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
- 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
- No colors shall be considered that cannot be reproduced on the output
- 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.