Colour gamut analysis

ISO TS 18621-11 Graphic Technology - Image quality evaluation methods for printed matter - Part 11: Colour gamut analysis describes methods of analysing and comparing the gamuts of colour prints. In Annex A it describes example test charts in RGB and CMYK which can be used in the analysis and volume calculation procedure described in the body of the standard.

Alternate test charts

An alternate patch set based on CMYK gamut boundary colours has been derived. The chart has 382 faces on the gamut surface. This patch set has been shown to give a gamut volume that is a close match to the reference values in Annex B when computed according to the method in ISO TS 18621-11.

The CMYK coordinates of the faces are given in a spreadsheet. This set of CMYK coordinates is present in a number of standard test charts used for printer characterizations:

ISO 28178:2009 Graphic technology - Exchange format for colour and process control data using XML or ASCII text
ECI2002 - see http://www.eci.org/en/projects/eci2002 ANSI/CGATS.17:2009 (R2015) Graphic Technology - Exchange Format For Colour And Process Control Data Using XML Or ASCII Text
ISO 12642-2:2006 Graphic technology - Input data for characterization of 4-colour process printing - Part 2: Expanded data set
ISO 12642-2:2006 and IT8.7/4.

Where the CMYK values are not present in the test chart used, colour values corresponding to these CMYK may be calculated by interpolation. Where this is done, details of the interpolation method should be provided.

Download alternate test chart spreadsheet

Gamut volume computation tool

An Excel spreadsheet is provided to compute the gamut volume from a characterization data set (formatted as specified in ISO 12642-2). The spreadsheet include an example data set, and other data sets can be copied in to give the gamut volume as defined in ISO TS 18621-11.

Download gamut volume computation tool

Errata in ISO TS 18621-11:2019

Users of ISO TS 18621-11:2019 should be aware of two issues in the published standard.

Equation 1 should read V=-(a·(b⨯c))/6 instead of V=(a·(b⨯c))/6
The RGB image in Annex A has 36 uniform steps in colorant combination ratios and 21 uniform steps in colorant amounts, while the CMYK image has 36 uniform steps in colorant combination ratios and 22 uniform steps in colorant amounts.

Colour gamut description in iccMAX

In the iccMAX specification a colour gamut is defined in terms of a face/vertex list, i.e. as an array of vertices on the gamut boundary in PCS colour space, together with an array of indices into the vertex array which give a set of triangles that connect to form a smooth and connected surface. Details are given here.

Basic vertex list and face list

A simple example is given here of the vertex and face lists. The 8 colours corresponding to the CMYK primaries, their overprints and the unprinted substrate from the FOGRA39 data set were used. The CMYK device data list is an 8x4 array where each row corresponds to one of the colours. The vertex list is an 8x3 array of CIELAB values corresponding to the CMYK values in the device data list. The face list is a 12x3 array of integers, in which each row contains the indices into the vertex list corresponding to a single face on the gamut surface. The example data is available to download as tab-delimited text files below.

Device data list

Vertex list

Face list

Perceptual Medium Reference Gamut
The PRMG is defined in Table 9 of the ICC.1 specification. For convenience it is provided here as a spreadsheet (.xlsx).
PRMG.xlsx