On Tue, Feb 5, 2013 at 12:05 AM, Richard Levitte <[email protected]> wrote: > In message > <canqv4twj9bt4zuqwjjyqka42bhtmcjazysgvrjtct5ba01v...@mail.gmail.com> on Mon, > 4 Feb 2013 22:04:21 +0100, Pascal de Bruijn <[email protected]> said: > > pmjdebruijn> On Mon, Feb 4, 2013 at 9:46 PM, Richard Levitte > <[email protected]> wrote: > pmjdebruijn> > In message > <CANqV4tVGpe0zqg4K1=shvvztah1z7sq4qhsoe7x1q6f3iqm...@mail.gmail.com> on Mon, > 4 Feb 2013 19:19:45 +0100, Pascal de Bruijn <[email protected]> said: > pmjdebruijn> > > pmjdebruijn> > pmjdebruijn> Double correction cannot happen (well unless you > have a really highend > pmjdebruijn> > pmjdebruijn> LaCie/EIZO display which can do hardware > correction or something). > pmjdebruijn> > > pmjdebruijn> > Ok, in that case I'd like an explanation to what's happening > to me. > pmjdebruijn> > This all started with me color characterising my monitor > (laptop LCD) > pmjdebruijn> > and creating a profile for it (using dispcalGUI), then loading > it > pmjdebruijn> > (using 'dispwin -L'). Display colors became much better (grey > is > pmjdebruijn> > actually grey, not something with a blue tint). > pmjdebruijn> > > pmjdebruijn> > Then I started doing some work on some images I had taken just > pmjdebruijn> > recently, and what I got was absolutely h-o-r-r-i-b-l-e. > Basically, > pmjdebruijn> > the three channels were kind of shifted in the histogram, and > what was > pmjdebruijn> > supposed to be a black background was now redish brown or some > such. > pmjdebruijn> > (I wonder if I could possibly produce a test, say by putting > together > pmjdebruijn> > a white .png, import it and see how it ends up). > pmjdebruijn> > Changing my display profile from "system display" to "sRGB" > gave me a > pmjdebruijn> > better look. > pmjdebruijn> > pmjdebruijn> Sounds like an invalid profile? > pmjdebruijn> > pmjdebruijn> Did you use GNOME Color Manager to generate it? > > Nope, I used dispcalGUI (a ArgyllCMS tool, and as I understand it, GNOME > Color Manager uses ArgyllCMS under the hood as well). > [note: I wrote this above, as well as how I load the profile]
I guessed as much. There is a reason why I recommend GNOME Color Manager for new users, as dispcalGUI leaves a lot of room for users to shoot themselves in the foot. I've invested some time in trying to get GCM to use to most robust defaults (at the cost of potential accuracy, so the worst case should be a little better and the best case should be a little worse). If you insist on continuing to use dispcalGUI try making a Single Gamma (curve) + matrix profile, with temperature set to 6500K and gamma 2.2 or 2.4 (which gamma is best is a difficult discussion, 2.2 seems standard in commercial software, while 2.4 seems technically more accurate). > pmjdebruijn> Which type of colorimeter did you use? > > ColorHug. > > pmjdebruijn> Do keep in mind that low end devices tend to be prone to > pmjdebruijn> get invalid reading on some display types, particular > pmjdebruijn> with newer LED displays. > > Mine is a standard backlit LCD display. > [note: I wrote this above] Right. Well, you may want to investigate if there's a CCMX for your particular display. But this is better discussed on the ColorHug mailing list. > pmjdebruijn> > pmjdebruijn> I have further details on anatomy of display > profiles here: > pmjdebruijn> > pmjdebruijn> > pmjdebruijn> > pmjdebruijn> > http://blog.pcode.nl/2012/01/29/color-management-on-linux/ > pmjdebruijn> > pmjdebruijn> > pmjdebruijn> > pmjdebruijn> If you still have any questions left, feel free > to give me a bump. > pmjdebruijn> > > pmjdebruijn> > Yup, I've read it and it makes a lot of sense. What doesn't > make > pmjdebruijn> > sense to me is why there's a need for a program like Gimp and > pmjdebruijn> > darktable to use the display profile to correct colors that are > pmjdebruijn> > corrected through the VLUT (by the X11 driver, I assume)... > pmjdebruijn> > pmjdebruijn> It's not the same thing. Think of cooking, if you make something > that > pmjdebruijn> needs to be more salty and sweet, only adding the salt won't do > the > pmjdebruijn> trick. You need to add salt _and_ sugar. (sorry for the horrible > pmjdebruijn> analogy :) > > Right, and that analogy doesn't explain anything to me, really. > > pmjdebruijn> The VCGT corrects for whitepoint and gamma. > pmjdebruijn> > pmjdebruijn> The matrix characterizes the displays gamut (which possibly means > pmjdebruijn> shifting hue's and scaling saturation). And mapping input gamuts > to > pmjdebruijn> display gamuts can be a dynamic process when using the (for > example) > pmjdebruijn> commonly used perceptual rendering intent. And the VLUT is > inherently > pmjdebruijn> static. > > Right. I've assume that the characterisation I've done, and loading > the profile I created is designed to make sure sRGB is rendered as > correctly as possible, is that assumption close to correct? Nope. Not at all. Your display profile has no relation to sRGB at all whatsoever. Your display profile describes the gamut of your display, so the application (ie Darktable) knows exactly (in absolute terms) how red your displays red is, and how green your displays green is, and how blue... A profile attached to an image (like sRGB) gives color encoded in a RGB form absolute meaning. So if we know what exact color is encoded into a file, and we have a display profile we can translate that into an exact color on the screen (assuming your display's gamut reaches that far, and if it does not rendering intents come into play). (to connect an input RGB data (sRGB) to output RGB data (your display profile), color management systems use a Profile Connection Space, which is usually XYZ or Lab, which define color in more absolute ways) > Furthermore, I assume that the display profile should make sure I see > what I will get when exporting the picture. Is that close to correct? Assuming the image viewer you use to view the exported image is color managed too (for example EOG), yes. Regards, Pascal de Bruijn ------------------------------------------------------------------------------ Free Next-Gen Firewall Hardware Offer Buy your Sophos next-gen firewall before the end March 2013 and get the hardware for free! Learn more. http://p.sf.net/sfu/sophos-d2d-feb _______________________________________________ Darktable-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/darktable-users
