[argyllcms] Re: Need help interpretating results of camera profiling
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 12 Jun 2014 12:48:15 +1000
Bernard wrote:
> Willing to process RAW images from my Canon EOS 600 without having to use
> proprietary
> software under MSWIN, I have attempted to generate an icc profile.
Hi,
Ben Goren is perhaps more of an expert in this area.
> After each session, I picked a
> properly exposed image (slightly overexposed), and adjusted the brightest
> white spot (R13
> on the 'CMP-DT3' target) using the exposure module, so as to obtain a 'L'
> (Lab) value of
> about 97-98, (RGB values close to the range 245-250 as prescribed by the
> manufacturer's
> notice). This was being done in 'darktable', with all plugins disabled except
> for high
> lights reconstruction , White Balance being set spotting 012 grey patch
> (resulting
> tint=1.006, input T=5035, output 5000, Red=2.326, Green 1.000, Blue=1.358).
It depends a lot on how you intent to use the profile, but I would have thought
that
changing the white balance of the test image was not the right thing to do.
Ideally
I think the target workflow should be to run the raw image through the profile
to the
working space, and then do any subjective adjustments in the working space.
Changing
the grey balance of the test image will completely mess this up, since for the
profile
to remain valid, you have to apply exactly the same adjustment every time you
use
the profile.
> Input profile
> was 'linear RGB', gamma correction OFF, no 'base curve', output profile set
> to 'absolute
> colorimetrics' and 'Linear RGB'.
Hmm. Is that Darktable setting ? How sure are you that it was doing no color
conversion ?
Ideally there would be a "calibrate" setting that disables all color conversions
adjustments.
> This being achieved, and prior to export to a 16-bit TIFF file in above
> mentioned settngs
> and profiles, curiosity lead me to measure a few spots, using darktable
> tools, and compare
> results with the values found in the manufacturer's file and in the Argyll
> CMP-DT3.cht
> file. The results surprised me... but please bear in mind that I know next to
> nothing
> about color measurement and standards ; I only learnt whatever one can grab
> reading a few
> dozens of pages. So, besides the white spot R13, I tested Q19 (a red patch),
> C10 (a green
> patch), and E4 (a blue patch). RGB measurements were a good surprise, since
> differences
> were found minimal between measured values and values stated in reference
> files for the
> same patches. Indeed it was so close that I could hardly believe it:
I guess I'm unclear which part of the process you are describing. Is this the
calibration
workflow of grabbing the test chart camera RGB values, or is this the "using
the profile"
workflow ?
I'm not sure what you mean by RGB measurements - these test charts have nothing
to do with
RGB - RGB is a device colorspace, either display, camera or standard. The
charts have a
spectral reflectance and XYZ value under a given lighting condition.
Verification of a
calibrated workflow involves comparing the XYZ values (or L*a*b* equivalent) of
the chart
with the XYZ values of the photo when converted through the input profile. What
you do
with the XYZ values after that is up to you. You can certainly convert them to
a working
RGB space if you like, using the appropriate RGB profile.
Note that the sort of "cooked" RGB values you get from a typical camera have
only an
indirect connection with the actual light measured by the camera, and a lot to
do with the
magic sauce the manufacturers add to give them an appealing "look". Duplicating
such
processing is a different category of task, and may or may not use any Color
Science.
> Now, let's see whether adding the Base curve would change anything. It is a
> default
> basecurve that Darktable proposes for my kind of Canon camera, according to
> the exif data.
> This default curve is not available when one opens a tiff image ; the one
> that is then
> proposed is a flat curve with slope=1, that does not change anything. But let
> us see what
> the one available in raw treatment does. Let us first test it WITHOUT the
> inprofile
> correction module.
I doubt that mixing other ad-hock modifications to a profiled workflow would be
of benefit. Either you are relying on the profile to convert from camera RGB
to device independent XYZ/L*a*b*, or you are not.
> Well... I don't know what to think ! The results are different still...
> Nothing close to
> so called expected values, not close either to the Canon JPG images:
Color management can be very complex. It's complexity is usually due to too many
fingers in the pie, and too little visibility as to what each component is
actually doing.
Trying lots of different combinations of switches is usually pretty futile -
there are
too many wrong combinations compared the number of right combinations. So in my
experience, the only way towards clarity is to break a process up into
individual
steps, and put each step under the microscope. Once you have your theory
matching your
reality at each step, then you have an overall workflow that works, and that you
understand.
> Now, Graeme Gill writes in his tutorial: 'Colprof: average errors of 5 or
> less and maximum
> errors of 15 would normally be expected'. My typical peak error is 29, and
> the average
> error of 8.53. Is this too much ?
Input profiles often have larger fitting errors than characterizing output
devices, but
DE of 29 is extreme. Most often this is due to a patch identification mixup.
> Does the elaborated icc profile take gamma correction into account ? Base
> curve ? Should
> one add either one or both of these to the raw treatment of CR2s ?
ICC profiling in its standard form isn't really appropriate for general
photography, due
to the highly varied lighting conditions encountered. The spectral interaction
of the
light source, colored surfaces and camera sensor cannot be anticipated by a
single
RGB to XYZ transform. There are ways around this limitation, but normal
profiling
is at its best under controlled lighting conditions, such as in a studio or on
a copy
stand.
An ICC profile takes everything into account, which is it's limitation
when it comes to general photography. In a general case you want to be able to
tweak various parameters independently, such as exposure and white point, that
are
assumed to be fixed for profiling.
Graeme Gill.
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