Graphics Programs Reference
In-Depth Information
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Editing in LAB:
I have nothing against the LAB color model. However, there are a group of
people who feel that editing in LAB is the only way to accomplish specific corrections, making
it sound like a macho editing space. It is true, there are a few correction techniques that
rely on a document being in LAB color space. The question becomes whether it's worth taking
the time or worse, producing image degradation to convert from a working space to LAB and
back. Every time a conversion to LAB is produced, the rounding errors and severe gamut mis-
match between the two spaces can account for data loss, known as quantization errors. The
amount of data loss depends on the original gamut size and gamma of the working space. For
example, if the working space is Adobe RGB, which has 256 values available, converting to 8-
bit LAB reduces the data down to 234 values. The net result is a loss of 22 levels. Doing the
same conversions from ProPhoto RGB reduces the data to only 225 values, producing a loss of
31 levels.
Bruce Lindbloom, a well-respected color geek and scientist, has a very useful Levels Cal-
culator, which allows you to enter values to determine the actual number of levels lost to quan-
tization (see the “Calc page” at http://www.brucelindbloom.com). If you do decide to convert
into and out of LAB, do so on a high-bit (16-bit per channel) document.
Another problem with LAB is that it has a huge color gamut, and if you're working in 24-
bit images, you have three channels, each containing 256 possible values. With large gamut
color spaces, these 256 data points are spread further apart than a smaller gamut space,
which can result in banding in certain kinds of imagery like smooth blue skies. LAB is derived
from CIE XYZ (1931), which defines human perception; it represents all the colors we can
see and its gamut is huge. Not as many tools or operations in Photoshop can operate on
images in LAB and the numeric scale isn't very intuitive for users to work with. Some applica-
tions we might want to work with can't accept a LAB file so an additional conversion is usually
necessary.
One advantage of LAB is that since the colors (the A*axis and B*axis) are separate
from the luminance (L*axis), it is possible to conduct tonal corrections that do not affect color.
Hue shifts are avoided when changing lightness. The other advantage of LAB is that it is
self-defining, meaning we don't need an embedded profile for files in LAB since LAB is truly
a device-independent color space. Some editing techniques can be conducted on an RGB file
and produce nearly identical results of using LAB by using the Luminance blend mode in
Photoshop. For example, instead of converting a file to LAB to sharpen just the L channel, apply
Unsharp Mask on your RGB file, then under the Edit menu, select the
Fade
command and
set the mode pop-up menu to
Luminosity
. Or apply the Unsharp Mask on a layer and set its
blend mode to
Luminosity
. This will produce the same qualities as sharpening on the L channel
without having to do a color space conversion, plus the
Opacity
slider is useful for fine-tuning
the effect.
Some users are under the impression that Photoshop does all its conversions to and from
LAB, converting on-the-fly. This is untrue as it would greatly slow down performance. Instead,
Photoshop uses LAB as a reference when conducting many operations. Photoshop is not actu-
ally converting pixel data between color spaces unless you, the user, actually ask for this. None
of these issues should be interpreted as implying that a conversion from working space to LAB
is bad. Just be aware of the issues involved with this kind of conversion and whenever possi-
ble, try to use similar techniques that can be conducted in the RGB working space.
