[argyllcms] Re: Can anyone help with a weird OSX problem?

  • From: Florian Höch <lists+argyllcms@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 25 Jul 2014 20:13:47 +0200

Am 11.07.2014 02:52, schrieb Quartz:
>> Another problem was that the MBPs of that generation had screen
>> backlights with primaries that were some distance from the sRGB
>> primaries ...
>  >[-]
>> one problem with the MacBook Pro display (as with a lot of
>> Notebook/Laptop displays) is that it has a fairly small gamut
>> (considerably smaller than sRGB).
> 
> I have a feeling that this is part of the problem. From other threads 
> I've been reading and from my own research, the screen's gamut is 
> generally smaller than sRGB, with the exception of the blue point, which 
> sticks out off to the side somewhat. However it's offset *away* from the 
> violet end of the spectrum (that is to say, it's somewhat more "blue" 
> than sRGB "blue").
> 
> For reference, here's a simple example of where I see the problem:
> In Photoshop I set my working space to my monitor's profile and create a 
> new unmanaged document, which I fill with blue (0,0,255). If I use the 
> DigitalColor Meter app to inspect the pixels in Photoshop's window, it 
> reports 0,0,255 as expected. However, if I set my working space to any 
> other color space (with the exception of ProPhoto RGB), and likewise 
> fill a new managed document with blue, DigitalColor Meter reports 96,0,255.

Makes sense. If your display profile says your blue primary is quite
cool (more towards green in the spectrum than towards violet), then red
needs to be added to approach a more saturated blue. Out of curiosity,
can you send me the display profile? (or alternatively upload it
somewhere and post a link)

> What's weird is that the color shift issue doesn't seem to affect OSs 
> running on the same hardware. It's like there's some sort of mapping 
> error within the OSX 10.6 colorsync subsystem or something.

Are you talking about non-Mac OS X systems? Only Mac OS X has full
desktop color management, so on those other systems you'll only observe
the problem if you install the same display profile and use a color
managed application (like Photoshop).

>> Another issue (although I'm not sure how much it contributes to the
>> problem) is that the old i1 Display2 is probably not very well suited
>> for a white LED backlit display (the i1 Display 2 originates from an era
>> when there were only CCFL backlights), at least without a correction matrix.
> 
> I'm not terribly familiar at all with how colorimeters work under the 
> hood, but doesn't it just average the spectrum frequency and pick the 
> closest K value?

All colorimeters use filters that are modeled after the CIE standard
observer spectral curves and with which they convert the measured light
to XYZ, but most of them aren't a good fit, and the filters also age
(deteriorate) over time.
This means there should ideally some sort of correction (usually in form
of a 3x3 matrix) be used for the type of screen you're measuring. The
older colorimeters all worked fairly well on CCFL backlit LCD displays
and CRTs, and had corrections for these types of screens built in.
Using one of these older colorimeters on a LED backlit screen will
probably not be very accurate if there's no correction used for that
type of screen.

> I know "white" LEDs have a sort of weird violet/yellow 
> response curve sometimes, but I wouldn't think that would affect things 
> much. Is there any way I can find or create such a matrix?

"White" LEDs are actually blue with a yellow coating. Their spectral
response does usually emphasize the blue part of the spectrum and has a
red/green deficiency.

> Basically, I'm trying to figure out what my next step should be here. I 
> don't want to drop several hundred dollars on a new sensor only to find 
> out that it's a software issue. I don't really want to upgrade the whole 
> OS if I don't have to either (lots of custom software that doesn't work 
> well on newer versions). I don't want to do either and then realize the 
> problem is me doing something dumb because I don't understand as much as 
> I think I do.

I don't think it's a software issue. The 2007 MacBook's small gamut is a
problem though (this is not uncommon of Laptop/Notebook displays in
general unfortunately).

One thing you could try is obtaining or creating a colorimeter
correction for your i1 Display 2 and MacBook screen and see if that
improves anything, but this would require you to lend a spectrometer.

-- 
Florian Höch


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