Fundamentals of Rendering Intents and Black Point Compensation
Rendering Intents
These
graphs illustrate one of the main characteristics of rendering intents:
brightness/luminance (L) scaling. This scaling occurs from an input luminance
dynamic range (L0-100) to an output device's exemplary L dynamic range
(L20-90).
In short:
1. The Absolute Colorimetric
RI is intended to reproduce an exact copy of the original. Whatever is in the
dynamic range of the printer will be replicated unchanged. Whatever is above or
below the device dynamic range, will be mapped to the maximum and minimum
luminance limits. Highlight and shadow details will be lost.
2. The purpose of the Relative
Colorimetric RI is very similar to Absolute. High color reproducibility
capabilities but mapping the source white point with the media white point. The
luminosity is linearly scaled. Shadow details will be lost, but details
highlights are kept.
3. The Perceptual and
Saturation RI, in terms of scaling the luminance/brightness dynamic range
are similar. A tone curve compresses details of highlights and shadows, usually
luminance values in mid-tones can be preserved. Losses in highlights and
shadows can be prevented.
4. Relative Colorimetric with
Black Point Compensation: In addition to the
mapping of the source and destination white points, also the black points are
mapped. Between the white and black point, the mapping is linearly scaled. No
details in highlights and shadows are lost but compared to the Perceptual Rendering
Intent the general contrast might be reduced.
ColorGPS - Rendering Intents
We know
that a conventional LUT-based ICC profile contains three dedicated tables:
Perceptual, Saturation, and Colorimetric LUT. The purposes of these four
Rendering Intents are common knowledge.
The Relative Colorimetric Rendering
Intent, along with its derivative Absolute Colorimetric (generated
by the CMS using the media white point), has gained increasing importance in
the printing industry.
After Adobe® invented Black Point Compensation (BPC) which is no official
standard, the relative colorimetric rendering with BPC became the preferred
choice of most users. It also plays an important role in the presets of today's
color management settings — Ergosoft is no exception.
It is
important to understand that, while CMSs are highly standardized and defined,
ICC profiles are also subject to standards that cover rendering intentions, although
in a more general manner. The application purpose of each one is specified, and
techniques are described in a general way. However, no strict rules or
mandatory algorithms are published. This means that the implementation of the Rendering
Intents and the calculation of their tables are the responsibility of the
software manufacturers of the ICC profile generators. The (Absolute and
Relative) Colorimetric Rendering Intent is the only one that clearly specifies
how to implement it. Depending on the ICC profile origin and its manufacturer,
there may be significant variations in both the Perceptual and the Saturation
Rendering Intent.
When
standard ICC profile generation settings are used, Ergosoft’s ColorGPS produces
distinct Rendering Intent tables with clearly noticeable differences.
- *Perceptual*rendering
differs significantly from *Relative Colorimetric with Black Point
Compensation (BPC)*. The smaller the output gamut, the more pronounced the
difference between these two intents.
- Differences
between *Perceptual* and *Saturation* intents are also visible.
However, from a technical perspective, the Saturation intent is derived from
the Perceptual intent within Ergosoft.
The purpose of offering multiple rendering intents is to give users a broader range of visual options. While color accuracy is essential, color perception in real-world applications is often highly subjective.
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