[Digikam-users] Re: SRBG, Adobe RGB, What to use ?

[Elle Stone]

Sorry to be so late posting to this thread. Sleepless, thank you for
pointing to my color management tutorial. Hopefully the following
might be useful:

"Tones for editing": imagine that you have 256 pebbles (8-bits), each
1 inch across, laid out so they touch each other. The total length of
your line of pebbles is 256 inches. Imagine that those pebbles are
then spread apart to fill a line that is 700 inches long. Now there
are gaps between the pebbles. Imagine instead that you have 65536
pebbles (16-bits). The gaps are smaller. (Careful, this analogy can
only take you so far.)

These "gaps between the pebbles" represents the effect of using a
larger color space (AdobeRGB) compared to using a smaller color space
(sRGB).

The gaps between the pebbles also represent the effect of using curves
to increase contrast between shadows and highlights - you spread the
pebbles farther apart regardless of what color space you are using.

sRGB and AdobeRGB are both RGB color spaces. RGB color spaces are each
defined by a red, a blue, and a green "primary" which dictate the
reddest red, bluest blue, and greenest green you can get from each
respective color space. (See
http://ninedegreesbelow/2011/imaging/color-management-tutorial/all-the-colors.html
for details.)

The red and blue primaries in sRGB and AdobeRGB are exactly the same.
Only the green primary differs. (See
http://ninedegreesbelow/2011/imaging/color-management-tutorial/pictures-of-color-spaces.html
for pictures comparing sRGB to AdobeRGB and also to a typical printer
color space and a typical camera color space.)

So the distance from reddest red to bluest blue, and also from
white/grey to reddest red and white/grey to bluest blue is the same in
sRGB and AdobeRGB. If you fill in the distance between reddest red and
bluest blue with your 256 pebbles (8 bits) or 65536 pebbles (16 bits),
the gaps between the pebbles is exactly the same for both color
spaces.

Your line of pebbles has to stretch farther in AdobeRGB to get from
reddest red to greenest green, from bluest blue to greenest green, and
from white/grey to greenest green.

So depending on how extreme of editing you plan to do, you are more
likely to get posterization, noticeable gaps between the pebbles, in
8-bits than in 16-bits, regardless of whether you are using sRGB or
AdobeRGB.

Likewise, you are more likely to get posterization in AdobeRGB than in
sRGB IF you are heavily editing areas that are predominantly green,
yellow, or cyan. Why yellow or cyan? In an RGB space, yellow is
composed of green and red - the yellowest yellow has 100% greenest
green plus 100% reddest red, and no blue at all). Pure cyan is an
equal mix of green and blue, with no red.

The sole advantage of AdobeRGB over sRGB is if your original or edited
image contains colors - certain greens, yellows, and cyans, that fall
OUTside sRGB color space and INside AdobeRGB color space.

If you want to post your AdobeRGB image to the web, you will need to
convert it to sRGB. Does this mean you have to throw all the extra
colors away? Yes, it does. But the trick is that you eliminate the
"out of gamut" colors away selectively and creatively, compromising
between conserving detail, brightness, and saturation. In short, you
use color management, specifically "soft-proofing" (which has its home
in making color conversions for printing, but applies equally well to
color conversions for display on the web) to do creative color
conversions.

See http://ninedegreesbelow/2011/imaging/color-management-tutorial/color-management-tutorial-introduction.html
and http://ninedegreesbelow/2011/imaging/color-management-tutorial/assign-assume-convert.html
for a bit more detail on the point of color management.

There is a question implicit in the above discussion regarding your
monitor profile. Every color-managed editing software that I've ever
encountered assumes that your monitor profile is sRGB UNLESS you tell
the software to use a different monitor profile.

If your monitor is not adequately characterized by sRGB, then you need
to either calibrate your monitor to match sRGB OR create and use a
monitor profile that actually characterized your monitor. Otherwise
the colors you see will be more or less wrong, depending on how poorly
your monitor "fits" sRGB.

If you are using an LCD monitor, it is NOT well-characterized by sRGB.
Why? Because sRGB describes the ideal CRT monitor in use in the
mid-1990s. And unless you have a wide-gamut LCD monitor you cannot
calibrate your LCD monitor to match sRGB primaries.

See http://ninedegreesbelow/2011/imaging/color-management-tutorial/srgb-no-color-management.html
for an example of what can happen when you "assume" your monitor is
well-characterized by sRGB.

See http://ninedegreesbelow/2011/imaging/color-management-tutorial/all-the-colors.html
and http://ninedegreesbelow/2011/imaging/color-management-tutorial/monitor-profile-calibrate-confuse.html
for more details about why sRGB is not a good monitor profile for LCD
monitors.

An aside: a 14-bit camera raw file has 16384 "pebbles" to spread
around in each Red, Blue, and Green channel, but your editing software
only does 8 or 16 bits, so you either have to collapse 16384 steps
down to 256 steps and thus throw information away, or increase it up
to 65536 steps.

A great discussion of sRGB vs. AdobeRGB can be found here:
http://www.cambridgeincolour.com/tutorials/sRGB-AdobeRGB1998.htm

For more information on the number of tones available for editing in
8-bit vs 16-bit images, see here:

http://ronbigelow.com/articles/posterization/posterization.htm
http://ronbigelow.com/articles/raw/raw.htm

Kind regards,
Elle Stone