David Neary wrote:

I may be misunderstanding things, but if the conversion from the
source colourspace to sRGB is done in lcms losslessly, then all
we're losing is the out-of-gamut colours from the colourspace
conversion. And, of course, the cost of discarding precision in
the data we get after the application of the profile. But I think
we still get the full 8 bits of data (they may not have the exact
colours in the source file, though).

As a (somewhat extreme) analogy, imagine applying a lightening gamma curve to an 8-bit data set (0,1,2,3,4.....253,254,255) you'd end up with not all codes being used at the dark end and codes being used multiple times at the light end - something like (0,2,4,6,7 .... 254,254,255,255). I.e. there are missing codes at the dark end, and reused codes at the light end, due to the limited dynamic range of the destination space. The problem I'm talking about will be something vaguely similar; there will likely be missing and reused codes in the working space.

Yes, but since this profile is applied once, on the projection
drawable, as the final step, its application doesn't present any
problems. But I see what you mean - we can go from the source

I wasn't suggesting this was a problem, merely trying to explain that profiles are only meaningful when used in pairs. People talk about applying a profile to an image; usually what they mean is applying a transformation between profiles to an image. It's an important distinction.

colourspace directly to the monitor's with one transformation.
This, however, poses problems for say the checkerboard pattern
(which will be transformed differently with different source
profiles), and for any occasion where different profiles get
mixed (cut & paste operations, for example).

It will indeed - I had already mentioned the cut-and-paste case as a possible pitfall. I hadn't considered the checkerboard pattern, but I'm not totally convinced that this is a critical problem!

I would have thought you would still have to have the 2 profiles
applied though... I'm sure I just don't know how lcms works.

Yes, you need 2 profiles - the key is that you don't use profiles directly, you use transformations built from a pair of profiles.

The proposed method of using a standard working space involves three profiles (and hence two transformations linking them) - the image's own profile, the internal working space profile and the monitor profile.

The "this" was referring to a passage that you cut - I think it
is probably a bad idea to have lots of image data in different
colorspaces. I can't put my finger on why, but I just have this
feeling that we will end up with a certain amount of confusion
when it comes to colour stuff (as you pointed out, the colour
picker is a good example, so is cut & paste).

I can certainly understand your gut feeling, and agree to a certain extent. The thought of destructive modification to the source data in just loading it makes me marginally queasier though!

I agree that copy-and-paste is a major potential pitfall, and will require at least a warning on mismatched profiles, and at best an option to convert as part of the paste operation.

I'm more than willing to defer to the many experts we have,
though. I wish I knew enough about the subject to consider myself

I'm no expert either (expert: n. from "X", meaning unknown, and "Spurt", meaning a drip under pressure!), but I have used colour profiles outside of the usual PhotoShop setting, and am just anxious that any colour management abilities the GIMP should sprout will be usuable for pre-press work in my job!

All the best,
Alastair M. Robinson
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