photogamut.org
reliability in digital image production

 

RGB working color space for photographers, photo labs and service bureaus

RGB data is usually available in a variety of different color spaces. Most commonly sRGB as a well introduced color space for most SOHO digital cameras and printer drivers, ECI-RGB or AdobeRGB as color spaces for highend retouching, creative digital composing, and professional output on photographic imaging devices.
Why then should a new working space be needed and which is best suited? Answering this question is not as easy as it may seem in the first place.

 

  • The most sophisticated workflow is to leave the file in its original color space throughout processing to finally convert it to the device's working space just in time for output. An obvious advantage is that there are no intermediate conversions prone to loss of information.
  • If more digital assets are to be integrated into a document you will have to settle for a uniform color space.
  • Color spaces of professional image capturing devices often tend to have a nonlinear gray axis which can introduce unexpected behaviours and are a source of inconsistencies when technical retouching becomes necessary. Converting the digital asset to a color space with a well behaved gray balance supports professional image editing.
  • In preparation for storing digital data in a digital asset management system it can be recommended to agree on a standardized color space even if its just for the sake of keeping things clearly structured and overseeable.
  • A working space which is dimensioned to precisely cover only those colors really printable on the existing variety of output devices makes output, especially of highly saturated colors, more predictable as the loss of brilliance can be foreseen on well calibrated and profiled displays.
  • It might also not be appreciated that the recipient of the digital asset can gain an insight to the origins of the data. This could be the case if the profile tag is named in some fashion so as to permit identification of the originator ("SmithsLab_Flextight_Pos.icc").
  • A working space that is uniformly applied to all assets increases the controllability of the process for the photographer and for the service bureau, since the reduced need for file conversions decreases the risk of introducing errors.


Requirements for a suitable working space

Thinking about the ideal properties of such a color space leads to the following results:

 

  • A harmonic gray axis from white to black is needed.
  • Shape and size of the color space comprises all photographic output color spaces.
  • No colors shall be considered that cannot be reproduced on the output device.
  • Best match between softproof and output on the best considered device.
  • Best possible output without colormanagement when output to photographic imagesetters and when output via standard printer drivers.
  • Implementation into ICC conforming colormanagement workflows for highest demands on quality.

Currently established working spaces like sRGB, ECI-RGB, AdobeRGB, ColorMatch RGB etc. are so- called matrix-TRC color spaces which indisputably offer the advantages of a perfect gray balance and smallest file sizes.
Since those color space profiles mostly represent virtual display devices that are self illuminant they mostly include wide ranges of nonprintable colors especially in brighter areas.
The aforementioned depicted the necessity for the development of a new color space which comprises solely reproducable colors thus being similar to most of the mentioned output devices in shape and size.

Major concerns are to reduce loss of information by concentrating not only on one output process i.e. on only one class of output devices, but to consider all output technologies currently available without introducing any restrictions.

Following steps are necessary to develop a working space to fullfill all mentioned criteria:

  • Generation of a smoothed profile from averaged samples measured using a number of representative output devices driven by RGB data.
  • Using that profile an RGB test chart is converted to Lab color space.
  • The resulting Lab data is then scaled so that all relevant RGB target color spaces plus offset printing are accommodated.
  • The gray axis is fitted for contrast and gray balance and a target gamma of 2.2 is to be met.
  • Colors of low to median saturation are slightly desaturated to assure a visually appealing output quality also when not using colormanagement.
  • Using those Lab values an RGB profile is rendered and - using special tools - compression is applied without impact on the quality. The final size shall be 189 kilobytes.