Hi, all

I wrote up a quick spec for how GIMP should deal with color profiles associated with files and images. The spec is attached, and is also attached to bug 608961. If you have any general thoughts/comments, I would love to hear them, but please note the scope of the document.

My main questions so far:
1) What should we do when exporting to formats that don't support color profile tagging? One option is obviously to convert to sRGB. Are there people who are familiar with formats like this? What is the standard thing to do? What do people usually use these formats for?
2) What should we do when an opened image contains an invalid profile?

Obviously, options for both of these things are "prompt the user." It seems like there should be better alternatives, but I'm not sure what they might be. guiguru? others?


[1] https://bugzilla.gnome.org/show_bug.cgi?id=608961

Author:  Omari Stephens <x...@xsdg.org>
Version: 1
Date:    3 February 2010

This spec is intended to define GIMP's color profile management behavior as 
pertains to the opening and exporting of image files.  How this is achieved or 
implemented, and especially the UI and UX considerations involved, is not 
currently in the scope of this spec.

Further discussion will happen on the mailing list: 

1) When an image is opened with no associated color profile, we assume that it 
is encoded in sRGB space.  In practice, I'll say that it has an implicit sRGB 
profile.  The user will have the following options:
  a) Assert (aka "apply") some explicit color profile
  b) Leave the image without an explicit color profile
  c) Convert the image from the implicit profile to some explicit profile 
(AdobeRGB, ProPhotoRGB, sRGB, etc.)

2) When an image is opened _with_ an associated color profile, the user will 
have the following options:
  a) Leave the image with the explicit color profile
  b) Convert the image to some other explicit profile

3) When a PNG is opened that is tagged with the sRGB chunk
  a) This is as-yet undecided.  See the later discussion about sRGB chunk vs. 
sRGB profile.

4) When an image with an explicit profile is exported
  a) It will be tagged with that profile in whatever way is appropriate for the 
file format.
  b) If this is an sRGB PNG, we need to decide between an sRGB chunk and sRGB 
profile.  See later discussion.
  c) If the file format has no way to embed color profile information, (FIXME!)

5) When an image with an implicit profile is exported
  a) The image is saved with no color profile information.  For PNG, this means 
no sRGB chunk and also no iCCP chunk.

PNG: sRGB chunk vs. sRGB profile
In theory, an untagged PNG and a PNG with the sRGB chunk should be treated 
identically.  In practice, and particularly among web browsers, this is not the 
case.  See [1].  Given this context, we face the question of what to do when we 
have the choice to use either an sRGB chunk or an sRGB profile in the iCCP 
chunk.  In theory, these two choices should have the same outcome.  However, it 
is not clear whether theory matches reality in this case either; I have very 
little data to judge either way.

[1] "Making the Color Spaces Match" section of http://hsivonen.iki.fi/png-gamma/

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