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I recently added support for digital camera raw
files to my photo editing software I've been working on, using LCMS. I too am
using DCRAW to get to the actual raw data. Here's what I've concluded up to this
point, concerning profiles:
It all depends on whether you scale the raw data
(after Bayer conversion) to some reasonable range before profiling, and it
depends on what software you use to build the profiles. And it's imperative you
use 16 bit color channels.
I purchased an IT8 target specially built for
digital camera profiling from the folks at Picture Window Pro, as part of their
Profile Mechanic package. However, I was not able to build a decent profile for
my Canon 20D camera with that package unless I scaled the image first, and even
then the resulting profile didn't work very well. It had a lot of posterization,
and other such problems. And if I didn't scale the data first, the
resulting profile was truly atrocious. I was however, able to profile both
my flatbed Canon scanner, and Nikon 8000 film scanner with great success. The
profiles were very good. But the digital camera profiles, sucked, to put it
lightly.
So as a goof, I tried using the free AIM profiler
(from the Timo guy) written in Excel. I wasn't expecting it to work, but
actually it worked quite well! While I was able to build a profile either with
or without pre-scaling the data, I got a better profile when I scaled the data
such that the maximum value encountered in any channel was set to 0.95, on a
scale of 0 to 1. This gave plenty of highlight headroom to work with when doing
color balancing later on, but also gave the profiling software reasonable data
to work with. It's imperative you work with at least 16 bit channels at this
point, or you'll have no hope at all of getting anything decent. Also, I find it
best not to touch the shadow point at all at this stage.
With the resulting device profile, you can at least
get a reasonable looking image in your favorite photo editor. You'll still
probably have to tweak the white points, color balance, etc, but the image
will be in the ballpark. While I'm still experimenting, I've found that it's
best to set the color balance and white point while in some sort of linear
space. From what I understand, most digital cameras are fairly linear in their
response to light, and by creating a device profile with AIM for example, you
basically are compensating for any deviations along the straight line. I believe
the AIM profile does not apply a "gamma" at all to the data, but I may be wrong
about that. (In other words, any gamma compensation happens in the conversion
from the device profile to the monitor profile.)
I seem to be getter better results doing
the color balance in the device profile space, but that's not
conclusive. (Can anybody shed any light on that?) Then I convert
to a linear space based on ProPhotoRGB
primaries or some other wide gamut space, but I don't use the 1.8 gamma of
ProPhotoRGB, but rather, stay in a linear space. From there, I set the white
point and basic midtone brightness. HOWEVER, I do NOT set the shadow point in
linear space. It's way too senstive, as all the shadow values are jammed up next
to each other. Convert to a profile that has a gamma of 2.2, or thereabouts, and
THEN set the shadow point. Note that you should only convert to 8 bit AFTER
changing the gamma. I've also discovered that it's sometimes better to set the
midtone brightness after changing the gamma. It seems to pack more punch in
gamma 2.2 space than in linear space. I'm not sure about that yet,
though.
I've used this workflow successfully with a Canon
20D and also a Canon G2, but I'm still experimenting and there's a lot to
learn.
As others have mentioned, the problem with using
profiles that aren't built specifically for your DCRAW processed data, is
that you have no way of knowing what assumptions are being made about scaling,
gamma, etc, when those profiles were built. I've found that using generic
profiles I've gotten of the web, or from, for example, Capture One, result in
dull lifeless and sometimes incorrect color. There's nothing wrong with these
profiles per se, but they were written with different assumptions in mind than
what I use in my DCRAW converted data. I haven't been able to determine
what those assumptions are.
Bryan
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