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REPORTS

Wide Color Gamut vs. sRGB colours brought to heel

By Denis Freund
28.01.2009, 10:00

Users who switch to a display with an extended colour space (often advertised as "Wide Color Gamut") will often use the adjective "colourful" when they first switch on their new monitors. In fact, the colour space, which is larger than sRGB, does give rise to a visible over-saturation in the display of relevant source material.

It is possible to find an elegant way round the problem in colour management software with a calibration by colorimeter and a consequent measuring of the monitor colour space in the course of the profiling. This transforms the RGB values so that colour-accurate work is possible if the source profile is stored correctly. Generally, it is also possible to work in AdobeRGB without problems, which would not be possible using screens without extended colour spaces.

Unfortunately, however, the colour management cannot have an effect everywhere. The two colour management-enabled browsers (Firefox 3 and Safari) are naturally limited to the correct display of integrated images (where these are in sRGB or where the correct colour profile is stored), whilst graphical elements of the website are simply "passed through". Games, too, have to manage without colour management and present themselves brightly accordingly. Depending on the game and the viewer, this need not necessarily be distracting – but sometimes it is. However, video playback is affected particularly acutely. Thus, Rec.709 defines a colour space for HDTV which demonstrates the same primary colour positions as sRGB. Brightly coloured playback is guaranteed, but has little to do with the required image display.

What to do?

In the TV sector, extended colour space screens have now become the norm. Even with regard to xvYCC, this only well thought out at first glance, since it will still be some time until relevant material, e.g. on Blu-ray, becomes available, if at all. However, every TV does in fact offer the option to limit its colour space to the colour spaces that are in use today.

This option is lacking in the computer display sector. A small number of high-end models such as those marketed by Eizo offer a way out. The CG series realises colour space emulation via the programmable LUT. This allows for accurate display, even in unmanaged environments.

6-axial colour correction

Current NEC and Eizo screens have a hidden function in their OSD which is easily overlooked when the specifications are scanned. Here, "6-axial colour correction" is a tried and tested method for carrying out a "light colour space emulation". However, a colorimeter is required as well as the freely available "HCFR" software, which is actually intended for the calibration of LCD, plasma TVs and projectors.

6-axial colour correction on the Eizo SX3031W.

Below, we wish to limit an Eizo SX3031W to sRGB as an example. As well as the screen, our test set-up consists of the x-rite DTP94 colorimeter and the HCFR software and iColor software from Quato

The procedure

First, we would like to mention that the following workflow is not a replacement for a professional colour space emulation ist. However, the results achieved are astonishingly good and are completely sufficient for the home user sector.

Step 1: We achieved the best results by carrying out a calibration with iColor before using HCFR. The settings we chose were: white point 6500K, brightness 140 cd/m² and gamma 2.2.

Explanation of 3D views: The black grid represents the respective standard colour space and the white grid represents the monitor’s colour space. The coloured cube depicts the actual intersection between both colour spaces. Where the black grid protrudes beyond the cube, the monitor’s colour space can no longer display the standard colour space. If the monitor’s colour space is larger than the standard colour space, the white grid will protrude beyond the cube.

After calibration and profiling, we save the ICC profile we have created. The monitor’s colour space is represented as follows in a 3D view as compared with sRGB:

Monitor colour space of the Eizo SX3031W compared with sRGB

Without colour hue mapping in colour management-enabled software, the usual colourful display arises with sRGB as the source colour space.

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