Re: [Gimp-user] ICC Profiles: UFRaw + GIMP => 2x correction?

[Gary Aitken <gimp dreamchaser org>]

Humor me, and try the following:

Scenario 1:

Load a raw image into ufraw with some feature for which you can tell a 
difference when a profile is applied.

Apply the profile in ufraw.
Save the file as .tif

Start gimp.
Go to Edit/Preferences/ColorManagement
  Make sure RGB Profile is set to "None"
Load the .tif image.
It should look as the corrected image, i.e. profile applied.
Go to Edit/Preferences/ColorManagement
  Check RGB Profile
    It still says "None"
ok so far
  Presumably the profile used is the one embedded in the .tif image.
  But there is no way of knowing what that profile is or whether it has 
been applied that I can figure out.

Now set the RGB Profile to the one you used in ufraw when generating the 
.tif image.
  You will see the image transformed again.
  In effect, the profile is being applied twice.

Discard the image from gimp.
Leave the RGB profile set.
Load in the tif image.
You will see that the color profile has been applied twice, once from 
the one embedded in the image, and once from the one specified in the 
color management preference.

Scenario 2:

Start gimp
Edit/Preferences/Color Management
  Set RGB profile to none
Read in a non-raw image with no profile attached
Edit/Preferences/Color Management
  Set RGB profile to something where you can notice a difference.
Export the image as .tif
Change the gimp color profile back to none.
Load the exported image back in.
The image appears as it did when first read in, without any applied profile.
Apparently, the profile was not attached to the exported image.

Scenario 3:

Start gimp
Make sure no profile is set.
Drag a raw file onto gimp
  Should start up ufraw
  Make sure ufraw applies a color profile to the image.
  Click ok to end ufraw and send the image back to gimp
Image shows up in gimp with profile applied
Edit/Preferences/Color Management
  Shows no RGB profile
Export the image as .tif
In this case the exported image still contains the color profile.

In scenario 2, gimp exports a .tif image without the applied color 
profile.  In scenario 3, gimp exports the image with the applied color 
profile, presumably because it was embedded in the image when it got it 
from ufraw.

It appears that gimp only exports a profile with an image if the profile 
came with the image when it was loaded into gimp.

Unless you know in advance whether or not an image has a color profile 
attached to it, you can't know how to process the image, given the above 
issues.  Furthermore, you can't produce an output image with a profile 
attached, such as adobe rgb for printing.  You may be able to achieve 
that result if the printer you are targeting is reachable from your 
system, but that doesn't cover the case where you want to put an image 
on a thumb drive or transmit it to a different place for printing.

I claim this a bug.  I would think it should track which profile was 
applied to each image, and
  1.  Not apply it twice if it is the same as the one which came with 
the image
  2.  Ask for verification if trying to apply a profile to an image 
which is different from the one the image came with (Assuming it came 
with one).  If applying a new profile over an existing profile, I can 
think of two possible scenarios.
     2.a  You believe the input profile is incorrect or inappropriate. 
 In this case you want to unapply/replace the input profile with the 
new one.
     2.b  You accept the input profile as correct, but want to remap to 
a different output space, such as adobe RGB.  In this case you want to 
keep the transformation of the input but attach the new profile to the 
output.  I presume this is what the CMYK profile is for, but it appears 
to me that profile only applies when executing code in the print command 
path.  It does not appear to be applied when exporting an image.  It 
also appears that this is what the RGB profile in the properties 
actually does (apply a profile in addition to any input profile that 
came with the file); however, as noted earlier, the profile is not 
embedded in the output.  I do not know if it is permissible to apply 
multiple profiles to a file; my impression is not.  If it is not, then 
in order to produce a file suitable for printing, gimp (and any other 
image manipulation program) has to be able to read a file with a 
profile, apply that profile to transform the input pixels to the 
reference space, and then output that file from the reference space with 
a new profile attached.  Otherwise, all of the processing of the image 
has to be redone to produce images for different targets (e.g. web, print).
  3.  Export the profile with the image when one has been applied, if 
possible.

BTW, I'm using gimp 2.7.5 (2.8 preview); I don't know if this is 
specific to 2.7.5 or not.  It may also be that these are temporary 
issues with the transition to full color management and 16bit colors. 
But if they are issues not already on the gimp development plate, they 
should probably be raised so they are at least well-known.

Or I need to be educated if they are not considered bugs.  Sticking to 
sRGB doesn't solve these issues.  I'm more than willing to accept that I 
need an education, so any and all clarifications or welcome.

Gary

On 1/30/2012 6:07 PM, Frank Gore wrote:
> On Mon, Jan 30, 2012 at 7:50 PM, Gary Aitken<garya dreamchaser org>  wrote:
> > I have a question regarding the use of ICC profiles in UFRaw + GIMP.
> >
> > If I load an image into UFRaw and specify an ICC profile for my camera,
> > then output a .tiff image, that image is color-corrected.  At least that is
> > my assumption...
> >
> > Now, if I load the .tiff image into GIMP, and again specify an ICC profile
> > for the camera, the image is corrected again, I think.  Which amounts to
> > over-correction.  Correct?
>
> ICC is not a correction. It is a profile. It tells the application how
> to interpret the colours that are encoded in the file. If you specify
> an ICC profile in UFraw, that profile gets saved into the image file,
> and Gimp uses that profile information when it opens the image file.
> If you re-specify a new ICC profile within Gimp after you've opened
> the image, you're not "over-correcting" anything, you're just doing an
> extra useless step that results in nothing happening.
>
> In your case, I'd say you've got absolutely no reason whatsoever to be
> doing colour management. Sticking to standard sRGB for your entire
> workflow would be entirely acceptable and would give you the best
> results.
>
> --
> Frank Gore
> THE place to talk photography!
> www.FriendlyPhotoZone.com