I personally see no value in aRGB at all. The fact is that aRGB
effectively steals bits from the dynamic range to provide greater bit
depth in the midtones to extend colors beyond the normal 100% red or
100% green or 100% blue. With sRGB, 100% red is all you'll ever get,
but with aRGB, you can get like 110% red. To provide this greater than
red, bits are taken from the lower tones.
All this is fine and dandy, but unless the output mechanism is able to
reproduce these extended color ranges, you are just beating yourself.
It also makes "ETTR" all the more important because you need to keep
your important data above the bit-bending point.
So, let me approach this from a different perspective: The camera or
scanner is digitizing the analog world in somewhere around 12-14 bit
per color channel depth. Image data is stored as 16-bit. Is the RAW
file any particular colorspace? Well, actually, yes it is. It's mapped
in a form of sRGB. The mapping of the response curve follows standard
bit assignment. When you convert the file from RAW to TIFF (or
whatever your working format is) if you use aRGB, the resulting file
is remapped data. When you remap data, one of two things happen:
Information is either thrown away (lossy), or it is reassigned. It is
reassigned to provide greater CONTROL over color and tonal curves in
some parts of the tonal/color spectrum at the expense of other parts
of the tonal/color spectrum.
At issue with these colorspace things is the misunderstanding that you
can actually create an extended color range with aRGB than sRGB. This
would be only true if the output medium has means of producing colors
outside of the RGB color range. Also note that current sensor/scanner
technologies are hamstrung by their very filter packs from ever
capturing an aRGB range. Why? Because the camera cannot see reds
beyond 100% red. The human eye IS able to, only because human vision
uses subtractive processing. We literally cannot see red. We see a a
wide spectrum of reddish-orange colors, but red is determined by the
absence of green. When you look at that red rose, we are actually
seeing an orange rose, but since there is no green in the rose, the
human vision decodes this as red.
So, aRGB is a fine colorspace when you want to create, bend, and
artificially alter an image from a standard sRGB color selection to a
wider colorspace. But that wider colorspace does absolutely come with
a price. One such price is being limited in being able to print it.
The more inks the printer has, generally the wider the colorspace it
can reproduce. If your printer is a standard CYMK inset, you'll be
limited to sRGB colors since it is impossible to produce a redder red
than what yellow-magenta is going to give you. However, with the
addition of a Red color cartridge, which is a redder red, we can
extend that range. Same with Green and Blue color cartridges. The Red,
Green and Blue inks being added to the printers are beyond what you
can achieve through CMYK color mixing. Same with blacks. Who knew that
we needed three different kinds of black?
Since nearly 100% of my usage is sRGB, I see no value to using aRGB,
expect in extremely rare circumstances where I'm struggling with an
image and can't get the subtle color transitions (microcontrast) that
I'm seeking. But then the fun part is converting back to sRGB for
output. At this stage, there is a huge loss of data. Data loss which
never occurs when working exclusively in sRGB.
--
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
--
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