Re: Support for color profiles?

[Stephane Delcroix <stephane delcroix org>]

f-spot doesn't support color profiles (yet) and no one is actively
working on this right now, but it's definitely something we're looking
for.

regards

s

On Mon, 2007-10-29 at 20:53 +0100, Christoph Moench-Tegeder wrote:
> Hi,
> 
> Is there any ongoing work for supporting color profiles in f-spot?
> I'm aware there's some code in src/Imaging/, but I already learned
> it does "not enough".
> Things I'd like to see:
> - support for embedded color (ICC) profiles as per JFIF or Exif 2.2
>   and for files using AdobeRGB as per Exif 2.21/DCF 2.0 when viewing
>   files.
> - conversion from the profiles named above to sRGB when exporting
>   (optional, so you can use sRGB for web galleries, photo sharing
>   sites etc and the native color space of the file whenever needed).
> 
> This obviously does not result in a "color managed workflow" but
> will help dealing with color profiled images.
> 
> I'm willing to help coding and testing, but I did not want to
> duplicate work already done. Can someone of the developers
> tell me which parts of the color profile code is working?
> 
> Below I have compiled some points about color profiles and
> file formats, in random order, for those who didn't already
> dive into the gory details (sorry, it's a little lengthy):
> - Do not expect to get a "color managed workflow" and such by
>   just having those profile thingies around.
> - The two most important color spaces today are sRGB and AdobeRGB.
>   The former was supposed to be a "good guess" for most monitors
>   (roughly speaking) when it was designed, the latter was designed
>   by Adobe as enhanced "working color space" for preprint stages.
>   It has a somewhat better color reproduction in cyan and green
>   areas.
> - A lot of software does not handle color profiles, but just
>   assumes sRGB on all images. This includes all major web browsers
>   except Apple Safari (and perhaps the upcoming Firefox 3)
>   and even a lot of image viewers (gqview-devel 2.1.5 does
>   support embedded profiles, but not Exif 2.21 as explained below;
>   additionally, gqview development seems dormant). Gimp 2.4
>   supports color profiles, but not Exif 2.21.
>   For this reason images to be posted on the web should be in
>   sRGB color space.
> - If files recorded in AdobeRGB are viewed as sRGB files (without
>   conversion), colors look very dull and grey.
> - Strictly speaking, there is no such thing as a "JPEG file".
>   Today, whe have JFIF (JPEG File Interchange Format) and
>   Exif (Exchangeable Image File Format). The former does always
>   and the latter does often carry their "payload" (the image)
>   as JPEG compressed data. According to the papers, they require
>   slightly different metadata handling (almost(?) every application
>   does handle both file types just fine transparently to the user).
> - if a (newer than 2003) camera records in AdobeRGB, it will
>   hopefully do so in compliance with Exif 2.21/DCF 2.0. Exif 2.21
>   is an update to the widely known and used Exif 2.2 standard.
>   The most important part of the update (at least for this case)
>   is the support for color spaces besides sRGB and "uncalibrated".
>   So, if the Exif version of an photo file is 2.21 (0x0221 in tag
>   0x9000 ExifVersion in APP1), it may have an color space of
>   "uncalibrated" (0xFFFF in tag 0xA001 ColorSpace in APP1 (Exif))
>   and the string "R03" in Exif.Iop.InteropIndex (that's tag 0x001
>   in the APP1 section), which marks it as a "DCF option file" with
>   AdobeRGB color space. An sRGB file has 0x0001 in ColorSpace and
>   "R98" in InteroperabilityIndex) and is called a "DCF normal
>   file". Also, a "DCF option file" is supposed to have an underscore
>   as the first character of it's file name (for example "_mg_1234.jpg"
>   when using Canon gear).
>   The string "R03" is not defined in Exif 2.2 but only in Exif 2.21.
> - Some software indicated the use of AdobeRGB on Exif 2.2 files by
>   putting 0x0002 into Exif.Photo.ColorSpace. This is not covered by
>   the standard, and (hopefully) not to common.
>   The "right way" to indicate color profiles besides sRGB in Exif 2.2
>   is to set Exif.Photo.ColorSpace to "uncalibrated" and supply the
>   profile in Exif.Photo.ICC_Profile (the profile itself is stored in
>   an APP2 section).
> - Files created by other software (Gimp, Photoshop) will be in JFIF
>   format can carry an embedded profile in an APP2 section, but there
>   are no "real Exif tags".
> - For correctly interpreting "DCF option files" with AdobeRGB color
>   space one needs to have the AdobeRGB profile around. You can download
>   these at http://www.adobe.com/digitalimag/adobergb.html but I think
>   their license is too tight for including the profile in open source
>   software. Alternatives are using a homebrewed profile "compatible
>   with Adobe RGB (1998)" (I think I've seen one in src/Imaging/Tiff.cs)
>   or instructing the user to download a profile on his own. There
>   have been suggestions from the OpenICC project to put color
>   profiles in /usr/share/color/icc/, /usr/local/share/color/icc/ or
>   ~/.color/icc/.
> - Some reading:
>   the exiftool documentation http://www.sno.phy.queensu.ca/~phil/exiftool/
>   the Exif 2.2 standard http://www.exif.org/specifications.html
>   a tutorial on color management basics
>     http://www.cambridgeincolour.com/tutorials/color-management1.htm
>   the most comprehensive freely available official source on
>     Exif 2.21/DCF 2.0 (no joking here)
>     http://www.cipa.jp/exifprint/contents_e/01exif2_2_e.html
> 
> Regards
> Christoph
>