Rendering intent

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With the rendering intent ( rendering target or also referred to as "priority"), the user of a color management system determines which conversion method is to be used. The conversion of an image from one color space to another is called gamut mapping . It should be mentioned that two color spaces are not converted directly into one another, but instead take the detour via a media-neutral connection color space (PCS = Profile Connection Space) in order to avoid additional color distortions.

Four different rendering intents (RIs) were defined by the ICC (International Color Consortium) : perception-oriented (= perceptual = photographic), relative colorimetric, absolute colorimetric (or colorimetric) and saturation-preserving (= presentation). In order to understand why, despite the same RIs, different results occur when using different programs for profiling, one must know that the RIs are not standardized. They do not correspond to any specific calculation rule or formula. So are z. Z. (mid-2008) predominantly mixed forms of RIs in circulation, which can best be described as intermediate stages of the individual RIs.

The ICC divides the RIs into two categories: A) re-purposing (new purpose, this includes perceptual and saturation-maintaining) and B) re-targeting (other print edition / proof , this includes the two colorimetric).

The names are in flux, as the subject is still relatively young.

use

New application

(Re-purposing, especially conversion from a larger to a smaller RGB color profile)

Perceptual : This RI transforms the image in a visually appealing manner by maintaining the relative visual relationships between the source colors. The human eye reacts more to shifts between the source colors than to a smooth change in which the relationships of the source colors remain. This is the ideal method for documents with a large color gamut, as the relationship between the colors inside and outside the color gamut is preserved.
When converting from a larger to a smaller color space , all color distances are compressed, but the saturated areas are compressed more than the less saturated ones, since the human perception of color perceives differences between less saturated or neutral (gray) hues much more strongly than between saturated colors same shade. So the compression is non-linear. The gamut mapping is determined when the ICC profile is created, so it is independent of the motif. Under certain circumstances, this can lead to subjects with little chroma being unnecessarily compressed. This RI is particularly useful when converting images to sRGB for web output.

Saturation-preserving (saturation): This conversion is primarily used to obtain the most saturated colors possible in presentation graphics (pie or bar charts, etc.). Maximum saturation is sought, even with the acceptance of hue shifts, but the color accuracy is considerably impaired.

Different printing method

(Re-targeting, used for converting RGB to CMYK, and for digital proofing )

Absolute colorimetric (ICC-absolute colorimetric): When converting from a larger to a smaller color space, the colors that are outside the target color space are mapped onto its envelope (clipping). The colors within both color spaces are not changed. This process is particularly interesting for digital proofs, wherein the proof printer (typically an ink jet printer having 6 or more colors) other printing processes (eg. CMYK - offset ) simulated. Absolute colorimetric reproduction ensures that the proof shows all colors exactly as the subsequent print result. This process is mainly used when the proof is sent to the printer as a template. Since the white space must not be adjusted, ideally the edition paper must be used as proof paper or have the same degree of whiteness. A reliable proof must not have any clipping. The gamut of the printing process to be simulated must therefore lie completely within the proof printer gamut. Output from file to printing plate is also carried out using this method.

Relative colorimetric (media-relative colorimetric): The conversion is carried out in a similar way to the absolute colorimetric rendering intent, but here there is a paper white adjustment. All colors of the source color space are first shifted so that the white points of the two media coincide. Colors that lie outside the target color space are then - similar to the absolute colorimetric intent - cropped, but while maintaining the relative color location distances and differences in brightness. This method is mainly used for layout prints and when converting images from a larger to a smaller gamut, such as from RGB to CMYK. In contrast to perceptual, the brightness differentiations are better preserved. This method is therefore characterized by the lowest loss of quality and is the default setting in current image processing programs.

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