many images, as rendering has more impact on colors that fall outside the gamut of the target output device, and less on those colors inside the gamut. Rendering intent can have a profound effect on one image while having a more subtle (or invisible) effect on another image with different color content.
Compare these two poppies: the one on the left shows an original photograph with a full range of oranges and yellows; that on the right represents a possible rendering on a device with a much reduced color gamut. Colors that cannot be represented faithfully are replaced by the closest match. Of course, the effect here is exaggerated. The result is that the image on the right has lost much of the subtle detail.
The International Color Consortium (ICC) has established four rendering intents for processing images in a color managed work flow. The four rendering intents provide for proofing, reproduction of artwork, precision in color processing, and saturation for business graphics. Each rendering intent has a purpose.
All colors are mapped to points within the destination gamut. The source gamut is compressed linearly or nonlinearly to fit into the destination gamut. All colors are affected, not just out-of-gamut colors.
Perceptual rendering does not render colors as precisely as relative colorimetric rendering does. Instead, perceptual makes 'pleasing color' images from source to destination, possibly compromising hue, brightness and saturation slightly in the process. Colors are mapped smoothly, avoiding the loss of distinction between similar shades, which other rendering intents may map to the same color.
It is a useful default rendering when the precision of color is not as important as the overall appearance, for example for photographic images.
Perceptual rendering
Relative colorimetric
Relative Colorimetric rendering is designed to map out-of-gamut colors to their closest equivalent that is in-gamut, while sacrificing hue, lightness and saturation as necessary. The effect of relative colorimetric rendering is more precise color rendering for in-gamut colors, as they remain unchanged. However, out-of-gamut colors may incur a heavy penalty in terms of accuracy Where Perceptual rendering yields an image that 'looks good' both in terms of hue and saturation, Relative Colorimetric rendering will map the out-of-gamut colors to the closest in-gamut equivalent.
Relative Colorimetric renders the 'white' of an image to absolute (media) white, making the resulting image appear as 'bright' as possible. In this respect, slight changes may occur to the lightness of all colors, as the lightness is 'stretched' or 'compressed' to fit the gamut of the output media.
Relative colorimetric rendering
Absolute colorimetric
Absolute Colorimetric rendering behaves the same as Relative Colorimetric rendering except that no compression or stretching of the lightness occurs to fit the gamut of the output media. If the source image contains colors that are lighter than the media's white, they will be clipped to the output media's white. If the source images white is darker than the media white, it will be printed with some ink in order to simulate the input white as precisely as possible.
This means that the whitest 'white' mapped by the absolute method is the white of the media measured when the profile was made. Proofing for printing processes that use non-white media (like newsprint) is more effective with Absolute Colorimetric rendering because the measured paper white is the 'whitest' color rendered in the resulting image.
The resulting proof is more representative of the actual printed product than any of the other rendering intents. It is the most effective rendering intent for spot colors, as long as they are within the output media's gamut.
Due to the potential for clipping or printing image white, Relative colorimetric is preferred over Absolute colorimetric for comparable input and output white points.
Absolute colorimetric rendering
Saturation
Saturation rendering has as its purpose the rendering of saturated color to the most saturated equivalent within the gamut of the output profile. Designed mostly for business graphics (illustrations, charts, graphs, etc.), saturation rendering maps out-of-gamut colors to the most saturated color, occasionally sacrificing the accuracy of the color in the process in order to maintain saturation. The saturation rendering intent is often used to process images where solids are favored by the software. Color precision is sacrificed for color saturation.