Hi Hedley,

On Fri, Jul 18, 2008 at 12:06 PM, Hedley Finger
> David, Sven:
> On Thu, 2008-07-17 at 16:52 +1000, Hedley Finger wrote:
> I know just enough about colour management to get myself into trouble.
> On Gimp 2.4.6, there are now some new options under the Image > Mode >
> menu path, namely
> @   Assign to Color Profile // Actually uses Little CMS (lcms)
> @   Convert to Color Profile // Actually uses Little CMS (lcms)
> @   Assign colorspace  // Hmmm, should be 'Color Space' for niceness?
> @   Convert colorspace  // Likewise!
> Sven replied:
> If you see four options there, then you have installed some additional
> third-party plug-ins for color management that you should not need with
> GIMP 2.4. It's probably best to get rid of those extra plug-in(s).
> Yes, a respondent on another list said something similar -- the last two
> options were probably inherited from a previous Gimp 2.x release, because
> the Windows installer by default installs into the c:\Program
> Files\Gimp-2.0\ folder created by previous installers.  The first two
> options actually call Little CMS to do the work.
> @   What does *assigning* an image to a colour profile do and what
> effect does it have on colour management downstream?
> It changes the meaning of the pixel values. Generally this means the
> image looks different.
> David, I see no difference when importing an image with no attached/assigned
> profile.  But you are saying that the pixel numbers do not change, so no
> harm done if an inappropriate profile assigned -- just assign a more
> appropriate one?

Yes, no harm done.
Images always use a color profile, even when there is none assigned.
In GIMP preferences,  'RGB profile' determines the color profile used
if the image has no profile assigned. By default this is set to None;
None is treated as if it were sRGB.

> @   How is *converting* an image to a colour profile differ from
> "assigning", and what effect does it have on colour management downstream?
> Converting, instead of assigning a different meaning to pixel values,
> converts the pixel values from the previous color profile to the
> profile you specify, so that the result is as close to identical as
> possible to the original..
> So this is an irreversible step -- you may not be able to convert back to a
> previous profile because the pixel values may not contain enough information
> to allow this to happen?
Yes. With GIMP's current limitation of 8bit/channel, some data loss is
virtually guaranteed. After GIMP supports floating point images, data
loss should become negligible when using floating point.

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