Gamut comparisons with the HLC Colour Atlas

Gamut comparisons with the HLC Colour Atlas – which colours can be produced in which process?

The free PDF version of the HLC Colour Atlas shows the gamuts of different output situations. The CIELAB colour space is systematically run through in H/L/C with a constant step width. This method is exact, practical and meaningful. This article shows some interesting facts about the gamut comparison, which can be deepened much further with the pdf version.

CIELAB coordinate space and CIELAB color bodies

With the free colour HLC Colour Atlas XL the CIELAB colour space is divided into its polar coordinates HLC. If one divides H (interval 0..360), L (interval 0..100) and C (interval 0..125) into increments of 5, 32851 coordinates result.

CIELAB HLC coordinate space
View from above of the CIELAB HLC coordinate space used in the HLC Colour Atlas.

Approx. 40% of this coordinate space, however, are not color tones! The range of real existing color tones, the CIELAB color body, is limited by the so-called optimal or rectangular colors. These are spectral curves which show a reflection of 100% in one wavelength interval and 0% in all other intervals. In the article “Borders of the CIELAB color space” the facts are explained in more detail.

CIELAB rectangular colors
The optimal colors (here: 10nm intervals) form the outer shell of the CIELAB color space. The CIELAB color space is not only a part of the coordinate space, but also protrudes beyond it in some areas.
CIELAB color space
CIELAB color space in finer resolution (1nm optimum colors) with additional division into 20 brightness levels (L=5..100). (Image: Prof. Bernhard Hill, Aachen)

The CIELAB colour body has a very irregular shape. In the green area it reaches a maximum a-value of approx. -160, which goes beyond the a-interval in software products mentioned above, so this colour range cannot be set in the usual products (Adobe CC and others). However, this circumstance is usually not serious, since the highly intensive color areas in question can hardly be displayed in any output process close to the rainbow colors.

Gamuts of important output variants

“Gamuts” answer the question: which colors can be displayed in a certain output situation – where are the limits? It is clear, for example, that printing on newsprint can produce far less vivid colour tones than printing on a good monitor. The gamut production of the HLC Colour Atlas happened for most output variants in Photoshop: from a bitmap with the entire CIELAB coordinate space, the non-gamut areas were removed by “Select colour range, colours outside colour range”. This is quite simple and comprehensible for everyone with the Photoshop file below, but the underlying gamut algorithm remains unknown. In the case of the Epson proof printing variant, which is also available in printed form, the gamut was determined using Colorlogic’s ZePrA color management tool, which allows finer adjustment of parameters.

The HLC Colour Atlas compares the following output methods:

CIELAB-color space and sRGB-gamut
CIELAB-color space and sRGB-gamut contained therein (Image: RWTH-Aachen University, Research Group Color Science).
  • sRGB: Standard RGB according to HP/Microsoft: the international standard IEC 61966-2-1 is also adopted by ICC and W3C and has therefore established itself in the RGB area.
  • ISO coatedV2: the profile “ISO Coated v2 (ECI) 300%” is published by the European Colour Initiative (ECI) for the characterization data FOGRA39 developed by FOGRA. It is advantageous compared to the Adobe profile for FOGRA39 (see below).
  • FOGRA39: the profile “Coated FOGRA39 (ISO 12647-2:2004)” is provided by Adobe in the CreativeSuite/Cloud and is therefore used.
  • FOGRA51: the profile “PSO Coated v3 (FOGRA51)” is the offset printing standard for coated papers as proposed by FOGRA and ECI since 2016. It takes better account of the usual optical brighteners in papers.
  • FOGRA52: the profile “PSO Uncoated v3 (FOGRA52)” is the offset printing standard for uncoated papers, as proposed by FOGRA and ECI since 2016.

Circumference of color spaces

The table shows the calculated share of the HLC coordinates contained in the respective gamut in the entire CIELAB coordinate space. The CIELAB color space has an estimated share of approx. 60% of the CIELAB coordinate space, so that the values must be multiplied by 1.67 to obtain the share of the CIELAB color space.

Variant relative colorimetric absolutely colorimetric
sRGB 39.1% 39.1%
Coated FOGRA39 (ISO 12647-2:2004) 34.4% 26.9%
ISO coatedV2 37.3% 27.2%
Epson_SCP7000_720x1440dpi10c_GMGsemimatte250_V1_Photoshop 44.2% 40.4%
PSOcoated_FOGRA51 39.0% 27.7%
PSOuncoated_v3_FOGRA52 37.2% 17.0%

 

Which orange shades can be created by sRGB (left) and by offset printing on uncoated paper (right)?

Approximately 64% of the CIELAB color space, i.e. almost two thirds of all colors, can be produced both in proof printing and in sRGB. Nevertheless, there are big differences between the two gamuts:

The Epson proof print (left) can reproduce different color areas in a consistent manner, while sRGB cannot.

In offset printing on coated paper, another third of the color tones cannot be reproduced cleanly. Offset printing on uncoated paper cannot even reproduce half of the colours that can be printed on a proof printer.

The use of the relatively colorimetric rendering intent results in larger gamuts, but this software distortion of the original data does not correspond to the reality of the above question. The HLC Colour Atlas therefore shows the gamuts with absolute colorimetric rendering intent.

Further results

In which colour ranges can a process produce more or where can it produce fewer colour tones? Here are some important results:

Even if sRGB has a good colour range, it can only represent very few colour tones in the blue range. Here, even the offset printing is superior to it and even more so the proof printing, whose gamut is almost twice as big in this area.

sRGB is able to produce light colours a little better than proof printing, especially in the yellow and orange range, which brings out even more strong nuances in darker colours.

There are many more differences in detail, the data of the HLC Colour Atlas XL can be used to examine these differences in detail.

HLC-Colour Atlas XL Gamuts (Photoshop PSD)
The best way to do this is with the pdf version of the atlas, which you can download here shortly.

Comments and open questions

More uniformity and transparency would be desirable with gamut algorithms. In contrast to Photoshop, one can, for example, freely define the desired maximum DeltaE00 deviation from the target value in the color management program ZePrA from ColorLogic; this definition then has a decisive influence on the gamut limits. A short comparison between the Photoshop and ZEPRA generated gamut (the latter with maximum DeltaE00=1) shows that the Photoshop gamut is somewhat smaller.

Comparison of the gamut of the hue H50 generated in Photoshop and in ZePrA.

The question of a suitable algorithm for the limits of the CIELAB color space based on the surface points defined by the rectangular spectra is open. If these limits were fixed, the question could finally be answered whether a CIELAB coordinate represents a color at all (= lies within the body spanned by the rectangular spectra) or not. Non-color could then be excluded from the representation. A suitable approach might be the wireframe surface generation using “Meshing”, where polygon surfaces are placed on these points. Only points in the inner area of the body defined by the polygon surfaces are colors, external points are excluded from the color calculation or traced back to a surface point.


author: Holger Everding

Translated with www.DeepL.com/Translator