Re: Color management in ubuntu

[Pascal de Bruijn <pmjdebruijn pcode nl>]

On Thu, Jul 1, 2010 at 6:18 PM, Marek Matulka <marek matulka gmail com> wrote:
> Hello,
>
> This is my first post on the group, but I've been following a group for
> some time. Well done guys for a rocking bit of software giving us a
> great colour management tool!
>
> On Thu, 2010-07-01 at 17:58 +0200, Pascal de Bruijn wrote:
>> > So, to test how color management works in a browser, I found this web
>> > site: http://www.gballard.net/psd/go_live_page_profile/embeddedJPEGprofiles.html.
>> >  Using google chrome (a non color managed browser) this page is a mess of
>> > incorrectly rendered photo's - I expected this.  Using Firefox, things are a
>> > lot better - imbedded color profiles are now honored and the photos display
>> > fine. However, about half-way down the page at the heading "sRGB / Standard
>> > RGB 2.2 gamma" I've come across a problem.  This photo is a tagged/untagged
>> > rollover of an sRGB image.  It says that "If your monitor is profiled to 2.2
>> > gamma and D65- 6500 kelvin, there should be minimum change in the Un-tagged
>> > sRGB rollover."  In my Firefox window, the change is not minor, and I'm
>> > trying to figure out why this is.
>> > In my workflow all exif data is stripped from my photo's before I upload to
>> > the web.  I thought that this wouldn't matter, as long as my final image was
>> > created in an sRGB colorspace.  However, I have now found that my photo's
>> > look subtly but definitely different in the software I use (gimp, digikam)
>> > and my browsers.
>> > My limited understanding of color management led me to believe that
>> > un-tagged sRGB images should look the same in a browser on a color-managed
>> > system.  However, on my system they don't and its causing a problem for me -
>> > my photo's don't display the way I intended.  How can I resolve this?  Is my
>> > understanding wrong, should I educate myself a bit more?  Did I do something
>> > wrong when I calibrated my monitor?
>>
>> The problem is that being color managed, and being properly configured
>> is not always the same...
>>
>> All color managed applications checked for an embedded profile when
>> loading images and will for example convert an Adobe RGB image to sRGB
>> on request. If an image is untagged sRGB will be assumed.
>>
>> However, if you do not properly configure the application to use your
>> display profile it will be displayed as plain srgb without your
>> display profile applied, hence wrong colors (since only the videolut
>> will have effect).
>>
>> I think GIMP doesn't use the system display profile by default.
>>
>> Firefox also requires the display profile to be configured through
>> "about:config" search for "display_profile".
>>
>> Same goes for lots of other applications. So you will still need to
>> check each an every applications configuration. Since they might use
>> poor defaults.
>
> I had similar issue - when I created three images: 1. tagged with Adobe
> RGB, 2. tagged with sRBG, 3. untagged - all three images were correctly
> displayed in GIMP, but not in any desktop viewer.
>
> I think the problem lies in assumption, that for untagged images viewer
> should not apply monitor profile, while it should assume sRGB and
> convert it accordingly. It seems GIMP is working that way hence all
> three images are correctly displayed.

The tagging of an image has _NOTHING_ to do with applying a display
profile or not... If this really is the case, then someone should
really be shot for that (not dead... the knees will do just fine :).

The application of the display profile (should) only depend on the
application settings not on the input image...

All programs assume untagged images are sRGB, (even though tagging
images with an sRGB profile _is_ a good idea, this will prevent future
ambiguity).

Programs which aren't color managed (or where color management is
disabled) are just RGBin (from file) == RGBout (to display)

That said... Please also note that not all forms of sRGB are equal,
there are several version of the sRGB profile that might differ a
bit... I think at one time there even was a simplified sRGB with a
real 2.2 gamma curve... which is wrong, since sRGB is gamma 2.4 with a
small dead area in the blacks.

Regards,
Pascal de Bruijn